What would be the best choice, if money not in first perspective, for deep-sky use, not planets, moon or sun.
I’m into a 25”, and look over several choices, as fast, and slow focal whit a small diagonal in thicker ( or thinner plano ) in Zerodur or Quartz.
Idea of no coma corrector( or a PII ) and there are producers that can do this mirrors very nice.
But it gets a tall construction.
If going bigger or max pupil observation on low power ( and high powers to ) a fast focal is needed and that also put feet on ground.
-Example can be ;
( not pocket milled Zerodur MRF polished, as way to expensive )
Ex 1, Quartz plano at 1.5” at weight around 24 kg, tested via normal test as ex Focault, but no other tolerance more than ‘trust me’.
This will has some cool-off issues if temperature change.
Few will do this under f/3, but there are producers.
Ex 2, Cast Pyrex at 5 mm ribs/surface and ratio at 6;1, IF tested to diffraction ( 20 nm RMS on the surface or L/4 ) and 8-10 Å surface and weight around 16-17 kg ( very stiff, and fast cool-off )
Ex 3, Very thin ( around 1/2” ) meniscus in fused Quartz/Silica, Tosoh at N material star-tested thru system whit primary/diagonal/coma corrector/eyepiece, and weight under 10 kg.