[PRejto (Peter)]

I've recently set up a new system using my TEC140 + new TEC FRC (a .9 reducer/corrector) that has a back focus of 115 mm. Initial tests may be indicating a slight error in getting the 115 mm correct though it isn't at all obvious as to where or how the calculation might have been in error. (The camera is an SX-46 with a KAF 16200 CCD. Resolution is 1.43 arcsec, FOV 104 x 85 arcmin) TEC indicates that the corrected circle of the TEC140 is 52mm. The 16200 CCD is 22 x 27mm so I think my results should be better....

I seem to remember reading somewhere that it is possible to calculate how the distance might need to vary by comparing focus at the center and at the edges of the FOV. I presume knowing the distance between the two focus points could reveal the correction amount and direction of the correction. Can anyone point me to some written instructions?

Many thanks,

Peter