Impact of tilt on a telecentric assembly

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christian viladrich
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Impact of tilt on a telecentric assembly

Post by christian viladrich »

Hello,
In a former post, Minh asked the question of the impact of the tilt of the telecentric assembly (including the etalon) on solar images (FWHM et Center Wavelength):
http://www.solarchatforum.com/viewtopic ... 0&start=25
It was the case of the focuser "droop" with the telecentric and etalon aligned on the same optical axis, but at a tilt with the telescope optical axis.

I had a new look at it and I am afraid I was wrong in the answer I gave to Minh. So here is the correct answer.
There is indeed a blueshift of the CWL and a broadening of the FWHM when the telecentric is tilted. Still, the impact is smaller than if the etalon alone is tilted (i.e. the telecentric is not tilted and is on the same optical axis as the telescope).

The following table is the results of Zeemax simulations on AiryLab Hat 8"+ 3.7 x telecentric (courtesy AiryLab) and OSLO simulations on a 300 mmf/5.5 Newtonian + 5x telecentric.
http://astrosurf.com/viladrich/astro/in ... lation.jpg

The first column gives the angle of tilt (0, 1, 2°). The last column gives the angle of the chief ray (angle of the axis of the cone of light falling on the filter).We can see that the angle of the chief ray is smaller that the angle of tilt.

An interesting point is that image quality is hardly affected by the telecentric tilt.

More information there:
http://astrosurf.com/viladrich/astro/in ... ntric.html


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Re: Impact of tilt on a telecentric assembly

Post by minhlead »

christian viladrich wrote: Sat May 29, 2021 11:50 am Hello,
In a former post, Minh asked the question of the impact of the tilt of the telecentric assembly (including the etalon) on solar images (FWHM et Center Wavelength):
http://www.solarchatforum.com/viewtopic ... 0&start=25
It was the case of the focuser "droop" with the telecentric and etalon aligned on the same optical axis, but at a tilt with the telescope optical axis.

I had a new look at it and I am afraid I was wrong in the answer I gave to Minh. So here is the correct answer.
There is indeed a blueshift of the CWL and a broadening of the FWHM when the telecentric is tilted. Still, the impact is smaller than if the etalon alone is tilted (i.e. the telecentric is not tilted and is on the same optical axis as the telescope).

The following table is the results of Zeemax simulations on AiryLab Hat 8"+ 3.7 x telecentric (courtesy AiryLab) and OSLO simulations on a 300 mmf/5.5 Newtonian + 5x telecentric.
http://astrosurf.com/viladrich/astro/in ... lation.jpg

The first column gives the angle of tilt (0, 1, 2°). The last column gives the angle of the chief ray (angle of the axis of the cone of light falling on the filter).We can see that the angle of the chief ray is smaller that the angle of tilt.

An interesting point is that image quality is hardly affected by the telecentric tilt.

More information there:
http://astrosurf.com/viladrich/astro/in ... ntric.html
Christian,
Can not thank you enough for the time you have put into finding the answer for this. And as someone who have tilted the telecentric and etalon intentionally, I can confirm that with a 1 degree tilt, the CWL shift about 0.15-0.2 A (2 clicks of the tuner on the Quark).
But I do not see the effect of FWHM widening. According to your calculation, the tilt will introduce huge widening effect into FWHM. a 0.5 A etalon will behave like a 0.95A at F/30 when there's a tilt of 0.5 degree (which most focuser have, as Daystar stated). If this is true then most of the Quark that's on the market won't be able to see any chromosphere detail due to the tilt alone.
Also, after introduce a 1 degree tilt into my optical stack (about 0.2 degree tilt at the etalon according to your calculation), I do not notice significant lost of contrast (suggesting that there aren't significant FWHM widening).
Thanks again for shedding light on this. Really appreciate it!


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Re: Impact of tilt on a telecentric assembly

Post by christian viladrich »

Hello Minh,
Thanks for your comments. I think your are refering to the center figure (0.5 A etalon) here :
http://astrosurf.com/viladrich/astro/in ... -angle.JPG
If so, it says that if the etalon is tilted by 0.5°, then the resulting FWHM is equal to 0.95A at F/30. The important thing here, is that the etalon is no tilted. Only the etalon is tilted.

However, I understand you are refering to another case, which is a tilt of the telecentric of 0.5° on the optical axis, with the etalon square to the optical axis of the telecentric (=focuser "droop"). If so, the effective tilt at the etalon is about only 0.1 to 0.15°, so the effective FWHM is now about 0.67A instead of 0.60 A without any tilt (for a nominal 0.5A FWHM). It makes very little difference visually.

In your other example (1° tilt of the telecentric resulting in a 0.2 °tilt at the etalon) the effective FWHM will go from 0.6A to about 0.75A at f/30, assuming a nominal 0.5A. At this stage, it is difficult to assess what is the nominal FWHM of your Quark. A 0.3A etalon would be much more sensitive to these issues than a 0.6A etalon.


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Re: Impact of tilt on a telecentric assembly

Post by Merlin66 »

Interesting.
I found that any tilt of the CaK filter assembly, as expected, moved the CWL towards the blue.
Using a spectroscope to measure the CWL and the band pass FWHM removes any ambiguity.


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Re: Impact of tilt on a telecentric assembly

Post by minhlead »

christian viladrich wrote: Sat May 29, 2021 3:44 pm Hello Minh,
Thanks for your comments. I think your are refering to the center figure (0.5 A etalon) here :
http://astrosurf.com/viladrich/astro/in ... -angle.JPG
If so, it says that if the etalon is tilted by 0.5°, then the resulting FWHM is equal to 0.95A at F/30. The important thing here, is that the etalon is no tilted. Only the etalon is tilted.

However, I understand you are refering to another case, which is a tilt of the telecentric of 0.5° on the optical axis, with the etalon square to the optical axis of the telecentric (=focuser "droop"). If so, the effective tilt at the etalon is about only 0.1 to 0.15°, so the effective FWHM is now about 0.67A instead of 0.60 A without any tilt (for a nominal 0.5A FWHM). It makes very little difference visually.

In your other example (1° tilt of the telecentric resulting in a 0.2 °tilt at the etalon) the effective FWHM will go from 0.6A to about 0.75A at f/30, assuming a nominal 0.5A. At this stage, it is difficult to assess what is the nominal FWHM of your Quark. A 0.3A etalon would be much more sensitive to these issues than a 0.6A etalon.
I'll try a bigger tilt (2 degrees) and see if there are lost in contrast.
Merlin66 wrote: Sat May 29, 2021 11:03 pm Interesting.
I found that any tilt of the CaK filter assembly, as expected, moved the CWL towards the blue.
Using a spectroscope to measure the CWL and the band pass FWHM removes any ambiguity.
I wish I have one. Anyone here with a spectrometer with enough resolution to measure this?


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Re: Impact of tilt on a telecentric assembly

Post by Merlin66 »

I have various spectrographs, my hi-res Littrow will resolve to better than 0.5A. With a narrow slit and bright target, down to around 0.4A.
Getting spectrographs together with solar filters and optics is the challenge.


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Re: Impact of tilt on a telecentric assembly

Post by minhlead »

Merlin66 wrote: Mon May 31, 2021 6:14 am I have various spectrographs, my hi-res Littrow will resolve to better than 0.5A. With a narrow slit and bright target, down to around 0.4A.
Getting spectrographs together with solar filters and optics is the challenge.
Could you please design a test for this? I really curios.
christian viladrich wrote: Sat May 29, 2021 3:44 pm Hello Minh,
Thanks for your comments. I think your are refering to the center figure (0.5 A etalon) here :
http://astrosurf.com/viladrich/astro/in ... -angle.JPG
If so, it says that if the etalon is tilted by 0.5°, then the resulting FWHM is equal to 0.95A at F/30. The important thing here, is that the etalon is no tilted. Only the etalon is tilted.

However, I understand you are refering to another case, which is a tilt of the telecentric of 0.5° on the optical axis, with the etalon square to the optical axis of the telecentric (=focuser "droop"). If so, the effective tilt at the etalon is about only 0.1 to 0.15°, so the effective FWHM is now about 0.67A instead of 0.60 A without any tilt (for a nominal 0.5A FWHM). It makes very little difference visually.

In your other example (1° tilt of the telecentric resulting in a 0.2 °tilt at the etalon) the effective FWHM will go from 0.6A to about 0.75A at f/30, assuming a nominal 0.5A. At this stage, it is difficult to assess what is the nominal FWHM of your Quark. A 0.3A etalon would be much more sensitive to these issues than a 0.6A etalon.
So I tried a really drastic tilt: almost 3 degree (I had to use a longer screws to achieve a higher tilt) and it shift the CWL further blue (the best setting now seems to be 0 or 1) but I still do not notice a very big differences in contrast other than some halo that I think due to internal reflection rather than FWHM widening. The best contrast seems stable in all 3 tests: zero tilt, 1 degree tilt and 3 degree tilt (before the telecentric).
I know this is not a scientific test but it's does seems strange.


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Re: Impact of tilt on a telecentric assembly

Post by christian viladrich »

As explained by Ken, it is very challenging to measure narrow band filters.

One nice approach, valid for air-spaced etalon, is the one using a Ha lamp :
http://astrosurf.com/viladrich/astro/in ... sting.html
Still, it is restricted to measurements at normal incident.

I am currently testing a Sol'Ex spectrograph. The resolution is 0.1 A/pixel which is a nice fit to Ca K filters. I am having nice results with Ca K filters with FWHM = 2 to 4 A, both in terms of CWL and FWHM measurements. The impact of tilt is obvious.

Measuring Ha filters down to 0.3 A would ask for 5 to 10x more spectral resolution. This is much more difficult.

BTW Minh, what part do you tilt ? Is it the whole Quark (i.e. Quark mounted on a tilt plate), or only the etalon part (i.e. tilt plate between the telecentric and the etalon) ?


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Re: Impact of tilt on a telecentric assembly

Post by minhlead »

christian viladrich wrote: Mon May 31, 2021 11:07 am As explained by Ken, it is very challenging to measure narrow band filters.

One nice approach, valid for air-spaced etalon, is the one using a Ha lamp :
http://astrosurf.com/viladrich/astro/in ... sting.html
Still, it is restricted to measurements at normal incident.

I am currently testing a Sol'Ex spectrograph. The resolution is 0.1 A/pixel which is a nice fit to Ca K filters. I am having nice results with Ca K filters with FWHM = 2 to 4 A, both in terms of CWL and FWHM measurements. The impact of tilt is obvious.

Measuring Ha filters down to 0.3 A would ask for 5 to 10x more spectral resolution. This is much more difficult.

BTW Minh, what part do you tilt ? Is it the whole Quark (i.e. Quark mounted on a tilt plate), or only the etalon part (i.e. tilt plate between the telecentric and the etalon) ?
I tilt the Quark and everything behind it. I use a quark with intergrated barlow so titling is always tilt both the telecentric barlow and the etalon together


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Re: Impact of tilt on a telecentric assembly

Post by christian viladrich »

minhlead wrote: Mon May 31, 2021 2:35 pm I tilt the Quark and everything behind it. I use a quark with intergrated barlow so titling is always tilt both the telecentric barlow and the etalon together
Thanks for the clarification. So, I would say in your setup :
- the telecentric in tilted with respect to the telescope optical axis,
- the etalon is not tilted with respect to the telecentric (= the optical axis of the telecentric is normal to the etalon surface).


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Re: Impact of tilt on a telecentric assembly

Post by minhlead »

christian viladrich wrote: Mon May 31, 2021 4:23 pm
minhlead wrote: Mon May 31, 2021 2:35 pm I tilt the Quark and everything behind it. I use a quark with intergrated barlow so titling is always tilt both the telecentric barlow and the etalon together
Thanks for the clarification. So, I would say in your setup :
- the telecentric in tilted with respect to the telescope optical axis,
- the etalon is not tilted with respect to the telecentric (= the optical axis of the telecentric is normal to the etalon surface).
Yes. That is correct.


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