500mm Lens

I hit an senior NIkon and it takes enthusiastic pics, 35mm. I poverty a photo lense for wildlife etc. There is a mirror lense I crapper intend which is 500mm. What specifically is the determine of the mirror lens?

http://cgi.ebay.co.uk/Paragon-500mm-Telephoto-lens-M42_W0QQitemZ290290912199QQcmdZViewItemQQptZUK_CamerasPhoto_CameraAccessories_CameraLensesFilters_JN?hash=item290290912199&_trksid=p3286.c0.m14&_trkparms=72:1690|66:2|65:12|39:1|240:1318

I already hit a fivesome cardinal millimetre Centon mirror lens, but as this uses an M42 mount, I could confiscate a 2X photo convertor and would hit a 1000mm lens, which is what I rattling want. Would that be worth it?

So, if im hunting at these drawing correctly, this Bower lense needs a LOT of reddened to duty properly, correct? (f8.0)

im intelligent for a photo lens. something same 500mm-1800mm (or anything around that range). im not a professed YET!. in college and im hunting for a decorous lense to intend me started in the woods. hunting in the CHEAP to low $500 range. not hunting for a randoms lense i could encounter myself online im hunting for ones that grouping hit and would recomend me getting. gratify and thanks.

I am intellection most effort that for my camera XTI module this impact well?