M13 - 18 Sept 05

By the way Mr. Grennan, why has the star in the top left corner have a 'jet' coming from it? :

By the way Mr. Grennan, why has the star in the top left corner have a 'jet' coming from it? :

Thats coma at the very edge of the field, look at the star in the top right corner too. Thats one of the characteristics of the SCT design, you need to look at designs like a ritchie-chretien or other field flattened cassegrains to avoid coma like this

Ah right! I know what you mean (I didn't spot the star in the top right doing the same!).

the 2 coma stars look like someone is shining 2 stage spot lights on the cluster, nice effect. :lol:

By the way Mr. Grennan, why has the star in the top left corner have a 'jet' coming from it? :

Thats coma at the very edge of the field, look at the star in the top right corner too. Thats one of the characteristics of the SCT design, you need to look at designs like a ritchie-chretien or other field flattened cassegrains to avoid coma like this

Dave,
Fine photo! I'm dying to get the observatory and get back into the astrophotography!
As for the star trails, I'm not sure it is coma. However, I'm not an expert on optics. I just haven't noticed anything like that on any of the images I took with my SCT, with or without focal reducer. However, to be honest, I've no idea as to what else it could be? Just seems excessive to be coma, but mabey I'm wrong.

I'm dying to get the observatory and get back into the astrophotography!.

Aren't we all !

I agree with Michael on closer inspection, it does seem very severe for coma, I've never seen anything as strong as that in my usage of the canon on the lx200.
Its probabily due to the fact that those 2 stars are just unusually/unluckily bright near the edge of the field of view.

I'd bet the colours are due to some sort of chroma effect, like not having the field of view exactly square with the optical axis, this might not be the cameras fault, if the primary mirror is not perfectly square with the secondary, I can imagine you might see the same colour effect as the focal plane might not be square with the camera chip?!?

To test this try taking an image and finish the focus the focusing anticlockwise, and then take another image focusing clockweise, if you see a differance, that its the primary mirror.
No matter what, I really wouldn't worry about it anyway.

I might be totally wrong, but I did see a similar effect with a webcam using a simple incorrectly placed adapter, where the camera was definitely not square with the optical axis.