Does a reducer work to achieve F10?

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Does a reducer work to achieve F10?

Post by bart1805 » Fri Oct 27, 2017 7:14 pm

I learned here that an air spaced etalon in a telecentric beam is not a good idea. But how would this solution work?
For example a skywatcher Mak 180 mm F15. If you use a 0,67x focal reducer and then attach the PST etalon, would that work?
Thanks! Bart.



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Re: Does a reducer work to achieve F10?

Post by marktownley » Fri Oct 27, 2017 7:59 pm

The maths is never that simple - what spacings would you use?


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Re: Does a reducer work to achieve F10?

Post by bart1805 » Fri Oct 27, 2017 8:30 pm

Don't know Mark. Would have to calculate that. But would it theoretically work and has anyone ever tried it?



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Re: Does a reducer work to achieve F10?

Post by marktownley » Fri Oct 27, 2017 8:32 pm

In theory, as always, yes. But to work best you need to know the numbers...


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Re: Does a reducer work to achieve F10?

Post by bart1805 » Fri Oct 27, 2017 8:50 pm

For sure Mark, you need to calculate the correct numbers. But that can't be as difficult as landing a man on the moon (-;
But it seems to be a rather simple solution to use a pst etalon with for example a Mak 180. On this forum a lot of different mods, but have not read about this one.



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Re: Does a reducer work to achieve F10?

Post by marktownley » Sat Oct 28, 2017 8:09 am

Try it. Just need that reducer the correct distance infront of the PST collimating lens. A bit of trial and error should do it.


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Re: Does a reducer work to achieve F10?

Post by bart1805 » Tue May 01, 2018 7:58 pm

marktownley wrote:
Sat Oct 28, 2017 8:09 am
Try it. Just need that reducer the correct distance infront of the PST collimating lens. A bit of trial and error should do it.
Hi Mark, this time a question not about the SW Mak 180 but about the Intes 703. Just realised that I made a mistake in thinking I could just put the PST etalon behind the 703 because it is a f10 telescope. Yes, it is a f10 scope, but the beam will not be a f10 beam....
Am I right that I need a telecentric 3x? Baader, Airylab and Beloptik are then the usual suspects.
Then remove the front lense of the etalon and also remove the back lens? I am a bit confused because on the Airylab site I read that one should remove both lenses and allways thought it was only the front lens which had to be removed.
Thanks in advance! Bart.



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Re: Does a reducer work to achieve F10?

Post by marktownley » Tue May 01, 2018 9:40 pm

Hi Bart, In this instance I think I would not bother with a telecentric but run the pst etalon (with collimating / re-collimated lenses) on the back of the Intes. If you don't like you can always get a telecentric afterwards and try it that way. Mark


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Re: Does a reducer work to achieve F10?

Post by bart1805 » Tue May 01, 2018 9:46 pm

Thanks Mark, will see what happens.



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Re: Does a reducer work to achieve F10?

Post by Merlin66 » Wed May 02, 2018 1:40 am

Bart,
The 703 is an f10 Mak, why do you think it will not provide an f10 beam to the PST etalon assembly?


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Re: Does a reducer work to achieve F10?

Post by GreatAttractor » Wed May 02, 2018 7:24 am

I believe the f/10 beam from the Mak should be fine, but the real problem is having enough back-focus (~200 mm for PST). You'd need to check how far you can pull the focal plane out (plus there's the effect of changing effective f.l. when the distance of Mak's mirrors changes).


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Re: Does a reducer work to achieve F10?

Post by bart1805 » Wed May 02, 2018 8:06 am

Merlin66 wrote:
Wed May 02, 2018 1:40 am
Bart,
The 703 is an f10 Mak, why do you think it will not provide an f10 beam to the PST etalon assembly?
Hi Merlin, is the secondary f10? Back to primary Maksutov school for me maybe... Thanks! Bart.



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Re: Does a reducer work to achieve F10?

Post by bart1805 » Wed May 02, 2018 8:18 am

GreatAttractor wrote:
Wed May 02, 2018 7:24 am
I believe the f/10 beam from the Mak should be fine, but the real problem is having enough back-focus (~200 mm for PST). You'd need to check how far you can pull the focal plane out (plus there's the effect of changing effective f.l. when the distance of Mak's mirrors changes).
Backfocus is huge. About 250 mm. So that should be enough.



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Re: Does a reducer work to achieve F10?

Post by GreatAttractor » Wed May 02, 2018 10:27 am

bart1805 wrote:
Wed May 02, 2018 8:06 am
is the secondary f10?
Actually, f/10 is the effective focal ratio of the primary-secondary combination (so you're good to go). E.g. in an f/15 MCT there may be an f/3 primary and an f/5 secondary.


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Re: Does a reducer work to achieve F10?

Post by Astrograph » Wed May 02, 2018 3:25 pm

PST etalon with its collimating lens on the back of an F10 scope should allow it to work correctly. Obviously a front ERF is needed on the Mak.



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Re: Does a reducer work to achieve F10?

Post by RodAstro » Wed May 02, 2018 8:25 pm

I realise you have moved away from the first question will a reducer work but I will just add to this first question, most reducers are based on a simple petzval (not a true one) lens and although it may work be aware that the elements in the reducer tend to be bonded and will separate if they get near 100deg before that the glue will be damaged if the temperature gets high.

For a simple alternative use a reasonably long focal length air spaced lens 60mm or above inside the tube.
It does work I am using a 80mm f7 inside my scope a 150mm f15 to bring it to f8 you also have less distortions and no vignetting.
Petzval did play with air spacing to improve the image and corrections but I find no need if you use a longer focal length lens it just needs to be bigger and further inside focus but much better than any commercial 48mm reducer I have tried.
I'm sure there are more expensive reducers that are more complex and will work well but they are designed for set f ratios and would you like to chance them in a solar scope?
The other thing to remember it will amplify any distortions your scope already has as the field gets wider



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Re: Does a reducer work to achieve F10?

Post by bart1805 » Thu May 03, 2018 7:37 am

@rodastro, greatattractor and astrograph.
Thanks! Hope to be able to test it on Sunday. Will let you know the results.



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Re: Does a reducer work to achieve F10?

Post by RodAstro » Thu May 03, 2018 8:45 am

Good luck Bart hope it works well



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Re: Does a reducer work to achieve F10?

Post by christian viladrich » Thu May 03, 2018 6:27 pm

I can't make an extensive answer here, but you can dig the question here :
http://astrosurf.com/viladrich/astro/in ... lar/FP.htm

If we go back to the initial question : is an F/15 scope + reductor giving F/D 10 OK with a PST (and its collimating lens) ? Then, the straight answer is yes. In other words, it does not make any difference if you have a native F/10 beam, or if you reach F/10 by the addition of Barlow lens or a reducer.

Then, there is another question : is a telecentric beam "better" than a "collimating beam" ? You can find some elements of answer in my web pages.

If you have a telecentric beam, it should be > F/30 for mica-space etalon and > F/45 for airspace etalon (e.g. PST without its collimating lenses)

There is also the question of back focus available which might make some setups feasible or not.

Hope this helps.


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Re: Does a reducer work to achieve F10?

Post by bart1805 » Thu May 03, 2018 8:43 pm

christian viladrich wrote:
Thu May 03, 2018 6:27 pm
Hope this helps.
Thanks Christian, yes it helps! And my luck that the love of my life studied French, so she has got some work to do. (-;



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Re: Does a reducer work to achieve F10?

Post by bart1805 » Sun May 06, 2018 3:56 pm

Hi all,
Yes, it works! Really bad seeing, but I could get fairly easily in focus.

The set up.
Image20180506_112342_resized by bart moors, on Flickr

On the front the 160 mm D-Erf. I made an adaptor out of wood and an old dew cap. And can screw it on to the front of the M703.
Image20180506_112321_resized by bart moors, on Flickr



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