LS50 at he BACK of a scope?

[Creativspelerr]

This is a relatively minor modification but since it's "off label use" I thought it should go here.

The LS50 front mount etalon is a good bit cheaper than the internal pressure-tuned etalon that goes at the back. Someone in another forum suggested I mount the LS50 at the back of the scope with a 4x baader telecentric (to get the f-ratio). It would need some adapter rings. Does any see a flaw in that set up?

I guess this needs a little more context as to the motivation for doing this (other than to save a buck). I am trying to make a carry on suit case sized h-alpha scope with the best quality filters that money can buy, as long as that money is in the $3k range, $4k as a stretch. The size limit rules out a nice long refractor. That leaves me with either a dedicated 50mm solar scope (that would be much smaller than I could potentially put in a suitcase) or adding filters to an APO. It would be something like a 90mm, f5.5 (I know that's faster than ideal for a solar scope, but again, that's what fits in the suitcase and it's what I would want for night observing). It would be easy enough to put the 50mm etalons on the front of a 90mm scope, but I want to explore the back mount option just to know how viable it is, and because the back mount would let me move the system over to other scopes.

Q1) Are there any fundamental problems with putting a LS50 style etalon at the back?

Q2) Let's talk about f-ratios. The 4x telecentric gets the APO overall f-ratio up to 22. I've heard some people say f14 as a minimum and some say f30 as a minimum for good quality. I wanted to compare the fwhm of the set up to the dedicated Lunt 500mm scope (or to a cornoado PST, 40mm f/10, which I currently own) using this chart:
https://www.cloudynights.com/uploads/mo ... 269279.jpg
But then I realized it's the f ratio (angle of the light) at the etalon I would need to know in order to compare apples to apples. The lunt scope is f/7 but it could be higher at the etalon. Any ideas how to assess how the back-mount system compares to other options? Is this f/22 set up going to be on-band enough to get good contrast?

Q3) LS50Fa or LS50c?

[Dennis]

Hi.
About Q2: the graph you reference is for mica based etalons. The Lunt is a air spaced etalon (apples and oranges) and in combination with a telecentric beam it would need to see a minimum of f/40 to show something. For solar a f/5.5 instrument is pretty useless in my opinion. typical seen is f/7 - f/10 for to reach the needed low angles of incident at those filters in combination with telecentric or collimated arrangements.

[Creativspelerr]

That's what I was afraid of. Good to know. That basically means my night astrophotography APO can't do double duty as my solar scope. I'm currently looking for an f/10 refractor that will fit in a suitcase. Assuming I find one, that leaves questions 1 and 2.

[marktownley]

Welcome to the forum!

You are going to end up with a physicaly large scope here - long with large turning moment due to weight of etalon,(assuming you want reasonable aperture) - as such a similar such mount will be needed. Things like focuser arrangements need to be beefy and ideally screw thread. HAve a look at Petr threads in the main section, he uses a similar arrangement to what you propose.

Personally I would look at a Lunt 50 or a Lunt 60MT for a travel scope.

[christian viladrich]

Hi there,

The mica graph is a bit oldish. You can have a look at the upadated graphs for mica-spaced and air-space etalons here (near page bottom) :

http://astrosurf.com/viladrich/astro/in ... P-Coro.htm

Full data for mica-space etalons (text file) :
http://astrosurf.com/viladrich/astro/in ... _ratio.txt

and air-spaced etalon :
http://astrosurf.com/viladrich/astro/in ... _ratio.txt

Hope this helps !

[KMH]

Hi there,

The mica graph is a bit oldish. You can have a look at the upadated graphs for mica-spaced and air-space etalons here (near page bottom) :

http://astrosurf.com/viladrich/astro/in ... P-Coro.htm

Full data for mica-space etalons (text file) :
http://astrosurf.com/viladrich/astro/in ... _ratio.txt

and air-spaced etalon :
http://astrosurf.com/viladrich/astro/in ... _ratio.txt

Hope this helps !

Christian,

Thanks for posting those links. I did not appreciate that there was such a big difference between mica and air gap etalons in a 4x telecentric beam.

Kevin

[pupak]

Please explain me this problem. If we talk about the effect of F on FWHM, then it is quite clear that as F increases, FWHM decreases because the beam angle is closer to perpendicular, which is the ideal condition.
What I don't understand is the connection between F and the telecentric beam. relative to the FWHM. If the beam is once telecentric (perpendicular to the etalon), then it must not matter what the F of the lens is. If I have an F10 and TZ3x lens, there is a perpendicular beam at the output, just like with F8 and TZ4x, or F15 and TZ2x.
My experience is that if I use TZ3x on AR100/F15, the result is worse, even if it is F=45. So far I've had the best results where I've used the appropriate TZ for a given F. I don't know what better conditions anyone could create than a telecentric beam. It doesn't make sense to me.
The AR150/F6 with the telecentric Barlow TV5x works just as well as the AR100/F15 with the TZ2x. Personally, I see no reason to change a telescope with F>6 for another, when it is enough to use a suitable telecentric Barlov or TZ.

[christian viladrich]

Please explain me this problem. If we talk about the effect of F on FWHM, then it is quite clear that as F increases, FWHM decreases because the beam angle is closer to perpendicular, which is the ideal condition.
What I don't understand is the connection between F and the telecentric beam. relative to the FWHM. If the beam is once telecentric (perpendicular to the etalon), then it must not matter what the F of the lens is. If I have an F10 and TZ3x lens, there is a perpendicular beam at the output, just like with F8 and TZ4x, or F15 and TZ2x.
My experience is that if I use TZ3x on AR100/F15, the result is worse, even if it is F=45. So far I've had the best results where I've used the appropriate TZ for a given F. I don't know what better conditions anyone could create than a telecentric beam. It doesn't make sense to me.
The AR150/F6 with the telecentric Barlow TV5x works just as well as the AR100/F15 with the TZ2x. Personally, I see no reason to change a telescope with F>6 for another, when it is enough to use a suitable telecentric Barlov or TZ.

Hi,

The explanation is very simple : the light beam is not perpendicular to the etalon in a telecentric beam.

Only the chief ray is perpendicular to the etalon.

A drawing is better than any long explanation. Have a look here :
http://astrosurf.com/viladrich/astro/in ... tage-5.jpg

Some more drawings and explanations here :

http://astrosurf.com/viladrich/astro/in ... icity.html

When a non telecentric system is used (for example a Barlow lens), then the chief angle is no longer perpendicular to the etalon, and the broadening of the FWHM is even more larger, and there is de drift of the center wavelength accross the field of view :
http://astrosurf.com/viladrich/astro/in ... o-tele.htm

[KMH]

Christian,

That's weird - in your post above, I (KMH) was listed as the author of the question posed by pupek. In any event, thanks for the additional explanation.

Kevin

[pupak]

Hi,
The explanation is very simple : the light beam is not perpendicular to the etalon in a telecentric beam.

Only the chief ray is perpendicular to the etalon.

A drawing is better than any long explanation. Have a look here :
http://astrosurf.com/viladrich/astro/in ... tage-5.jpg

Some more drawings and explanations here :

http://astrosurf.com/viladrich/astro/in ... icity.html

When a non telecentric system is used (for example a Barlow lens), then the chief angle is no longer perpendicular to the etalon, and the broadening of the FWHM is even more larger, and there is de drift of the center wavelength accross the field of view :
http://astrosurf.com/viladrich/astro/in ... o-tele.htm

I really don't understand. I've been reading for years that a collimated beam only has the main beam perpendicular, whereas a telecentric system has the entire beam perpendicular. It's written by Baader, TV and now you tell me that everything is different. I'm going to shoot myself and drown.
Why is there no difference in image quality with the 150/F6 system with telecentric TV5x and 100/F15 with TZ2x? Surely there should be a noticeable difference when there is such a significant difference in F.

[Dennis]

Hi,
The explanation is very simple : the light beam is not perpendicular to the etalon in a telecentric beam.

Only the chief ray is perpendicular to the etalon.

A drawing is better than any long explanation. Have a look here :
http://astrosurf.com/viladrich/astro/in ... tage-5.jpg

Some more drawings and explanations here :

http://astrosurf.com/viladrich/astro/in ... icity.html

When a non telecentric system is used (for example a Barlow lens), then the chief angle is no longer perpendicular to the etalon, and the broadening of the FWHM is even more larger, and there is de drift of the center wavelength accross the field of view :
http://astrosurf.com/viladrich/astro/in ... o-tele.htm

I really don't understand. I've been reading for years that a collimated beam only has the main beam perpendicular, whereas a telecentric system has the entire beam perpendicular. It's written by Baader, TV and now you tell me that everything is different. I'm going to shoot myself and drown.
Why is there no difference in image quality with the 150/F6 system with telecentric TV5x and 100/F15 with TZ2x? Surely there should be a noticeable difference when there is such a significant difference in F.

Im not sure if the field is large enough to see much of a difference between the two cases - with close up imaging and the sensors we typically use. How many mm do we get away from the optical axis at f/30, idk. I would assume too that the tz2x setup would give a narrower bandpass and more even field. Also since i think the TV 5x is not much telecentric, is it?

[Creativspelerr]

Those are great ray diagrams. I guess my original question about whether it would even work to put a LS50 at that back could be rephrased as this: Given that the ray diagram of a telecentric is different than the rays coming from the sun (which is what the LS50 is expecting to receive) does that create an incompatibility (whereas an etalon designed to be rear-mounted has already planned around that configuration), or are the angles on an f30 or f40 so shallow that the etalon will function as intended?
In an attempt to answer that for myself, I figured the angular size of the sun is 0.5 deg, so the incoming rays are out of parallel but up to that much. A long horizontal triangle with an angle of 0.5 deg has Tan(0.5) = 0.0087. If that same triangle were inside the telescope it would correspond to aperture / focal length, so 1/0.0187=114 indicates a f-ratio of 114.
I don't think any etalon demands f/114 so I take that to mean they are tolerant of incident angles greater than 0.5 deg, but I guess that does still leave the question open of whether front etalons are less tolerant than rear ones. My suspicion is that they're made using the same process, thus equally tolerant.

[pupak]

I will detach myself from all theory and share my knowledge from practice.
I use a DSII stacked with a Quark combo and a tiltable LS40 in that order. This rear combination works with the appropriate telecentric element on all solar telescopes I have. AR150/F6, AR100/F15, N180/F7, AR210/F8.8, AR150/F24. In practice, there is no need to tune anything, but perhaps better results could be achieved by changing the settings. The LS40 itself as a rear etalon does not seem usable to me. It's definitely a matter of entitlement, but not for me. The contrast is very low. I also tried DSII from two LS40s and that only worked acceptably on an AR150/F24 with a telecentric Barlow 2x. This corresponds to the graph presented here, where a high F= 48 will provide good conditions. I got the best results by adding the BF before the Barlow. With two BFs, the contrast was quite decent and the attenuation at H-a was acceptable.
But when using Quark, the situation is completely different. The combination of Quark and LS40 works without any problems since F24. I tried it on the AR150/F24 and the contrast is very good. No other optical element was used.
Since I still haven't found the time to study at least the basics of optics, I keep coming across situations where something works that theoretically shouldn't work and vice versa. I guess I have no choice but to start studying. Optgeo is obviously not enough.

[christian viladrich]

I really don't understand. I've been reading for years that a collimated beam only has the main beam perpendicular, whereas a telecentric system has the entire beam perpendicular. It's written by Baader, TV and now you tell me that everything is different. I'm going to shoot myself and drown.

Don't shoot yourself It simply means that many people talk while they don't know what they are talking about

More seriously, it takes more than 140 characters to explain what is a collimated beam or a telecentric beam, and much more to explain the implications for solar observations.

Still, a good explanation is given in Baader description of the TZ system. I am talking of the pdf file. There is a figure and a legend explaining :

"This ray trace shows an f/30 telecentric design. The rays are uniform across the image plane. This does not mean that they
are all going through the filter perpendicular to the image plane but the rays are parallel to the central rays."

This is not very accurate from the optical point of view, still it gives the general idea.
The associated document is right on spot. It takes 15 lines. Everything is there :

"The telecentric is often confused with a barlow. Both can be used to magnify the focal
length. "Telecentric" is not a synonym for parallel beam. Telecentrics are designed so that
the exit pupil lies at infinity, which means that the center ray from any point in the field
appears to come from infinity and is therefore perpendicular to the image plane and
parallel to the optic axis. This means that the off axis beam arrive at the image plane with
the same angular geometry as the axial rays. All field elements look as if they where like
they are on axis, across the image plane and unlike a barlow, the edge field rays are not
tipped bundles.

Because all the principal rays across the image plane are perpendicular to the image
plane. The rays at the edge of the field will pass through an etalon just in front of the focal
plane with exactly the same geometry as the rays on axis. So in a f/30 telecentric refractor
the etalon sees the exact same 2.5 degree geometry clear across the field, and the
spectral bandpass does not shift the wavelength across the field."

I present more information on telecentric, near-telecentric , collimated or not system here, as well as implication for solar observation :

http://astrosurf.com/viladrich/astro/in ... lar/FP.htm

I have to admit this might a bit difficult to read. This is more bulk information and figures. One day, I'l try to re-write all of this, but I have many other projects in the pipeline before ...

Solar Astronomy book presents this (and more) in a more pedagogical way and easier to read (well, I hope ...).

Regarding theory and experiment : if theory is different than experiment, it simply means that theory and/or experiment is wrong.

In the case of concern, theory and experiment are quite consitent (including lot of imaging and spectro measurements).

[christian viladrich]

Those are great ray diagrams. I guess my original question about whether it would even work to put a LS50 at that back could be rephrased as this: Given that the ray diagram of a telecentric is different than the rays coming from the sun (which is what the LS50 is expecting to receive) does that create an incompatibility (whereas an etalon designed to be rear-mounted has already planned around that configuration), or are the angles on an f30 or f40 so shallow that the etalon will function as intended?
In an attempt to answer that for myself, I figured the angular size of the sun is 0.5 deg, so the incoming rays are out of parallel but up to that much. A long horizontal triangle with an angle of 0.5 deg has Tan(0.5) = 0.0087. If that same triangle were inside the telescope it would correspond to aperture / focal length, so 1/0.0187=114 indicates a f-ratio of 114.
I don't think any etalon demands f/114 so I take that to mean they are tolerant of incident angles greater than 0.5 deg, but I guess that does still leave the question open of whether front etalons are less tolerant than rear ones. My suspicion is that they're made using the same process, thus equally tolerant.

The answer is given hereafter.

When an etalon is placed in front of a telescope, it is in a "collimated "beam. The two implications are :
- the effective FWHM of the etalon is equal to its nominal FWHM => this is the good news,
- the CWL (center wavelength) changes when going away from the optical axis => this is the sweet spot effect, and ... the bad news.

When an etalon is placed in a telecentric beam (a true one, like with Baader TZ-x), the implications are :
- the effective FWHM and CWL are constant accross the field of view => this is the good news,
- but the effective FWHM is larger than the nominal FWHM => this is the bad news,
- the CWL is drifted to the blue => this is usually not a problem since the etalon is themo-regulated.

In a telecentric beam, because of the broadening of the FWHM :
- if you use a mica-spaced etalon (DayStar, Solar Spectrum), you have to use an f-ratio larger than about 28,
- if you use an air-spaced etalon (Coronado, Lunt), you have to use an f-ratio larger than about 40-50.

You can see for yourself what is the effective FWHM according f-ratio :
- for an mica-spaced etalon :
http://astrosurf.com/viladrich/astro/in ... spaced.jpg
- for an air-spaced etalon :
http://astrosurf.com/viladrich/astro/in ... spaced.jpg

If you are interested in getting of full understanding of the reasons why of all of this, you can have a look here :
http://astrosurf.com/viladrich/astro/in ... lar/FP.htm

[pupak]

So I finally took the few hours and studied the basics of optics, especially Christian's materials, which are very informative.
The question marks melted away and I only regret not doing it sooner.
But the issue of solar etalons will require a little deeper dive.
Christian is doing a great job here.

[christian viladrich]

Hi Peter,
I am glad you've found these pages usefull.
There is a bit of a learning curve there. Nothing complex, but it takes some times to go through it.
And I have to re-organize this pages to make them easier to read.

[Creativspelerr]

Thanks for all those insights and resources!

[DavidP]

Yes! This discussion has help me tremendously.

[AndiesHandyHandies]

Hi

Why not buy one of these?

https://www.teleskop-express.de/shop/pr ... raphy.html

I assume that will fit in your suitcase.

Use the LUNT 50mm front etalon at the front, stops it down to 50mm F7.2. No field angles. 3.6mm image size for small CCD. 10mm eyepiece.

3x Vision King TE gives F20 10.8mm image. 20mm eyepiece.

Add 2x ES TE for F40 21.6mm image. 40mm eyepiece.

Working at F40 I find a DS PST 20mm etalon works fine behind a Quark.

Image size will affect blocking filter cost.
Look at Bobs posts on blocking filter options.
Need a UV-IR, RG630 or CCD red 100nm for etalon leakage, and IR block out to 1200nm+ KG3 or in blocker.

Once you get to F40 mica or air-spaced atalon work fine for me.

Cheers. Andrew.

[Creativspelerr]

I return to you older and wiser (having continued to do some more research).

Raf Camera has a custom mount already designed for attaching a Lunt 40mm front etalon at the rear of the scope (in a way that allows the etalon to tilt without putting a kink in the optical train).
https://rafcamera.com/adapter-lunt-40mm

Which was based off this design:
viewtopic.php?t=37624&sid=0f4decc3c9bfd ... dd3cd4eafa

And a discussion here about double stacking with pressure tuned rear/internal filters vs adapting a front/external filter.
https://www.cloudynights.com/topic/7015 ... nt-filter/

Now here's the next question I need to solve. I'm asking it as a continuation of this thread rather than a new thread because it still comes down to whether it's better to use a front etalon in front, or in back. (I'm including the explainers for anyone who is new to the hobby).

Option 1 listed from object side to image side: Lunt 60mm front etalon, Askar FRA500, TZ-4, Solar Spectrum Suna.
The 60mm front aperture makes f/33. I could add an iris or mask to go down to f/40 at 50mm aperature.

Option 2: Askar FRA500 masked to between 60mm and 50mm (f/33 to f/40), TZ-4, 40mm Lunt front etalon, Solar Spectrum Suna.

(Thanks AndiesHandyHandies-Andrew for the suggesting another version of this set up)

Additional details just for anyone not familiar with this or that piece of equipment. Solar Spectrum Suna is a mica spaced etalon, similar to a Daystar Quark (but with a better quality control reputation). 0.65A FWHM band pass. Askar FRA500 is a 500mm focal length petzval. TZ-4 is a 4x telecentric, because unlike the Quark, the Suna does not have that built in.

So the specific question is, given that all the rest of the equipment would be identical, would the optics have any better imaging quality by double stacking with a tilt tuned etalon where God... I mean Lunt... intended it, at the front, or moving it to the back?

There are some reasons to prefer putting it at the back. Raf has that adapter already available, and when I eventually graduate to high resolution imaging with my C8 I would put the etalon at the back anyway which would be a little harder to do with the 60mm etalon. But for the FRA500 set up, if the image quality would be better double stacking at the front rather than the back, maybe it's worth the extra hassle of designing my own mounts.

[christian viladrich]

Now here's the next question I need to solve. I'm asking it as a continuation of this thread rather than a new thread because it still comes down to whether it's better to use a front etalon in front, or in back. (I'm including the explainers for anyone who is new to the hobby).

Option 1 listed from object side to image side: Lunt 60mm front etalon, Askar FRA500, TZ-4, Solar Spectrum Suna.
The 60mm front aperture makes f/33. I could add an iris or mask to go down to f/40 at 50mm aperature.

Option 2: Askar FRA500 masked to between 60mm and 50mm (f/33 to f/40), TZ-4, 40mm Lunt front etalon, Solar Spectrum Suna.

(Thanks AndiesHandyHandies-Andrew for the suggesting another version of this set up)

Additional details just for anyone not familiar with this or that piece of equipment. Solar Spectrum Suna is a mica spaced etalon, similar to a Daystar Quark (but with a better quality control reputation). 0.65A FWHM band pass. Askar FRA500 is a 500mm focal length petzval. TZ-4 is a 4x telecentric, because unlike the Quark, the Suna does not have that built in.

So the specific question is, given that all the rest of the equipment would be identical, would the optics have any better imaging quality by double stacking with a tilt tuned etalon where [no religion please]... I mean Lunt... intended it, at the front, or moving it to the back?

There are some reasons to prefer putting it at the back. Raf has that adapter already available, and when I eventually graduate to high resolution imaging with my C8 I would put the etalon at the back anyway which would be a little harder to do with the 60mm etalon. But for the FRA500 set up, if the image quality would be better double stacking at the front rather than the back, maybe it's worth the extra hassle of designing my own mounts.
Re: LS50 at he BACK of a scope?

Hi,

I copy/paste the answer already given on Cloudy Nights Forum :

*****************************************************************************************************************************

The precise answer might be a bit long to explain, but the general idea is as follows :

- an etalon placed in "front" position is an etalon in collimated beam,
- a etalon placed in "back" position is an etalon in a telecentric beam (when things are donne proprely), or in collimated beam (rarely) or in a non collimated nor telecentric beam (which is bad).

Now, let's take an air-spaced etalon in collimated beam (for example in a "front" position)
- its FWHM is equal to its name plate FWHM : which is excellent news,
- but its CWL (center wave length) changes with the distance to the optical axis : this is bad news, and the explanation of the "sweet spot".

Let's take this same air-spaced etalon in telecentric beam (back position) :
- its FWHM is larger than its its name plate FWHM : which is bad news, the effective FWH depends on the f-ratio :

http://astrosurf.com/viladrich/astro/in ... spaced.jpg

- but the CWL is constant all over the field of view : there is no sweet spot.

There is no one fit all solution wink

All in all, the best solution is to have a front etalon (air-spaced) with the largest possible diameter. But is comes to a cost ...

In your case, Lunt 60mm front etalon > Askar FRA500 > TZ-4 > Solar Spectrum Suna would give better contrast than Askar FRA500 masked to between 60mm and 50mm (f/33 to f/40) > TZ-4 > 40mm Lunt etalon > Solar Spectrum Suna.

This is because, in the 2e case, the Lunt 40 mm is an air-spaced etalon placed in a telescentric beam and not in a collimated beam.

NB1 : if you want to push further your undestanding of this, you can have a look here (it is a bit technical):
[url]http://astrosurf.com...nt/solar/FP.htm[/url]
You can also get a copy of Solar Astronomy where all all this is explained more in detail.

**********************************************************************************************************
NB2 : regarding the specific question of tuning the etalon by tilting. In fact, it would be better to tune it by pressure. Explanations is a bit technical, and too much information at once could be misleading. But you could get a feeling of it here (from section D) :
http://astrosurf.com/viladrich/astro/in ... lar/FP.htm


Hope this helps.

[christian viladrich]

Askar FRA500 is a 500mm focal length petzval.

Just for the record, a Petzval is made of two doublets. So, the FRA 500 is definitely not a Peztval given it is a triplet + doublet. On top of that, Askar placed a patent on the design, which won't be possible if if was a Petzval.

On the other hand, the Takahashi FSQ is indeed a Petzval.

To check this out, you can easily find the schematic diagrams of both designs on the web.

[Creativspelerr]

Thanks for the Petzval correction!

Thanks for explaining the FWHM vs sweet spot trade off - that's exactly the kind of thing I was trying to come up with.

It sounds like you are recommending the front mount to favor better contrast, but just to play devil's advocate a moment. The front mount would get better contrast but not a uniform image and there's not much that can be done about that. The instances when I've heard people say "the sweet spot effect is not a problem" were usually talking about full disk imaging where the not-so-sweet region was off the edge of the image. I would want to be able to do close-up feature images and cropping is just a waste of all that equipment. The rear mount option (with a proper telecentric) would have better uniformity, and because it's double stacked, it would get a good FWHM (not as high as it could be if front mounted, but still good).

The graphs taken from http://www.astrosurf.com/viladrich/astr ... aystar.htm say that a double stack 0.6 has about the same or slightly wider FWHM than a single 0.3, but flatter tails to let in less of the far off-band continuum (so might even perform better).




I don't know if that analysis is exactly applicable to this scenario, but it's the closest I could find.

My moto is a problem that can be solved is always better than one that can't be solved, so I'm leaning in favor of the uniformity of the rear mount, and call the double stacking and post processing the "solution" to the sacrifice in contrast. But I'm open to other input if other's have a different opinion.

[marktownley]

If you are leaning to a rear mounted solution then I would encourage you to consider how it will all be mounted, as all that weight will impart a turning motion on the focuser that will cause sag. The effect of this will be a blue shifted image that is banded. Everything needs to be orthogonal to the optical axis, an extended dovetail with guidscope rings can help with this but will not be a grab and go setup.

[christian viladrich]

I would want to be able to do close-up feature images and cropping is just a waste of all that equipment.

Hi,
I am no sure what you mean by this. Are you considering a third option, in addition to the two options you've described above :
- Option 1 : Lunt 60mm front etalon > Askar FRA500 > TZ-4 > Solar Spectrum Suna
- Option 2 : FRA500 masked to between 60mm and 50mm (f/33 to f/40) > TZ-4 > 40mm Lunt etalon > Solar Spectrum Suna.

If so, can you describe this third option ?

Regarding the sweet spot effet, have a look at the detailed explanations and calculations here :
http://astrosurf.com/viladrich/astro/in ... -spot.html

As a summary :
- if we consider the sweet spot as the area within 0.25 A of Ha line center, then - for a front mounted air-spaced etalon - the diameter of the sweet spot is twice the diameter of the Sun. So no problem.
- but ...this is true only if the center wave length (CWL) of the etalon is right on Ha at normal incidence, which is not generally the case.
- usually, the CWL is shifted to the red, so some tilt is to be applied to have the CWL center on Ha. Doing so, the sweet spot becomes a "sweet ring". Hence the "banding" observed in some pictures.

Christian

[christian viladrich]

The graphs taken from http://www.astrosurf.com/viladrich/astr ... aystar.htm say that a double stack 0.6 has about the same or slightly wider FWHM than a single 0.3, but flatter tails to let in less of the far off-band continuum (so might even perform better).




I don't know if that analysis is exactly applicable to this scenario, but it's the closest I could find.
My moto is a problem that can be solved is always better than one that can't be solved, so I'm leaning in favor of the uniformity of the rear mount, and call the double stacking and post processing the "solution" to the sacrifice in contrast. But I'm open to other input if other's have a different opinion.

You are forgetting an important point : a 0.6 A air-spaced etalon has an effective FWHM of 0.6 A only if it is placed in a collimated beam.
But if becomes a 1.1 A etalon if placed in a f/30 telecentric beam.

In your two options, the effective FWHM of the two etalons are :
- Option 1 : Lunt 60mm front etalon (0.6 A, if the etalon is good, effective FWHM = nominal FWHM) > Askar FRA500 > TZ-4 > Solar Spectrum Suna (0.6 A nominal FWHM becomes 0.7 A effective FWHM in a telecentric f/30 beam)
- Option 2 : FRA500 masked to between 60mm and 50mm (f/33 to f/40) > TZ-4 > 40mm Lunt etalon (0.6 A nominal FWHM becomes 1.1 A effective FWHM) > Solar Spectrum Suna (0.6 A nominal FWHM becomes 0.7 A effective FWHM in a telecentric f/30 beam).

This is why option 1 provides the best contrast.

[AndiesHandyHandies]

Hi

Surely a Quark is a more appropriate rear etalon for a small telescope?

And if you want to use the Lunt 50mm just use where its designed to go on the front of a telescope.

With the single etalon you will only see prominences well and some surface details may improve with a polariser.

Cheers. Andrew.