Most catadioptric telescopes are between f/10 and f/15, so these lenses are average to fast for that application, and yes a tracker is a typical requirement. As to everybody already having one, Covid has inspired a large number of folks to take up back yard astro observation and photography, and that has contributed to the shortage of telescopes, so no, not everyone has one. Fast lenses are desirable for short exposure wide angle shots, but very few longer telescopes are fast.I know nothing about astro. Most of the posts I see here about astro want fast lenses. Am I correct to assume that these mirror lenses need to have a tracker for astro? I'm guessing the guys real serious about astro already have one?
Sadly, don't quite think it will get that far!
I reckon the new 400/4 will be popular if they can make it even more compact, and no doubt performance is going to improve significantly with the RF converters as well, so it may work with a higher pixel density APS-C body as well.I am a very happy RF 100-500mm user, as evidenced by the number of shots with it and the RF 1.4x and 2x I’ve posted. But that Nikon 500mm f/5.6 PF is the best lightweight lens I‘ve owned and I’d rather buy the Canon equivalent of it than a 400/4 even if they could reduce the weight right down. The range with TCs of 500-700-1000mm (like on the 100-500mm too) is more useful to me than 400-560-800mm. My 400mm DO II spent most of its time with a 1.4 or 2xTC on it.
Actually, you get get some surprisingly GOOD images out of a well made pinhole camera.Very interesting what Canon would make out of this old lens design. As others have stated here, the classic mirror lenses have very special characteristics, in fact you could use them for arts photography, like some people use pinhole or cheap Lomo cameras for their arts prints. Here is a good article, based on those classic mirror design lenses - btw astronomical optical telescops basically use the same principle:
This is a short primer on mirror, or as they are also known - catadioptric lenses, which are long telephoto lenses that are light, small, and do not cost a fortune.www.bhphotovideo.com