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I forgot to post this yesterday. It's in black and white because there was horrible purple fringing. Anyone know of a way to get rid of it? This is the best image I have yet taken through my 75-300mm zoom lens.

A great attempt!

The stars around the edges of the image have got fish tails. These are usually caused by a poor quality lens. If you look carefully you can see that the fish tails of the stars on the left hand side of the image go to the right, and the fish tails on the right hand side of the image go to the left (and up to down and down to up), though the stars on the left hand side have much worse fish tails than those on the right hand side. There are two likely reasons for this - the first is chromatic aberration whereby the lens is unable to focus different wavelengths of light at a single point (different wavelengths are refracted by different amounts), do your fish tails show a rainbow effect on the coloured prints? - the second is an alignment issue whereby the elements of the lens and the sensor surface are not truly parallel to each other, this is a common problem with cheap zoom lenses because of something called barrel slop - when the lens is zoomed out to its maximum focal length it flops downwards slightly in the main barrel, and the objective lens is no longer aligned with the sensor, this causes fish tails all going the same way on the image. I think you have a combination of these two problems.

clop

The lens I got as a gift. I'm not sure whether the purple fringing was from chromatic aberration on the lens or from the CMOS sensor's microlenses. Part of it certainly has to do with the lens; I don't get any of these problems with my other lenses or 6'' Schmidt Newtonian. Oh well, the lense was free, and it works pretty well anyway.

Another thing, I have a 50mm f/1.8 lense and the first time I tried it the stars at the center of the field had concentric bands around them, and the stars at the edge of the field seemed to be displaying coma. It looked horrible.

Later, I forgot to remove the stop on the aperture when I was using it again, and it was at f/3.2 or something as opposed to 1.8. The problem wasn't there and the stars were relatively sharp across the field. Is there something about the wide aperture that causes this?

Yes. Almost all lenses, primes and zooms, are sharper when stopped down a couple of stops. In addition, stopping down increases the depth of field and makes it easier for you, and the lens, to get the stars in focus. There's a limit to how far you can go though - above f/16 things usually start getting worse again.

Here's a link to the resolution chart for the lens I use.

http://www.photozone.de/images/8Revi...200_28/mtf.gif

You can see that even this L-designated prime lens gets sharper when stopped down to f/5.6, though it's not a huge improvement. What model is your zoom lens? Maybe we could look at the test scores and see how good we can expect it to be?

clop

I own these lenses:
Canon 50mm f/1.8
Canon 75-300mm f/4-5.6
Canon 28-90mm f/4-5.6
Canon 18-55mm f/3.5-5.6
The last two are kit lenses that don't work so great, but are okay for versatality.