Coma Corrector

[PhilDowns]

Hi All,

I couldn't find a topic on Coma Correctors, sorry if I missed it, if so please move this question to the right place.

I'm looking at installing the F4 Aplanatic coma corrector into my Orion 200 astrograph (Focal length 806.9mm) and I'm sorting out the spacers needed for the back focus to my CCD camera.
It states a back focus of 55mm (which seems standard) however I think thats for a 1000mm focal length, I think I need to reduce it slightly to 53.66mm (seen this number somewhere but cant find it now).
I know I need to be as accurate as possible to get the best out of my set-up, can anyone help me work out the exact back focus length I need before I order a load of spacers?

Many thanks

Phil.

    

[rl]

I've got the Skywatcher f/4 aplanatic coma corrector which I think is the same as yours from the description. Baader do a useful variable spacer #2956929 which covers 20-29mm and works fine on my OSC camera...assuming your camera focal plane is the standard 18mm deep, and you're not having to squeeze in a filter wheel as well.  The OAG is 11mm thick so the spacewr is set to 55-18-11 = 26mm. I think they do other ones as well. Just tweak for best results. 

Edited August 6, 2020 by rl
more info

[PhilDowns]

Thanks for the reply, yes thats the CC I have on order, unfortunately I don't have the room for a variable spacer.

My camera QHY183M (17.5mm BF) plus rotator (12.5mm) plus Filter wheel (14mm having removed the 2" receptacle so using M54 threads both sides) plus the M54/M58 adapter (2mm) to go between the CC and the Wheel (not shown), gives me slack of 9mm (at 55mm BF from the CC) for the M54/M42 adapter that I haven't ordered yet to go between the rotator and wheel.

If the BF on the CC needs to be reduced from the stated 55mm for my FL, then I will reduce the 9mm adapter accordingly (I'm getting this made so I can order any length I like as long as it's over 1mm) 

Once it's all screwed together there won't be much I can tweak, apart from adding spacing shim washers into one or more of the threads, but that can only add small amounts. 

It's not easy to tell from your pic but it looks to me like you're a little under 55mm. 

 

Edited August 7, 2020 by PhilDowns
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