PST etalon (or similar) in a telecentric beam

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

Post by marktownley » Wed Aug 13, 2014 6:25 am

What sort of usable field of view can you get out of the SCT system Frederic?
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Re: PST etalon (or similar) in a telecentric beam

Post by Valery » Wed Aug 13, 2014 7:36 am

fjabet wrote:
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.
If there any chances you show us the photos you take with this PST etalon in it's normal configuration (say with refractor F/10) and with SCT Ha?
Also the pictures through LS50F Ha + SCT Ha ?

Thanks,

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

Post by mdwmark » Wed Aug 13, 2014 7:56 am

Hi Guys,
You missing why they don't use a telecentric beam on a air spaced etalon.
If you started out with a .5Ang air spaced etalon and put it in a F/30 telecentric the etalon would broaden out to about 1.2 Ang.
It will be uniform but much broader.
Like stated earlier, It,s the same bandwidth just the wavelength shifts from the center blue from all sides, and you get your sweet spot.
When designing the collimated lens. You should get one of those free lens programs to play with for your spacing. If you just put a -300mm lens in front of prime focus you will find that you need the thickness of the lens. And the back lens spacing is important too. And the direction the lens are facing makes a big difference. It can take you from 1/4 wave to over 1 wave just by the direction the lens are facing.
Unless you are going to be around F/70 with an air spaced etalon your are going to have to live with a sweet spot.
Remember the broader the filter the smaller the sweet spot.
Mark W.

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

Post by fjabet » Wed Aug 13, 2014 8:13 am

marktownley wrote:What sort of usable field of view can you get out of the SCT system Frederic?
That depends on your etalon size.
With a 50mm etalon and without the SCT thread that limits to 44mm clear aperture, the system delivers the full sun. But you would need a focal reducer + a long FL eyepiece. It's possible, but I don't see the point. But you could use a full frame sensor.
With a Quantum (32mm diameter), I see 1/5 of the solar surface with a 35mm Eudiascopic or a XW30. I use this configuration with a CMOSIS CMV4000 (11,2mm square sensor) and with a Meade 6.3 reducer when the seeing is very bad.
With a 20mm PST + BF15, the field just fill up the 35mm Eudiscopique. But with the binovue the AFOV limitation isn't an issue.
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Re: PST etalon (or similar) in a telecentric beam

Post by Valery » Wed Aug 13, 2014 8:20 am

mdwmark wrote:Hi Guys,
You missing why they don't use a telecentric beam on a air spaced etalon.
If you started out with a .5Ang air spaced etalon and put it in a F/30 telecentric the etalon would broaden out to about 1.2 Ang.
It will be uniform but much broader.
Like stated earlier, It,s the same bandwidth just the wavelength shifts from the center blue from all sides, and you get your sweet spot.
When designing the collimated lens. You should get one of those free lens programs to play with for your spacing. If you just put a -300mm lens in front of prime focus you will find that you need the thickness of the lens. And the back lens spacing is important too. And the direction the lens are facing makes a big difference. It can take you from 1/4 wave to over 1 wave just by the direction the lens are facing.
Unless you are going to be around F/70 with an air spaced etalon your are going to have to live with a sweet spot.
Remember the broader the filter the smaller the sweet spot.
Mark W.
Hi Mark,

I believe that the narrower the filter, the smaller the sweet spot.

I also don't believe that any of these air spaced etalons mentioned (PST and LS50) work fine because all air spaced etalons
are MUCH more sensitive to converging of the beam that the solid spaced etalons. Where the solid spaced etalons will work fine
with about F/30, all air spaced etalons will fail.
Walter Koch already tried the telecentric baader (TZ4) with his LS50F Ha with fail.

This is the reason why Lunt Engineering use a collimation - refocusing system in their telescopes with internal etalons.


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

Post by fjabet » Wed Aug 13, 2014 8:21 am

Valery wrote:
If there any chances you show us the photos you take with this PST etalon in it's normal configuration (say with refractor F/10) and with SCT Ha?
Also the pictures through LS50F Ha + SCT Ha ?

Thanks,

Valery.
On this link, the first one is PST DS, the second a single PST. The seeing was very poor and there are some fringes.
http://www.astrosurf.com/ubb/Forum3/HTML/041024.html
I didn't take any image with the PST in its standard configuration.

The LS50F + HaT test has been done in Serbannes with Oliver's filter, I didn't have the time to take any image. But we had a very nice view in it with Oliver and Christian Viladrich. If Oliver passes by, he would comment.
I ordered a LS50FHa and I will test that has soon as I receive it and I have the adaptation rings done.
If the Lunt is good (i.e. if I'm lucky), there will be a Quantum 0.6 PE in the classified soon...
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Re: PST etalon (or similar) in a telecentric beam

Post by fjabet » Wed Aug 13, 2014 8:28 am

Valery wrote:
Hi Mark,

I believe that the narrower the filter, the smaller the sweet spot.

I also don't believe that any of these air spaced etalons mentioned (PST and LS50) work fine because all air spaced etalons
are MUCH more sensitive to converging of the beam that the solid spaced etalons. Where the solid spaced etalons will work fine
with about F/30, all air spaced etalons will fail.
Walter Koch already tried the telecentric baader (TZ4) with his LS50F Ha with fail.

This is the reason why Lunt Engineering use a collimation - refocusing system in their telescopes with internal etalons.


Valery.
In theory you're right, but if the telecentricity is very good it just works. The bandwidth may be a bit broadened, but I just can't tell the difference.
Jean Pierre also used his LS35 etalon with his 230mm in a telecentric configuration with success.

And the great advantage on the collimated solution is that there is no soft spot as there is just the marginal angle and no field angle.
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Re: PST etalon (or similar) in a telecentric beam

Post by fjabet » Wed Aug 13, 2014 8:37 am

Here is a better one with the PST single stack.
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Re: PST etalon (or similar) in a telecentric beam

Post by marktownley » Wed Aug 13, 2014 8:40 am

Thanks for the info guys! :)
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Re: PST etalon (or similar) in a telecentric beam

Post by swisswalter » Wed Aug 13, 2014 7:30 pm

fjabet wrote:Here is a better one with the PST single stack.
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hi Frédéric

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

Post by marktownley » Wed Aug 13, 2014 8:40 pm

mdwmark wrote:It,s the same bandwidth just the wavelength shifts from the center blue from all sides, and you get your sweet spot.
Hi Mark, Frederic et al,

I don't know if anyone can answer this, but as the wavelength shifts from the centre to blue at the edges does it do it in a linear way, or does it do it non linearly? i.e. the further you get from the centre the more progressively blue it becomes - kind of like with the inverse square law and how gravity falls away with distance...
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Re: PST etalon (or similar) in a telecentric beam

Post by fjabet » Thu Aug 14, 2014 8:36 am

Intuivively, I would say a sine law ?
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Re: PST etalon (or similar) in a telecentric beam

Post by Valery » Thu Aug 14, 2014 6:58 pm

fjabet wrote:
Valery wrote:

And the great advantage on the collimated solution is that there is no soft spot as there is just the marginal angle and no field angle.
Frederic,

All depends. Have you seen my photos here? They all were made with the telescope equipped with a specifically designed collimation system.
Also, look at photos taken with Lunt dedicated solar telescopes (with internal collimated system)?

Have you seen soft spot on them?
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Re: PST etalon (or similar) in a telecentric beam

Post by fjabet » Thu Aug 14, 2014 8:43 pm

I agree, there is no soft spot if the etalon is large enough to keep the field angle below 0.5°. But with a FL long enough to aim at high resolution, the telecentric seems to me the best way to go : I prefer to have only some controlable marginal angle than field angle and a non homogeneous bandpass over the FOV. I'm using quite large sensors for imaging (CMOSIS CMV4000, and the IMX174 pretty soon), sometimes with a focal reducer, so I'm very concerned with the field consistancy.
BTW I've seen in a quite large number of Lunt with internal etalon, and I've not been thrilled by the result. I've found the quality very inconstant, including a pair of LS152 with the field not being on band completly.

I think that Lunt and Coronado have chosen the collimated path to preserve a short FL and the capability to see the full disk and not because it's better than the telecentric solution.
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Re: PST etalon (or similar) in a telecentric beam

Post by fjabet » Thu Aug 14, 2014 8:52 pm

Just another advantage of the telecentric solution : you can use a mirror telescope. With the internal collimated etalon I'm not sure it's possible (unless having a huge baffle to place the etalon in). And over 200mm aperture a refractor is clumsy and more expensive.
Now for smaller intruments you may be right, the collimated option has advantages starting with the shorter FL.
I thought all this in a >200mm perspective. Actually my next project is a 350mm aperture Ha+CaK telescope (not a C14, that's a completly different design from the HaT and a SCT couldn't cope with UV bandwidth).
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Re: PST etalon (or similar) in a telecentric beam

Post by marktownley » Fri Aug 15, 2014 12:30 pm

fjabet wrote:I think that Lunt and Coronado have chosen the collimated path to preserve a short FL and the capability to see the full disk and not because it's better than the telecentric solution.
I completely agree Frederic
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Re: PST etalon (or similar) in a telecentric beam

Post by marktownley » Fri Aug 15, 2014 1:09 pm

fjabet wrote:I agree, there is no soft spot if the etalon is large enough to keep the field angle below 0.5°.
How do we do this with a PST mod and a collimating lens system, or is it just not possible with the small size of the PST etalon?
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Re: PST etalon (or similar) in a telecentric beam

Post by fjabet » Fri Aug 15, 2014 3:13 pm

Hello Mark,

as a rule of the thumb, the chief ray field angle for a 0,25° field (full sun) is -0,25*F/f in degrees where F is the refractor FL, and f the negative lens FL.
For example for the PST : -0,25*400/-200 = 0,5° => we are within the etalon tolerance (that is less than 0,5° in the case of a collimated beam, the telecentric solution is 2x more tolerant regarding the angles as mentionned in a paper I've seen some times ago).
If you want this for a 100/1000 refractor, then you must increase the negative lens to -500mm. In that case, to keep the F/10 ratio, the negative element aperture must be (-)500/10 = 50mm. Thus you need a 50mm etalon as well. And the etalon should be quite deeply installed in the refractor.
In fact the real issue with this is to find the negative lens, they are scarcer than converging lenses in manufacturer portfolio.
But with a 100/1000 refractor and a 50mm non obstructed etalon, you have a home brewed Lunt 100 :)


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

Post by Valery » Fri Aug 15, 2014 6:56 pm

fjabet wrote: If you want this for a 100/1000 refractor, then you must increase the negative lens to -500mm. In that case, to keep the F/10 ratio, the negative element aperture must be (-)500/10 = 50mm. Thus you need a 50mm etalon as well. And the etalon should be quite deeply installed in the refractor.
He don't need to go with 100mm F/10 refractor. He can go with 100mm F/5.
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Re: PST etalon (or similar) in a telecentric beam

Post by Valery » Fri Aug 15, 2014 7:01 pm

fjabet wrote:
I think that Lunt and Coronado have chosen the collimated path to preserve a short FL and the capability to see the full disk and not because it's better than the telecentric solution.
It is indeed better than telecentric solution if an etalon is air spaced. Lunt did his job nicely with no mistakes. In the same time I know two cases when Lunt airspaced etalons were used in a telecentric configuration. Both failed due to lowering of the contrast (bandpass broadeing).

Valery.
Last edited by Valery on Sat Aug 16, 2014 1:08 pm, edited 2 times in total.
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Re: PST etalon (or similar) in a telecentric beam

Post by marktownley » Sat Aug 16, 2014 6:23 am

Valery wrote:He don't need to go with 100mm F/10 refractor. He can go with 100mm F/5.
Yes, that's another way of doing it...
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Re: PST etalon (or similar) in a telecentric beam

Post by fjabet » Sat Aug 16, 2014 7:40 am

I wonder if the field angles increasing as the FL shortens ?

Anyhow as I said the collimated beam preserves the FL and maybe the way to go for aperture <180mm. But for larger telescope, you don't really have a choice and you don't want to make full sun but rather high resolution.

As for Lunt performances, we are several observers who don't share you enthousiasm but maybe we have been unlucky :)

Do you have any specs and picture of your commercial system ?
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Re: PST etalon (or similar) in a telecentric beam

Post by Valery » Sat Aug 16, 2014 1:16 pm

fjabet wrote:I

As for Lunt performances, we are several observers who don't share you enthousiasm but maybe we have been unlucky :)

Do you have any specs and picture of your commercial system ?
Yes, definitely, you, guys, were extremely unlucky because all of you own bad Lunt scopes(???). First - why not ask Lunt to replace the telescopes of fix the problems? They seems to have excellent customer service. Secondly, what are real problems with all your, guys, Lunt telescopes with collimated internal etalons?
What the telescopes you were unhappy with?

BTW. I wan't share any details about my commercial products. Hope this understandable.
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Re: PST etalon (or similar) in a telecentric beam

Post by fjabet » Sat Aug 16, 2014 3:43 pm

Let us say that a lot of the Lunt I've seen on the field were below expectations in term of contrast and/or uniformity. I know very well a LS152 that just can't show more than 40% of the field on band.
Now it seems that there was (is?) an issue with the Lunt BF as replacing it with a Coronado would increase the contrast.

Anyway we will have to use Lunt now as Coronado is gone, at least for a while...

I was not asking for your design, but just the commercial product presentation...
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Re: PST etalon (or similar) in a telecentric beam

Post by Valery » Sat Aug 16, 2014 7:00 pm

fjabet wrote:Let us say that a lot of the Lunt I've seen on the field were below expectations in term of contrast and/or uniformity. I know very well a LS152 that just can't show more than 40% of the field on band.
Now it seems that there was (is?) an issue with the Lunt BF as replacing it with a Coronado would increase the contrast.

Anyway we will have to use Lunt now as Coronado is gone, at least for a while...

I was not asking for your design, but just the commercial product presentation...

Andy Lunt himself has said that his LS152T is purposed for high resolution views and imaging.
Such a telescope is not for full disk viewing - there are a lot of alternatives in smaller sizes - 50mm, 60mm, 80mm and 100mm size - with front mounted and with internally mounted etalons. However even with LS152T our friend Pedro easily imaging the Sun in full disk size. Same with several other owners of LS152T.
If correctly adjusted, LS152 shows full sun disk in Ha, may be with some contrast loss near the edge of the sun disk. I saw enough photos where all is OK with sun disk in band. More so, I can tell you, that my solar kit has been designed for using in 100mm F/5, 120mm F/5 and 150mm F/5 telescopes.

More developments will follow soon.
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