Speed question.

[Mr Moff]

Sorry for yet another basic question.

When people refer to the speed of say a Dob being faster than something else,what are the referring to?

Danny.

[Si W]

Sound like you asking about focal length ?, the lower the F ratio the faster the speed (photographical), also the lower the focal ratio the wider the field of view of a give patch of sky, how it works, for instance my scope is an F4, it's worked out by the width of the primary mirror, in my case 254mm with a focal length of 1016mm, so 1016 mm divided by 254mm is 4, so the focal length is f4.

http://www.chuckhawks.com/telescope_focal_length.htm

[Mr Moff]

Thanks for the description and the link mate.

It's starting to make sense now.

Much appreciated.

Danny.

[rowan46]

f number refers to focal ratio which is the focal length divided by the apparture. For visual work it makes little difference in the view other than faster scopes (lower f numbers ) need better eyepieces, give less magnification on a given eyepiece and give a wider view. On imaging rigs however F number is vitally important as the lower the the f number the shorter the exposure. a short exposure is much more forgiving of a mount's inadequacies than a long one.

[MikeP]

My simple way of thinking about the difference between a "fast" scope and a "slow" scope is to visualise how quickly incoming parallel rays of light are brought to a focus.

A small f number means the light cone is "short and fat" whereas a bigger f number means it is "long and thin".

That means a fast scope can be physically shorter than a slow one.

I'm a simple chap.

Mike

[Si W]

I'm a simple chap.

Mike

I like simple, simple is good, no war and peace. that's what links are for.

[Mr Moff]

Great stuff.

All this new info you guys have been arming me with is making me think about what my next scope should be.My first one hasn't even arrived yet.

I can see where this is leading.

Danny.

[MikeP]

Yes, I read the linked page before posting.

Mike

[Si W]

Yes, I read the linked page before posting.

Mike

I use links cos I'm to bone idle to type