From the Workshop...#3 by Tim C

From the Workshop...#3 (from SBAU newsletter, March 2014)
Tim Crawford
     From the first two issues of “From the Workshop” we now know what it takes to begin grinding a mirror blank against its tool: the center of the mirror deepens and the edges of the tool wear as they become a perfect complement to the mirror. Now you may ask yourself "How deep do I make the center of my mirror?" In other words, “When should I stop grinding?” This, of course is up to you! What focal length do you want mirror to have? The focal length, F, is the distance from the mirror where parallel (distant) light comes into focus. Moreover, the depth of your mirror at its center determines F. It will also drive some of the design characteristics of your finished telescope, as we will see in future issues. In short, when you reach the desired depth at the mirror’s center, it is time to stop deepening your mirror! What follows are some straightforward “mirror mathematics.”
     (1) The Focal Ratio. This number is nothing more than the ratio your mirror’s focal length, F, to your mirror’s diameter. For example, if you are working on an 8” (200mm) diameter blank and you want a focal length of 1000mm, the focal ratio is 1000/200 = 5. You would say that the focal ratio of your desired mirror is 5. Or, equivalently, you are crafting an F/5 mirror.
     This ratio has a number of important consequences.  One is that a smaller-focal-ratio mirror will have a shorter focal length and, as a result, a deeper center. It may not be obvious at this point, but this type of mirror will have a wider field-of-view. When used with shorter focal length eyepieces, a short focal length mirror will capture larger areas of the night sky. And it will do so with low power. Often this kind of mirror is used in what’s called a “richest field” telescope. Open star clusters are wonderful targets for this kind of telescope!
     On the other hand, if you plan to do a lot of planetary viewing, a longer focal ratio mirror will suit you better. Here, with a comparable eyepiece, the longer focal length will supply the higher power needed in seeing details in, say, Jupiter or the Moon.
     Keep in mind that there is no focal ratio that “does it all.” A comfortable focal ratio of F/4 to F/6 with focal lengths between 750mm to 1500mm both (a) keep you off tall ladders and (b) allow you to enjoy both ends of the “Planet-to-Cluster” viewing spectrum. I crafted and use a 10" f/5.2 scope and find it easy to use and carry about. It yields fine views of planets and deep sky objects that are within its grasp. So...you pick a number! How about an 8" mirror with an f/6 focal ratio?
     In my next essay, I will discuss the mirror’s “radius of curvature" and some other terms that define how our finished mirror will behave optically. Till then, I wish you "clear skies," but only after we get some much needed rain!