183GT Backfocus on ZS73 with ZFlat73A

Awesome, thanks John!

What’s the 55mm relative to / why 55? Just an industry standard?

Been out of the astrophotography game for over a year so I’m putting the pieces back together.

Thanks!

No worries.

The 55 mm is the back focus of Canon and Nikon SLRs and DSLRs and hence it became a datum for CCD imaging. ZWO supply all their cameras with the spacers etc to get you to 55 mm whether or not you have an OAG filter wheel etc.

So add the spacers to the camera loosen the lock ring on the flattener so you can see the calibration marks. Screw out the outer section until it sits at 11.2 mm then tighten the locking ring back up and screw on the camera M48 to M48. 11.2 comes from distance A above 66.2 less 55.

Thanks everyone!

Excited to start narrowband imaging!

Any tips? Check out my thread here:

https://www.astrobin.com/forum/c/astrophotography/deep-sky/new-to-narrowband-sho-suggested-exposure-times-calibration-frames-etc/

Thanks.

John Noble:

I have to say that I find the image of WO quite confusing. They do a much better job for specifying backfocus for FLAT73R. Why specify two values A and B for backfocus? Why 66.2 and not 66.4 if they mention 11.4mm further up on the page (55 + 11.4 = 66.4) ? The 0.2mm may not matter but it's confusing anyway.

On a side note Canon has a flange distance of 44.1 mm, Nikon 46.5 as you can see from the back focus specifications of e.g. the ZWO Canon EOS adapter.

Clear skies
Wolfgang

Good catch on the 55 mm Wolfgang, the 55 is the approximation of the 44.1 and the 44.6 plus a standard T adapter ~ 11 mm - at least that's my understanding.

55mm is the flange distance according to the specs of the T-mount defined in 1957. It seems the T-mount then became the standard mount for telescopes and that's why we have 55mm as the standard backfocus (with plenty of exceptions though ...).

https://en.wikipedia.org/wiki/T-mount#

Clear skies
Wolfgang