Originally posted by wanderer2 ..................On Amazon and B&H I also found many non-mirror f8 - 32 manual lenses in 500 mm and several of these include a doubler for 1000mm reach for a bit less than $100. A description is below (I assume these are all made by the same manufacturer). These would seem ideal for the eclipse but I wonder about the IQ. When a 500 mm standard type lens from Sigma, etc. costs $5-6000 it's difficult to imagine a $90 lens would provide much quality. I'm not knowledgeable about optics and have no idea what "catadioptic mirror lens" or " achromatic refractor design" mean. The description:......................
Those are the ones.
The design uses a thin lens up front with a small circular spot in its middle that's aluminized to reflect light from the main mirror at the back. The rear mirror is spherical which is low cost to make and the lens corrects for most of the spherical aberrations of the mirror. These are also referred to as a "folded" optical system and are shorter than just having the main mirror in a tube like a Newtonian (standard type) reflecting telescope. It allows a focal length of 500mm in a lens much shorter than 500mm.
If the manufacture does their job well, this can be a pretty good lens (though less contrast than a refractive lens but that can usually be corrected in post-processing). The "CATS" that B&H carry are of Asian origin and may or may not be constructed so well. I really can't advise since I haven't used any and there may be variations between brands or even individual lenses, but there are some promising reports elsewhere (some in the Pentax forum) you might check if interested. Try Googling also.
The image quality would probably be on a par with most low cost telescopes and maybe even better since the "CAT" has better aberration correction (theoretically). They are certainly easier to handle because of their compact design. As you surmised, they probably won't stack up to a Sigma prime 500mm and they are slow compared to f5.6 or f4 (and have fixed aperture and no autofocus capabilities).
I would get or make a solar filter that fits over the front of the CAT. Alternately, you can buy a skylight or UV filter, remove the retaining ring with a spanner wrench and put a disc of solar filtering material in place of the glass, making sure there aren't any gaps at its edges when the retaining ring is screwed back in. I just mention the latter because of the short time, you may want to order a filter mount at the time you order the CAT if you choose to do so. The slip-on type filter changes faster than a screw-in filter though.
Last edited by Bob 256; 07-25-2017 at 10:04 AM.