Telescope Basics: "Fast" and "Slow" Focal Ratios – Pros and Cons

I wanted to go through some "telescope basics" and thought that gathering "pros and cons" regarding focal ratio would be a good start. This table applies if comparing scopes that differ only in focal ratio, but are of the same telescope type (Newton, Fraunhofer, and so on) and have the same aperture – e.g. comparing a 60/350 refractor with a 60/900 refractor, or comparing an 114/500 Newton with an 114/900 Newton.

Judging by the number of points, the result look like they favour "slow" telescopes – and sure enough if you want to cheaply get a good scope, it is easier to get a "slow" scope (e.g. a 60/900 refractor or a 114/900 Newton) which is cheap and good. Getting a "fast" scope (e.g. an 60/350 refractor or an 114/500 Newton) which is cheap and good is a bit harder. Either you pay more for a faster scope, or you (usually) have to accept less quality.

The main point against slow telescope is length of the OTA, with the problems following from this (needs better mount, more difficult to transport, takes up more space when stored).

	Advantage	Disadvantage
Faster	- Shorter tube (less demanding on the mount, easier to transport and store) - Easier to achieve wide-field and rich-field views	- More aberration (chromatic, spherical, coma, and so on) - More expensive solutions needed to combat aberrations (e.g. parabolic primary mirrors for Newtons, or APO lenses for refractors) - More demanding to manufacture - More suspectable to collimation errors - Selection of "proper" size of focuser (for refractors) or secondary mirror (for Newton scopes) is more challenging (vignetting and/or reduced aperture can easily become a problem) - More stray-light problems - High magnifications are more difficult - More demanding on the eyepiece  - Usually more expensive
Slower	- Less aberration (chromatic, spherical, coma, and so on) - Less demanding to manufacture - Less suspectable to collimation errors - "Correct" size of focuser or secondary less critical - Less stray-light problems - Easier to achieve high magnifications - Less demanding on the eyepiece - Usually cheaper	- Longer tube (more demanding on the mount, more difficult to transport and store) - Wide-field views are more difficult to achieve

One more point arises when one wants to do prime focus astrophotography, which is that the faster scope allows shorter exposure times (being "faster") – with an increased FOV, but decreased resolution (less magnification). When using eyepiece projection or afocal astrophotography, this advantage is no longer of relevance and then the slower scope usually is better suited (with less aberrations).

(If you compare a catadioptric Newton with an non-catadioptric Newton – e.g. an 114/1000 with an 114/900 – then usually the catadioptric version has most of the advantages and disadvantages of a fast Newton. In this example the catadioptric 114/1000 is in most regards more on par with an Newton 114/500.)