Which negative lens for collimated beam.

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Which negative lens for collimated beam.

Post by GUS »

Thinking of making a collimated beam DS unit for my LS100 scope using a SM11 40mm Etalon filter.What type of Negative Lens-i.e-Plano, bi-concave,Meniscus, or negative achromat is best to use.The LS100 is f7, so will also purchase an 40mm f7 achromat as the re-focus lens. I figure the negative lens will also have to be f7, is that correct. Thanks for any help.


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Re: Which negative lens for collimated beam.

Post by swisswalter »

Hi GUS

a negativ Achromat f/7 does the job


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Re: Which negative lens for collimated beam.

Post by Valery »

GUS wrote:Thinking of making a collimated beam DS unit for my LS100 scope using a SM11 40mm Etalon filter.What type of Negative Lens-i.e-Plano, bi-concave,Meniscus, or negative achromat is best to use.The LS100 is f7, so will also purchase an 40mm f7 achromat as the re-focus lens. I figure the negative lens will also have to be f7, is that correct. Thanks for any help.
The problems are:

1. There are no such ready negative achromats on the market.
2. Even if you find it, there is no warranty that it will be corrected for spherical aberration and coma
3. There is no warranty that the whole system will be corrected for SA.
4. You will sure (for 99%) have a banding as the SM etalon require some tilt to come to CWL of 6562,8A
Internally mounted etalons are MUCH more sensitive to tilt (bending problem) than front mounted etalons.

Out of five my etalons, only one is well suited for internal use and one more is barely acceptable. All other
etalons suffer from a banding.

Hope this helps.

Valery
Last edited by Valery on Tue Oct 14, 2014 4:42 am, edited 3 times in total.


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Re: Which negative lens for collimated beam.

Post by GUS »

Thanks Walter and Valery, getting the lenses of the right diameter and focal length isn't a problem, as there is a local lens manufacturer. I have the Richview tuner which is similar to the PST tuner, so I'm hoping it works better than the tilt tuned units as an Internal etalon. I'm getting a price on the lenses tomorrow, and if they are reasonable, I wouldn't mind trying it anyway.Do I need to use a negative achromat or can I use a meniscus or a plano.


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Re: Which negative lens for collimated beam.

Post by Merlin66 »

Gus,
For dedicated Ha it doesn't have to be a doublet.....
Your optician should be able to recommend whether a simple plano negative can do the job.


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Re: Which negative lens for collimated beam.

Post by Valery »

Merlin66 wrote:Gus,
For dedicated Ha it doesn't have to be a doublet.....
Your optician should be able to recommend whether a simple plano negative can do the job.
If the refocusing objective is an achromatic lens, corrected for coma and SA, then negative collimator MUST be corrected
for these aberrations as well.

Only the systems which were designed straight trough - from the first element to the latter can use singlets as collimator and
refocused components.


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Re: Which negative lens for collimated beam.

Post by Valery »

GUS wrote:Thanks Walter and Valery, getting the lenses of the right diameter and focal length isn't a problem, as there is a local lens manufacturer. I have the Richview tuner which is similar to the PST tuner, so I'm hoping it works better than the tilt tuned units as an Internal etalon. I'm getting a price on the lenses tomorrow, and if they are reasonable, I wouldn't mind trying it anyway.Do I need to use a negative achromat or can I use a meniscus or a plano.
Three PST etalons I experimented with, all worked poorly. The CWL and the sweet spots always change unsmoothly, jumping forth and back and only one of these etalon I was able to adjust for more or less uniform picture over ICX445 chip at optimal sampling scale. With some oversampling the uniformity is better.


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Re: Which negative lens for collimated beam.

Post by GUS »

Thanks Ken and Valery, am I wasting my time attempting this, I'm only interested in visual use.


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Re: Which negative lens for collimated beam.

Post by Valery »

GUS wrote:Thanks Ken and Valery, am I wasting my time attempting this, I'm only interested in visual use.
Why do you think you wasting the time. Better to make it properly - then your time will be well spent.

Make one of two designs:

1. Both lenses - collimation and refocusing, as singlets. The SA and coma can be corrected. But all spaces are tight.

2. Both - collimator and refocuser - as doublets. More freedom for design and will work better.

In my solar telescope I have triplet collimator and doublet refocusing objective. I can place the latter component even in 1m from the
designed point with no visible image dgradation even in computer simulation. And I can use ANY achromat as re-focusing lens - depends of my goals.

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Re: Which negative lens for collimated beam.

Post by GUS »

Thanks for the information Valery.


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Re: Which negative lens for collimated beam.

Post by marktownley »

You should look at the diameter of the exit beam - will you be able utilise the full 40mm aperture of that sm40? or if you can't then you are using less of the aperture of the sm40 and its central obstruction will start to be a higher % obstruction relatively. Also, can you get a 40mm f7 lens 280mm inside the focal point of the Lunt? That's where it will need to be to get a collimated output.


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Re: Which negative lens for collimated beam.

Post by GUS »

Thanks for the input Mark.


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Re: Which negative lens for collimated beam.

Post by michael.h.f.wilkinson »

As I understand it, a negative meniscus lens is generally better corrected for SA than a plano-convex lens. So if the front lens is a best-form lens, a meniscus lens might be the best bet. However, as has been said, for top performance, you need to match the lens designs carefully.


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Re: Which negative lens for collimated beam.

Post by Brett »

Why not use a barlow as a negative lens? This is much better corrected for aberrations in my opinion. I may be wrong though...


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Re: Which negative lens for collimated beam.

Post by marktownley »

A barlow will not produce a collimated beam which the etalon needs.


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Re: Which negative lens for collimated beam.

Post by Brett »

Mark, if I may ask, what do you mean by the barlow will not produce a collimated beam that the etalon needs? I am probably missing something, but shouldnt the etalon only need parallel light? When a barlow is placed its focal length inside the light cone of the primary telescope objective, this will cause the light rays to become parallel again, would it not?


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Re: Which negative lens for collimated beam.

Post by marktownley »

No, a barlow will not make the light rays parallel. If you use a 2x barlow say, it will just halve the angle of the light cone, eg make it to f20 not f10 rather than make it parallel, or collimated.


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Re: Which negative lens for collimated beam.

Post by Merlin66 »

Mark,
I think you're incorrect...
Re-read the replies - a barlow at it's focal length inside the focus will give an almost collimated beam.


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Re: Which negative lens for collimated beam.

Post by marktownley »

It would have to have the same focal ratio as the scope output beam it was being used with, and then used inside it's focal position, in which case it would. I think this would be a very fast focal ratio though...


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Re: Which negative lens for collimated beam.

Post by Brett »

What is the focal length of 2x 2 inch barlows normally?

One would need to stop it down to around 10mm on an f/7, 700mm scope, so the minimum focal length of the barlow lens should be about 70mm.


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Re: Which negative lens for collimated beam.

Post by Valery »

Merlin66 wrote:Mark,
I think you're incorrect...
Re-read the replies - a barlow at it's focal length inside the focus will give an almost collimated beam.
To not produce the vignetting it must has at leas the same F/D as the objective.

Another thing is that at such position the barlow lens will have unknown spherical aberration and coma which will be
introduced in the system. The collimator must be specially designed for a given objective.


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Re: Which negative lens for collimated beam.

Post by marktownley »

Brett wrote:What is the focal length of 2x 2 inch barlows normally?

One would need to stop it down to around 10mm on an f/7, 700mm scope, so the minimum focal length of the barlow lens should be about 70mm.
That is the thousand dollar question...

However, like Valery says it would being used out of specification and so SA will not be behaving itself.

I've always used negative achromats from Edmonds Optics or Thor Labs to produce a collimated beam, as these have known optical parameters, and while they may not be exclusively designed to operate at 656nm they will certainly be within a 'tolerance' at Ha wavelengths.


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Re: Which negative lens for collimated beam.

Post by Brett »

All true points made. However, if one should end up using a 50mm negative lens and stop it down to say 13mm or so, then the center 13mm of the bigger negative lens should have a good optical quality.

BTW, Surplus shed has some achromats designed for optimal performance at 656nm.


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Re: Which negative lens for collimated beam.

Post by marktownley »

Brett wrote:All true points made. However, if one should end up using a 50mm negative lens and stop it down to say 13mm or so, then the center 13mm of the bigger negative lens should have a good optical quality.
With only the centre 13mm of the lens being used the sweet spot effect would be huge.


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Re: Which negative lens for collimated beam.

Post by GUS »

Thanks all for the useful information, this has been interesting to follow. To be able to adjust the Etalon, I want to keep it outside the LS100 body As mark suggested I would need the collimator lens inside the tube, ie. 280mm inside of focus to get a collimated beam. Is there any reason why I couldn't have the spacing between the negative achromat collimator and the etalon long enough to get the etalon outside the tube, if the beam is collimated I can't see why not, or am I missing something.Also to keep the whole assembly short, I want to fold the recollimated focus back above the main tube using either two Newtonian secondary mirrors or two refractor diagonals which would be sized accordingly, and have a focuser for the fine focus adjustment at the end as well as the main focuser ,which will primarily be used to get the right position for the collimator lens. The etalon will be housed independent so as to allow the necessary tilt for removing the ghost image and free movement of the RV Tuner as well Can anyone see any problems with this idea.


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Re: Which negative lens for collimated beam.

Post by Merlin66 »

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 »

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 »

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 »

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 »

Thanks Valery and Ken.


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Re: Which negative lens for collimated beam.

Post by Bob Yoesle »

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 »

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 »

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 »

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.

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Re: Which negative lens for collimated beam.

Post by marktownley »

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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