10" dob focal length

[davidpretorius]

Hi all, what are those important facts like focal length and F/X for my 10"dob

[Striker]

Not sure what Dob you got but being a 10" it will be more likely a 1250mm focal length at F5....the F number is just the ratio of the aperture to the focal length.....meaning your FL of 1250mm divided by the aperture 250mm = F5

[davidpretorius]

thanks, that makes sense. where does the term slow or fast come from and how does it apply?

[rmcpb]

Fast and slow are old photography jargon. In cameras a lense with a smaller f ratio (f1.4) effectively "grabs" more light in a given time and allows for shorter exposure times but a shallower depth of field, hence you use a "faster" shutter speed. Conversely the larger f ratios (f15) do the reverse and are often used in macrophotography where depth of field is ultra-important.

In telescopes the shorter focal lengths, f5 and below, allow for wider fields of view BUT as they have to bend the light more to achieve focus you pay the price by needing better quality eyepieces, better collimation. Longer f ratios, f6 and above, are easier on eyepieces and the collimation is not as critical but, of course the better you set it up the better views you get.

Hope this helps.

[davidpretorius]

so the good old refractor will have a long focal length and the reflectors ie dobs etc will be shorter focal lengths because of the larger aperture per length of scope.

thanks again

[asimov]

There is short focal length refractors available these day's too, mainly APOs (apochromatic) rather than achromatic.