Astro modded Canon back focus

[CedricTheBrave]

I have been battling with coma on my Canon 700D but now I realise that it maybe the position of my coma corrector

the BF of the corrector I have states 55mm and the T Ring appears to be 10mm so with the 44mm of the Canon I am 1mm out to start with!

would the fact that my Canon is also astro modded make the BF further back or no different?

 

Dave 

[bottletopburly]

I have a baader mk3 which I tweaked to 57.1 mm 

[CedricTheBrave]

Just now, bottletopburly said:

I have a baader mk3 which I tweaked to 57.1 mm 

ok thats interesting thanks

[michael8554]

Post a jpg of a problem sub.

Specify which OTA and CC.

Michael

 

[CedricTheBrave]

4 hours ago, michael8554 said:

Post a jpg of a problem sub.

Specify which OTA and CC.

Michael

 

This was a few nights ago. its a bit messy but shows the issue.

OTA is my VX14 and the CC is the Skywatcher 0.9 reducer one

i dont get this issue with my Nikon only the Canon but the nikon is no astromdded and its BF is 46.5

 

M_27_Light_180_secs_2021-08-11T00-37-55_009.cr2

[michael8554]

I couldn't find redetails of a Skywatcher 0.9 Reducer, I could find a Skywatcher 0.9 Coma Corrector.

To my eyes the bottom-half of the image isn't too bad, the top-half has noticeable Coma.

Suggesting tilt - either through the connections to the focuser, or the sensor after the Mod.

I believe that a Nikon with a T2-Adapter, and a Canon with a T2-Adapter, are both 55mm spacing from the T2 thread.

So if the coma is fine with the Nikon, and if you are using the same connections, sounds like sensor tilt.

But try with the CC unscrewed slightly, to give you a mm or so extra spacing.

Michael

[CedricTheBrave]

It is the SW Coma corrector but its also a 0.9 reducer 

Nikon F Mount is 46.5mm focal flange length so as long as the T Ring is 8.5mm then the Nikon should also be 55 

I have 3d printed a mixture of spacers to try out so now await a clear night