FR/FF = focal reducer/flat fielder.
Knowing the effective f/ratio of the telescope with the FR/FF, allows calculation of the effective focal length
(E.F.L.) of the optical system by multiplying the f/ratio by the aperture.
Example:
Lens Aperture X Effective f/ratio = E.F.L.
Using my 10" (254mm) lens aperture and an f/5 focal ratio, produces the following.
254 X f/5 = 1270mm E.F.L..
Knowing the E.F.L. allows calculation of the true field-of-view in arc minutes at the image plane of the chip
by 3438 (the radian constant in terms of arc minutes), and then dividing that number by the focal length in millimeters.
Example using a SX MX916 CCD chip (dimensions 6.5 X 8.7mm) and a telescope with an E.F.L. of 1270mm:
3438 x 6.5mm = 17.59 arc minutes.
1270mm
3438 x 8.7mm = 23.55 arc minutes.
1270mm
Then the true field-of-view for the MX916 CCD chip and a telescope of this focal length is calculated to be:
17.59 arc-mins x 23.55 arc minutes for this CCD camera and telescopic configuration.
John R Fletcher. M.T.O. UK.