To focus or not to focus - the creation of a build set

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Bastelhannes
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To focus or not to focus - the creation of a build set

Post by Bastelhannes »

Hi folks,

the title is a bit misleading, but it summarizes my question.

I have seen three versions of a PST40 stage 2 mod:
1. using two focusers: one between telescope and Etalon, one between Etalon and blocking filter
2.using one focuser:
version a) etalon is a fixed position related to the telescope and the focuser between etalon and the BF
version b) focuser between telescope and etalon, length between etalon and BF fixed.

I assume the different advantages:
Version 1: the ideal position of the Etalon can be adjusted. It should be 200 mm (plus or minus some 20 mm do not hurt, as I evaluated). Switching between different scopes would be easy (I am currently testing several scopes here). The focus for visual and photographic use can be adjusted easily with the second focuser.

Version 2.a) This would be a cost effective version, if you are using one scope and shortened the scope correctly.

Version 2.b) This seems to work as well. But here I am not sure, if my assumption of a +/- 20 mm range of the ideal PST location is correct. I would move the etalon very time I am focusing...

The reason why I am asking:

I am going to create some sets for users to build their own modifications. I want to put spacers, adapters and focuser into these sets to make life much more easier.

Therefore any wishes, hints, suggestions and even crazy ideas are more than welcome. (e.g. I am currently creating a setup using the Baader flip mirror 2 with two BF5 to switch easily between cam and visual. I am just curious, if it going to work).

Karsten aka Bastelhannes
Last edited by Bastelhannes on Mon Sep 13, 2021 6:29 am, edited 1 time in total.


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Re: To focus or not to focus

Post by Bastelhannes »

Here are some of my crazy sketches...
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Re: To focus or not to focus - the creation of a build set

Post by AndiesHandyHandies »

Hi

Backfocus is an issue with the Collimator lens needing to be 200mm inside of focus, at F10.

Do you have adapters for the front and back of the PST etalon?

I use a T2 to 28.5mm Bluelightning plate at the front of the PST and 1.25" and/or T2 extenders to position the collimator lens inside the focusser.

I have the collimator lens in an old barlow nose-piece.

If you position the focal plane 200mm after the re-focus lens you do not need any extra focusser.

I just use a UV-IR in front of the PST because of reflection issues off a Baader 35nm and the etalon.
Also I use a circular polariser between the etalon and comb-filter.

I use cheap Chinesse filters with 20mm aperture as stops in several places. Minus pretty coloured glass. The Bluelightning adapters are thick enough to screw something on either side.

Cheers. Andrew.


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Re: To focus or not to focus - the creation of a build set

Post by Bastelhannes »

Hi Andrew,

I have got the adapter between scope and etalon laying around here. So no prob.

The extra focuser between etalon and BF might be nice, if you are using different cams, eyepieces and other stuff... The ZWO ASI cams have different backfocus.

I want to create some universal sets of adapters and focuser to allow people building up their own PST mods stage 2...

Cheers...


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