On the other hand, as a well-known and "infamous" "anti-Pentax" Pentax user and guy, what I had to do is of course in the opposite direction than Pentax! As a result, I decided to use Pentax lenses as telescope and what I needed is this Chinese-made (US)$65 converter! :-D
(Above: The (glass?) Prisms of the converter can be seen from the mount side; and most of the lens elements are multi-coated - the quality of the coatings is not bad.)
I then put some of my "obsolete" K-mount lenses and tried. This is one of the very first K-mount Sigma AF lenses that I purchased back to 1988 at about $130:-
Now, what I have is 7X to 21X zoom telescope with my 70-210mm zoom which was originally meant to be a camera lens!
Manual Focusing is very easy as this adaptor has not much light loss and is very bright by itself. It is good for lenses at F4 or slower, though, as the "eyepiece" seems to suffer more with some kinds of aberrations at larger apertures.
In fact, I have not used my this lens for a long time for taking photographs as it comes no close to any SMC Pentax zooms in colour rendition and flare resistance! It is tack sharp, though, even it is wide opened, and through out the whole zoom range! But then the colours it produced scare me every time when I used it, no matter with films or when it was put on DSLRs! :-(
After all, the results are really great! I can safely tell that it is not different from a $1000 telescope with that high image quality that delivered but with the cost of $65 only! I like also the push-pull zoom mechanism of my this lens (which is a rare design for an AF lens) which is very convenient to be used for this specific application and with MF needed at the same time. In addition, this telescope has another special feature of letting the user to stop down the aperture when the viewing scene is just too bright, which is very useful IMHO and protects the eye! And, the magnified image could be even sharper when the lens is stopped down! :-D
Furthermore, this accessory could also be used to inspect the IQ of a lens at the image centre, which is another great news for measurebators like me! I can see that some of my lenses have more purple fringing than others after magnified and some lenses needed to be stopped down more and some required less before the IQ is the best. I am sure that this is yet another very nice feature of it!
Btw, I also tried this with my beloved F*300/4.5 lens, which I found the optimal apertures starts from f/5.6 to f/11 something (but not smaller). Well, I have a tripod mount for this 30X telescope, too, which at this high magnification is a very useful feature then (the FA*300 does not have one.):-
(Gear Picture via Original Blog Post Here.)
Last but not least, adding an 1.4X TC to this combo which made up a 42X telescope which I also tried and the IQ was found to be acceptable. All in all, it is really funny but yet useful to adapt all different lenses of mine like this. This story tells us once again that for most of the time innovative ideas and clever minds produce great products that can sell and high tech is not always needed! :-o