Internal etalon size

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JesusRL
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Internal etalon size

Post by JesusRL »

I guess that like most, or at least many, of the amateur astronomers I am limited in my budget, but, at the same time, I would like to take the best from it.

The fact is that I would like to have the maximum resolution (aka telescope diameter) with the minimum cost (aka etalon diameter) in my Ha setup.

I have some ideas on how to do it, but may they be misconceptions, I would like to be corrected by who knows about this.

If I were to use an external etalon there is no discussion: etalon diameter == telescope diameter == €$£¥

But If I use an internal etalon these are my thoughts (please correct me):

The etalon works at it's best when parallel rays of light cross it so it has an input lens that converts from the predefined f ratio to parallel and an output lens that does the opposite, converting from parallel to the f ratio again. The distance covered between these 2 lenses does not count in the focal length.
Image

If I use an etalon bigger than the path of light at its current position I am wasting my money: If I put it smaller the image will suffer vignetting.
Image

When double stacking the conclusions are the same.
Image

The actual use that one of the most known companies in the sector makes of this second stack internal unit creating a single etalon that can be used with different diameters (80&100 0r 130&152) makes me think that conditions for a proper use of the ethalon may be more related to using a proper f ratio than any other thing (rays of light do not suffer bottle necks inside the etalon).

NOW: Could I use an etalon from a smaller telescope in a bigger one as long as it has exactly the same f ratio and is placed in the light path more in the back to avoid vignetting but using its full diameter?
OR IN OTHER WORDS: What is the advantage of using a bigger etalon placed more in the front? Is there any?
Image

I am sure all this has been already discussed here; excuse me if asking redundant subjects.
Thanks in advance for any enriching idea or comment


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Bob Yoesle
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Re: Internal etalon size

Post by Bob Yoesle »

What is the advantage of using a bigger etalon placed more in the front? Is there any?
It is best to use as large an etalon as possible, due to the size and focal length (and related f ratio which should be as close as possible to the objective's) of the collimator lens in order to minimize field angle magnification and give you the largest Jacquinot ("sweet") spot diameter.

The lens used to collimate the light presented to the etalon can only provide parallel rays through the etalon on-axis. Off-axis, since the Sun is not a point object but has a substantial diameter, and the off-center field rays are magnified. The placement of the collimator lens's focus must be coincident with the objective lens's focus to produce collimated light rays. However, the shorter the focal length, the greater the field angle magnification - this is essentially what a simple telescope and negative eyepiece does.

For example, if you have an objective with a 1000 mm FL, and the collimator is half the diameter with half the focal length (500 mm with the same same f ratio to prevent vignetting), the field angle magnification is 1000/500 = 2, and you would use an etalon with a clear aperture of 1/2 the objective.

If you use and etalon with 1/4 the diameter of the objective, the collimator lens FL would also have to be 1/4 the objective FL (i.e. 250 mm) to keep the f-ratio the same, and the field angle magnification is 1000/250 = 4.

A single air-spaced etalon with a band-pass (FWHM) of 0.7 A has a Jancquinot spot diameter of 1 degree. The is etalon on the front of the telescope therefore will take in the entire diameter of the Sun and another 1/2 solar diameter around it in which it which will be on-band. An air-spaced etalon with 1/2 the objective diameter placed internally will need a collimator with 1/2 the objective FL to create a collimated output to the etalon, and the field angle magnification will reduce the Jacquinot spot by 1/2 to 0.5 a degree, which will just fit the disc of the Sun. A smaller etalon of 1/4 the diameter of the objective will need a collimator of 1/4 the FL of the objective, and the field angle magnification will reduce the Jacquinot spot to 0.25 a degree, and only half the Sun will be in the "sweet spot." Outside this the image will become progressively off-band. If you're only interested in viewing or imaging a small portion of the Sun, this may be acceptable.

If you are double stacking internal etalons, the FWHM of ~ 0.5 A reduces the "sweet spot" to about 0.8 degree, and the same principles above result in even smaller portions of the Sun's disc being on-band at the narrower band-pass.

Once you get to less than 1/4 the diameter of the objective, viewing the Sun's disc becomes more like looking through a straw - OK for imaging at high resolution, but poor for visual observing. At this point it's probably better to go to a small solid-spaced etalon in a telecentric lens system, and this is why all the small near the focus etalon systems are of this design.

The Sun's diameter subtends 0.5 degree, and therefore field angle of the Sun's limb is 0.25 degree, equivalent to about an f108. This is as small as it can be...

Etalon filter systems.jpg
Etalon filter systems.jpg (137.07 KiB) Viewed 1798 times


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Re: Internal etalon size

Post by JesusRL »

Thank you Bob for your extensive  explanation. If I am not wrong I understand the following from your words:

* An internal etalon placed half the way in the focal length of the telescope would reduce the sweet spot to 0.5
* A double stack unit placed even closer to the focus plane would reduce this sweep spot even lower, thus smaller than the disk.

This defines the commercial solar internally double stacked telescopes. Is their sweet spot smaller than the full disk when double stacked?

T&R


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Re: Internal etalon size

Post by Bob Yoesle »

Your first statements are correct.
This defines the commercial solar internally double stacked telescopes. Is their sweet spot smaller than the full disk when double stacked?
That depends on the particular telescope. For example, the original LS80 telescope had an internal 50 mm etalon, and the Jacquinot spot was almost as large as a front mounted etalon's. Double stacking it still took in the entire solar disc.

Other telescopes with larger objectives and smaller etalons are not intended for full-disc viewing/imaging but rather high resolution close-up viewing/imaging, and therefore have smaller "sweet spots," such as the LS152.


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