back focus and adjustable coma corrector

[Ulysses31]

Hi, guys i have a Opticstar ARX200 f/3.9 astrograph which came with a f/3.9 adjustable coma corrector and I am struggling to work out the back focus! I am using a Nikon D5100 DSLR which I know the the air gap is 48mm from sensor to imaging plane.

When I use the corrector I can only gain focus with it when it is 0mm but still get major coma around edges of frame. I have tried at every point from 0-55mm and cant gain focus anywhere else??

Anyone any ideas what I am doing wrong??

[Lolly Eater]

I might be talking out of my hat, but I think the idea is you move the focus point in front of the corrector, by the same amount on the scope, until you minimise the coma. So try 10mm then focus the telescope by the opposite amount 10mm. Let me know if it works. I'm guessing you will go through the sweet spot, then halve the correction each time. Once done, I doubt you will have to do this again.

 

[alacant]

14 hours ago, Ulysses31 said:

what I am doing wrong?

Hi. Remove the helical focuser and screw the coma corrector into yout Nikon t2 adaptor. You do have a Nikon t2 adaptor? That gives you the 55mm you need for sensor-cc.

HTH and clear skies.

[Ulysses31]

Cheers guys will give this ago