Any clues as to the reason why?....

[Peter]

 I have started to notice a strange instance to my bias frames. Namely the central section seems to be noticeably different from the outer section. The central section seems noisier and less smooth than the outer section. Does anyone know thew cause or at least point me in the right direction as to the cause?

[vicent_peris]

Hi,

Is it also visible at 1:1 zoom level?

V.

[Peter]

Hell Vincent, yes it is visible at 1:1 level, that is how I first noticed the problem. Will take more bias shots this evening to see if the problem recurs. Thanks for replying.

[Peter]

Sorry Vicent for calling you Vincent, very English of me. I believe the problem may have been due to light leakage from my telephoto lens giving me an enlarged central portion of my frames. I tried again without the telephoto lens attached and glad to report that everything is fine again. So note to oneself, take calibration frames in the dark and not in the daytime.