I have a similar one (Tamron 400mm f6.3), I got it locally for $30..
It is a Tamron, but no good at all.
But for guiding (and terestrial probably), it's OK.
It seems the optical quality of those lenses are all over the place.
http://www.iceinspace.com.au/forum/s...ight=f6.3+lens
I removed the field lens group (the one closest to camera) and it become ~300mm..
Could you do some tests on stars? Middle, of the frame, corners..