Has anyone here had any experience, either good or bad, using telephoto mirror lens for photometry or amateur astronomy in general?
I have had fine experience with at telephoto lens 200 mm on a Canon 550D camera.
The mount is the new mount from Sky-Watcher Az-GTI.
I get fine results down to 10.magnitude 32x 30 sec.
Is this what you are asking about?
From a formal point of view, the lesser f-number, the better. If you mean under "mirror telephoto lens" catadioptric systems (something like this https://www.dpreview.com/forums/post/51568774), you can see that minimal f-number is rather big, so you would make long exposures to gather enough light.
I think for photometry of point-like sources (rather than extended sources like nebula) the f-number is less important in terms of exposure time, and the aperture area/$ ratio is rather ok for those lenses. I was about to buy a legacy lens like this for testing photometry but found out it just wouldn't fit physically on my DSLR because the lens was so huge at the base that it bumped against the flashlight "nose" of my DSLR :-(, so this is something to be aware of.
In real sky photometry, I doubt you would find any true "point sources" ! When you combine the effects of:
2. Optical distortions/imperfections
3. Tracking errors
Stars will become quite "extended sources" just like small nebulae...So, F/ratio will matter quite a bit! Faster is definitely better.
Ok, but we are talking here about lenses with a focal length of (say) 500mm (as in the example above) to perhaps 800mm max. I'm quite confident that with these optics you would even have to defocus a bit intentionally to get the best photometry results on stars, rather than beeing limited by seeing, optical imperfections and tracking errors, so you can get all the light from the aperture into the number of pixels that you want. The only drawback that I see compared to faster optics with the same effective apperture (the obstruction from the secondary mirror needs to be taken into account of course) is a smaller FOV which can make it difficult to get comp stars in the field for bright stars. But in terms of SNR, I'm quite sure you can get results on par with optics with the same aperture area.
And I guess there are quite a few people here who do photometry with SC optics (typically f/10) .