C8 + PST?

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C8 + PST?

Post by Wah »

Hi all,

I have thought of modifying my C8+PST to achieve higher resolution solar images.
Before buying big ERF, I used a 40mm mask in front of C8 to see if it can focus well.

After the test, it can focus with C8+PST. But it seem the sweet spot become very small, which is unexpected for me, because the system is f/50, which is much slower than original PST lens.

Could anyone tell me what's wrong with the system?
Thanks!

Wah!


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Re: C8 + PST?

Post by swisswalter »

Hi Wah

interesting. Have you some pics, also from the setup


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Re: C8 + PST?

Post by Valery »

The rule of thumbs tell us that if the internal etalon has 0.7A FWHM the sweet spot has the angular diameter
of d/D degree. Where the "d" is etalon diameter and the "D" is telescope aperture diameter.
If the FWHL is narrower, the sweet spot is also narrower.

Let define, that the PST etalon has these 0.7A FWHM. The sweet spot will be 1/10 degree in diameter or
1/5 of the solar disk diameter. And this is the ideal case. Any misalignment in optics will makes the sweet
spot size smaller.

When you stop your telescope by off-axis mask you also stops the working size of the etalon in the same
proportion and so, the sweet spot size will remain the same - about 1/5 of the solar disk or smaller.

Hope this helps.

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Re: C8 + PST?

Post by marktownley »

I guess the lens configurations just doesn't match up wah.


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Re: C8 + PST?

Post by Merlin66 »

Wah!,
How were you achieving focus? With a Stage1 mod (ie with the Black Box)??
The reason I ask is that achieving the -200mm on an SCT can be problematic.


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Re: C8 + PST?

Post by Bob Yoesle »

There are only two parameters you really need to know when determining the extent of a "sweet spot" - the objective focal length, and the collimator lens focal length.

The sweet spot originates from the magnification of field angles. If the sun were a point source, a collimator based system would have no problem with field angles (there are none). However the sun is a rather large extended object spanning a half degree. When its center lies on axis, the limb has a field angle of 0.25 degree, and prominences beyond the limb have an even greater field angle.

You can think of a collimator based system as a simple telescope with objective and "eyepiece" giving a collimated output to the etalon, just as it would to your eye. The magnification you see is actually the magnification of the field angles, and this is simple to determine: divide the objective focal length by the collimator focal length. The C8 has an EFL of 2000 mm. The PST collimator lens has a focal length of (-) 200 mm. 2000 / 200 = 10. Therefore the field angle magnification is 10 x, and the limb of the sun's field angle will subtend 0.25 x 10 = 2.5 degrees.

The "acceptance angle" of the PST etalon is about half a degree (0.5) -- this is a good figure to use for most air spaced etalons. Anything greater than a half a degree will begin to fall off-band as the the CWL shifts across the etalon. So in the case of the C8, the sweet spot indeed is about 1/5th the diameter of the sun (2.5 degrees), but this really doesn't depend on the diameter of the objective or the etalon, it only has to do with the field angle magnification, which again is precisely optically defined by the focal length of the objective divided by the focal length of the collimator lens.

So now you can see how to get a larger sweet spot: Find a longer focal length collimator lens. As an example if you used a collimator with a 1000 mm focal length lens, the field angle magnification will be 2000 / 1000 = 2. The limb of the sun will then have a field angle of 0.25 x 2 = 0.5 degree, and the entire disk of the sun should about fit within the acceptance angle of the etalon. There are other geometrical concerns that come into play, and this is why Vallery's method is an OK "rule of thumb," but it is not as precise as the actual optical physics, as I have described.

The EFL of your resulting telescope will also be defined by the field angle magnification factor. For example, if your refocusing lens (located after the etalon) has a focal length of 300 mm, and the field angle magnification is 2, the EFL will be 300 x 2 = 600 mm. For the FA magnification of 10, it would be 300 x 10 = 3000 mm, etc.

Christian Viladrich posted this diagram which shows the origin of the sweet spot in a collimated beam: as the field angles increase, the CWL (center wave length) of the transmission cure is blue shifted away from the H alpha emission line. Disk chromospheric detail will vanish and be replaced by continuum, and prominences will disappear.
Daystar-tilt.JPG


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Re: C8 + PST?

Post by Valery »

Bob Yoesle wrote:There are only two parameters you really need to know when determining the extent of a "sweet spot" - the objective focal length, and the collimator lens focal length.

The sweet spot originates from the magnification of field angles. If the sun were a point source, a collimator based system would have no problem with field angles (there are none). However the sun is a rather large extended object spanning a half degree. When its center lies on axis, the limb has a field angle of 0.25 degree, and prominences beyond the limb have an even greater field angle.

You can think of a collimator based system as a simple telescope with objective and "eyepiece" giving a collimated output to the etalon, just as it would to your eye. The magnification you see is actually the magnification of the field angles, and this is simple to determine: divide the objective focal length by the collimator focal length. The C8 has an EFL of 2000 mm. The PST collimator lens has a focal length of (-) 200 mm. 2000 / 200 = 10. Therefore the field angle magnification is 10 x, and the limb of the sun's field angle will subtend 0.25 x 10 = 2.5 degrees.

The "acceptance angle" of the PST etalon is about half a degree (0.5) -- this is a good figure to use for most air spaced etalons. Anything greater than a half a degree will begin to fall off-band as the the CWL shifts across the etalon. So in the case of the C8, the sweet spot indeed is about 1/5th the diameter of the sun (2.5 degrees), but this really doesn't depend on the diameter of the objective or the etalon, it only has to do with the field angle magnification, which again is precisely optically defined by the focal length of the objective divided by the focal length of the collimator lens.

So now you can see how to get a larger sweet spot: Find a longer focal length collimator lens. As an example if you used a collimator with a 1000 mm focal length lens, the field angle magnification will be 2000 / 1000 = 2. The limb of the sun will then have a field angle of 0.25 x 2 = 0.5 degree, and the entire disk of the sun should about fit within the acceptance angle of the etalon. There are other geometrical concerns that come into play, and this is why Vallery's method is an OK "rule of thumb," but it is not as precise as the actual optical physics, as I have described.

The EFL of your resulting telescope will also be defined by the field angle magnification factor. For example, if your refocusing lens (located after the etalon) has a focal length of 300 mm, and the field angle magnification is 2, the EFL will be 300 x 2 = 600 mm. For the FA magnification of 10, it would be 300 x 10 = 3000 mm, etc.

Christian Viladrich posted this diagram which shows the origin of the sweet spot in a collimated beam: as the field angles increase, the CWL (center wave length) of the transmission cure is blue shifted away from the H alpha emission line. Disk chromospheric detail will vanish and be replaced by continuum, and prominences will disappear.
Daystar-tilt.JPG

Hi Bob,

My method is strictly correct. Some facts:

1. Telescopic system magnification is M = F / f where F - objective focal length, f - eyepiece focal length

2. Telescope system magnification M = D/d where D - aperture diameter, d - exit pupil diameter

3. An etalon in a collimated system is placed at the exit pupil and it's working diameter will be the same as exit pupil diameter

4. PST collimator lens has f=-200mm and d=20mm, C8 has D=200mm F=2000mm

5. the telescopic system ( C8 + PST collimator) has M = F / f = 2000mm / 200mm = 10x
Also this telescopic system has M = D /d = 200mm / 20mm = 10x

A collimator with f=1000mm can work only as positive lens and it must be 100mm diameter and the same PST etalon (20mm)
will use only 1/5 of aperture of the exit pupil and so, only 40mm of the entrance aperture will work.
More so, in the case of axial position of the PST etalon it will not work at all because it will be shadowed by the central obstruction
of the system. It then must be placed off-axis, but this will not help much - the working aperture will still be the same 40mm.
No gain vs PST telescope itself. For increasing a sweet spot size (or acceptance angle) you also need to increase an etalon size
proportionally to the collimator focal length increasing. So, we come to conclusion that for a collimated system the stweet spot
size is a function of D/d (and etalon bandpass).


Summarize: for the collimated system with the 0.7A FWHM etalon at the exit pupil of this system, the sweet spot size is D / d (degree).
where "d" is a working diameter of the etalon and "D" is system aperture diameter.

Hope this helps.

Valery.


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Re: C8 + PST?

Post by marktownley »

Another complication here chaps - the 'overall' focal ratio of the C8 is f10, but it's main mirror is f2 and the secondary f5 (to give the f10 total) meaning it's not as straight forward as in a refractor system.


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Re: C8 + PST?

Post by swisswalter »

that's like a crime

I love to read it, thanks all for the informations provided


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Re: C8 + PST?

Post by Bob Yoesle »

Hi Valery, great information as usual!

Indeed, all of what you state is of course strictly correct.
Could anyone tell me what's wrong with the system?
I have noted many questions and design misconceptions like this on the forum, and a general lack of understanding of the origins of sweet spots and how to address them.

I believe it might be better to give the questioner the information by which to understand how the sweet spot effect originates (field angle magnification), and how it can be addressed through changing the focal length of the collimator lens -- I have found this of the most importance for doing my own modification calculations and designs. Admittedly my example of using a very much longer FL is not practical with a PST etalon -- it was just intended for illustrative purposes related to the field angle magnifiction.

Best wishes,

Bob


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Re: C8 + PST?

Post by Merlin66 »

Bob,
I think the comment ""The "acceptance angle" of the PST etalon is about half a degree (0.5)""
is part of the stumbling block in the understanding of the "sweet spot" (Jacquinot spot).

A recent paper:
http://www.astro.umd.edu/~veilleux/mmtf/basic.html
explains the sweet spot mechanism...
To quote:
"" The monochromatic, or Jacquinot, spot is defined to be the region over which the change in wavelength does not exceed √2 times the etalon bandpass.""

I made contact with the author of this paper in an effort to define the sweet spot for the PST etalon but we were unsuccessful.
The search continues....


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Re: C8 + PST?

Post by swisswalter »

Hi Ken

more interesting information, thank you very much


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Re: C8 + PST?

Post by Bob Yoesle »

Hi Ken,

I found this paper with a reference to the Jacquinot spot on page 78. It seems to be a more complete description, and shows that the "sweet spot" (Jacquinot spot) is tied to field angles:

http://articles.adsabs.harvard.edu//ful ... 2.000.html
The Jacquinot spot is defined as the field about the optical axis within which the the peak wavelength variation with field angle does not exceed √2 of the etalon bandpass... This angular field can be used to perform close to monochromatic imaging... [Emphasis added]
So if my calculation method is correct (?!), with this we can see that for apparently ANY etalon (air spaced or solid) the sweet spot will be identical and is defined by the bandpass: √2 x FWHM. For a 0.7 Å FWHM etalon, the Jacquinot spot is 1.4142 x 0.7 = 0.9899 degree. Therefore for a front mounted etalon, the entire disc of the sun, and prominences within a half solar radius, will fit easily within the Jacquinot spot. Perhaps an etalon's "acceptance angle" is therefore by definition 1/2 the Jacquinot spot size, as this seems to correlate perfectly with the off-axis Jacquinot spot field angle...

For an etalon placed in a collimator system with a magnification of 2 - where the collimator has 1/2 the objective focal length - the Jacquinot spot stays the same, but the disc of the sun has been magnified from 0.5 degree to 1.0 degree, and perhaps just barely fits inside the "sweet spot." Any higher magnification and one can not fit the entire disc of the sun in the "sweet spot."

It is also known that due to refractive index of the spacer, air spaced etalons are more sensitive to CWL shift with tilt compared to solid spaced etalons with their higher refractive index. Thus the issue of field angles becoming more critical with tilt-tuned etalons placed internally within the optical system, and the need to keep field angle magnifications low for good contrast performance, especially with air spaced etalons.

Christian Viladrich posted this change in CWL verses tilt (or field angle) diagram for a solid spaced etalon:
CWL-tilt1-zoom.JPG
CWL-tilt1-zoom.JPG (79.86 KiB) Viewed 5312 times
It is also interesting to consider the implications for double stacking. When I DS my two front etalons, the bandpass is reduced to ~ 0.5 Å. From the Jacquinot formula, one would expect the "sweet spot" to be reduced to 1.4142 x 0.5 = 0.707 degree. Moreover, the transmission profile is narrowed even further than a single filter with an identical bandpass - again from Chrsitian's paper:
Daystar-double-stack.JPG
Daystar-double-stack.JPG (188.67 KiB) Viewed 5312 times
Therefore the disc of the sun should still fit within the sweet spot, but prominences (or choromspheric detail) only a little over 3/4 (or perhaps even less) a solar radius off-axis should begin to show evidence of a "sweet spot." I may be wrong, but this is not what I recall seeing when I shift the optical axis off the center of the sun, and I will have to explore this further when my weather improves. As with some other parameters, one may be limited in extrapolating the results of DSing to single filter performance, or vice-versa, or perhaps my simple calculation method is in error...
Last edited by Bob Yoesle on Sun Jan 25, 2015 5:54 pm, edited 3 times in total.


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Re: C8 + PST?

Post by Bob Yoesle »

After pondering the above, and reviewing Christian's notes and graph, it does look like all is performing as the Jacquinot spot formula would suggest -- but CWL shift is independent of FWHM (or double stacking), as the FWHM parameter is missing in the formula for calculating the CWL shift:
formula-2.JPG
formula-2.JPG (29.63 KiB) Viewed 5293 times
In Christian's graph, we can see the CWL is blue shifted ~ 0.10 Å at a field angle (tilt) of about 0.5 degree. At a field angle (tilt) of 1.5 degrees, the CWL has shifted 1.0 Å. As Gene Baraff noted in his paper on telecentric performance ( http://home.comcast.net/~g2baraff/Solar ... g/Tele.pdf ), this degradation due to CLW shift will increase quadtratically with off-axis distance. With an air spaced etalon, the off-axis degradation will be greater.

I was curious to see what this looks like, so thanks to a little sunshine, I grabbed some quick DSLR images. I used my DS filter system, and centered the primary image. I then adjusted the second etalon so that the secondary "ghost" reflection(s) just touched each other. I made lengthening exposures in order to capture the details due to the ghost images decreased brightness, and so one can get a "feel" for how detail changes as one moves farther off-axis and the CWL shifts:
IMGP3500 sm.jpg
IMGP3500 sm.jpg (153.58 KiB) Viewed 5293 times
IMGP3497 sm.jpg
IMGP3497 sm.jpg (250.68 KiB) Viewed 5293 times
IMGP3496 sm.jpg
IMGP3496 sm.jpg (211.37 KiB) Viewed 5293 times
IMGP3506 sm.jpg
IMGP3506 sm.jpg (183.54 KiB) Viewed 5293 times
A better or and more valid comparison would have been to use 1) a better camera system, and 2) adjust the primary image off-axis to see how it changed, but my window of opportunity did not allow for this :( . Perhaps others can give that a try...


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Re: C8 + PST?

Post by Bob Yoesle »

Here's my rough subjective estimates of details visible. I've combined similar exposures of the primary image and a similarly exposed secondary image adjoining it:
IMGP3498-3500 lableld sm.jpg
Click for larger version.

Arc A represents the extent of good contrast performance, and appears to be at about 0.4 degree off-axis, which seems to correspond with the Jacquinot spot formula for the ~ 0.5 Å FWHM calculation and a 0.3535 degree acceptance angle. Inside this radius both disc features and prominences (1) appear as they do on the primary image - e.g. this is the "monochromatic" region. Outside of this radius features become more and more blue-shifted.

Arc B represents where significant chromospheric detail on the disc begins to disappear. Between A & B prominences (2), plage features (3), and fainter filaments (4) are becoming more obscure. Beyond this arc continuum begins to dominate, and halfway through the next reflection disc, only continuum is visible.
Last edited by Bob Yoesle on Sun Jan 25, 2015 5:40 pm, edited 2 times in total.


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Re: C8 + PST?

Post by swisswalter »

Hi Bob

A fine test. Thank you very much.


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Re: C8 + PST?

Post by Valery »

Bob Yoesle wrote:Here's my rough subjective estimates of details visible. I've combined similar exposures of the primary image and a similarly exposed secondary image adjoining it:
IMGP3498-3500 lableld sm.jpg
Click for larger version.

Arc A represents the extent of good contrast performance, and appears to be at about 0.4 degree off-axis, which seems to correspond with the Jacquinot spot formula for the ~ 0.5 Å FWHM calculation and a 0.3535 degree acceptance angle. Inside this radius both disc features and prominences (1) appear as they do on the primary image - e.g. this is the "monochromatic" region. Outside of this radius features become more and more blue-shifted.

Arc B represents where significant chromospheric detail on the disc begins to disappear. Between A & B prominences (2), plage features (3), and fainter filaments (4) are becoming more obscure. Beyond this arc continuum begins to dominate, and halfway through the next reflection disc, only continuum is visible.
Hi Bob,

Your results are somewhat contradict with my experiments I did with SM40 + LS35.

Note that SM40 is at CWL with no tilt at all at my location (40m above sea level). LS35 give the best contrast with SM40
when the sun and it's secondary image are just in touch. The full sun disk is excellent DS image, very contrasty.
But ONLY if the sun is exactly in the center of the FOV. If I move the DS sun image only very slightly from the center,
the sun edge becomes significantly less contrasty.
So, the sweet spot in such my combination is exactly 0.5 degree. Prominences still better if I move them closer to the center
of the FOV.

Hope this is useful.


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Re: C8 + PST?

Post by Merlin66 »

Hmmmm
need to check the equations re units for field angle - radians??


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Re: C8 + PST?

Post by marktownley »

Loving the way this thread has developed chaps, very interesting indeed.


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Re: C8 + PST?

Post by Bob Yoesle »

Hmmmm
need to check the equations re units for field angle - radians??
Hi Ken - you got me thinking, and so I have made a new set of calculations. Perhaps someone can check my logic and math on the following:
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...
Therefore the Jacquinot spot peak wavelength variation limit can be mathematically expressed as:

Equation 1: Δλ = √2 x FWHM

The tilt (field) angle verses wavelength change can be found with Christians formula above 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 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 value for a 0.7 Å FWHM etalon.

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. This seems fairly close to my subjective value as noted above.

Again I would appreciate if others could examine these assumptions and figures.


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Re: C8 + PST?

Post by Bob Yoesle »

LS35 give the best contrast with SM40
when the sun and it's secondary image are just in touch.
Hi Valery,

This is the usual case, and if you think about it, the theoretical maximum contrast of the DS system would occur (and you would have better image brightness), if no tilt was required for the second etalon. In this case the primary image and reflection(s) then would overlap each other exactly - something one can not obtain.


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Re: C8 + PST?

Post by Valery »

Bob Yoesle wrote:
LS35 give the best contrast with SM40
when the sun and it's secondary image are just in touch.
Hi Valery,

This is the usual case, and if you think about it, the theoretical maximum contrast of the DS system would occur (and you would have better image brightness), if no tilt was required for the second etalon. In this case the primary image and reflection(s) then would overlap each other exactly - something one can not obtain.
I know this, Bob. But I am wonder why your results are more optimistic than mine.

1. You are liviing (as I conclude from the photo of your observatory) at significantly higher altitude than me.
So, it is very much unlikely that your first etalon is exactly squire to the optical axis and has not tilt to be in a CWL.
It must have some tilt. This will narrow it's sweet spot and influent to the DS sweet spot.

2. A second etalon in your DS should require more tilt than my second etalon. This should make a DS sweet spot smaller.

3. Despite these two reasons your sweet spot is larger than mine.

This is the questions: how this possible? May be your etalons are wider than mine?


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Re: C8 + PST?

Post by Valery »

Bob Yoesle wrote:
Next, for a double stacked etalon system with a 0.5 Å FWHM, and assuming a DS system follows the same rules,
I think the sweet spot will be more distinct with sharper edge (larger contrast gradient).


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Re: C8 + PST?

Post by Bob Yoesle »

Hi Valery,
This is the questions: how this possible? May be your etalons are wider than mine?
I don’t really know. It’s too bad we live so far apart, it would be nice to be able to compare directly. I can only compare to other filter systems I have encountered at star parties and the like, rather infrequently.
You are liviing (as I conclude from the photo of your observatory) at significantly higher altitude than me. So, it is very much unlikely that your first etalon is exactly squire to the optical axis and has not tilt to be in a CWL.
I can tell you the following:

My observatory is at 612 meters. My first/primary etalon requires very little if any tilt, and my second DS etalon also requires less than others I have tried (only a degree or so depending on my need to remove the secondary image from the field of view) - both were originally intended to be used as single etalons, and were produced long before offset CWL DS etalons were the norm.

At HOTS 2005, many told me that they thought I had one of the best filter systems present, comparing it to Markus’ Tak with a 140 front and 90 internal DS. Markus had much better resolution, I had a bit better contrast/uniformity – though overall I would have picked Markus’ !!!

I recently visited OPT in California - one of the largest sellers of H alpha solar filters in the US at an elevation ~ 15 m - to confirm the performance of a prestige set of filters. Mike, OPT’s most experienced solar filter sales associate, stated these were working within spec and doing a fine job. A frequent contributor to the forum has won many an accolade and imaging awards using an identical set of filters.

Next. I sent up my DS system, and Mike asked me where I got my filters - I replied I ordered them from him over 10 years ago - off the shelf. He stated he had not had such a good view through a double stacked filter system since David Lunt paid OPT a visit and he viewed through David’s personal set of SM90's. Mike asked if I wouldn't mind leaving the filters with him for awhile, but I declined ;-)
A second etalon in your DS should require more tilt than my second etalon. This should make a DS sweet spot smaller.
I don't think more tilt will change the overall size of the sweet spot, but will result in increased banding. A slight bit banding can seen on both on the primary image and brightened secondary image in this greatly reprocessed DSLR image (first color image above):
IMGP3500 sm grey adj.jpg
IMGP3500 sm grey adj.jpg (101.36 KiB) Viewed 5159 times
I think the sweet spot [for a narrower bandpass] will be more distinct with sharper edge (larger contrast gradient).
Seems that way to me as well. Christian’s graph for bandpass broadening vs. filter bandpass and F/D (tilt angle change) indicates this lager gradient for narrower bandpass as well. But the transition from on-band to blue shifted might be a bit subtle. More definitive might be the transition from any blue-shifted detail to continuum. For me this appeared near the middle or so of the second reflection solar disc -- about 1 + degree off-axis. Where does this occur for your filter system?

So “how is this [e.g. my results] possible?” I wish I had the expertise to give you a definitive answer. Christian’s paper http://www.astrosurf.com/viladrich/astr ... aystar.htm definitively states:
More interesting, it can be noticed that the shape of the transmission (and the FWHM) does not change with the tilt... This is because the incoming beam of light is collimated. [As would apply to front-mounted etalons]
My guess at this point is that maybe that etalon finesse plays a significant role in the off axis tilt angle performance. The lower transmission in the tails of a high-finesse filter system - which limits out-of band parasitic light more than a lower finesse filter - may result in a larger perceived sweet spot (less off-band degradation) compared to a lower finesse filter system...?
Last edited by Bob Yoesle on Tue Jan 27, 2015 10:13 pm, edited 1 time in total.


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Re: C8 + PST?

Post by Valery »

Hi Bob,

Thanks a lot for your info.

My SM40 has no tilt at all to be in CWL or very near it. And what I have found is that this etalon
starts to work at the very limit of usefulness if the atmospheric pressure reached 772mm Hg.
It works best at 762mm Hg and require a bit of tilt at 755-760mm Hg.

So, I see that 15mm Hg is almost a limit for CWL shift to the blue wing. If your first etalon does not
require any tilt at all at the 612m elevation, then it should not work at 15m elevation - too much
blue shift and no any tilt will help, just opposite. May be it still require a bit of tilt.

To realize if it require a tilt or not, peform the easy test: use a magnification when the FOV is about
1.5 degree. Better 90 or 100 degree eyepiece. Adjust the etalon for maximal contrast in the center of the FOV.
Then move the sun across the FOV in perpedicular directions - along with tilt mechanism and perpendicularly.
See the size of the sweet spot. If no tilt, the sweet spot will be equal along both directions. If there is a tilt,
then you have a banding and the sweet spot size will be different - smaller along of the tilt mechanism direction.

If you ever decide to sell your first etalon SM90, I will be the first in the turn for it.

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Re: C8 + PST?

Post by Merlin66 »

If both etalons are designed to be tilt tuned ( I had a pair of external SM60 etalons with T max tuning), then I'd also assume that the "design" wavelength would be slightly above Ha to allow the tilt tuning into "band" and then onto the blue.
I have use the etalons as single stacks (on the ED80) and with the PST as a double stack, and this seems to be the case for me.
What this infers to me is that when the second etalon is tilted to remove the "ghosts in the machine" we must be tuning it, probably into the blue wing, and when combined as a DS ( with the other etalon on band) the overall FWHM will still be there, very slightly towards the blue, but the intensity of the image reduced.
Does this make any sense?


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Re: C8 + PST?

Post by Valery »

Ken,

In the ideal pair of etalons for DS both etalons must require a little tilt - 2x smaller than than in the second etalon
in the case of first etalon has no tilt and second etalon has a minimal tilt when the ghost is a bit farther than a
touch (to allow full disk and prominences be not blended with the ghost).
And need to be clocked so, that they move the ghost in the same direction. In this given case the resulting delta Lambda
will be smallest.

As for your second case with blue shift for a second etalon. You will not reach narrower FWHM. It will stay the same,
but the S/N will be worser.


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Re: C8 + PST?

Post by Merlin66 »

Valery,
We had a spreadsheet (probably still in the files area) which shows the reduction in FWHM with etalons slightly off-set to each other. Such an arrangement of, say, double 0.7A etalons does indeed give a FWHM of around 0.4A. The intensity peak is reduced, so I suppose you could say there's a lower SNR.
Added: If you tilt the front element say 1 degree to push the "ghosts" from the FOV, this would, based on Bob's graphs, move the CWL towards the blue by 0.4A.


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Re: C8 + PST?

Post by Valery »

Merlin66 wrote:Valery,
We had a spreadsheet (probably still in the files area) which shows the reduction in FWHM with etalons slightly off-set to each other. Such an arrangement of, say, double 0.7A etalons does indeed give a FWHM of around 0.4A. The intensity peak is reduced, so I suppose you could say there's a lower SNR.
Added: If you tilt the front element say 1 degree to push the "ghosts" from the FOV, this would, based on Bob's graphs, move the CWL towards the blue by 0.4A.
Not exactly 0.4. And if to take in to account the S/N decreasing, there will be no gain vs standard DS when both peaks are at the same WL.

I experimented a lot with all my etalons and always the best results were when both etalons transmission curves were coincide - when the image was maximally bright and contrasty.


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Re: C8 + PST?

Post by Merlin66 »

I don't disagree....
"the best results were when both etalons transmission curves were coincide "
The question was, when one element is tilted (to remove ghosts), how do you get it and the other back "on-band"


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Re: C8 + PST?

Post by Valery »

Merlin66 wrote:I don't disagree....
"the best results were when both etalons transmission curves were coincide "
The question was, when one element is tilted (to remove ghosts), how do you get it and the other back "on-band"
The second etalon is typically designed so, that it come to the CWL H-a when it tilted enough to place the ghost out of
the main solar image. I do have such a pair of LS50 etalons - the main etalon does not require any tilt to come to the CWL H-a
(at my typical atmospheric pressure and temperature) and the second etalon when adjusted to the CWL places the ghost
sun image outside or main solar image. Of course, when used as an internal etalon it produce severe banding and sweet
spot is not round, but a band - right enough to cover my camera chip.


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Re: C8 + PST?

Post by Merlin66 »

Ah....
You're using different etalons within the DS.....
What about the case when you use two identical etalons??? Same CWL and bandwidth?


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Re: C8 + PST?

Post by Valery »

Merlin66 wrote:Ah....
You're using different etalons within the DS.....
What about the case when you use two identical etalons??? Same CWL and bandwidth?

Then we need circular polarizator between the etalons, cause we can't get rid of the ghost or
we need to change the elevation (higher) - and then tilt the etalons slightly and in opposite directions.


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Re: C8 + PST?

Post by Bob Yoesle »

What about the case when you use two identical etalons??? Same CWL and bandwidth?

...cause we can't get rid of the ghost or we need to change the elevation (higher) - and then tilt the etalons slightly and in opposite directions.
Hmmm. That's VERY interesting Valery.

Perhaps why despite my elevation with my two nearly identical etalons I'm rarely tilting the primary etalon when double stacking - since I can't actually tilt it in the opposite direction (it's been a long time since I've used it single stacked to know the tilt needed when used by itself). All I can do is leave it in the "flat" TMax position and apply the necessary tilt to the second etalon. Most of the time the second etalon requires only about 1 mm of opening at the thumb wheel of the TMax tilt to remove all ghost reflections from the field of view of a low power eyepiece. Since the TMax has a diameter of 135 mm, 1/135 = tan θ, and therefore θ = 1.3 degrees.

I've also wondered why despite my custom clocking adapter I always find the overall best clocking position is with the TMax tilt hinge plates oriented 180 degrees from each other...

When left in the ~ 1.3 degree tilt position I had used frequently for visual observing, I found my images showed a bit of contrast gradient, even thought this was difficult to see visually:
5-6-13 Ha full disk proc jpg sm 2.jpg
5-6-13 Ha full disk proc jpg sm 2.jpg (263.4 KiB) Viewed 4297 times
More recently for imaging I have taken to leaving the first ghost just off the CCD sensor for minimum tilt, and this does give better uniformity to the disk contrast:
3-11-14 Full Disk SM.jpg
3-11-14 Full Disk SM.jpg (269.81 KiB) Viewed 4297 times
I have PM'd a couple of people including Christian to see if they can look at my Jacquinot spot rationale and calculations -- I really would like to see how DSing affects this, especially because of the narrowed transmission profile over that of a nominally identical FWHM single stacked filter...


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Re: C8 + PST?

Post by Merlin66 »

Bob,
I still can't get my head around the issue of the CWL shifting when the etalon is tilted (to remove ghosts)
With identical etalons surely this must lead to a situation where the CWL's are NOT coincident....
(I used the tune my external double stack to give maximum on band brightness, then effectively lock them together to form a "single" 0.4A bandwidth unit. To see the red and blue wings the whole assembly was tilted - the relative positions of the etalons being unchanged)


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Re: C8 + PST?

Post by marktownley »

I think the premise here is wrong; there is no such thing as two identical etalons. They may be 'close' but they are not identical. 'Identical' etalons occur in a textbook and an equation but not in day to day use.


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Re: C8 + PST?

Post by Merlin66 »

Mark,
Yeah, but.....
A 1 degree ghost tilt (+/- 0.4A shift) is still a hellava shift...
To me it would mean that the etalon, by design, would have to have a CWL of at least +0.4/0.5A above Ha to allow ghost tilting back on band.


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Re: C8 + PST?

Post by Bob Yoesle »

I think the premise here is wrong; there is no such thing as two identical etalons. They may be 'close' but they are not identical. 'Identical' etalons occur in a textbook and an equation but not in day to day use.
Then add to the mix that each etalon is not heated identically in series, or held absolutely firm in a constraining cell, but 'floats' on a sponge-foam ring and is radially supported by buttons of silicone, and then add in some changing ambient temperatures from morning to afternoon, followed by changes in gravitational pulling as the scope tracks from horizon to horizon... you get the idea.


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Re: C8 + PST?

Post by Valery »

And the last and the most influent factor for air spaced etalons - a barometric pressure.


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Re: C8 + PST?

Post by Bob Yoesle »

Christian has now added a section on air spaced etalons with the calculation of the Jacquinot spot diameter:

http://www.astrosurf.com/viladrich/astr ... P-Coro.htm


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Re: C8 + PST?

Post by Valery »

Bob Yoesle wrote:Christian has now added a section on air spaced etalons with the calculation of the Jacquinot spot diameter:

http://www.astrosurf.com/viladrich/astr ... P-Coro.htm
Thank you, Bob, for this useful link.

Valery.
Last edited by Valery on Mon Feb 02, 2015 8:25 pm, edited 1 time in total.


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Re: C8 + PST?

Post by swisswalter »

thanks Bob


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Re: C8 + PST?

Post by marktownley »

excellent indeed, thanks!


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Re: C8 + PST?

Post by Merlin66 »

Some great stuff being discussed here.
The etalon as we know is only part of the Ha filter system.
I think the ITF and final sorting (blocking) filter supress the etalon "wings"


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Re: C8 + PST?

Post by Merlin66 »

The final section:
""Here we assume we have an incident convergent beam with no field angle. This is the situation when using a F-P etalon in a telecentric system"
I don't think this is correct...
Even with an external mounted etalon, the field angle will be 0.25 degrees. We have seen from previous discussions that the telecentric lenses can change this field angle, but never to zero.


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