|
Most people don't realize how quickly things become sensitive once we up the ante in f/ratio domain. For a given camera (chip size and pixel size) sensitivity goes up with fourth order (*) when we drop the f/stop. (coma goes up with square of f-ratio, but Airy disc also gets smaller and angular field of view also gets larger for given chip)
In plain terms, f/4 scope will be nearly 2.5 times more sensitive to collimation misalignments than f/5 one. f/3 (geometric) as in Veloce astrographs becomes 2 to 3 times more sensitive again compared to f/4 (I'm sure Bert can confirm that!).
And I do agree, no matter who makes the scope, you will have to be confident you can collimate it yourself to perfection. Big primaries, big secondaries, big correctors and focusers simply make things if not impossible, at least extremely hard to keep collimation 100% in check when traveling around the world.
(*) this only applies if pixels are small enough to sample Airy disc; otherwise rule becomes more like cube of f/ratio difference
Last edited by bratislav; 20-07-2018 at 11:31 AM.
|