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Causes for non uniformity of Front Etalon Solar Systems

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#1 Dennissloan

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Posted 10 August 2020 - 02:55 PM

Greetings ,
What are the causes for lack of full disk uniformity in front etalon solar systems such as Coronado and Solarscope? Can their be solutions to remedy this problem?
Thanks
Dennis

#2 PETER DREW

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Posted 10 August 2020 - 03:09 PM

I think the causes are likely to be many and varied and not necessarily confined to the front etalon, the blocking filter can also have its variations.  You need a "perfect storm" of compatibility for the highest quality performance.  Solar Ha components are very specialised, hence the high prices.


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#3 Dennissloan

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Posted 10 August 2020 - 03:49 PM

I do agree the blocking filter must not be overlooked.
More time should be spent in matching individual BF with the front etalon in daytime environment.

#4 Dennissloan

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Posted 10 August 2020 - 03:49 PM

I do agree the blocking filter must not be overlooked.
More time should be spent in matching individual BF with the front etalon in daytime environment.

#5 briansalomon1

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Posted 10 August 2020 - 04:04 PM

I'm assuming you're talking about the fact that only a portion of the image that appears in the eyepiece is sharply "on band" (displaying a sharp detailed image) in a tilt tuned etalon. I believe it's caused mainly by the way the etalon tunes.

 

A tilt etalon tunes because the distance the light has to travel in the etalon changes (becomes longer) as the etalon is tilted. That means the sharp on-band part of the light coming through the etalon only appears across a portion of the displayed image.

 

Here's a diagram - https://www.cloudyni...998-tilt-tuned/

 

Assuming that's what you mean, it's a design limitation and can't be eliminated. Adding a second etalon at 90 Degrees to the first will extend the on-band area. Pressure tuned etalons have a circular, rather than rectangular on-band transmission, which may be better for imaging. I prefer tilt tuned for their simplicity but I think the new Lunt double stack pressure tuned units probably perform really well....

 

I'm just a visual guy myself and don't know if it's possible to take a succession of images as you tune through the band and edit out all the off-band portions.

 

I haven't researched this very much but believe it's accurate.



#6 BYoesle

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Posted 10 August 2020 - 04:21 PM

Lack of uniform etalon plate flatness.

Lack of uniform gap thickness (spacer thickness in an air-spaced etalon).

Wedge - lack af etalon plate parallelism.

Etalon CWL too high off-band, which requires excessive tilt and creates the "banding" non-uniformity.

Allowing insufficient time for the etalon to reach thermal equilibrium.

 

These factors are exacerbated with double stacking which enhances the presence of any contrast uniformity issues.

 

If well made and properly tuned, the filter(s) will have near perfect contrast uniformity - but this requires time and great effort - and a commensurate price tag for the quality. 

 

Due to the relatively large FWHM of the blocking filter compared to the etalon itself, this would be the least significant factor.

 

An internally placed etalon will add the reduced size of the Jacquinot spot, which depending on field angle magnification, may or may not affect full disc uniformity. For full discs, the general rule is no more than a two (2) times for the field angle magnification. This also means the collimator lens' focal length should not be less than half the objective's focal length.


Edited by BYoesle, 10 August 2020 - 04:32 PM.

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#7 Dennissloan

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Posted 10 August 2020 - 05:34 PM

Thanks Bob. I’m concern with a single etalon less than .7A. If properly manufactured should have uniformity across the entire solar disk. Quality control and more testing through fabrication of the etalon

#8 BYoesle

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Posted 10 August 2020 - 09:01 PM

 

Can their be solutions to remedy this problem?

 

You can return the etalon for a replacement which hopefully performs better.

 

Try before you buy is the best advice.

 

Other than that, getting a custom made etalon from a professional level supplier would be your best - and a very expensive - alternative.



#9 Dennissloan

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Posted 11 August 2020 - 06:05 AM

The main feature I found with front air etalons is a preferred orientation between the etalon and BF. In other words, rotating the etalon respect to BF , variations in passband across the solar disk is seen. One chooses the best orientation. I feel it’s the wedge issue non parallel of the two plates.

#10 briansalomon1

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Posted 11 August 2020 - 09:47 AM

Lack of uniform etalon plate flatness.

Lack of uniform gap thickness (spacer thickness in an air-spaced etalon).

Wedge - lack af etalon plate parallelism.

Etalon CWL too high off-band, which requires excessive tilt and creates the "banding" non-uniformity.

Allowing insufficient time for the etalon to reach thermal equilibrium.

 

These factors are exacerbated with double stacking which enhances the presence of any contrast uniformity issues.

 

If well made and properly tuned, the filter(s) will have near perfect contrast uniformity - but this requires time and great effort - and a commensurate price tag for the quality. 

 

Due to the relatively large FWHM of the blocking filter compared to the etalon itself, this would be the least significant factor.

 

An internally placed etalon will add the reduced size of the Jacquinot spot, which depending on field angle magnification, may or may not affect full disc uniformity. For full discs, the general rule is no more than a two (2) times for the field angle magnification. This also means the collimator lens' focal length should not be less than half the objective's focal length.

Thanks for the explanation and the links. I'll read through them and see if I can improve my understanding.



#11 briansalomon1

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Posted 12 August 2020 - 07:50 AM

I have reviewed "everything you ever wanted to know about etalons but were afraid to ask" as well as some of the other links.

 

I now understand an air spaced tilt tuned etalon actually should produce an on-band image across most, if not all of the entire etalon and when it doesn't it's due to imperfections in the manufacturing process, tuning, environmental conditions or use. It should also tune uniformly from red to blue with very little tilt.

 

What I had described as a design limitation is actually an imperfect and/or incorrectly tuned etalon. I apologize to the forum for the misleading post.



#12 Dennissloan

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Posted 12 August 2020 - 01:24 PM

It’s so true that front air etalon used a single stack .7A should be uniform across the entire solar disk. Alignments with the ERF tilt, and tilt of the wedge plus centering the CWL all are non uniformity factors. With BF at 5-6A, is not a factor.


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