Back-focus issues with a collimated etalon

Frankenscope? Let's see it!***be advised that NOTHING in this forum has been safety tested and you are reading and using these posts at your own peril. blah, blah, blah... dont mess around with your eyesight when it comes to solar astronomy. Use appropriate filtration at all times...
Post Reply
AndiesHandyHandies
Im an EXPERT!
Im an EXPERT!
Posts: 350
Joined: Fri Mar 30, 2018 6:46 am
Location: Derbyshire UK
Has thanked: 450 times
Been thanked: 165 times

Back-focus issues with a collimated etalon

Post by AndiesHandyHandies » Mon Nov 23, 2020 5:53 pm

Hi

Using the 200mm focal length and 24.5mm diameter lens with the PST etalon pushed forward to cope with back-focus issues may not be able to be moved far enough forward.

You could use a shorter focal length pair say 100mm nearer to the PST etalon and you will just be using 10mm of the etalon aperture rather than 20mm. A 4x better ERF will keep the heat load to be the same.

I have tried a collimated beam on the 127mm Triplet with my Prominence Quark and it seems to work just fine.

I have a Prominence Quark which has mica faults in one sector so I thought of a way to reduce the scatter by using only a part of the etalon.

Using a 60mm focal length lens pair on my 127mm F7.5 gives me a 8mm collimated beam. Setting the 1.25" eyepiece holder 5mm off axis means I can rotate the Quark in it to pick out a clear 8mm circle around the edge. Or 6mm with my Mak180 F10. I will add a 7nm Baader filter to the Quark, after the front 35nm Baader and Beloptik KG3/IR/UV ERFs to reduce the heat load on the etalon.

Cheers. Andrew.



User avatar
marktownley
Librarian
Librarian
Posts: 29767
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Has thanked: 2837 times
Been thanked: 1143 times
Contact:

Re: Back-focus issues with a collimated etalon

Post by marktownley » Thu Nov 26, 2020 4:24 pm

The only thing I would say is the less of the etalon that is used the smaller the sweet spot....


Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!

AndiesHandyHandies
Im an EXPERT!
Im an EXPERT!
Posts: 350
Joined: Fri Mar 30, 2018 6:46 am
Location: Derbyshire UK
Has thanked: 450 times
Been thanked: 165 times

Re: Back-focus issues with a collimated etalon

Post by AndiesHandyHandies » Sat Nov 28, 2020 10:14 am

Hi

Surely the size of the angle of the sweet spot will not change?

The collimator merely changes a converging beam with phase encoded information into a parallel beam with location encoded information and then back into a converging beam with phase encoded information at the same F No so the apparent image will have the same solid angle and the sweet spot be as before?

In professional optics collimators are used all the time.

Cheers. Andrew.



User avatar
marktownley
Librarian
Librarian
Posts: 29767
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Has thanked: 2837 times
Been thanked: 1143 times
Contact:

Re: Back-focus issues with a collimated etalon

Post by marktownley » Sun Nov 29, 2020 11:06 am

In a collimated system the size of the sweet spot is a ratio of free aperture of the scope to the used aperture of the etalon. As the aperture gets bigger, or the etalon smaller then the size of the sweet spot gets smaller too. Increase the size of the etalon or reduce the size of the free aperture of the scope and the sweet spot will be larger.

Mark


Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!

Post Reply