There's a whole host of inexpensive mirror lenses available in the 500 mm - 800 mm range (this for example). They have slow max apertures (f/5.6-8) and a donut shaped bokeh (not a problem for astronomy work), but it seems that with suitable tracking they could be excellent for DSLR astrophotography.
However, I can't find anything one way or the other in regards to this.
Are these lenses appropriate for DSLR astrophotography work?
Apart from longer (and more accurate because of the focal length) tracking times because of the slow aperture, is there some glaring issue with them?