Here is a stack of 8X120sec at 2000mm F10 Tal200k, no filters, Moon 90% taken with 5DH @ ISO 1600. Autoguiding has a slight problem with a focal length of 2000mm but I chose the best of the worst. Just a test.
http://users.bigpond.net.au/avandonk/Car_Tal.jpg
Bert