Coma

[200f4 (Geoff)]

Hi all
Have been trying out photography with a canon 60d camera attached to my 200mm F4 gso - I have a coma corrector (GSO model) but am wondering about the stars on the edges of my frame - coma? or tracking error? (this is a full frame - not cropped) -- would this be normal for a short scope like this or should I be able to get better?

the image was a stack of 5x 4mins and 5x 1 mins shots at 1600 iso
Any thought gladly received
Geoff

[cometcatcher (Kevin)]

Looks kinda like coma. I would have thought the GSO corrector would have fixed that. The Baader MKIII coma corrector makes for pin point stars from edge to edge.

What's your CC to focal point spacing like?

[MrB (Simon)]

Definitely coma. You will likely need to play with the spacing. Hopefully the spacing needs increasing, if it needs reducing you could be in trouble. Try adding a cardboard shim between the corrector and the EOS adaptor to see if it reduces the severity of the coma. If it does, it's then time to play even more to determine how much spacing you need. Hopefully it's not much. When you work that out, try to source some brass or stainless shim stock with the correct thickness or combination of thicknesses.

[200f4 (Geoff)]

Ok thanks will work on the spacing I can get two distances with the existing spacers 50 mm and 95 mm - I think the optimum is 65 mm so will look at obtaining a ring or making a card one to test and see if it will help -

[jsmoraes (Jorge)]

The distance between sensor and the coma corrector GSO 2" is 70 mm. You have plus 5 mm or minus 5 mm of tollerance. Despite of I use it with plus 10 - 13 mm: my distance is 83 mm with razonable performance. And my telescope is f/4.9

If your coma corrector is the same... check that distance.

And really it is coma. Not tracking error. The direction of drift is from periphery to the center.

[200f4 (Geoff)]

Thanks guys - I made a spacer and ended up with 75mm spacing - much better results -- excuse attached photo color as high misty cloud tonight but I am much happier with the stars - much better out to the edge - will look a finding the correct spacer rings and see how it comes out
Hopefully a bit more tuning will see it great - maybe the 70mm will be spot on jsmoraes

Geoff

[cometcatcher (Kevin)]

A big improvement.

[jsmoraes (Jorge)]

Yes, very better.

My experience says:

1) this coma corrector has relative low tolerance. For example: I can not reduce from 83 mm to 80 mm, because of filter well. But the performance with 80 mm is better than 83 mm. I have some reflex because of this plus 3 mm. Despite of it works with plus 10 - 13 mm. My telescope is f/4.9, so less critical than yours.

2) I percept that always there is a side of the photo with less performance. I thought that it was by bad axis optic centralization. Your photo seems to have the same appearance: the right side is better then left side. Exactly like mine.

If this is true... don't worry. It's the same with me.

[200f4 (Geoff)]

Thanks
I am hoping the slightly worse side is due to the adapter as I made it from tape and some metal packing rings -- was worried the weight of the camera may pull it apart but lucked out and it held together - after I obtain some threaded spacers I will know for sure if it is something else I need to chase.

Geoff