PST etalon (or similar) in a telecentric beam

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Re: PST etalon (or similar) in a telecentric beam

Post by swisswalter » Tue Apr 15, 2014 3:44 am

Hi Mark

dead on. At least not for the moment ;-)
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Re: PST etalon (or similar) in a telecentric beam

Post by Merlin66 » Tue Apr 15, 2014 4:55 am

Mark, et al,
I'm really having difficulty here....why would the "sweet spot etc." be better at f40 (or so) compared with say, the fully collimated beam directly from the Sun (i.e. an external etalon).
The field angle is minimal, equivalent to around f 230. (1/tan 0.25 deg)
Why is this so......
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Re: PST etalon (or similar) in a telecentric beam

Post by swisswalter » Tue Apr 15, 2014 5:01 pm

Hi Ken

I have to wait for Mark to have an answer to your question :oops:
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Re: PST etalon (or similar) in a telecentric beam

Post by marktownley » Tue Apr 15, 2014 7:29 pm

Merlin66 wrote:Mark, et al,
I'm really having difficulty here....why would the "sweet spot etc." be better at f40 (or so) compared with say, the fully collimated beam directly from the Sun (i.e. an external etalon).
The field angle is minimal, equivalent to around f 230. (1/tan 0.25 deg)
Why is this so......
With an external etalon the field angle is indeed 'minimal', however with the telecentric design the field rays appear to come from infinity (not 150,000,000km as with an external etalon) and so are perpendicular to the image plane and parallel to the optical axis, as a result there is no sweet spotting with a telecentric. There are no field angles with a telecentric.

Also, you wouldn't have much aperture with a PST etalon mounted externally ;)
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Re: PST etalon (or similar) in a telecentric beam

Post by Merlin66 » Tue Apr 15, 2014 11:58 pm

""the off axis beam arrive at the image plane with the same angular geometry as
the axial rays"" -Baader TZ webpage
OK I think we are saying the same but different...
The angular geometry of the solar disk will still give a "field angle" - around +/- 0.25 deg., but there will be no additional "instrument" angle(??)....
I'm sure a similar result (re Marcon's positive lens collimator) would be achieved if we could find an aberration free collimating system.
In the meantime the TZ system looks the most promising......
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Re: PST etalon (or similar) in a telecentric beam

Post by Merlin66 » Wed Apr 16, 2014 12:27 am

http://www.edmundoptics.com/technical-r ... entricity/

In the mark up #1 is the prime image of the solar disk, #2 is the etalon in position behind the telecentric lens assembly and #3 shows the angles into the etalon....
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Re: PST etalon (or similar) in a telecentric beam

Post by marktownley » Thu Apr 17, 2014 7:41 am

Edmunds has some useful stuff. The video is interesting, just wish it didn't have the annoying music in the background. Will be ordering the TZ4 next week, so that should herald weeks of cloud and rain, but as soon as the weather gods have forgotten something astronomical I will report back on how I get on with it. :)
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Re: PST etalon (or similar) in a telecentric beam

Post by swisswalter » Fri Apr 18, 2014 6:12 am

Hi Mark

good luck on the TZ4. I had yesterday some first try. It works fine, but I had not enough time to add the blocking filter. Just ended the try with WL.
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Re: PST etalon (or similar) in a telecentric beam

Post by Valery » Fri Apr 18, 2014 11:47 pm

marktownley wrote:Edmunds has some useful stuff. The video is interesting, just wish it didn't have the annoying music in the background. Will be ordering the TZ4 next week, so that should herald weeks of cloud and rain, but as soon as the weather gods have forgotten something astronomical I will report back on how I get on with it. :)

TZ is not the best possible solution. It is just the most universal solution.
Last edited by Valery on Tue Feb 06, 2018 3:02 am, edited 1 time in total.
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Re: PST etalon (or similar) in a telecentric beam

Post by marktownley » Sat Apr 19, 2014 10:29 pm

I think you should send me a prototype to test Valery :D :bow2

I also rediscovered this article on telecentrics and etalons, and also on it a design for a telecentric for a 100mm f10 frac... Interesting... http://www.sonnen-filter.de/TZS/index-tzs.html
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Re: PST etalon (or similar) in a telecentric beam

Post by Merlin66 » Sun Apr 20, 2014 1:43 am

Good "re find" Mark!
I also had that saved but had "lost" it.
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Re: PST etalon (or similar) in a telecentric beam

Post by swisswalter » Sun Apr 20, 2014 6:40 am

Hi Mark

very interesting read. I can't wait to use the TZ4/LUNTLS50 combo
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Re: PST etalon (or similar) in a telecentric beam

Post by Valery » Sun Apr 20, 2014 10:50 am

Looking for results with the LS50F in a telecentric TZ4!
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Re: PST etalon (or similar) in a telecentric beam

Post by swisswalter » Sun Apr 20, 2014 8:37 pm

Hi Valery

I guess you are spot on judging on my first experience with the TZ4/LuntLS50F Combo. What has to be done to get it narrower ?
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Re: PST etalon (or similar) in a telecentric beam

Post by marktownley » Sun Apr 20, 2014 8:49 pm

Valery wrote:In such telecentric system the band wide will be about 0.85 - 0,9A which is significantly wider than in a native scheme with F/109 beam (in front of an objective).
The benefit is that it works with a much larger telescope = higher resolution. PST mod is much better.
The sweet spot effect of the PST mod just frustrates me now though Valery, for me it's a case of improving the matter, while hopefully retaining the pst etalon, even if the collimating optics are not kept. How would you improve the sweet spot issue?
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Re: PST etalon (or similar) in a telecentric beam

Post by Valery » Sun Apr 20, 2014 11:05 pm

marktownley wrote:
Valery wrote:In such telecentric system the band wide will be about 0.85 - 0,9A which is significantly wider than in a native scheme with F/109 beam (in front of an objective).
The benefit is that it works with a much larger telescope = higher resolution. PST mod is much better.
The sweet spot effect of the PST mod just frustrates me now though Valery, for me it's a case of improving the matter, while hopefully retaining the pst etalon, even if the collimating optics are not kept. How would you improve the sweet spot issue?
Mark,

We all know well the rule: there is no free lunch. Right? In the PST mod for higher resolution we should pay. And we pay - losing the useful field size.
The only method to solve the sweet spot problem is to pay money - buying a larger etalon (and blocker). As lower the ratio Dob/Det - as more even the field of view is.
Without additional money investment and without band wide increasing, the only method to get rid of a sweet spot is to use a larger scale - so the camera chip will be fully within a sweet spot. Getting rid of a sweet spot in the case of no money investment still has it's price - band wide increasing with all these contrast losing.



Walter,

The only method to get the narrowest possible band wide with a front air-spaced etalon ( Lunt LS35F, LS50F and LS60F) is to mount it in a collimated beam.
To narrow the band wide in a tele centric scheme you need to add another etalon and you will back to it's native 0,7A, but will have problems with reflections (ghosts) and banding. The useful FOV within sweet band will be significantly smaller.
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Re: PST etalon (or similar) in a telecentric beam

Post by swisswalter » Mon Apr 21, 2014 5:59 am

Hi Valery


thank you very much for your explanation. That sounds frustrating
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Re: PST etalon (or similar) in a telecentric beam

Post by Merlin66 » Sun Apr 27, 2014 6:58 am

http://articles.adsabs.harvard.edu/cgi- ... etype=.pdf
also
http://adsabs.harvard.edu/full/2006ASPC..358..155F
The above makes very interesting reading - collimated v's telecentric....

For completeness in this thread I copy a recent message from Valery re the D/d ratio...
""
1. Acceptance angle for a front mounted etalon is about 1 degree or two solar disk diameter. In the telescopes with collimated beam we have increased angle of incoming light. The magification is D/d where "D" is the objective diameter and "d" is collimator diameter. So, for a telescope with D/d=2 we have 2x smaller acceptance angle on the sky. About 1/2 degree or one solar diameter. With the D/d ratio increasing the acceptance angle on the sky will proportionally decreased. For wider band pass the acceptance angle is larger, for narrower band pass the acceptance angle is smaller.
For D/d=3 the acceptance angle is somewhat smaller than a solar disk, but because we tolerate some bandpass increasing and shifting, it will work despite that D/d is more than 2. But 3 is the limit and works only if you have etalon 0.7 - 0.8A and which does not require a tilt for a given conditions (barometric pressure and temperature).
""
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Re: PST etalon (or similar) in a telecentric beam

Post by swisswalter » Sun Apr 27, 2014 7:19 am

Hi Ken

thank you very much. That gives a good sunday reading in the rain
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Re: PST etalon (or similar) in a telecentric beam

Post by marktownley » Sun Apr 27, 2014 9:23 am

A couple of very interesting papers there Ken, thanks for posting. Each method (telecentric & collimated) both have their pros and cons - no best of both worlds scenario. A good learning experience though.

The telecentric option could be good for mounting a PST or other external etalon in a setup with a front mounted etalon to double stack... Would tighten up the bandpass of the front mounted etalon a bit, but still keep the even field - problem is working at at least 3x the native focal length of the scope (without using a reducer).
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Re: PST etalon (or similar) in a telecentric beam

Post by fjabet » Tue Aug 12, 2014 5:02 pm

I use this set up with the HaT telescope with very good results.
The HaT delivers F/10 flat correctged field, then our telecentric amplifier bring the F/D at 27,59 (more or less 1° marginal angle) with less than 10e-2° field angle at 0,25".
I used the following etalon with very good results :

- Daystar Ion 0,5A
- Daystar Quantum PE 0,6A
- PST single (single good etalon without its collimating lenses of course) +BF15
- PST DS + BF15 (very nice, but rather dim and with an uneven illumation, for visual only)
- Lunt LS50Fha + BF30 (very good, best bang for the bucks, that is my new reference design)

In all the cases the bandwidth was as expected without ANY soft spot, except some vignetting and banding in the PST DS due to less than perfect second PST. So to me the F/D >27 image side telecentric design is just the best solution when compared with the soft spot plagued collimated PST mod, unless using the right elalon size.
But you need a real telecentric, I'm not sure that the Tevevue falls in this category.
And remember that a telecentric lens design depends on the aperture, the FL AND the design (a 200/2000 SCT has more field angle than a 200/2000 doublet for example).

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Re: PST etalon (or similar) in a telecentric beam

Post by Valery » Tue Aug 12, 2014 7:49 pm

fjabet wrote:
a 200/2000 SCT has more field angle than a 200/2000 doublet for example).
Can you prove this statement with math?

Thanks.

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Re: PST etalon (or similar) in a telecentric beam

Post by Valery » Tue Aug 12, 2014 7:54 pm

fjabet wrote:I use this set up with the HaT telescope with very good results.
The HaT delivers F/10 flat correctged field, then our telecentric amplifier bring the F/D at 27,59 (more or less 1° marginal angle) with less than 10e-2° field angle at 0,25".
I used the following etalon with very good results :

- Daystar Ion 0,5A
- Daystar Quantum PE 0,6A
- PST single (single good etalon without its collimating lenses of course) +BF15
- PST DS + BF15 (very nice, but rather dim and with an uneven illumation, for visual only)
- Lunt LS50Fha + BF30 (very good, best bang for the bucks, that is my new reference design)

In all the cases the bandwidth was as expected
OK, can you be more specific with your expectations in each of these cases - #3 and #5? And how have you measured them?


Thanks,

Valery.
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Re: PST etalon (or similar) in a telecentric beam

Post by fjabet » Tue Aug 12, 2014 8:08 pm

Valery wrote:
fjabet wrote:
a 200/2000 SCT has more field angle than a 200/2000 doublet for example).
Can you prove this statement with math?

Thanks.

Valery.
200/2000 doublet : 2,4° at 25', half sun field angle
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Re: PST etalon (or similar) in a telecentric beam

Post by fjabet » Tue Aug 12, 2014 8:14 pm

Valery wrote:OK, can you be more specific with your expectations in each of these cases - #3 and #5? And how have you measured them?


Thanks,

Valery.
I don't have a HR spectrophotometer. Nevertheless the PST etalon behaves like in its nominal full PST outfit, and by the way this one is very good, it is close to a single stack Coronado SM front filter. I use it also for imaging with the HaT and it is better than my Quantum for this application.

In the case of the Lunt it was Oliver's set up and the result was pretty much the same as my Quantum.

I have seen in a lot of Ha intruments over thge years and I can judge quite well the filter quality.
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