Correct me if I'm wrong.

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pupak
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Correct me if I'm wrong.

Post by pupak »

Situations are often solved here when I have an F6 telescope and an etalon with collimating lenses at F7, or F6/F0, F8/F10. Sometimes someone recommends setting the lens to the appropriate F.
But I think it's pointless. After all, the aperture of a collimating lens cuts the light cone to its F, if it is correctly positioned at the focal point.
I did a test with the 150/F6 and the LS80 etalon for f7 with an aperture of 128mm. As a result, there is no noticeable difference in contrast, but there is a visible drop in detail.
What do local experts have to say about it?


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Re: Correct me if I'm wrong.

Post by Valery »

pupak wrote: Wed Mar 08, 2023 3:18 am Situations are often solved here when I have an F6 telescope and an etalon with collimating lenses at F7, or F6/F0, F8/F10. Sometimes someone recommends setting the lens to the appropriate F.
But I think it's pointless. After all, the aperture of a collimating lens cuts the light cone to its F, if it is correctly positioned at the focal point.
I did a test with the 150/F6 and the LS80 etalon for f7 with an aperture of 128mm. As a result, there is no noticeable difference in contrast, but there is a visible drop in detail.
What do local experts have to say about it?
Buy and use a refractor with F/7-8, add a LS80 etalon and observe/image the sun. It will work at full aperture, full resolution and maximal possible sweet spot at F/7. With a little smaller sweet spot at F/7.5 and even a little smaller at F/8. But still at a full resolution.


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Re: Correct me if I'm wrong.

Post by pupak »

Valery wrote: Thu Mar 09, 2023 12:14 pm Buy and use a refractor with F/7-8, add a LS80 etalon and observe/image the sun. It will work at full aperture, full resolution and maximal possible sweet spot at F/7. With a little smaller sweet spot at F/7.5 and even a little smaller at F/8. But still at a full resolution.
What you write, Valery, is true, but I am dealing with the F5-6 aperture for the F7 etalon. That's a different situation.
It is about whether the F7 aperture of the standard collimating lens is equivalent to the make F7 aperture of the F5-F6 lens. If so, then the lens aperture seems completely unnecessary to me.


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Re: Correct me if I'm wrong.

Post by christian viladrich »

Hello,
I am not sure this is helpfull or not, but we have the following :

- if the f-ratio of the telescope < f-ratio of collimator, then the objective of the telescope is vigneted. This is like having a telescope with a smaller aperture (= D * f-ratio telescope / f-ratio collimator),

- if the f-ratio of the telescope > f-ratio of collimator, there is no problem. The only thing is that you would have the same result with a smaller (=less costly) etalon.


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Re: Correct me if I'm wrong.

Post by pupak »

I guess I can't express myself correctly, but I have often been advised to stop the F6 telescope at F7 if I want to use the LS80 standard, which is at F7. What I'm talking about is that the collimator lens will adjust F to F7 regardless of whether I stop the lens or not.


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Re: Correct me if I'm wrong.

Post by christian viladrich »

Yes indeed, the collimator will have an f/7 beam whatever the f-ratio of the refractor.

And as the refractor is at f/6, only part of its lens will be used. You won't need to stop it down "mechanically" with an aperture stop. This will be done "optically" by the collimator.


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Re: Correct me if I'm wrong.

Post by pupak »

Thank you for confirmation.


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Re: Correct me if I'm wrong.

Post by Bob Yoesle »

- if the f-ratio of the telescope < f-ratio of collimator, then the objective of the telescope is vigneted. This is like having a telescope with a smaller aperture (= D * f-ratio telescope / f-ratio collimator),

- if the f-ratio of the telescope > f-ratio of collimator, there is no problem. The only thing is that you would have the same result with a smaller (=less costly) etalon...

Yes indeed, the collimator will have an f/7 beam whatever the f-ratio of the refractor.

And as the refractor is at f/6, only part of its lens will be used. You won't need to stop it down "mechanically" with an aperture stop. This will be done "optically" by the collimator.
"A picture is worth a thousand words" and really makes it easy to understand:
Internal etalon geometry.png
Internal etalon geometry.png (100.95 KiB) Viewed 1710 times

You can see as Christian has described the objective focal ratio being larger than the collimator simply results in a smaller portion of the etalon being used - which may actually improve performance due to there being less overall CWL variation over a smaller diameter of the etalon diameter.

With a shorter objective focal ratio compared to the collimator, you are effectively reducing the objectives diameter and the overall system resolution can be negatively impacted.

On the other hand, the "sweet spot" is completely dependent on the ratio of the focal lengths (magnification) of the objective and the collimator lenses: FL Obj / FL col. The larger the magnification, the smaller the sweet spot. If for example we assume an air-spaced 0.7 A etalon has an acceptance angle of 0.5 degree, it will have a Jacquinot (sweet) spot diameter of 1 degree. A collimator of 1/2 the objective's FL will reduce this to 0.5 degree. Anything shorter will reduce the Jacquinot spot to being smaller than the disc of the sun. A collimator with 1/4 the objectives FL will have a Jacquinot spot 1/4 a degree = 1/2 the sun's disc diameter, etc.

Bob


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Re: Correct me if I'm wrong.

Post by AndiesHandyHandies »

Hi

My solution for using a F7.5 at full resolution with a PST Mod2 was to buy new collimating and re-focus lenses.

150mm FL 25mm Opto-Sigma from Laser 2000. I do not know what the neighbours who took the package thought I was up to.

20x the FL is the majic formulae.

For my Rumak F10 PST Mod2 I moved the collimating lens forward as its not possible to chop the back off a Maksutov to reach focus.

Cheers. Andrew.


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