Which negative lens for collimated beam.

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Re: Which negative lens for collimated beam.

Post by Merlin66 » Tue Jan 27, 2015 9:59 pm

Gus,
The further the barlow is from the etalon, the greater the spread from the edge of the sun's image, and hence vignetting.
I'm getting my head around the fact that FOR A STELLAR IMAGE on axis the exit beam from the barlow is parallel and collimated. For an EXTENDED object, like the Sun, the exit beam is collimated BUT NOT PARALLEL to the optical axis - the edges sit at an angle - the field angle, in the case of the solar disk +/-0.25 degree. So the greater the distance, the greater the edge appears from the optical axis.
Hope this makes some sense.
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Re: Which negative lens for collimated beam.

Post by GUS » Wed Jan 28, 2015 2:37 am

Hi Ken, is there any benefit in using a positive achromat as a collimator lens instead of the negative lens, I understand that the optical path would be longer. Another option I was looking at was using a SM40 Ha scope turned backwards on top of the LS100 and fed from the LS100 scope through two diagonals and then through an F7 40mm positive achromat, which would then feed the necessary collimated light beem into the SM40 etalon, but the re collimating lens( the SM40 objective is an F10, would this cause any issues.I did see this done by someone on this site a few years back, but couldn't find the post.

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Re: Which negative lens for collimated beam.

Post by Valery » Wed Jan 28, 2015 4:17 am

GUS wrote:Hi Ken, is there any benefit in using a positive achromat as a collimator lens instead of the negative lens,
No benefit.
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Re: Which negative lens for collimated beam.

Post by Merlin66 » Wed Jan 28, 2015 5:38 am

Valery is correct. You face the same field angle issues.
I tried something similar with my EC90 - a PST mounted with double diagonals with initially a 2" Big Boss TV Barlow then later a 40mm achromat.
It worked.....but.....I was not over excited with the results.
(I've mentioned before trials when I used a couple of 50/50 beamsplitters was pretty hopeless.)
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Re: Which negative lens for collimated beam.

Post by GUS » Wed Jan 28, 2015 6:30 am

Thanks Valery and Ken.

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Re: Which negative lens for collimated beam.

Post by Bob Yoesle » Fri Jan 30, 2015 9:00 pm

This modification is not as simple or straightforward as it might seem.

Having a great deal of space between the negative collimator and the etalon will be less than ideal, as marginal rays might be missed. Then you need another refocusing lens, and focuser...

You can also use a positive collimator lens system, which would likely place the entire assembly outside of the tube -- on or in the focuser. Placing a positive lens after the focus also allows one to use a simple field lens near the focus to better constrain the marginal rays, and just as with eyepieces better controls both aberrations and field angles. It is usually easier to find positive achromats to begin with. However, a second collimator, etalon, refocusing lens, and focuser, would be very problematic. And just as with telecentric systems, the mechanical constraints of the focuser(s) will come into play for on-axis optical alignment -- think optical bench.

But because you are already using an internal etalon system with the LS100 to begin with, it might be very difficult to get any such system to perform well, as the refocusing lens system is more difficult to account for, and the SM40's relatively large central obstruction will possibly have some other untoward effects. Therefore I would use something like the Lunt 35 or 50 unobstructed etalons.

To be honest I don't think the optimization would be very easy in any case - especially using tilt tuning, and in the long run you'll probably wish you just saved up for a Lunt DS pressure tuned module. Just sayin' .

The best choice would be to DS with a front etalon (SM90 or LS100F). Start saving.
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Re: Which negative lens for collimated beam.

Post by marktownley » Fri Jan 30, 2015 10:55 pm

GUS wrote:Hi Ken, is there any benefit in using a positive achromat as a collimator lens instead of the negative lens, I understand that the optical path would be longer. Another option I was looking at was using a SM40 Ha scope turned backwards on top of the LS100 and fed from the LS100 scope through two diagonals and then through an F7 40mm positive achromat, which would then feed the necessary collimated light beem into the SM40 etalon, but the re collimating lens( the SM40 objective is an F10, would this cause any issues.I did see this done by someone on this site a few years back, but couldn't find the post.
Been there, tried it, as has Walter, gave it up because it just isn't workable...

Sorry, this mod is one of those that is 'theoretically' possible, but in reality there are numerous practical hurdles that make it far from straight forward.

Bobs advice is the best, an external DS unit. I appreciate this may not be the best solution on the bank balance, but it is the most realistic.
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Re: Which negative lens for collimated beam.

Post by GUS » Fri Jan 30, 2015 11:11 pm

Thanks Bob and Mark, looks like I'll be saving then, suppose I could sell the DS SM40 to finance it.

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Re: Which negative lens for collimated beam.

Post by Valery » Sat Jan 31, 2015 6:58 am

Bob Yoesle wrote:
To be honest I don't think the optimization would be very easy in any case - especially using tilt tuning, and in the long run you'll probably wish you just saved up for a Lunt DS pressure tuned module. Just sayin' .

The best choice would be to DS with a front etalon (SM90 or LS100F). Start saving.
+100! Very true recommendation.

Valery.
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Re: Which negative lens for collimated beam.

Post by marktownley » Sat Jan 31, 2015 9:24 am

A less ideal solution that would be easier on the wallet would be to double stack using a Quark... Several people are doing that with some success.
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