Sweet spot, banding and tilted etalons ...where are the limits ?

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Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

Hi,
**************************************************************************************************************************************
To the moderators, I am not quite sure to what section this post belongs to, so feel free to move it in the correct one.
******************************************************************************************************************************************

As you might know, most front side etalons are to be tilted in order to tune them on the Ha line. With some etalons, the tilt is small (a few 0.1°), with others the tilt is "larger" (from 0.6° to more than 1°).
At last Zoom meeting, Bob Y rose the issue of "banding" formation when the tilt of the etalon gets large. Let's have a look at it.

First, let's remember the definition of the sweet spot : this is the area projected on the sky where the center wavelength of the etalon is equal to Ha +/- a given tolerance. Let's take a tolerance of 0.25 A.

If the etalon center wavelength is precisely at Ha at normal incidence, the"sweet spot" is a circle of 1° diameter. This is good news since the solar disk is only 0.5°.
http://astrosurf.com/viladrich/astro/in ... t/SS-2.png

Now, let's assume the etalon is 0.6A from Ha at normal incidence (this was the case of the SM III 60 I tested recently). Then the "sweet spot" is no longer a spot but a "sweet ring". Tilting the etalon on Ha moves part of the ring on the solar disk. In other words, the part of the Sun tuned on Ha is a "band"(so the term "banding")
http://astrosurf.com/viladrich/astro/in ... t/SS-4.png

The bad news is that the width of this band is only 0.33°, which is smaller than solar diameter ....

Let's assume a more extreme situation with an etalon 1.25 A away from Ha (this was the case of the SM III 60 RichView I tested). Then, the width of the "sweet ring" is only 0.22° (smaller than the solar radius). This is not exactly good news ...
http://astrosurf.com/viladrich/astro/in ... t/SS-5.png

Indeed, as soon as the center wavelength of the etalon at normal incidence is farther than 0.3 A from Ha, the width of the "sweet ring" is smaller than the solar diameter.

At this end of the day, it is much better to have etalon very close to Ha at normal incidence. But ... this is more difficult and more expensive to do.
http://astrosurf.com/viladrich/astro/in ... t/SS-6.png

The same holds (with different figures) for narrowband filters (1 to 2 A FWHM Ha, Ca K or H filters).

The whole page is there :
http://astrosurf.com/viladrich/astro/in ... -spot.html

Christian


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

Great information Christian, thanks!

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by marktownley »

Thanks Christian! Very well put JAPP.


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

justapictureposter wrote: Fri Feb 26, 2021 8:05 pm Very well reported Christian, and this is unfortunately the humpty dumpty story of solar filters!

companies will not throw underperforming glass filters in the trash unless there is a severe defect; companies will attempt salvage of the rejects by allowing an extended acceptable tolerance range.

This is why all of our filters are different, it is impossible to conform to such strict standards as designated by these requirements. IT is much easy to increase the acceptable tolerance.


with such extreme circumstances, a company must produce 9 bad filters so the 10th can be good. The 9 bad filters will be sold to a group of unsuspecting individual's (US) ; when the users realize their filter is not performing as it should; the warranty/return period has expired.

:( Sadface :(
I am afraid you are probably right :-(


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by krakatoa1883 »

Very informative reading, Christian. Btw something like the sweet ring you described can be observed in some pressure-tuned etalon, may be worth of investigation.


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

Some questions, if they may be entertained:

1) What's the ideal way to get a larger sweet spot on a rear-mounted etalon (or at the very least, a sweet spot large enough for a narrow FOV to not see the non-sweet spot)?

2) Is it possible to get a full disc inside of the sweet spot in Lunt's 60mm modular HA scope's etalon?

3) What would happen if one were to mount the etalon from a pressure tuned Lunt 60mm modular scope on the back of a longer scope in terms of how the sweet spot would change?

4) Is there any way to improve the sweet spot or preserve the sweet spot of a PST etalon?

From what I'm reading, it looks like the best way is essentially to get an etalon that is as close to on-band with the least amount of tilt needed to get there in the first place (which is near impossible for most of us), or basically a perfectly even pressure tuned etalon with the largest aperture (also near impossible to find).

It seems the legendary gear is already out there, and no new legendary class gear is being made, with respect to etalons?

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

justapictureposter wrote: Sat Feb 27, 2021 1:59 pm
MalVeauX wrote: Sat Feb 27, 2021 1:03 pm Some questions, if they may be entertained:

1) What's the ideal way to get a larger sweet spot on a rear-mounted etalon (or at the very least, a sweet spot large enough for a narrow FOV to not see the non-sweet spot)?
I successfully enlarged the sweet spot on my pst etalon; simply by removing the collimator and placing a much larger one deeper into the optical tube.

On my 127mm x 1200mm telescope i had installed an 80mm x 800mm plano concave; minus 800mm into the ota.

Next i took the original negative planoconcave lens out of the pst etalon, and left the face of the etalon bare.

. After further investigation, I had learned that the system was no longer using the full 127mm aperture; because the parallel beam output from the 80mm lens was 80mm in diameter; and the face of the etalon is only 21mm. The system loses input light by the overlap of the beam around the etalon. See deiagram.

To further on this concept, we may be able to squeeze the 80mm output down using mulitple lens elements.; I.e -800 to 800 , -400 to -400, -200 to 200. This would be a very complex beam expander; operating in reverse; however I do not know about optics to understand if the sweet spot would shrink to original size on the stock PST.

So in theory, yes it is possible to expand the sweet spot by enlarging the circumference of the solar image with a sacrifice of light; and further investigation is needed to determine if advanced optics will allow full objective power to be obtained.

This is the real world result, the sweet spot is no longer visible across the entire full disc;
I would be very interested to try this on either my 80mm F7.5 (masked to 60mm F10, or whatever is needed) or my 120mm F8 (I usually mask to 100mm F10 for this, or whatever is needed). Do you recall where you got the different lenses? Even just getting a little more FOV within the sweetspot would be a huge help.

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by p_zetner »

Hi Christian.

I'm assuming banding wouldn't be an issue for etalons that are tuned slightly to the red for normal incidence? Here is a series of interferograms taken in H alpha (discharge tube + 10nm fwhm interference filter) for the Lunt LS50FHa etalon.
Montage_131626-742-821_levels.jpg
Montage_131626-742-821_levels.jpg (61.93 KiB) Viewed 7658 times
From left to right: Etalon tilt adjustment wheel is turned completely CCW (left image), adjusted to "maximize" central spot (centre image) and completely CW (right image) with wheel rotation direction as viewed from the etalon back end. I didn't measure the etalon tilt by any independent means although this could be done optically or via a precision tilter like that used in the Cyril Bazin and Serge Koutchmy paper. The central spot in the centre image has a 1.5 degree fwhm which seems a bit high but is very advantageous if true!

It's apparent from the figure that the fringe pattern shifts in the field of view with changing etalon tilt. In the centre image there is a 0.6 degree offset between the centre of the fringe pattern and the centre of the fov. The left hand image gives a 2.5 degree offset! These interferograms were taken with an Edmund Optics 1" interference filter which constricted the field of view but allowed me to roughly measure the centre of the fov by using the "Measure" function in ImageJ to return centre coordinates for a circular selection, as shown in this figure:
131626 fov coords siz.jpg
131626 fov coords siz.jpg (37.09 KiB) Viewed 7658 times
I'm guessing the etalon plates are slightly wedge shaped in order to account for this shift?

Cheers.
Peter


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by marktownley »

Interesting discussion, i've put in the mods section, not because was in the wrong place, but will get lost very quickly on the main board many posts down.


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

Any thoughts on the Lunt 60mm modular pressure tuned etalon they have available now? It's on an F7 setup. Thumb screws hold it on the back of the OTA. Should be fairly easy to configure to mount on the back of any OTA. I'd like to see if this is possible to mount on the back of big refractors (with proper DERF of course). But I'm curious, how will the sweet spot be on such an etalon? Is this worth exploring? Better than rear mounted PST etalon? Same? The main goal is getting a bigger sweet spot.

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by marktownley »

MalVeauX wrote: Sun Feb 28, 2021 3:53 pm Any thoughts on the Lunt 60mm modular pressure tuned etalon they have available now? It's on an F7 setup. Thumb screws hold it on the back of the OTA. Should be fairly easy to configure to mount on the back of any OTA. I'd like to see if this is possible to mount on the back of big refractors (with proper DERF of course). But I'm curious, how will the sweet spot be on such an etalon? Is this worth exploring? Better than rear mounted PST etalon? Same? The main goal is getting a bigger sweet spot.

Very best,
I bet the free aperture of the etalon is less than you would think. Hopefully somebody will measure one.


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

marktownley wrote: Sun Feb 28, 2021 6:46 pm
MalVeauX wrote: Sun Feb 28, 2021 3:53 pm Any thoughts on the Lunt 60mm modular pressure tuned etalon they have available now? It's on an F7 setup. Thumb screws hold it on the back of the OTA. Should be fairly easy to configure to mount on the back of any OTA. I'd like to see if this is possible to mount on the back of big refractors (with proper DERF of course). But I'm curious, how will the sweet spot be on such an etalon? Is this worth exploring? Better than rear mounted PST etalon? Same? The main goal is getting a bigger sweet spot.

Very best,
I bet the free aperture of the etalon is less than you would think. Hopefully somebody will measure one.
Agreed, I'm sure its not 60mm. It's internal. So maybe 18 to 26mm? But my question is, what sweet spot could be figured from this if setup on an F7 instrument such as up to a 150mm F7?

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by marktownley »

MalVeauX wrote: Sun Feb 28, 2021 7:36 pm
Agreed, I'm sure its not 60mm. It's internal. So maybe 18 to 26mm? But my question is, what sweet spot could be figured from this if setup on an F7 instrument such as up to a 150mm F7?

Very best,
Yes, it's all just scaling


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

marktownley wrote: Sun Feb 28, 2021 7:42 pm
MalVeauX wrote: Sun Feb 28, 2021 7:36 pm
Agreed, I'm sure its not 60mm. It's internal. So maybe 18 to 26mm? But my question is, what sweet spot could be figured from this if setup on an F7 instrument such as up to a 150mm F7?

Very best,
Yes, it's all just scaling
I'm trying to see if the sweet spot would be larger potentially than that from the PST etalon in a similar configuration?

I currently use the PST etalon but I lose a ton of FOV due to sweet spot, on all configurations. I'd like to expand the sweet spot somehow (if possible). Or just go for a different rear mounted option with a bigger sweet spot (that is NOT a powered option that needs F30+ like Daystar/SolarSpec/SolarScope. So just exploring internal Lunt pressure tuned etalon options.

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

Hello Marty,
There is a simple rule for internal etalon in collimated beam :
- in an "optimal" system, the sweet spot diameter is proportional to the ratio d/D, where d is aperture of the etalon (and collimating lens) and D the aperture of the refractor.
Accordingly, the maximum diameter is acheived when d= D, i.e. when you have the etalon in front position.
Hope this helps


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

christian viladrich wrote: Sun Feb 28, 2021 8:58 pm Hello Marty,
There is a simple rule for internal etalon in collimated beam :
- in an "optimal" system, the sweet spot diameter is proportional to the ratio d/D, where d is aperture of the etalon (and collimating lens) and D the aperture of the refractor.
Accordingly, the maximum diameter is acheived when d= D, i.e. when you have the etalon in front position.
Hope this helps
Yikes...

So, a PST etalon at aprox. 20mm clear aperture, without any refractor or anything (just the opening into the etalon and the rear lens) would produce an equal d/D ratio of 1 for sweet spot size relative to the etalon. And a refractor objecitve of 40mm means the PST was at a d/D of 0.5. And so putting it into a 120mm F10 configuration, its only 0.16? And on my C8 Edge, only 0.1? That's why the sweet spot is so tiny on my C8 configuration with PST etalon then!

So to compare, the Lunt 60mm internal pressure tuned etalon (I don't know the size, let's say its somwhere between 18~26mm?) operating around F7 with a clear aperture collimating lens that is the same size or slightly larger to accept the light cone to it, means that a little 60mm aperture would be from 0.3 to 0.43 d/D sweet spot size ratio (smaller than the original PST?). And if placing it into a larger instrument, say 120mm, it would be anywhere from 0.15 to 0.22 basically? So potentially smaller, to potentially a little larger than the PST etalon in the same configuration?

Does that sound about right?

If so, then the idea is to get the biggest clear aperture etalon with biggest collimating lens possible for this, to ensure the largest possible d/D ratio, 100% limited by the aperture of the etalon.

I need to figure how to calc how that d/D ratio sweet spot then can be related to focal length and disc image size to see if I can see the optimal configuration to generate a full disc image within the sweet spot size of the etalon's configuration with the OTA?

Thanks!

++++++++++++++++
++++++++++++++++

Illustrating real world:

100mm F10 refractor (aperture 100mm); focal length 1,000mm; disc image size 9.09 to 10mm in size
PST etalon with F10 collimating lenses (aperture 20mm)
10mm blocking filter
d/D ratio is 0.2
sensor is 1.1" IMX253

Looking at the preview here, it looks like the sweet spot is around 1/5th the size of the solar disc in this case. But I'm not sure how to translate this to disc image size in mm or how to relate it so I can reverse calc what is needed to achieve a full disc in the sweet spot of a 20mm to 26mm etalon aperture?

FC_Preview_100mm_PSTetalon_IMX253.jpg
FC_Preview_100mm_PSTetalon_IMX253.jpg (100.61 KiB) Viewed 7597 times

And here:

60mm F10 refractor (aperture 60mm); focal length 600mm; disc image 5.45mm to 6mm in size
PST etalon with F10 collimating lenses (aperture 20mm) trailing a SM60mm II series DS etalon (front mount)
10mm blocking filter
d/D is 0.33
sensor is 1.1" IMX253

Here, the sweet spot seems to be around 2/3rds of the solar disc, you can see it obviously falls off, it wasn't exactly center, likely due to the minor tilt or bandpass of the SM60II DS etalon on front, but was just to see.

PST_600mm_Test.jpg
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Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by marktownley »

MalVeauX wrote: Sun Feb 28, 2021 9:12 pm
Does that sound about right?
Yup!


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

MalVeauX wrote: Sun Feb 28, 2021 9:12 pm Does that sound about right?
Yes indeed !

As an example, you might remember Antonello Satta's refractor (not sure of his pseudo here). He made a 150 mm f/10 refractor with an internal Lunt 60 mm etalon. It was a kind of truss tube OTA.
This is also the principle at work in the 140/90 mm famous Ha refrator of Bob Y, with a 90 mm internal etalon.
For a given aperture, the larger the internal etalon the larger the sweet spot is.


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

BTW, there is also a possible combinaison with a double stack Lunt 80 mm internal etalon. I don't remember what is the actual diameter of this etalon.


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

christian viladrich wrote: Mon Mar 01, 2021 8:52 am
MalVeauX wrote: Sun Feb 28, 2021 9:12 pm Does that sound about right?
Yes indeed !

As an example, you might remember Antonello Satta's refractor (not sure of his pseudo here). He made a 150 mm f/10 refractor with an internal Lunt 60 mm etalon. It was a kind of truss tube OTA.
This is also the principle at work in the 140/90 mm famous Ha refrator of Bob Y, with a 90 mm internal etalon.
For a given aperture, the larger the internal etalon the larger the sweet spot is.
Interesting thanks; so realistically the 35mm, 40mm, 50mm, 60mm front mounted etalons are going to be larger etalons and if they were mounted collimated rear-mounted they would provide a much larger sweet spot then since they are all going to be larger than the internal designed etalons.

I'm curious now what the internal clear aperture is of the 60mm pressure tuned, and really interested then in how large the 80mm internal double stack by Lunt is. If it's even bigger or not.

I'm very interested to learn more about the 150mm truss frac with an internal 60mm Lunt etalon.

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

justapictureposter wrote: Mon Mar 01, 2021 2:08 pm ;i made the LS50C modkits, with the intention of doubling the size of the etalon for least cost. The closer you get to the limb it goes off band because the solar image size on the etalon requires an -800mm collimator or greater. SO i then developed the -800mm collimator.

The deeper you get into the tube, you require a larger diameter collimator. an 80mm collimator requires an 80mm diameter etalon or else you vignette the aperture by light not entering the etalon.

I advise against wasting your money on a mod for a full disk, they are only good for high resolution high magnfication.

Too many sacrifices to make it viable over what lunt already sells as a complete optimized telescope, intended for full disk out of the box.

Get the lunt 40mm.
Interesting thanks!

If we put a full disc image aside, and just consider it for a rear mounted application for moderate to high resolution use on larger apertures and longer focal lengths, getting into 0.5" to 0.3" per pixel scales basically, I'm curious how the sweet spot size would be with these options (larger since the etalon is larger) and how simple it would be to mount and collimate it. The 40mm and 50mm etalons are pretty affordable for this. My PST etalon works already, I'm just looking to increase the sweet spot size, which may simply call for a new etalon to do it.

It would be great if a modular etalon like the new Lunt 60mm modular one could work, but its a smaller etalon so it probably will not be a bigger sweet spot, at least, not significantly so (I don't know its clear aperture).

With regards to the full disc, mainly I would like to be able to use an etalon that is modular like this with a small short scope for full disc within the sweet spot (like a 400mm to 600mm focal length) and then plug it in behind a bigger scope for high res with small FOV and a smaller sweet spot, but hopefully a bigger sweet spot than the PST provides, so I need a bigger etalon to do that.

But a 40mm~50mm aperture etalon would greatly increase my sweet spot. Even if I had to get a separate one, I wouldn't mind.

Anything to avoid getting an electric powered F30+ Daystar or similar option again; I'd like to avoid this. I sold my Quark after my PST etalon was working as my PST etalon is better than my Quark in all respects except the sweet spot size.

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by marktownley »

Hi Marty,

My mindset went this way a few years ago, yes, use the larger etalons to increase the sweet spot, the problem you will have is finding commercially available collimating lenses, hence JAPP producing his own 80/-800 lens.

I went the route of using my good Lunt50mm etalon on a 60mm/f6 frac with a Quark behind - the Quark see's an f7.2 beam before the 4.3x takes the beam through the >f30, I then use the 0.7x baader telecompressor which fits it onto the camera chip. This has given me the most even disk. This one from sunday, open in a new window for full size. As is from camera, just ImPPG.
Ha-FD-DS50.jpg
Ha-FD-DS50.jpg (2.38 MiB) Viewed 7544 times
Naturally, which ever way you go relies on getting good etalons in the first place, be they external. internal or rear mounted.

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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

marktownley wrote: Mon Mar 01, 2021 4:06 pm Hi Marty,

My mindset went this way a few years ago, yes, use the larger etalons to increase the sweet spot, the problem you will have is finding commercially available collimating lenses, hence JAPP producing his own 80/-800 lens.

I went the route of using my good Lunt50mm etalon on a 60mm/f6 frac with a Quark behind - the Quark see's an f7.2 beam before the 4.3x takes the beam through the >f30, I then use the 0.7x baader telecompressor which fits it onto the camera chip. This has given me the most even disk. This one from sunday, open in a new window for full size. As is from camera, just ImPPG.

Naturally, which ever way you go relies on getting good etalons in the first place, be they external. internal or rear mounted.

Mark
Thanks Mark,

Interesting setup. A lunt 50 on a 60mm frac with a Quark. I'm surprised that produced a full disc. 1548 effective focal length before the blocking filter, disc size is pretty big, it passed the 12mm blocking filter of the Quark and then was reduced by the 0.7x reducer? I did full discs with Quarks a bit in the past, was able to do it with a 400mm focal length base scope, and a reducer spaced away from the sensor, but it definitely didn't look like that which you posted, it was tiny and had bad aberrations from the reducer and was really low res. I'd like to avoid the Quark if I can though at this point.

I'm curious what the internal etalon size is of the 50mm and 60mm etalons (internals; pressure tuned ones).

Ultimately I would be fine with the resolution and disc size produced from a typical 60mm solar scope like a Lunt 60. I'm just seeing if it's possible to use that etalon rear mounted on another scope and if it would have a bigger sweet spot than the PST etalon, on the larger aperture scopes I use since the PST etalon's sweet spot is so small. So if it were 26~30+mm in size that would be great, but it likely is not, so I'm curious just how much bigger, if it would be worth it. The idea being to use it on its 60mm scope and do a full disc with that, stock, and remove it and put it on the back of a bigger scope for high res.

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by Bob Yoesle »

Thanks Christian and Peter!
At this end of the day, it is much better to have etalon very close to Ha at normal incidence. But ... this is more difficult and more expensive to do.
You've confirmed what I have observed - namely that tilt-tuned etalon performance suffers if their CWL is too high and requires excessive tilt. And the production realities of keeping the spacer thickness within a very narrow range of tolerance to keep CWL close to the H alpha emission is more difficult to attain and therefore expensive. This explains the general better performance of pre-Meade Coronado etalons, and their and Lunt's higher relative cost. Air pressure tuned etalons are more immune to spacer thickness induced variations in CWL due to the uniform air density/refractive index tuning. Unfortunately these do not exist for front etalon filters.
If so, then the idea is to get the biggest clear aperture etalon with biggest collimating lens possible for this, to ensure the largest possible d/D ratio, 100% limited by the aperture of the etalon.
I just want to note that the objective & etalon diameters are proportional for the sweet spot size due to geometry, but may be imprecise. The actual sweet spot optical size is defined via field angle magnification, which is solely defined via the focal lengths, and the acceptance angle of the etalon.

The field angle at the Sun's limb is ~ 0.25 degree. This is what a front mounted etalon sees, and why it will generally offer the largest "sweet spot" of on-band performance (given the previous constraints noted for CWL).

For an internal etalon, if one uses an objective with a 1000 mm FL, and the collimator lens (such as the PSTs) has a FL of 200 mm, the field angle magnification will be 1000 / 200 = 5, and the limb of the Sun will have a field angle of 5 x 0.25 = 1.25 degrees. If a 0.7 angstrom air-spaced etalon has an acceptance angle of 0.5 degree to be on band, we can see that the extent of on band performance will be smaller - like looking through the narrow tube. A narrower bandpass etalon will have a smaller acceptance angle, and the sweet spot will therefore be even smaller.

If we use a collimator lens with a longer focal length (which also requires a proportionally larger etalon to prevent vignetting) we get better performance: Using a collimator with a 500 mm FL results in a magnification of 1000 / 500 = 2, and a field angle of the Sun's limb is now 2 x 0.25 = 0.5 degree, and most if not all of the disk will be within the sweet spot of the etalons acceptance angle.

The issue of lack of large negative COTS lenses can be overcome by using positive collimation lenses, but obviously the physical size of the OTA will become longer.

Bob


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

Well, dang, now I'm confusing myself.

My PST etalon is a 20mm clear aperture.
My old Quark etalon is 21mm clear aperture.

So they should have a very similar sweet spot?

I seem to recall the Quark having a larger sweet spot on the same aperture scope setup. I may be imagining this. Or is it due to the telecentric taking care of the off-axis rays?

Otherwise, it has me thinking about all of this with respect to a 60mm F15 refractor with a Quark Combo (21mm clear aperture, 25mm blocking filter), allowing a full disc to be imaged with a larger sensor. The d/D is 0.35. Despite me not wanting to have a Daystar again, this at least is something to think about on my end, unless I'm missing something (which is likely!).

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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by hk160 »

> So they should have a very similar sweet spot?

The effective (refractive) index of Mica is 1.6 vs. 1.0 for the air spaced etalon. Therefore the quark can accept beams more inclined, which is equivalent to a larger sweet spot.

Another way to see it is that a 21mm quark is equivalent to a 1.6x21mm=33.6mm air spaced etalon.


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

hk160 wrote: Tue Mar 02, 2021 12:05 am > So they should have a very similar sweet spot?

The effective (refractive) index of Mica is 1.6 vs. 1.0 for the air spaced etalon. Therefore the quark can accept beams more inclined, which is equivalent to a larger sweet spot.

Another way to see it is that a 21mm quark is equivalent to a 1.6x21mm=33.6mm air spaced etalon.


Klaus
Thanks! That perfectly explains what I was remembering. Quite interesting.

So does this mean that a 60mm F15 aperture refractor with a Quark Combo (21mm aperture air, but 33.6mm mica?) mean it has a d/D ratio of 0.56 and therefore can put a full disc in the entire sweet spot? This seems too easy. Probably can't be true.... but I gotta ask!

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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by AndiesHandyHandies »

Hi

What does the size of a sweet spot say about the configuration?

I have a Rumak180mm F10, perfect flat field 'orthoscopic', with 1.5A Omega seconds as a Blocker and a Meade PST which looks 0.75A.

Tuning the PST I get a sweet spot about 1/4 diameter of the field and then it goes into a doughnut.

As its only showing proms its well big enough.

Cheers. Andrew.


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by hk160 »

So does this mean that a 60mm F15 aperture refractor with a Quark Combo (21mm aperture air, but 33.6mm mica?) mean it has a d/D ratio of 0.56 and therefore can put a full disc in the entire sweet spot? This seems too easy. Probably can't be true.... but I gotta ask!
Well, in a telecentric design you need f/#=30, so with a 60mm aperture, that is a fl=1800mm, giving a disc size of 17mm so that would fit. There is no sweet spot in that case, just the fwhm goes from 0.7A to 0.8A.

For a collimated design, (which is discussed here), you need to create a pupil image of a certain size and turn the beams parallel. By geometry, if I make the pupil 1/n th the size, the angles get n times larger. So if you take the sun as an example, you start with .5 deg, and you need to go from 60mm aperture to 20mm pupil=20mm etalon aperture --> angle increases to 1.5 deg. Now you can dial in that the cwl is right half way out from the center, so really you need to accommodate 0.75deg.

For the mica etalon, this leads to a CWL shift of 0.2A, occurring in the center and at the edge of the disc. Whether this is acceptable or not depends on how picky one is.


I also want to point (again) to Christian Viladrichs excellent analysis on his page here:

http://www.astrosurf.com/viladrich/astr ... lar/FP.htm

I took the took the cwl shift and FWHM value from there, too lazy to calculate them myself ... ;)


Hope that helps,
Klaus
Last edited by hk160 on Tue Mar 02, 2021 3:09 pm, edited 1 time in total.


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by Bob Yoesle »

My PST etalon is a 20mm clear aperture.
My old Quark etalon is 21mm clear aperture.

So they should have a very similar sweet spot?

I seem to recall the Quark having a larger sweet spot on the same aperture scope setup. I may be imagining this. Or is it due to the telecentric taking care of the off-axis rays?
As Klaus describes, you can't mix the imprecise rule-of-thumb about objective and etalon diameters used for collimated systems with telecentric systems, which ideally keep the converging cone field angles below the acceptance angle threshold to maintain the specified bandpass. However, in my experience you can also get a Jacquinot spot (in addition to bandpass widening) if the telecentric system isn't optimized to produce a truly telecentric axial cones normal to the etalon.

Etalon locations.jpg
Etalon locations.jpg (114.48 KiB) Viewed 4582 times

Again, the Jacquinot spot is a result of what the etalon acceptance angle is (varies based on etalon gap refractive index), and the field angle of the extended object you are observing:

The Jacquinot spot is defined as the field about the optical axis within which the the peak wavelength variation [ Δλ ] with field angle does not exceed √2 of the etalon bandpass. This angular field can be used to perform close to monochromatic imaging.

So you can see that monochromatic imaging can only be accomplished to the degree in which the field angle doesn't exceed the acceptance angle.

Equation 1: Δλ = √2 x FWHM

The field angle verses wavelength change can be found with formula for the CWL shift:

Equation 2: Δλ = ½ (CWL / n^2) θ^2

We can now solve for θ:

√2 x FWHM = ½ (CWL / n^2) θ^2

θ^2 = √2 x FWHM ÷ ½ (CWL / n^2)

For an air spaced etalon (n = 1.00) with a FWHM of 0.7 Å at the H alpha line (6563 Å), with θ in radians (1 radian = 57.2957795 degrees):

θ^2 = 1.4142 x 0.7 ÷ ½ (6563 / 1.00)

θ^2 = 0.98994 ÷ 3281.5

θ^2 = 0.000301673

θ = √0.000301673

θ = 0.017368736 (radians) x 57.2957795 degrees

θ = 0.9951553 degree

Therefore the Jacquinot spot is ~ 1.0 degree, and the “acceptance angle” (field angle) for this size a spot would be ~ 0.5 degree, as is the frequently cited acceptance value for a 0.7 Å FWHM etalon. Outside this "sweet spot" radius H alpha detail will begin to fade into continuum.

Next, for a double stacked etalon system with a 0.5 Å FWHM, and assuming a DS system follows the same rules, we get the following;

θ^2 = 1.4142 x 0.5 ÷ ½ (6563 / 1.00)

θ^2 = 0.7071 ÷ 3281.5

θ^2 = 0.00021548

θ = √0.00021548

θ = 0.0146792 (radians) x 57.2957795 degrees

θ = 0.841058 degree

Therefore the Jacquinot spot is ~ 0.84 degree, and the “acceptance angle” (field angle) for this size a spot would be ~ 0.42 degree. Christian Viladrich produced a great graph of these relationships for various band-passes and refractive index materials which is quite useful:

Jacquinot spot.jpg
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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

Thanks guys, the math helps a lot!

:bow :bow

It really helps to emphasize that large etalons that are front mounted are rather important for a double stack with respect to the full disc FOV to ensure that a double stack sweet spot keeps a full disc within the sweet spot FOV.

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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by marktownley »

It's finding that winning combination Marty, just takes time!


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

marktownley wrote: Tue Mar 02, 2021 4:57 pm It's finding that winning combination Marty, just takes time!
Ain't that the truth.

The problem is there's very little selection and no standard for quality. I'm not willing to drop $6k on a Quantum (or more) and really don't feel like doing $8k~16k on a SolarSpectrum, looking at the rear mica spaced etalons with larger apertures for high res. I realize there's no free lunch, so I'm always looking for what's a good balance between affordability and functionality and being able to be modular/flexible.

I've never had a good full disc HA setup. I tried a few different double stacks, went through SM40, SM60 double stacks, pre and post Meade at this point, and none of them were good. Even my pre-Meade had problems. It's sad that my PST etalon (pre-Meade) is better than all those etalons I had and my past Quark. Sold them all, kept the PST etalon and I still have a Sm60II double stacking etalon laying around just because its near worthless to sell and I can double stack with it.

Based on this thread, I realize I probably should just get a Lunt 80mm for full discs. But, that price tag is not happening for me right now. And I'm not gambling on Solarmax since there's no support and I cannot test them or see them before buying, so the SMIII series 90's, while more affordable, are just not likely going to cut it without cherry picking one somehow. And I don't have high expectations for them.

So, I've been toying with the idea of how to optimize some other affordable options until I can get into a Lunt 80 or something.

So then I wanted to explore, what would it take to explore optimizing a PST 20mm etalon for a full disc and potentially when using it in a large system for high res, to get a bit bigger sweet spot. The PST etalon I have is great, good finesse, great contrast, but a small sweet spot. So I was just seeing what can I do to get a bit more out of this etalon for now. If I could use it with a collimator system to get a full disc with something that has about a 60mm aperture in terms of resolution, that would be great.

Then I started exploring, what if I got a Quark Combo unit to hold me over a while, and just use it with F15 refractor at 60mm. Even if I had to put a 2x powermate between it and the frac, a reducer would bring it back down to F15 and the 25mm blocking filter and a big sensor should be able to full disc. But, I didn't know how to best calculate the sweet spot to know if the disc would be within the sweet spot on such a system. The benefit here would be the ease to double stack a Combo unit and the ease of use on larger apertures with my big scopes and DERF since I already have those working.

The other thing I'm looking at is the Lunt 60mm modular that has a pressure tuned etalon and is operating at F7, being able to set this up in a rear mounted configuration with a larger etalon aperture than the PST and being pressure tuned might be interesting, while also being competent for full discs on its native 60mm scope. So this has me highly interested as a versatile mod. I was just exploring what the sweet spot would be in its 60mm form and when mounted behind a 120mm or 150mm refractor at F7 with a DERF of course. I'm not sure what the etalon clear aperture is on this though.

If anyone has experience with the above 3 or similar options, I'm all ears, eager to know your experiences. This thread with sweet spot calcs got my attention since it directly is in line with what questions I've had about some of this equipment and how it could potentially be used, without extreme re-modeling.

Part of it, as you said is getting a good combination, since every single one is unique and requires some tweaking. Few are excellent stock. Few are excellent at all. So I'm trying to learn more about the geometry of this stuff to better select potential options to tweak in the first place.

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MapleRidge »

Hi all...

This is quite the read, and need to go over it a few more times.

I have a question about the sweet spot ration from the d/D ratio. Does the result, say .2, have any units associated with it or is it 0.2 of the actual field of view, or some other relative detail?

Just trying to make sense of it all.

Thanks,
Brian


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

MapleRidge wrote: Tue Mar 02, 2021 9:42 pm Hi all...

This is quite the read, and need to go over it a few more times.

I have a question about the sweet spot ration from the d/D ratio. Does the result, say .2, have any units associated with it or is it 0.2 of the actual field of view, or some other relative detail?

Just trying to make sense of it all.

Thanks,
Brian
Hi Brian,
The calculation is straigtforward, the radius of the sweet spot is given by :

theta = sqrt (delta lambda) x f/F
with theta in degree,
delta lambda = CWL offset accepted at the edge of the sweet spot, for example 0.25 A, units = A,
F = focal of the refractor
f = focal of the collimator

In order not no have vigneting of the refractor aperture, we need to have :
F/D = f/d
with F and D = focal and aperture of the refractor
f and d = focal and aperture of the collimator

As the apertures of the collimator and etalon are identical (and equal to d), the sweet spot radius is given by:
theta = sqrt (delta lambda) x d/D
theta in degrees

=> this is valid only for Ha and air-spaced etalon. For Ca K (or other wavelengths) or mica-spaced etalon, we need to use a more general formula..


Hope this helps


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

Hrm,

Any help with this?

Mica spaced etalon, 21mm aperture (Quark Combo)
60mm aperture F16.67 telescope (1000mm focal length)

Sweet spot size and general thoughts?

I just rough calced it with n=1.6 and the result was 1.59 degrees approximately? This seems to agree with the graph above?

I'm curious how this would work, without anything, at native F16.67, and how it would work with a 2x powermate followed by a 0.5x reducer and if the disc image would fit within the sweet spot on a larger sensor, such as IMX253 (1.1") or IMX183 (1")?

So the above setup, native F16.67, on IMX183 1" sensor would be 0.75 degree x 0.5 degree
Same above with IMX253 (1.1") sensor would be 0.81 degree x 0.59 degree
Disc size should be between 10mm and 9.09mm
Blocking filter is 25mm
1.1" sensor is 14.2mm x 10.4mm (17.6mm diagonal)

I'm missing something.

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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by Bob Yoesle »

Hi Marty,

You can not determine much because the Quark does not specify a FWHM - you'd have to measure this for your filter before anything else could be determined. Once you do this, you can use the equation I posted above to determine the size of the Jacquinot spot for this filter - which again is dependent on filter FWHM and gap refractive index (e.g. mica).

With a simple objective-etalon system at f16.7, you'd have significant bandpass widening and poor contrast. Generally, the field angle of the Sun's disc of 0.25 degree is equivalent to about a f108 converging cone, so a rough estimation is f108/f16.7 = 6.46 - or about a 6.5 times increase in field angle for the objective etalon-system, and a significant sweet spot therefore seems inevitable.

I you use the Powermate, you will not have a truly telecentric system (converging light cones normal to the etalon) and compound the bandpass widening with increased field angles. Therefore you may have a "sweet spot" develop as well, although it won't be near as bad because the filter now sees a f33.4 converging cone. This is about a 3 x increase in the 0.25 field angle of the solar disc edge (e.g. ~ 0.75 degree) and depending on the FWHM, this might allow a full disc to have uniform contrast at the widened FWHM of the filter at f33.

For any system, this would be independent of a further downstream focal reducer, which only affects the final image size, not the "sweet spot" originating with the increase ray angles presented to the etalon.

Without understanding the mechanisms of the Jacquinot spot, as well as collimator and telecentric optics, it will be difficult to get ideal performance from a home-brewed DIY solar telescope design. Air spaced etalons generally are better designed for objective to collimator systems with larger etalons and shorter EFLs, and smaller mica etalons work better with telecentric systems and longer EFLs.
Last edited by Bob Yoesle on Wed Mar 03, 2021 3:51 pm, edited 1 time in total.


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

Bob Yoesle wrote: Wed Mar 03, 2021 3:43 pm Hi Marty,

You can not determine much because the Quark does not specify a FWHM - you'd have to measure this for your filter before anything else could be determined. Once you do this, you can use the equation I posted above to determine the size of the Jacquinot spot for this filter - which again is dependent on filter FWHM and gap refractive index (e.g. mica).

With a simple objective-etalon system at f16.7, you'd have significant bandpass widening and poor contrast. Generally, the field angle of the Sun's disc of 0.25 degree is equivalent to about a f108 converging cone, so a rough estimation is f108/f16.7 = 6.46 - or about a 6.5 times increase in field angle for the objective etalon-system, and a significant sweet spot therefore seems inevitable.

I you use the Powermate, you will not have a truly telecentric system (converging light cones normal to the etalon) and compound the bandpass widening with increased field angles. Therefore you may have a "sweet spot" develop as well, although it won't be near as bad because the filter now sees a f33.4 converging cone. This is about a 3 x increase in the 0.25 field angle of the solar disc edge (e.g. ~ 0.75 degree) and depending on the FWHM, this might allow a full disc to have uniform contrast at the widened FWHM of the filter at f33.

For any system, this would be independent of a further downstream focal reducer, which only affects the final image size, not the "sweet spot" originating with the increase ray angles presented to the etalon.
Thanks Bob, that puts that to rest, I figured something had to be fundamentally wrong with this, it just seemed too easy of a solution. Will dump the concept all together.

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

christian viladrich wrote: Wed Mar 03, 2021 9:38 am
MapleRidge wrote: Tue Mar 02, 2021 9:42 pm Hi all...

This is quite the read, and need to go over it a few more times.

I have a question about the sweet spot ration from the d/D ratio. Does the result, say .2, have any units associated with it or is it 0.2 of the actual field of view, or some other relative detail?

Just trying to make sense of it all.

Thanks,
Brian
Hi Brian,
The calculation is straigtforward, the radius of the sweet spot is given by :

theta = n x sqrt (delta lambda) x f/F
with theta in degree,
n : index of the etalon gap (n=1 for air-spaced etalon, n=1.6 for mica-spaced etalon)
delta lambda = CWL offset accepted at the edge of the sweet spot, for example 0.25 A, units = A,
F = focal of the refractor
f = focal of the collimator

In order not no have vigneting of the refractor aperture, we need to have :
F/D = f/d
with F and D = focal and aperture of the refractor
f and d = focal and aperture of the collimator

As the apertures of the collimator and etalon are identical (and equal to d), the sweet spot radius is given by:
theta = n xsqrt (delta lambda) x d/D
theta in degrees

=> this is valid only for Ha. For Ca K (or other wavelengths), we need to use a more general formula.
=> this is valid only in a collimated beam.


Hope this helps


Christian Viladrich
Co-author of "Planetary Astronomy"
http://planetary-astronomy.com/
Editor of "Solar Astronomy"
http://www.astronomiesolaire.com/
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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

I updated the formulae for mica-spaced etalon.


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

MalVeauX wrote: Wed Mar 03, 2021 2:55 pm Hrm,

Any help with this?

Mica spaced etalon, 21mm aperture (Quark Combo)
60mm aperture F16.67 telescope (1000mm focal length)

Sweet spot size and general thoughts?

I just rough calced it with n=1.6 and the result was 1.59 degrees approximately? This seems to agree with the graph above?

I'm curious how this would work, without anything, at native F16.67, and how it would work with a 2x powermate followed by a 0.5x reducer and if the disc image would fit within the sweet spot on a larger sensor, such as IMX253 (1.1") or IMX183 (1")?

So the above setup, native F16.67, on IMX183 1" sensor would be 0.75 degree x 0.5 degree
Same above with IMX253 (1.1") sensor would be 0.81 degree x 0.59 degree
Disc size should be between 10mm and 9.09mm
Blocking filter is 25mm
1.1" sensor is 14.2mm x 10.4mm (17.6mm diagonal)

I'm missing something.

Very best,
Hello Marty,
I 've updated the formulae for mica-spaced etalons.
Regarding your configuration, I have not understood if you are using a collimated beam or not ? If so, what is the focal length of the collimator. These are the data you need to calculed the radius of the sweet spot. The formulae I gave are valid only for a collimated beam.
BTW, what would you use a 2x Powermate followed by a 0.5x reducer ?


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

christian viladrich wrote: Wed Mar 03, 2021 5:10 pm
MalVeauX wrote: Wed Mar 03, 2021 2:55 pm Hrm,

Any help with this?

Mica spaced etalon, 21mm aperture (Quark Combo)
60mm aperture F16.67 telescope (1000mm focal length)

Sweet spot size and general thoughts?

I just rough calced it with n=1.6 and the result was 1.59 degrees approximately? This seems to agree with the graph above?

I'm curious how this would work, without anything, at native F16.67, and how it would work with a 2x powermate followed by a 0.5x reducer and if the disc image would fit within the sweet spot on a larger sensor, such as IMX253 (1.1") or IMX183 (1")?

So the above setup, native F16.67, on IMX183 1" sensor would be 0.75 degree x 0.5 degree
Same above with IMX253 (1.1") sensor would be 0.81 degree x 0.59 degree
Disc size should be between 10mm and 9.09mm
Blocking filter is 25mm
1.1" sensor is 14.2mm x 10.4mm (17.6mm diagonal)

I'm missing something.

Very best,
Hello Marty,
I 've updated the formulae for mica-spaced etalons.
Regarding your configuration, I have not understood if you are using a collimated beam or not ? If so, what is the focal length of the collimator. These are the data you need to calculed the radius of the sweet spot. The formulae I gave are valid only for a collimated beam.
BTW, what would you use a 2x Powermate followed by a 0.5x reducer ?
Hello,

I was curious about both. Daystar claims you can use their combo version on F15 refractors, and then of course Mak/SCT. So I was curious why they would claim you could just use the Combo on a F15 refractor in general and what it would produce, knowing full well that it's not likely going to be excellent contrast due to the way the mica-spaced etalon works. However, I was still curious since it wasn't being explicitly stated to use a telecentric with them.

So I was curious how the following configuration would function and the result and the sweet spot size and disc size:

60mm F16.67 refractor with Quark Combo and IMX253 sensor (1.1", 3.45um)

60mm F16.67 refractor with 2x powermate then Quark Combo then 0.5x focal reducer then IMX253 sensor

For a larger aperture, no expectation of full disc, the Quark Combo with a 4x telecentric approximately would be used, as one would expect and reduced on the camera side for sampling.

But I was more interested in how this would function and sweet spot size would work with the above small aperture long focal-ratio refractor approach, but only with the Combo version (not the one with the built in telecentric 4.2x).

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

Hello Marty,
Ok, I see. Then the calculation is completely different since there is no telecentric or collimated beam falling onto the etalon.

In the first case, you just have a f/16.67 beam. It means you will have both:
- an enlargement of the FWHM (compared to its nominal value) because of the f/16 beam,
- a drift of the CWL away from the optical axis.
The formulae are there :
http://astrosurf.com/viladrich/astro/in ... lar/FP.htm
Most probably, it is not very good ...

For case #2, and if the Powermate 2x is telecentric, there is no sweet spot. Still, there is a broadening of the FWHM because of the f-ratio :
http://astrosurf.com/viladrich/astro/in ... FWHM-N.JPG


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

christian viladrich wrote: Wed Mar 03, 2021 7:01 pm Hello Marty,
Ok, I see. Then the calculation is completely different since there is no telecentric or collimated beam falling onto the etalon.

In the first case, you just have a f/16.67 beam. It means you will have both:
- an enlargement of the FWHM (compared to its nominal value) because of the f/16 beam,
- a drift of the CWL away from the optical axis.
The formulae are there :
http://astrosurf.com/viladrich/astro/in ... lar/FP.htm
Most probably, it is not very good ...

For case #2, and if the Powermate 2x is telecentric, there is no sweet spot. Still, there is a broadening of the FWHM because of the f-ratio :
http://astrosurf.com/viladrich/astro/in ... FWHM-N.JPG
Thanks Christian,

So it will be a bad way to go, no problem, was just curious how it would all work for my own curiosity. For a full disc, it's just way better to get a simple larger etalon smaller focal length non-mica setup, like a Lunt.

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MapleRidge »

Hi Christian....

Just looping back to the math and the additional formulas and terms you sent along...
Hi Brian,
The calculation is straigtforward, the radius of the sweet spot is given by :

theta = n x sqrt (delta lambda) x f/F
with theta in degree,
n : index of the etalon gap (n=1 for air-spaced etalon, n=1.6 for mica-spaced etalon)
delta lambda = CWL offset accepted at the edge of the sweet spot, for example 0.25 A, units = A,
F = focal of the refractor
f = focal of the collimator

In order not no have vigneting of the refractor aperture, we need to have :
F/D = f/d
with F and D = focal and aperture of the refractor
f and d = focal and aperture of the collimator

As the apertures of the collimator and etalon are identical (and equal to d), the sweet spot radius is given by:
theta = n xsqrt (delta lambda) x d/D
theta in degrees

=> this is valid only for Ha. For Ca K (or other wavelengths), we need to use a more general formula.
=> this is valid only in a collimated beam.


Hope this helps
Straight forward, OK :lol:
But walk me through the calculations for my setup if you would please:

The scope is a 150mm, f8 achromat using a Lunt pressure tuned etalon at the tail of the tube. The clear diameter of the etalon is 35.5mm, but I do not have any info on the collimation lenses used in it.

Do you have enough data to fill int he formula?

Please let me know if there are any additional measurement required that I have not provided.

Thanks for your assistance,
Brian
Last edited by MapleRidge on Sat Mar 06, 2021 10:23 pm, edited 1 time in total.


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Photos: https://www.flickr.com/photos/185395281@N08/albums

10'x15 Roll-off Roof Observatory
Takahashi EM400 Mount carrying:
C14 + Lunt 80ED
Deep Sky Work - ASI294MM Pro+EFW 7x36/Canon 60D (Ha mod), ONAG
Planetary Work - SBIG CFW10, ASI462MM

2.2m Diameter Dome
iOptron CEM70G Mount carrying:
Orion EON 130ED, f7 OTA for Day & Night Use
Ha Setup: Lunt LS80PT/LS75FHa/B1200Ha + Home Brew Lunt Double Stack/B1800Ha on the Orion OTA + Daystar Quantum
WL, G-Band & CaK Setup: Lunt Wedge & Lunt B1800CaK, Baader K-Line and Altair 2nm G-Band filter
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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MalVeauX »

MapleRidge wrote: Sat Mar 06, 2021 1:19 pm Hi Christian....

Just looping back to the math and the additional formulas and terms you sent along...
Hi Brian,
The calculation is straigtforward, the radius of the sweet spot is given by :

theta = n x sqrt (delta lambda) x f/F
with theta in degree,
n : index of the etalon gap (n=1 for air-spaced etalon, n=1.6 for mica-spaced etalon)
delta lambda = CWL offset accepted at the edge of the sweet spot, for example 0.25 A, units = A,
F = focal of the refractor
f = focal of the collimator

In order not no have vigneting of the refractor aperture, we need to have :
F/D = f/d
with F and D = focal and aperture of the refractor
f and d = focal and aperture of the collimator

As the apertures of the collimator and etalon are identical (and equal to d), the sweet spot radius is given by:
theta = n xsqrt (delta lambda) x d/D
theta in degrees

=> this is valid only for Ha. For Ca K (or other wavelengths), we need to use a more general formula.
=> this is valid only in a collimated beam.


Hope this helps
Straight forward, OK :lol:
But walk me through the calculations for my setup if you would please:

The scope is a 150mm, f8 achromat using a Lunt pressure treated etalon at the tail of the tube. The clear diameter of the etalon is 35.5mm, but I do not have any info on the collimation lenses used in it.

Do you have enough data to fill int he formula?

Please let me know if there are any additional measurement required that I have not provided.

Thanks for your assistance,
Brian
I am super interested in this Brian, especially how you mounted the Lunt pressure etalon back there and how the collimating lenses play into this (since there's F8.3 and F7 versions of the pressure tuned 60mm, if that's what you used). I'm not sure what collimating lenses are there, if any. Would love to learn more about this setup.

Very best,


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

Hello Brian and Marty,
We need an estimation of the focal length of the collimator ...
Is the 35.5 mm aperture the aperture of the Lunt 60 mm pressure tuned etalon ? If so, I understand there are two different versions of this filter: one for f-8.3 refractors and the other for f-7 refractors ?
Is there any way to measure the focal length of the collimator ? The front lens is a divergent lens, the second a convergent lens. Is it possible to detach one of these lens from the body of the filter ?


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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MapleRidge »

Hi Christian...

This is internal DSII etalon for the LS80T solar scope. I am only aware of the scope being made available as f7, and no options for different DSII units to match the OTA.

The system is undoubtedly able to be disassembled, but not comfortable doing so myself. The addition of the DSII unit to the single stack solar scope does not change the focus point much if at all. There is a spacer to use between the scope and focuser in single stack mode but this is removed and replaced by the DSII etalon which takes up the same space. Not sure if this speaks to the collimator lens net effect?

Brian


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Maple Ridge Observatory
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Photos: https://www.flickr.com/photos/185395281@N08/albums

10'x15 Roll-off Roof Observatory
Takahashi EM400 Mount carrying:
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Deep Sky Work - ASI294MM Pro+EFW 7x36/Canon 60D (Ha mod), ONAG
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iOptron CEM70G Mount carrying:
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Ha Setup: Lunt LS80PT/LS75FHa/B1200Ha + Home Brew Lunt Double Stack/B1800Ha on the Orion OTA + Daystar Quantum
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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by MapleRidge »

The clear aperture on the objective side is approximately 35mm. The calipers slipped when I was taking this pic and was trying not to touch the glass aggravating the measurement. If a better measurement is needed I can try this again...won't be -20C tomorrow :lol:

Or, am I measuring the wrong dimension? Is the thickness required?

Brian


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Maple Ridge Observatory
Cambray, ON Canada

Photos: https://www.flickr.com/photos/185395281@N08/albums

10'x15 Roll-off Roof Observatory
Takahashi EM400 Mount carrying:
C14 + Lunt 80ED
Deep Sky Work - ASI294MM Pro+EFW 7x36/Canon 60D (Ha mod), ONAG
Planetary Work - SBIG CFW10, ASI462MM

2.2m Diameter Dome
iOptron CEM70G Mount carrying:
Orion EON 130ED, f7 OTA for Day & Night Use
Ha Setup: Lunt LS80PT/LS75FHa/B1200Ha + Home Brew Lunt Double Stack/B1800Ha on the Orion OTA + Daystar Quantum
WL, G-Band & CaK Setup: Lunt Wedge & Lunt B1800CaK, Baader K-Line and Altair 2nm G-Band filter
ASI1600MM, ASI432MM, ASI294MM Pro, ASI174MM, ASI462MM
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Re: Sweet spot, banding and tilted etalons ...where are the limits ?

Post by christian viladrich »

So, we have :
- a refractor with focal length F = 150 x 8 = 1200 mm
- for the collimator, given that the free aperture is 35.5 mm, let's assume a full aperture equal to 40 mm. Given the f-7 ratio for the collimator, its focal is f = 7 x 40 = 280 mm

Now, if we consider an acceptable limit of 0.25 A shift of the CWL at the edge of the sweet spot, then the radius of the sweet spot is given by :
theta = f/F x sqrt (0.25) = 0.12°, or 0.24° in diameter. This is equivalent to the solar radius.

Hope this helps.


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http://planetary-astronomy.com/
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