PST etalon (or similar) in a telecentric beam

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PST etalon (or similar) in a telecentric beam

Post by Merlin66 » Sat Apr 12, 2014 10:35 pm

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 » Sat Apr 12, 2014 11:59 pm

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 » Sun Apr 13, 2014 12:04 am

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 » Sun Apr 13, 2014 12:17 am

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 » Sun Apr 13, 2014 2:37 am

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 191 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 » Sun Apr 13, 2014 2:44 am

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 » Sun Apr 13, 2014 3:50 am

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 » Sun Apr 13, 2014 4:17 am

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

Post by swisswalter » Sun Apr 13, 2014 6:38 am

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 » Sun Apr 13, 2014 7:20 am

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 » Sun Apr 13, 2014 8:08 am

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 » Sun Apr 13, 2014 9:28 am

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 » Sun Apr 13, 2014 1:33 pm

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 » Sun Apr 13, 2014 3:15 pm

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 » Sun Apr 13, 2014 9:23 pm

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 » Mon Apr 14, 2014 12:03 am

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 » Mon Apr 14, 2014 12:07 am

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 » Mon Apr 14, 2014 3:41 am

"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 » Mon Apr 14, 2014 4:20 am

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 » Mon Apr 14, 2014 7:10 am

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 » Mon Apr 14, 2014 7:13 am

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 » Mon Apr 14, 2014 7:15 am

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 » Mon Apr 14, 2014 3:58 pm

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 » Mon Apr 14, 2014 4:54 pm

Interesting! Let's see if it works! :)
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Re: PST etalon (or similar) in a telecentric beam

Post by marktownley » Mon Apr 14, 2014 6:09 pm

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