Originally posted by jcshellyg
SMC PENTAX FA 645 400mm.
Actually I am not using that lens. I am using the even faster SMC A* 400mm F/2.8 which is a regular (exceptional quality though) full frame K-mount lens. A 300mm f/4 lens would be a good starter lens or even one of the old Pentax 200mm f/4 lenses. The 300mm I have is the Sigma 300mm F/4 APO Tele Macro and I got it from my local camera shop at the stole it price of $130. When I suggest one of the old 200mm F/4 lenses I do mean old. I have and use the M42 mount SMC 200mm F/4 Takumar and I know others have used the M and A versions as well and those lenses really want to be run at F/5.6. I can usually find one locally for between $60 and $90 dollars in optically great condition with varying features and wear on the lens barrel. Lenses like a good 200mm or 300mm will open up a lot of good starter deep sky objects. Also if you want wider get one of the old Pentax 50/1.4 lenses, I run the M42 mount SMC 50/1.4 Takumar and like basically all the Pentax 50s it really wants to be run at F/2.8 for astro. Again old ones work just as well as newer ones and can be had for less than $100 in good shape. Wider and cheaper still you could get the old M42 mount SMC 28mm F/3.5 Takumar and for astro it gets great at f/4 and mine I got for $80 but it also had the case for it, the hood, and the hood case but was also basically immaculate.
Apart from the A* 400/2.8 the lenses I've mentioned above are not exceptional, are not hard to find, and are not expensive.
My other bit of advise is to get the biggest heaviest tripod you can find. My biggest one
is this wooden monster but that is probably overkill however it only cost me $50 and an afternoon with some power tools. The other tripod I use for astro is an old Manfrotto 3058 tripod with the Manfrotto 3057 head that I picked up for $110. For astro you really want big, heavy, and ridged as that minimized vibration and quickly dampens it.