Fast Scope, Slow Scope?

[johncbradley1]

What does this mean?

[Andymac79]

Dont get bogged down in the detail unless you are into photography but someone will offer a decent exp

Andy

[Moonshane]

hi John

fast and slow relates to the relationship between aperture (130mm for your scope) and focal length(again 650mm in your scope); this is called focal ratio and it is this that is fast or slow. your scope has a focal ratio of 650 / 130 = 5 (f5) and this and anything lower than f6 is considered fast. anything with f6 and higher is considered slow.

the faster the optical system, the more demanding it will be on eyepieces and therefore sometimes (but not essential) you need to spend more to get the same quality of view. as said though, don't worry about this.

slower scopes are often stated as having more contrast and detail on planets etc and this is somewhat true but more aperture will actually give you more detail on the best nights.

[Earl]

It refers to the focal ratio of the scope.

This is the focal length of the scope divided by the diameter of the scope.

For most imaging Faster is better so the lower the F ratio the better.

For visual it just depends on the Field of view you want, the Higher the F Ratio the smaller the field of view.

[jimmyjamjoejoe]

This pic explains it pretty well, it's basically how quickly the optics focus the light (in terms of how much distance it covers before doing so) -

[Moonshane]

"the Higher the F Ratio the smaller the field of view"

for John's benefit I know you mean for any given aperture. FOV is broadly the actual field of view of the eyepiece (e.g. 52 degrees for a plossl) divided by magnification (focal length of scope / focal length of eyepiece). so your scope with a 25mm plossl would give :

52 (degrees) divided by 26 (650/25) = 2 degrees field of view (or four times the width of the moon).

[johncbradley1]

Ahh, I see now. Thank's guys

[umadog]

There are various practical differences between fast and slow scopes. Slow scopes produce a light cone which is not steep (no large angles) and this makes it easy to design eyepieces which work well. Eyepieces will show little or no astigmatism, little or no chromatic aberration, etc. The drawback, however, is that focal length can get very long indeed with larger mirror sizes. As a consequence, we're now seeing a trend towards faster scopes for apertures above 10". The trend is nicely covered by this interesting article: ZipDob - the Zip Dobsonian - Mel Bartels

The main practical problems with scopes faster than about f/4.5 to f/5.0 is that you need expensive eyepieces to avoid astigmatic views caused by the steep light cone. You also need a device known as a coma corrector (Google "coma in newtonian telescopes"). Without those things the views will be nasty. Also, collimation becomes more stringent at faster focal ratios. This is somewhat balanced out by the fact that faster scopes are shorter and so have less flex, which influences the structure's ability to maintain collimation. To cap it all, fast primary mirrors are very much harder to make well compared to slow ones. That may all sound intimidating, but when it all comes together a fast telescope can perform spectacularly well. If it's well built, it will do so on a reliable basis.

[Merlin66]

Another point to note - the focus tolerance gets much much tighter with faster optics.

For an f10 system the "depth of focus" is about 0.22mm, whereas on an f4 system it is only 0.035mm - more accurate and more controllable focusers are required on faster systems.

HTH

[sublime_mick]

lovely thread - very straight forward. learnt alot here today

thanks all