There was a discussion a week or two ago about the
500mm mirror lens here
Is there a particular reason you want a zoom lens? A prime lens would most likely be sharper and provide a better image. For the prices you are looking at you could get an Orion ED80 and WO 0.8X FR which would give you 2˚ 08' X 3˚ with a D70 or you could get an FS60-C Tak with a slightly larger FOV