Quote:
Originally Posted by PSALM19.1
Hi malcolm, so my scope being an F6 (8" dob) would produce a comparable image to a slower scope of equal aperture? (Say Nexstar 8SE?)  |
The point about brightness was that the assertion was made that " a scope at f5 gathers light faster so it produces a brighter image". I suggest that is incorrect for visual use for the simple reason that your retina doesn't act like a sensor or a film in that the time you look at something doesn't make it brighter so the "speed" of a scope is irrelevant.
That is not to say that the focal ratio is not an important consideration in selecting a scope and there are factors that need to be considered and John (ausastronomer) has done quite a succinct summary in his post.
But what you are saying is essentially correct. At the same magnification, the images should be comparable. Of course a dob and an 8SE are different in a couple of ways. The 8SE is an SCT so it has a corrector plate at the front which will slightly reduce the light throughput and also has a larger secondary obstruction so that will also slightly reduce the light throughput and contrast.
Malcolm
