There is much prejudice against mirror lenses but although they have a bad reputation amongst some photographers for lack of sharpness and inferior resolution, astronomers seldom complain. Astro-photographers obtain wonderfully sharp and detailed astro-images when using their reflecting telescopes - but their technique is very different to that used by most terrestrial photographers.
Astronomers are very aware that for optimum imaging, their telescopes require time for the rear primary mirror objective to cool down (acclimate) to the ambient air temperature - thus astro imaging is not attempted immediately after setting-up.
An astronomical reflecting telescope’s rear primary mirror glass acts like a 'storage heater' when stored in a building and as soon as it's taken outside into the cooler atmosphere, the heat from the glass mirror radiates into the cooler air stream inside the 'scope - creating thermal air currents of unsteady/bouncing air with optical properties which can distort the image. Furthermore, the glass mirror objective lens continues to radiate heat for up to an hour during acclimation. If there is a temperature differential of just 3ºC between the glass and the air, imaging will be compromised. Unlike photographic mirror lenses, astronomical reflector 'scopes often include one or more cooling fans which assist cooling and considerably reduce the acclimation time.
Refracting camera lenses also need cool-down time after storage but because a refractor's prime objective is at the front of the lens, the temperature differential effect is minimal and they cool relatively quickly. During its longer acclimation period, the heat radiated from a reflecting camera lens' rear primary mirror can distort the image in the same way as that in a reflecting telescope.
Astronomers also use very rigid heavy duty tripods and mounts compared to the average camera tripod. Thus astronomers' 'apparatus shake' is minimal - compared to the camera/lens shake induced by a typical relatively lightweight photographic tripod used with a long focus lens.
So maybe camera/photographic mirror lenses (which are basically smaller versions of astronomical reflectors) are not the unsharp optics that many photographers purport them to be. With adequate pre-imaging cooling, and with the support of a very rigid tripod and mount, sharp and detailed images should be possible. But remember that a 500mm, 600mm or 800mm mirror lens will require a shutter speed of 1/500 sec to 1/800 sec to reduce the chance of camera shake and thus image blur. Furthermore, ‘in camera’ image stabilisation might not be of much help when the camera is on a tripod - and a steady tripod will be necessary for efficient and stable viewfinder composition.
Nearly all modern mirror lenses are manual focus designs - the exception is the Minolta / Sony AF mirror lens. Some photographers have never used a manual focus lens and not all modern digital interchangeable lens cameras are suited to manual focus lenses. However, many DSLRs and compact system cameras have a viewfinder magnification facility which can greatly assist manual focusing.
Unfortunately mirror lens images' 'doughnuts’ cannot be eliminated; photographers have to accept the fact that they will be present - but some consider they are a small price to pay for the convenience of using a relatively small and lightweight long focus lens. The severity of the ‘doughnuts’ in an image will depend on the subject, lighting and the mirror lens’ design - but all mirror lens’ images show the characteristic to a greater or lesser extent.
Judges are likely to comment regarding the ‘doughnuts’ phenomenon; this fact might affect your decision whether or not to enter a mirror lens image in an exhibition or competition.
The following fallow deer image taken with a Samyang 800mm f8 mirror lens shows the typical 'doughnut' phenomenon in the OOF background. Whether the doughnuts are too intrusive depends on personal preference but the relatively lightweight 900g lens fits into a small gadget bag - unlike an 800mm telephoto design which can weigh over 10 kg.
The accessory looks cumbersome but it works. Rubber bands provide sufficient pressure to attach and hold the rigid foam lever/wedge in place. The extra radius increases the focus travel and makes fine focus so much easier. I'm also hopeful that the lens might be usable with a 2x extender with the improved fine focus.
Perhaps the foregoing might encourage a few members to reconsider using a mirror lens - especially if you have a legacy mirror lens tucked away which can be easily adapted to a modern digital camera. Mirror lenses with focal lengths between 500mm and 1000mm are often available secondhand at bargain prices. The 900mm and 1000mm lenses can stick for months on dealers' shelves but they tend to be 'marque' brands rather than independent models - so are worth considering if you have the necessary tripod support.
Some photographers have 'given up' with their mirror lenses due to unsatifactory results. However, maybe their technique was lacking rather than the lens being a poor performer. Ebay can be an excellent source for discarded mirror lenses - and at bargain prices.