Astronomik EOS clip issue?

[jambouk]

I've got a CLS EOS filter clip on my Canon 600d. I've tried some widefield imaging with a 50mm f/1.9 Canon lens on my equatorial mount. But I am getting some artefact. Some of the stars in the peripheries (and maybe not evenly spread across the peripheries), are mis-shapen. See image below I took of Deneb and surround area. This was a 180 second exposure at ISO 200, f/1.8.

Is this related to poor polar alignment?

Dirty EOS clip?

Clip not being seated correctly in the camera body?

Dirty lens?

Recognised problem with this set up?

Something else?

One or more of the above?

Thanks for any replies.

James

[tdmodels]

Looks like more a feature of the lens, called "coma", which relates to the edges of the lens not bringing the stars to focus at the correct point.  I get this with my "Nifty Fifty" too.  Try stopping the lens down to something like f4 and you should see this issue disappear.  The only way to rectify this issue, is to either stop down, or buy the f1.2 version ;-)

[jambouk]

I had thought about coma but i didn't feel the artefact was evenly spread over the sensor, but i will give that a go.

When you say get the faster lens, do you mean the faster lens will be better quality, or with the faster lens i would still use it at f/1.8? Is coma always a risk in a lens when any lens is at its maximum aperture whether that aperture is f/1.2 or f/12?

James

[jambouk]

This link suggests most lenses will get some aberrations wide open:

http://www.astropix.com/HTML/I_ASTROP/LENSES.HTM

Thanks for pointing me in the right direction.

James

[jambouk]

And another useful page I've since found:

http://randombio.com/astrophotography-without-a-telescope.html

James

[lensman57]

Your lens is suffering from both coma and field curvature, this happenes a lot with fast lenses designed for terrestrial photography.

A.G