Quote:
Originally Posted by rally
In the real world it is often the case that the CFZ is up to a third larger than the theoretical figure. - Does anyone know why ?
Stating the obvious - the focal ratio of the system must include all optics in the imaging train.
Cheers Rally
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I'm not sure what you mean Rally. The CFZ is directly related to the f-ratio. The faster the f-ratio, the shorter/smaller the tolerances the CFZ will be.
As an example, the native FSQ-ED is 530mm @ F/5 which equates to a CFZ of 55 microns. With the F/3.6 reducer installed the CFZ drops down relative to the f-ratio, so 382mm @ F/3.6 becomes a 28 micron CFZ.
You are correct in that the imaging train will impact the f-ratio. Folks automatically believe that because they've got the Celestron/Meade F/6.3 reducer installed, that they will be imaging at F/6.3 on a F/10 SCT, but you need to take into consideration spacers and where the reducer is placed in the imaging train relative to the focal plane.
The Lumicon Giant Easy Guider (GEG) has a focal reducing lens that can be moved to provide F/10,F/6.5,F/5.5 and F/4. This is purely what the GEG is capable of, it is not an indication of what you'll be imaging at.
"F Ratio calculations: In the past I have unfairly critiscised Lumicon for quoting optimistic f/ratios for the GEG. In fact this is not true - I was neglecting two facts: the intrinsic f/ratio of my LX200 is actually f/10.7 (rather then the specified f/10), and the GEG itself adds a further 115mm to the focal length, bringing the system f/ratio to f/11.2. If the system f/ratio had been f/10 to start with, then the quoted Lumicon figures would be pretty much correct. F/ratios in all modes were calculated as accurately as possible by measuring the distance between star points at the extreme edges of the 35mm frame." Quoted from
http://www.astrocruise.com/geg.htm
So once you know your precise f-ratio, you can successfully calculate the CFZ. Plate solving (used for astrometric calculations/pointing) is an accurate way of determining what your true focal length is (and in turn your f-ratio).