Update on my experience with lenses and observing (I had a great News Year Night with the Mod 3).
I have maxed out my camera lens collection now, I have every sort of lens I might ever need and have verified them all as working at least acceptably.
Since the investment in the top-of-the-lime Mod 3 (gain control, white filmless screen) and the filters to go with it are so substantial I want to maximize its utility and also investigate the different types of lenses and mounts to investigate what works best. Nickle and dimeing on lenses when you have spent $5000 or so on the NV and filters makes no sense.
So I ended up with 15 camera lenses. For the ones I consider "astronomy related" (12 of them) I spent a total of $700 on them, or about $60 per lens, and $200 was for just one, my smallest, the 23FM25SP Tamron 2/3 25 mm F/1.4 for its special threading as others have done here. Leaving that aside it is $500 for 11 or about $50 each. Of this set I have 11 that are usable for astronomy - some were defective or could not reach focus (with no current fix), one was coming apart (cheap mechanical design), and one had damaged filter threads so I could not use a front filter. The set has focal lengths from 18 mm to 1000 mm.
Three lenses that have proven to be useful that were not astronomy related per se but can be used for that purpose are:
Canon Nifty Fifty F/1.8 (I already had it for an actual camera)
Cosmicar Asahi 15-180 mm F/1.9 TV (not CCTV) Zoom Lens C-Mount (which was surprisingly huge) which I am planning on using for unfiltered nature observing (these are still available on eBay)
Pentax Super Takumar Thorium lens 50 mm F/1.4
The Takumar I bought for my element collection because it has a lens that is 11% thorium. They don't make these now since they are mildly radioactive. The radioactivity creates problems for the lenses since it knocks electrons out of place and cause it to acquire a sepia tint over time. My lens, from the 80s, has the tint (proving it is what it is supposed to be). It turns out to be a very nice lens to use though. It shows what a real premium lens is like, sophisticated mechanical design, buttery smooth focusing which has plenty of out focus past infinity. The tint is no problem at all, the lens is still plenty fast (I actually dialed down its aperture a bit last night to darken the sky for star field viewing). BTW the tint can be removed by exposing it to intense light - I could if I want to let sunlight pass through it to knock the electrons back into place.
I poked around looking for adapters and was able to round up C-mount adapters for every type of lens mount I was at all interested in: T-Mount, Canon EF, Canon FD, Pentax K-Mount, Nikon F-Mount, 42mm screw (and C-mount of course). The different adapters allow me to explore the Mod 3 clearance problem (some are tapered and have no problem), whether I can put a 1.25" filter ring inside the mount, which are easiest to use, etc.
One discovery is that not all K Mounts work together. Older ones do not work with new ones, they do not engage securely. One older K-mount lens I bought I can't use for this reason.
I ended up with lenses of every mount type except the Nikon F, having the adapter collection allowed me to shop for lenses without worrying much about the mount - which was good since two lenses I bought advertised the wrong mount but I could use them anyway.
One advantage of having so many adapters and lenses is that I have can have a complete observing set for a night all with C-mounts ready, no adapter changes needed.
One type of lens that I encountered which is pretty neat is a long focus mirror lens - a catadioptric lens like an SCT. These are light and compact for the focal length (500mm) and are also inexpensive used. Seems these are not too popular with shutterbugs - the central obstruction (which our community knows all about) produces "bokeh" (blurring basically) but is fine for astronomy use.
The cheaper models of lenses do tend to have infinity focus problems since their max out-focus is "infinity" which it may not be quite for you. The better lens-makes provide out-focus past nominal infinity.
Two K-Mount lenses I have that did not quite come to focus I was able to resurrect with a cheap 2X teleconverter. Doubles their focal length but they now focus. I tried a T-Mount teleconverter but it did not work at all, led to optics that were way off.
I came up with filter rings for all of these lenses so that I can front-mount 2" filters, thanks to Raf Camera in Russia for five of them. Of the critical lens ring (that makes it possible to use cheap commodity rings sets to finish) three of them (the biggest, and the smallest two - extreme odd ball sizes) the rings were undersized and either engaged loosely or just barely did not engage. No problem, I just epoxied them on, since there is no situation where I would want to remove them ever.
Word of advice about working with threaded things and epoxy - if you are epoxying something that has a thread you intend to use later always coat that thread with petroleum jelly - just a thin wipe is all you need. You can say "I won't get any epoxy on the thread", and maybe you won't, but the jelly wipe guarantees that the thread will stay good.
Last night I was working with my shorter FL lenses, up to 135 mm and testing 2" filters. I have the Optolong 650 and 685 nm long pass, a cheap 590 nm long pass from eBay, the Optolong 7nm H-Alpha, and the Baader 35 nm H-Alpha. I found that I liked them all, for different effects. The 590 nm for $23 was a good deal that gives you more options.
I finally got the viewing dialed in for wide field star fields and was blown away in observing the Cassiopeia Window. The 50 mm lenses (of which I have four, with F ratios from 1.3 to 1.8) provided 20 degree TFOV with a high density of stars and many clusters visible simultaneously. I've never seen anything like it before.
I found the fastest lenses were a bit too fast, and liked to dial it down a bit for a darker sky background (using the long pass filters). Orion was nice with the Baader 35 nm, which showed nebulae, the constellation and surrounding stars in a "normal" like view (7 nm made the constellation largely disappear). I like the narrowest H-alphas when zeroing in on nebulas at higher power.
I will start posting lens pictures in the near future, and my further experiences when working with all this gear.
Edited by careysub, 01 January 2020 - 08:10 PM.