Back focus and extension tubes

[Adrian J]

Hi all,

Coming from a DSLR background I’m relatively new to astrophotography and still trying to get my head around everything.

I recently purchased a WO William Optics GT-81 IV and ZWO ASI 294MC-PRO which are both mounted on an HEQ5 Pro.

First thing I noticed was I couldnt achieve focus with the focus tube fully extended so I took the camera out, moved it further away by hand and found that I got a sharpish image about 2" inch further back so I bought a couple of 60mm extension tubes and believe these are needed as I'm not using a diagonal mirror?

I've achieved focus using the extension tubes but often read about back focus and how critical it is.

My images are okayish although stars sometimes appear to be a bit soft / bloated and this must be a dumb question but am I right to simply add the extension tubes or am I missing something regarding back-focus?

And if I add filters or a flattener / reducer (which I've already purchased but not fitted yet), how do I achieve perfect back focus?

Perhaps you folks can point me to some elementary education on the subject?

[Adrian J]

No worries guys, I've answered my own questions(s) ...

 

Edited November 24, 2020 by Adrian J