Variable coma

[markarian]

I've started using a Baader MPCC to help remove the coma on my 200P and it has made a huge difference.

However, I've noticed recently that it mostly fixes the coma on one side of the frame but less so on the other.

Below is a full-frame (Canon 350D) of M36 and then zoomed images of the top-left and top-right corners of the image. Top-left is good, top-right is much worse.

Any ideas why this might be?

Thanks

Mark

[andrew s]

Possible reasons include: collimation is off so more coma in one direction, camera not square on or coma corrector not square on. I would suspect the first - collimation - as the most probable cause. - Andrew

[brianb]

I would suspect the first - collimation - as the most probable cause.

Depending on how the fouser, coma corrector & camera are fixed together - the slop in 1.25" or 2" push fittings can be enough to throw things out enough to give the sort of effects you illustrate.

[markarian]

Thanks for your replies. I checked collimation and it's pretty good - could be better as always.

I have done some work on the focuser to try and improve the alignment but I suspect there's more to be done there. I have the single speed crayford which I had to get "shaved" to fit the coma corrector and reach focus. I know it's not perfect but wasn't expecting it to have that much effect on the image. I maybe need to look into a bit more.

Thanks