1st attempt at M57 - Ring Nebula. Focus or Coma or Collimation?

[Saketh]

I am more or less a newbie and started both observing and imaging less than a month ago. I have imaged the ring nebula for the first time last night and I would like to know what the problem is so that I can improve. My kit is:

SkyWatcher 150P-DS on HEQ5 Pro mount
Canon 600D unmodded with 2x Barlow (2" Explore Scientific)

Image settings:
ISO 800
Lights: 68 x 60s
Total Exposure: 1 hour 8 minutes
20x dark, 20x flat and 20x bias frames

It has been stacked using Deep Sky Stacker with recommended settings
Slightly processed in Gimp - very basic using curves and levels.

This is a cropped image almost at the centre of the frame. The left side of the image seems to have sharp (ish) stars, while the right side of the image are bit of a donuts. I tried to focus a lot, but couldn't get it right. Wondering it is my ability not to focus correctly or is it coma or is it my scope's collimation or something else?

 

Regards,

Saketh

Flickr: https://www.flickr.com/photos/26122118@N00

 

[Seelive]

What does the uncropped image look like?

[wimvb]

A newtionian with a large chip camera, a barlow, but no coma corrector is a sure way to get coma and colour abberation in your images. With this combination, you really need a coma corrector. Also, the Barlow, unless it’s a top quality one, will introduce som chromatic abberation.

To verify if collimation is a problem, take an image of a defocused star. The doughnut should be symmetrical with dark shadow accurately centered.

[Saketh]

Here is the uncropped version: