Curves on Stars - Really Struggling

[ribuck]

Hi All,

Towards the end of last season i bought a 10" Quattro and associated Sky watcher Aplanatic coma corrector and i've really struggled to understand what i'm seeing when i defocus the stars.  Basically the stars closer to the edge have severe light drop off in a curved fashion - but if you look at the curved light drop off - it follows a pattern almost like a circular patterns when you look at the stars in each corner.

 

My coma corrector is spaced at exactly 55mm - which is the correct spacing for the skyatcher apalantic coma corrector.  Collimation wise, everything should be ok as i collimated it with a Howie Glatter Laser and Cats eye Kit so it should be pretty darn close.

 

I think the secondary mirror rotation also looks ok as the secondary looks like a perfect circle in the 2" sight tube.  

 

Any advice would be greatly appreciated as i really dont know what i'm seeing.

 

Thanks in advance,

Cheers Rich.

 

 

Edited October 10, 2021 by ribuck
edit

[paul]

Bottom-line, I think this is an artefact of looking at out of focus stars where the OTA aperture is very close to the mirror diameter.

 

The last image I think helps the most. It shows symmetric clipping of the light path to the primary - that's good in one way because it shows the mirror is parallel to the tube and centred. In my view it also shows that the aperture is smaller than the non-parallel light cone when out of focus. If that is the case as you approach the actually focus point the light path of your imaging light is more parallel and I would hope to see the percentage of clipping reduce, tending to zero. Can you take an image nearer to focus?

 

If you have a dew shield try removing it.

 

the slight clipping on the right side of this star looks like the CC protruding into the cone of light. If moving the focuser out form where this image was taken gets you to the optimal focus point then it may not be relevant.  If the CC clipping is still present in focused long exposure stars then try moving the primary up a few millimetres. 

 

 

 

 

 

[ribuck]

Hi Paul thanks for the feedback - Having never imaged with a fast newt before i wasn't sure if this was normal or not when viewing stars out of focus.  For the clipping on the right is easy to fix as i have the primary racked in tight against the primary springs - i can easily loosen them off to push the the mirror a good 10mm up the tube.

 

In terms of the general results, things generally look ok - just ignore the star shapes as the scope was way out of collimation when it was taken. The out of focus star tests were done later after i had recollimated.

 

 

 

 

 

Edited October 11, 2021 by ribuck
update

[ribuck]

If anyone else has anything to add, then please feel free to comments as i would really appreciate as much input as possible.

 

[ribuck]

Paul - It looks like you were spot on with your analysis, as i've had a similar comment on Cloudynights, where i was told that this is perfectly normal behaviour when way out of focus with this coma corrector.  So just like you mentioned, it's just due to the critical nature of the F4 system & coma corrector.

 

So all is well, as it was not an actual issue, simply a lack of knowledge on my part. So I can sit back and relax now and stop chasing my tail over nothing.

 

Cheers,

Rich.

Edited October 12, 2021 by ribuck