Need some Help with Coma

[JSeaman]

I got myself into a pickle last night! 

The night before last I tried my new coma corrector (Baader 2" MPCC Mark III) and found I had lots of dust and marks in my image. I popped it apart, cleaned out all the dust and tested it last night. The first image I got was terrible:

 

After a lot of back and forth I ended up inverting the second element of the coma corrector and that seemed to improve things:

 

 

All four corners have the same issue though, coma with the tail pointing towards the centre of the image (this is the bottom left)

I double checked my spacing this morning and think I have a correct 55mm: 

 

So this morning I downloaded CCDInspector, a tool I don't fully understand, and produced this graph which may or may not be ok?!

 

I found the following image which shows what the FOV is like when the coma corrector is too close:

So I'm looking for a little guidance, should I just start increasing the spacing and trying my luck? I will check collimation again in a mo but I think that's OK. The focuser (stock 300PDS) doesn't appear to be sagging so I'm not entirely sure where to go next, thoughts welcomed!

James

 

Edited July 19, 2021 by JSeaman

[JSeaman]

Collimation looks very good, I did a tiny tweak on the primary but almost nothing

I've added about 0.5mm into the chain and will re-test that later but it's not a very scientific approach!

[bottletopburly]

The outer lens orientation is concave like a camera lens , the inner lens is concave both faces so shouldn’t matter which way you have it , I have my spacing at approx 57.1 mm using the m48 fitting I used a 2mm delrin washer in the base of the cc to push the lens further away , stars seem better though I have changed from dslr to a zwo533mc now on a 200pds.

[JSeaman]

Thanks that's interesting, I have just spaced mine out a little further and will check that. From the first picture to the second was night and day so I'm assuming I have the lens correct now and, as you say, the other one doesn't matter. I'll try this tonight and see what happens!

[michael8554]

 

 

[JSeaman]

Thanks, this is what I tried to show in the picture with the calipers above, 55mm to sensor