Need help with telescope mount; stars have tails

Question

If I take a shot longer than 20 seconds, the stars get a tail. I have a Celestron Advanced GT mount with starshoot autoguide scope. I use a cannon can on a 9" Celestron scope. I can't get a picture without having tails on my stars. It only looks good if I keep the pictures under 20 seconds.

EDIT :

I did the polar alignment. The mount has a polar alignment scope threw the center of it. It's also balanced, the mount has a brids eye level. I use the new PhD software with the starshoot autoguide.. There is no slop in the mount, the gears have almost no backlash.

Answer 1

If you can't get any longer than 20 seconds, chances are it's your polar alignment. The mount you have is a motorised GoTo EQ mount (I believe), but if these aren't aligned properly, then they can still have trouble.

Also, depending on your software, try a 2 or 3 star align to get a better accuracy.

Also check the scope is nicely balanced with the camera on the end, and that your mount can take the weight.

EDIT :

If you try all this and you are still getting trailing, it may be worth going to a professional to get the mount looked at. It may be faulty.

Answer 2

you need a motorized equatorial mount, which tracks the stars in such a manner as to cancel the rotation of the earth and eliminate the tails on your photos. These are commercially available for many common types of telescope mounts.