Focal plane on a C11

Hi,

I'm using a Nexstar 11 GPS and was wondering if anyone knows what Celestron means when they say F10/2800mm. I'm just starting in this astro-photography thing and it would help a lot. I know the F-ratio on SCTs is variable but I figure that the F10 ratio must have a defined focal plane somewhere, say maybe an Xmm eyepiece stuck into the diagonal holder on the visual back or something. Knowing where tha focal plane is would help me a lot in getting inside the ballpark for exposures. I can calculate the rest.

Take Care,

Jose Suro
Tierra Verde, Florida

Basically in an sct with its moving mirror design, the focal length is changed slightly as the primary mirror is moved closer or farther away from the secondary. Since the focal length changes, but the aperture does not, you get a slight change in f/ratio. basically, what it means is that when the focal length is 2800mm, the f/ratio will be f/10. The other thing that changes is the spherical aberation correction--the spherical correction is setup to be best at that sweet spot right at 2800mm. Bottom line, yes, things change at different focal plane distances, but unless you are near extremes of the range, you won't notice a huge difference. Another thing that comes into play is the use of focal reducers. a 'standard' f/6.3 sct focal reducer will only give f/6.3 at a certain 'sweet spot' distance from the reducer. closer or farther to/from the reducer will give different f/ratios. We could talk about the physics involved with this telescope design forever. Basically, knowing your exact f/ratio really isn't that important; as there are a number of things that will have a greater effect on your images than a slight difference in f/ratio. If you really want a precise measure of it, best bet is to take a photo of a star field, then compare it to a star chart to determine your focal length. Personally, I think it's all really not necessary; I'd suggest instead experimenting with different exposure times to see what is the longest exposure your film, skies, and equipment will allow.