"Affordable" is relative, of course, but the current, used prices I see for FA, D FA and DA 300mm f/4 lenses, which would seem to be very sharp indeed wide open, are for me much too expensive. A 300mm f/2.8 lens that is sharp at f/4 would be another (far less portable) option but It seems even less likely that I would find something like that I could afford. I am currently using an SMC Takumar 300mm f/4 but I need to stop it down to f/5.6 to get acceptable sharpness. It also has CA, of course, but with software I can at least minimize that. I can't really increase sharpness using software, at least in any major way. It would be real nice to use a faster lens that is at least as sharp.

Alternatively, a 300mm lens that is sharp at only f/4.5 would also be an improvement, although it would also be nice if such didn't have much CA. So, a Pentax F* 300mm f/4.5 would be fine, assuming I could find one for a reasonable price. The Sigma 300mm f/4 APO Tele Macro that MossyRocks has is a more affordable option that sounds like it would fit the bill nicely (reportedly sharp from f/4.5 onwards) but it is as rare as hen's teeth. I haven't seen one yet for PK mount on eBay. (Two on there now for Nikon.)

Note that I would likely use the lens only for astrophotography, so autofocus isn't strictly needed. However, AFAIK any lens that meets my criteria above is also AF.

Lew,

When I was looking, I saw seriously good prices for the 645 glass. You'd want to consider FOV, but the eBay "new" prices I see are:

SMC Pentax-FA 645 200mm F4 [IF] - $90-$100
SMC Pentax-A* 645 300mm F4 ED [IF] - $180-$250
SMC Pentax-FA 645 300mm F5.6 ED [IF] - $300-$400
SMC Pentax-FA 645 400mm F5.6 ED [IF] - $500-$1000

Appears a decent converter is in the $50 - $200 range, depending on which one you buy.

Thanks. I had not considered a 645 lens. Are the converters optically based or just mechanical? If the latter, what accounts for the 4x range in prices?

I believe they are just a mechanical coupler to convert the mount and get the correct register distance. A 300mm 645 lens will gibe the same FoV as any other 300mm (minor differences for lens distortion and the lens not being exactly 300mm not counted). By running a 6x7, 67, or 645 lens you would also get the benefit of only using the a much smaller central portion of the much larger medium format image circle.

As far as 300mm lenses I like I really liked my old Sigma 300mm f/4 APO Macro. I got a steal of a deal on it and paid $130. It is pretty good at f/4 for astro but gets great at f/5.6. It is a pain to nail the focus because at the infinity end of things a little movement makes a big difference so even with a bahtinov mask you have to play for a bit. In that regard it is nothing like the SMC A* 400/2.8 which has great manual focus ability. Don't get me wrong it is still easy to focus that Sigma 300 probably no different from other modern AF glass but I don't use much of that for astro so it is really hard to have a reference point to compare to.

This would be the candidate, but how do you mount it to a tripod? Does anyone sell a foot that would work? (Presumably Pentax never offered one.) I suppose a couple of oversized finder rings attached to a bar with a 1/4-20 threaded hole in it could work, like this guy did: Photo lenses for astroimaging | hhastrophoto. I have a pair of those I used to use with an 80mm guidescope. A little big to fit in my camera bag but....

Another question hopefully someone knows the answer to: If I wanted to add a 1.4x teleconverter to that (giving 420mm at f/5.6), would it be better to get a 645 one and put the 645-to-K adapter after it or get a K-mount one and put the adapter inbetween? The latter has the advantage that I can use the teleconverter with my other k-mount lenses. But maybe there is a good reason to use a 645 teleconverter, such as better optical performance.

If I were you, I would look at a 645 teleconverter, only because it would be bigger and you'd have a broader attachment for weight distro. The adapter to the converter would be a much thinner column. OTOH, there are many more K teleconverters.

I first considered then when I saw an article about the last Pentax telescope made (or one of them) and someone mentioned it was essentially this lens: SMC Pentax-M* 67 400mm F4 ED [IF] Reviews - 67 Telephoto Primes - Pentax Lens Reviews & Lens Database

See, e.g. here: Pentax 105SDP Apochromatic Refractor | Astromart

Rick suggested a mounting rig in his lens review of the 645 A*300/4, I found a slightly different model number that fit fine (see posts 11, 14 & 15): Grand Lens Test 10: The 645 A* 300mm f/4 - PentaxForums.com

Great info! Thanks. That would seem to solve the problem.

I used a long lens support like this:
Amazon.com : Neewer Telephoto Lens Support Bracket Quick Shoe Plate Long-Focus Stand Holder with 1/4inch and 3/8inch Screw Thread for Tripod Ball Head DSLR Camera Photography Compatible with Arca-Swiss : Camera & Photo

That looks well made and is surprisingly inexpensive at only $50US.