Back Focus

[Likwid]

Hello everyone,

So I just got an ASI294MM Pro in the mail today, and I have been learning about back focus. I have my appropriate 55mm from my focal reducer with the correct adapters, but it got me thinking and now I am confused. Before I got this camera, I never thought about back focus and I had my DSLR Adapter threaded right into the Focal Reducer. Things have been pretty good, and I can get focused fine with my bahtinov mask I have even gotten lucky and managed to get to 0.00 with the BackyardEOS Bahtinov Tool once or twice. So my questions are: What happens if you are not at the appropriate distance of back focus with a DSLR? Also, is there something inherent about a DSLR that doesn't require the same back focus?

I attached what I captured last night with the minimum distance possible between the focal reducer and DSLR sensor. Could I improve on that with my DSLR if I get the correct back focus?

Thank you!

 

[The Lazy Astronomer]

When using reducers/flatteners the back focus number is the optimum spacing required to get the best optical performance.

Some combinations of equipment are more sensitive to accurate back focus than others, but it's possible to get perfectly acceptable images with non-optimal spacing*, but you may get some strange star shapes around the edges of the frame and/or you won't get the advertised level of reduction. 

 

*when I say 'non-optimal', I mean slightly out. If you're a long way off, your image would probably look terrible.

Edited March 26, 2021 by The Lazy Astronomer

[Budgie1]

The reason the back focus is normally set to 55mm is because that is the standard back focus for the majority of popular DSLR cameras.

If you measure from the DSLR sensor face to the face of the T2 adaptor which butts up against the reducer, you'll find it's 55mm. It look like you've got a lot more on the ASI294 camera with the spacers, but it not.

You can still obtain focus in the centre of the frame if the back focus is slightly off, but you'll get a tunnelling effect with the stars as they get further away  from the centre of the image. There's a few examples on here if you do a search. 

[Likwid]

Interesting, thanks!