>I would like some comments/alternatives to calculating true f-ratios
>when using focal reducers. My approach is outlined below.
>
>I was playing around with some formulae I found to figure out the true
>f-ratio when using a focal reducer. The 80mm focal reducer in my
>Lumicon Giant Easy Guider (GEG) can be moved forward or back to vary the
>focal ratio. However, the real f-ratio of a 12" LX200 is something
>reater than f/10 to start, and the length of the GEG and resulting back
>focus means that the true f-ratios need to be calculated.
>
>With the GEG in place and the focal reducer in the rear position in the
>GEG, I placed the Meade 12.4mm Series 4000 eyepiece where the camera
>(35mm or CCD) would go using a 1-1/4" adapter. The *apparent* field of
>view for that eyepiece is 52 degrees. I timed how long a star (near the
>meridian and 0 degrees dec) took to drift across the field of view and>
>converted to degrees (time * 15) to get the *true* field of view>.
>
>Resulting magnification (M) = apparent field of view(AF)/true field of
>view(TF)

You are counting the number of angels on the head of a pin.

First of all, the specifications on ALL Maade scops are +-10%.
Second, the focal ratio of a typical Meade f/10 scope is f/8.9 to f/11.6.
Given these parameters, you simply cannot calculate accurate parameters.





Chris Frye
Silver Spring, MD.

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