Hi Shane
The term is f/
ratio, and is not a rating, as such, but the numeric raio of focal length divided by the diameter of the lens or iris opening. So, your example is correct in that a 200mm diameter, 800mm focal length scope is going to deliver an f/ratio of f/4.
Here's a paragraph off Wiki for reference:
http://en.wikipedia.org/wiki/F-numbe..._in_telescopes
Quote:
Focal ratio in telescopes
Diagram of the focal ratio of a simple optical system where f is the focal length and D is the diameter of the objective
In astronomy, the f-number is commonly referred to as the focal ratio (or f-ratio). It is still defined as the focal length f of an objective divided by its diameter D or by the diameter of an aperture stop in the system.
Even though the principles of focal ratio are always the same, the application to which the principle is put can differ. In photography the focal ratio varies the focal-plane illuminance (or optical power per unit area in the image) and is used to control variables such as depth of field. When using an optical telescope in astronomy, there is no depth of field issue, and the brightness of stellar point sources in terms of total optical power (not divided by area) is a function of absolute aperture area only, independent of focal length. The focal length controls the field of view of the instrument and the scale of the image that is presented at the focal plane to an eyepiece, film plate, or CCD.
For example, the SOAR 4m telescope has a small field of view (~f/16) which is useful for stellar studies, whereas the LSST 8.4m telescope, which will cover the entire sky every 3 days has a very large field of view (f/1.2), due to a special optical design.
|
Cheers
Chris