Thanks for the info folks. I'm starting to understand. I have a 115mm aperture x 900mm newtonian. This means the mirror in that is f/7.8 (900 / 115 = 7.8). Right?
I can purchase a 340mm (13.4") f/5 mirror (for my first build of a dob I'm thinking of). This will mean it has a focal length of 1700mm so I'll need to also build a ladder!
The smaller (faster) the f/ratio, the more "curve" there is in the mirror and therefore the smaller the focal length. So why do all these people build dobs that are 2+ metres long? Why not just get a faster f/ration mirror to reduce the length of the tube?
Quote:
Originally Posted by mental4astro
Fast scopes are considered better for seeing 'faint fuzzies' like galaxies, and other 'extended objects', that is faint objects that have a large surface area.
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So let's test my understanding. A 1cm square bundle of light beams come through the scope and hits the mirror.
- On a perfectly flat mirror, this will take up 1cm square of mirror surface exactly.
- On a slower less curved mirror, this will take up a little more than 1cm square of mirror surface.
- On a fast highly curved mirror, this will take up more than 1cm square of mirror surface = more light reflected to the ocular = better seeing.
I'm asking these questions because I'm thinking of building my first dobsonian. I'm pretty handy (woodworker as a hobby) so feel pretty confident that I can pull it all together. Just got to get a handle on the science behind it.
Thanks,
Af.