PST etalon (or similar) in a telecentric beam

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PST etalon (or similar) in a telecentric beam

Post by Merlin66 »

Following the very interesting discussions on the ARIES (Valery), the new DayStar QUARK and the recent PST mod ( http://solarchat.natca.net/viewtopic.php?f=9&t=11033) I thought it may be worthwhile starting a new topic to "capture" the ongoing discussions on "Generation 2" PST mods.

The concept seems to revolve around removing the front collimating lens (and probably the rear re-imaging lens) from the PST etalon assembly and using a telecentric lens to provide an input beam of >f25 (?) to the etalon. To me this places it "in the converging beam". A subsequent reducer could/would be used to reduce the final f ratio for visual/ imaging.
(I have a couple of TV Powermates, x2 and x4 which I'll have to try with the current TS102 PST mod - these would give f 22 and f 44 beams if positioned "normally" in the light path. Can they be "re-positioned" to give a "fully collimated" beam - not 100% sure, but it shouldn't take long to find out.)


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Re: PST etalon in telecentre beam

Post by thesmiths »

If to go down the route of removing the front and rear PST lenses, why not just start out with the Lunt 35? It's not that different in cost to PST and I assume very easy to remove the front etalon. If to add a dedicated front collimator and rear refocuser, it's more or less the design of Rogerio Marcon, who I believe used a Lunt 50 in his (very big) solar telescope.


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Re: PST etalon in telecentre beam

Post by Merlin66 »

Yes.....why not?
(Setting up a tuning mechanism may be "interesting")
The telecentric proposal is different from Marcon's - using a negative collimator rather than a positive lens and also (maybe) not going fully collimated but utilising an >f25 focused beam through the etalon.....


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Re: PST etalon in telecentre beam

Post by thesmiths »

I had always wondered why there were no Lunt 35 mods being done. The base scope is much easier to disassemble and you get a much better blocking filter, at least with the deluxe version. And a Sol Searcher thrown in!


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Re: PST etalon in telecentre beam

Post by marktownley »

Merlin66 wrote:Following the very interesting discussions on the ARIES (Valery), the new DayStar QUARK and the recent PST mod ( http://solarchat.natca.net/viewtopic.php?f=9&t=11033) I thought it may be worthwhile starting a new topic to "capture" the ongoing discussions on "Generation 2" PST mods.

The concept seems to revolve around removing the front collimating lens (and probably the rear re-imaging lens) from the PST etalon assembly and using a telecentric lens to provide an input beam of >f25 (?) to the etalon. To me this places it "in the converging beam". A subsequent reducer could/would be used to reduce the final f ratio for visual/ imaging.
(I have a couple of TV Powermates, x2 and x4 which I'll have to try with the current TS102 PST mod - these would give f 22 and f 44 beams if positioned "normally" in the light path. Can they be "re-positioned" to give a "fully collimated" beam - not 100% sure, but it shouldn't take long to find out.)
Good call Ken making this thread.

I think the thing with the powermates is the working distance (or lack of) behind the powermate lens assembly. All the TV documentation only go up to a working distance of 100mm http://www.televue.com/engine/TV3b_page ... 0nzb_ldWSo - which isn't much to get an etalon, blocking filter and eyepiece / imaging system in. Maybe they will work at greater distances than this, I don't know? My bug bear with the powermates, and I know others have had this too, is that can end up with a lovely hotspot / ghost in the fov as a result of reflections at monochromatic wavelengths. I sold mine as a result of this.

I think the Baader TZ system is the way forward in this respect: there are clear instructions as to the exact spacings you would need to use, and it is specifically designed for Ha systems
Telecentric System.pdf
(150.11 KiB) Downloaded 291 times
Gotta be careful with the terminology here Ken - "Can they be "re-positioned" to give a "fully collimated" beam" - Collimated is not the same as telecentric - infact if we were doing this collimated is what we're avoiding as this is what gives us the sweet spot with the PST etalon...


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Re: PST etalon in telecentre beam

Post by marktownley »

thesmiths wrote:If to go down the route of removing the front and rear PST lenses, why not just start out with the Lunt 35? It's not that different in cost to PST and I assume very easy to remove the front etalon. If to add a dedicated front collimator and rear refocuser, it's more or less the design of Rogerio Marcon, who I believe used a Lunt 50 in his (very big) solar telescope.
Rogerios design is not telecentric, it an etalon in a collimated light path. Completely different.

Why not start out with a Lunt 35? The reason the PST etalon is used almost unequivocally over other etalons in homebrew scopes is the fact it all just fixes together as an optical assembly quickly and cheaply using 'off the shelf' components. Yes, you could use a Lunt 35, Coronado 40, Lunt 50 or pretty much any etalon, but it is all going to be custom machining throughout, the removing the etalon is the easy part, the mounting it all together as an optical assembly is not.


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Re: PST etalon in telecentre beam

Post by Merlin66 »

Mark,
Interesting....
Why do you think a "bare" (tilt adjust) etalon would perform better in a f30 beam than the "original" (by design) fully collimated input i.e. the solar disk????
The TZ etc. provide this f30 to the front of the etalon and it's anticipated viewing/ imaging at the focus of this beam...


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

Post by marktownley »

The key here is that it has to be working in a telecentric beam, if this is the case then there will be no shift of bandwidth or centre wavelength across the field of view (sweet spot) that occurs as when these etalons are working in a collimated beam. The original design is a collimated beam, and I think the reason for this is that the PST was designed to a budget and hence design spec. If the PST had been designed to operate telecentrically it would be running at over a metre focal length and with this would be increased cost of larger OTA, blockers etc; also would people prefer the handy PST size we have now and its portability, or a skinny over metre long refractor? Also PST is designed as a quick view 'full disk' scope, it's telecentric equivalent would definitely be a high mag closeup view...

The PST is a great bit of kit that has brought so many people into solar, and then taken them further with the PST mods, but for the OCD obsessives like me I firmly believe the telecentric route is the next stage to improve things still further.

Here's an interesting, if heavy going, discussion on telecentrics...
Tele.pdf
(323.95 KiB) Downloaded 275 times


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

Post by swisswalter »

Hi Ken

thank you very much for opening that thread. That gives a great read


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

Post by Carbon60 »

Guys,

I'm right in the middle of a Lunt 35 mod.......

Basically a 150mm f10 objective with a 50mm diameter biconcave collimating lens with a -300mm fl set 300mm (of course) from the focal point of the objective to provide a parallel beam and a Lunt 35 mounted behind this. I realise that the collimated beam will only be 30mm at the inserted lens, but on my DMK41 chip that shouldn't matter and in any case the intention is to use my 2.5x Powermate for detailed magnified views. BTW I have a 90mm ERF positioned part way down the OTA to prevent meltdown :lol: .

I've had to design and get someone to make a lens holder to position the collimating lens within the rear of the scope and I've applied a means of securing the Lunt on the back end (photos to follow), with a slight mod on the rear of the Lunt to achieve focus and utilise my existing Coronado BF15, but hopefully it will work. The Lunt/Coronado combo works well on the Lunt alone, so it should be okay on the new set-up

The collimating lens is coming from the US next week by special order after a 6 week wait.

Despite the simplicity of the design and lens ray trace confirmation, I'm now feeling a little nervous with all the talk about collimated versus telecentric and what will and will not work with etalons. When the lens arrives and when I get some Sun, it won't take long for me to find out for myself. Hopefully the plan will come together and the time, effort and expense will be worthwhile.

We'll see and I'll let you know.

Stu.


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

Post by swisswalter »

Hi Stuart

thanks and good luck on your mod. I'm looking forward to hear from you


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

Post by marktownley »

Sounds a fun mod to me Stu! No reason at all why it won't work, I certainly wouldn't dwell on us talking about the merits of telecentric versus collimated. I look forward to seeing the results :)


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

Post by swisswalter »

Hi Mark

what is speaking against a LUNT LS50F mod, entering the etalon with a TZ4 beam not using lenses up front and the rear. Just following the baader distances to get focus with the right distance after the etalon ?
Last edited by swisswalter on Mon Apr 14, 2014 3:54 pm, edited 1 time in total.


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

Post by marktownley »

Hi Walter. No reason why it wouldn't work, the only complication is mounting the etalon and still being able to tilt tune it...


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

Post by swisswalter »

Hi Mark

thank you very much for your support. No problem with the tilt tuning. It is built in into the LS50F. I need only to have a decent sunshine


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

Post by Merlin66 »

Walter,
Bear in mind that the etalon would be mounted INSIDE the "optical assembly" between the telecentric and the imaging camera....unless you can come up with some very "special" adaptors to fit the front and rear of the etalon.. the tuning "tilt" should only apply to the etalon independent of the rest of the optical system.


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

Post by Merlin66 »

OK, I think I've done my homework "telecentric optics 101"
I THINK I know have a better understanding of how and what it does....
I do have a question....
The inference is that for a "air spaced" etalon, the higher the f ratio the better (possibly >f40), why then, not go for a fully "collimated" i.e. f ratio = infinity (!) before the "bare" etalon????


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

Post by Merlin66 »

"Telescopes, Eyepieces, Astrographs" by Hallock-Smith, Ceragioli, Berry, p383 describes the design and construction of a telecentric lens system.
Interestingly they warn about the "field curvature" (on a x4 version), radius of curvature 50mm....
(p 387, they also say at f32 the "sweet spot" would be 25mm diameter and "about half the moon could be imaged sharply (at full resolution)")


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

Post by swisswalter »

Hi Ken

thank you very much. Yes I have the Special constructions: scope, TZ4, Etalon, imaging camera, the Etalon sits in it's house and can be tilted. Now with your additional Information that we can only use 25 mm sweet spot less. I'm a little bit disapointed. Any way I'll give it a go.


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

Post by marktownley »

swisswalter wrote:Hi Mark

thank you very much for your support. No problem with the tilt tuning. It is built in into the LS50F. I need only to have a decent sunshine
Ahhh, yes, I forgot that. 92mm thread either end, just need the adapters to fix it all together.


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

Post by marktownley »

Merlin66 wrote:I do have a question....
The inference is that for a "air spaced" etalon, the higher the f ratio the better (possibly >f40), why then, not go for a fully "collimated" i.e. f ratio = infinity (!) before the "bare" etalon????
A collimated beam produces a sweetspot that varies in both bandpass and bandwidth across the fov, the reason to go for a telecentric beam is that this does not happen...


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

Post by marktownley »

Merlin66 wrote:"Telescopes, Eyepieces, Astrographs" by Hallock-Smith, Ceragioli, Berry, p383 describes the design and construction of a telecentric lens system.
Interestingly they warn about the "field curvature" (on a x4 version), radius of curvature 50mm....
(p 387, they also say at f32 the "sweet spot" would be 25mm diameter and "about half the moon could be imaged sharply (at full resolution)")
To me this just reinforces the importance of using a telecentric that is properly designed for the optical system it will be used with...


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

Post by swisswalter »

Hi Mark

as we have strong winds and clouds I found the time to shoot a pic of my latest combo to test on the sun, whenever she showes up again ;-)
TZ4_LUNTLS50F_combo.jpg


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

Post by marktownley »

Interesting! Let's see if it works! :)


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

Post by marktownley »

I take it this means the TZ4 is not for sale Walter :lol:


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

Post by swisswalter »

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 »

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 »

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 »

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 »

""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 »

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 »

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 »

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 »

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 »

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 »

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 »

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 »

Looking for results with the LS50F in a telecentric TZ4!
Last edited by Valery on Tue Feb 06, 2018 3:04 am, edited 1 time in total.


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

Post by swisswalter »

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 »

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 »

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.
Last edited by Valery on Mon Apr 21, 2014 7:38 am, edited 1 time in total.


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

Post by swisswalter »

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 »

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).
""
The search for the Holy Grail continues....


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

Post by swisswalter »

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 »

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 »

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).

Frédéric.


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

Post by Valery »

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 »

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,

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

Post by fjabet »

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
203mm edgeHD : 2,8°.

Calculated with Zemax MF RAID.


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

Post by fjabet »

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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