Fast & slow

[mattyb217]

I was just wondering what the term fast or slow telescope was referring too. I read something the other day, it said a fast telescope has a focal ratio of f/5 or lower & a slow telescope has a focal ratio of f/6 or higher.

[cometcatcher]

It's a photographic term used to describe how fast a telescope or lens can get a photo. All other things being equal, a 6 inch scope at F5 can get the shot in 1/4 of the time as a 6 inch scope of F10 and so forth so the F5 scope is considered "fast".

[raymo]

In case you weren't aware of it, the focal ratio of a scope is the distance
from the primary mirror or objective lens that an object comes to focus,
divided by the diameter of said mirror or lens. For example a scope with
a 200mm diam lens or mirror, and the object comes to focus at a
distance of 1000mm, is said to have a focal ratio of f/5.
The longer the focal length of a scope, the longer it takes to pass a given
amount of light to the camera's sensor, hence long focal length scopes
are called slow, and short focal length ones are called fast. That's it put simply.The main advantage of long focal length scopes is that they
display fewer optical aberrations such as coma and chromatic
aberration.
raymo

[Terry B]

Just note that the terms "fast and slow" really only apply to a scope from a photographic point of view. Visually it doesn't really apply as you can change the field of view by changing eyepieces.

[glend]

Well visually Fast and Slow do matter. For example, a fast dob or newtonian will be shorter than a slow dob and you may not need a ladder or stool. There are f3.7 fast dobs with large apertures which can be viewed with your feet on the ground. However, fast newts are more likely to need correction.

[barx1963]

I think the point Terry was trying to make is that as far as the brightness of the image in the eyepiece is concerned, fast and slow are not important. Yes it can affect the degree of coma and the length of the optical assembly and hence the ease of use, but that is a function of its focal length rather than it's speed. We still often refer to a scope as being fast or slow even when used visually as it is a simple shorthand way of describing it.

BTW Raymo in his post mentioned that longer focal length scopes will suffer less coma or chromatic aberration. That depends on the type of scope. For a newtonian (and dobs are a form of newtonian) only coma is affected as they do not suffer from chromatic aberrations.

Malcolm

[Pinwheel]

From my understanding Newts do suffer from-

1/ Spherical Aberration which causes a halo effect around bright objects
Eg Jupiter.

2/ Astigmatism which causes points of light to look like triangles or crosses.

3/ Coma which causes comet tails.

[barx1963]

Number 3 is correct, but the extent to which it is apparent is dependant upon the f ratio (speed). In most cases f6 or faster is is very hard to notice, at f5 really only noticeable with widefield eps, faster than f5 it starts to become significant. That said I have an 8" f4 and really doesn't bother me! It does arise from the nature of a parabolic mirror.

With the other 2, spherical aberration occurs if the mirror is spherical rather than parabolic. Some cheaper slower scope use spherical mirrors as they are cheaper and quicker to make and the slower the scope the less noticeable it is. With #2, astigmatism is as I understand, not inherent to newts but is a consequence of faults within the optical system. A good set of mirrors should exhibit no astigmatism. Any optical system can potentially be astigmatic, not just newts!

Of course I am no optician, so if I have this wrong feel free to correct me!

Malcolm