Need some help troubleshooting

[Savage234]

So... I have been off/on tearing my hair out and nearly giving up the hobby over this unknown issue for a while now. I have gone through the list and think I have narrowed it down to one thing,...hopefully. 

My photos taken with my reflector telescopes never seem to be in focus. There is either of duplication of everything, or what looks like really bad coma across the whole frame, but any distortion is just about perfectly uniform across the whole sensor. I think what is happening is that the coma corrector is not adjusted properly. I have made sure the two reflector scopes are culminated, made sure the optics are clean, made sure nothing is being pinched, I have since bought a new DSLR (not for this, I just wanted a new camera anyways). I think I have covered all the bases, the only variable is the coma corrector. 

The corrector I use is this https://optcorp.com/products/ba-rcc-i-rowe-coma-corrector , with the required backspace adjuster.  It calls for a total of 91.5mm of backspace. Assuming the camera has 55mm of backfocus the adjustment on the coma corrector is marked at 36.5mm...but that measurement does not appear to work for me.  I have played around with the +- 3mm of adjustment the directions allow for, but to no avail. 

Now, I also have a coma corrector for an F/4 refractor that calls for 55mm of back-focus that didn't work (made all the stars triangular) until I stuck a ~1mm ziptie between the coma corrector and the T-ring. That brought the focus to being nearly perfect....But adjusting the reflector coma corrector the same distance does nothing. Because nothing can be that easy. I unfortunately don't have a reference scope that I know for sure is good. The best I know is that the scopes seem to be fine without the coma corrector, albeit with terrible coma since they are very fast scopes (F/3.9) 

I have tried looking up my camera specs (Canon SL2) to get an exact figure for back-focus but I have not been able to find anything (backfocus and backspacing seem to have different meanings in the normal photography world).  Does anyone know of a resource where I can find these backspace specs? Or really just any advice or suggestions from a similar issue? I would really like to get to using my big scopes now that its getting colder out.  I am about at the point of getting a smaller reflector just to see if the issue is my scope or the coma corrector. 

Thanks for any/all help

[pete_l]

Teleskop Express (a highly thought of German supplier - ships to foreign countries) suggests a variable length extension for this corrector. Used with a 10 or 15mm fixed extension would give you a range that should cover all possibilities.

[michael8554]

Google for the back focus of an EF-S lens instead of for an SL2 - it's 44mm.

And a really precise search could be for Focal Flange Depth 🙂

A T2 Adapter will add 10 to 11mm, so your 55mm figure may be correct. 

Michael