Influence of different flat images on calibration

Here are two methods to create a flat field.

• An evenly illuminated cloudy sky

See here for an analysis on repeatability of this method.
The final flat is  the result of averaging the 25 exposures at the end of the sequence. The expectation is that the system has settled at that time. Exposure was 30 seconds. A 30 second dark field was subtracted from each image. Exposures were taken at 16C
• An evenly illuminated opal acrylic glass

See here for an analysis of the function and reliability of this method.
Exposures were 5 seconds and the glass was rotated 90deg every 5 exposures. Exposures were taken at 18C

Flat from clouds (16C)                                                    Flat from milky glass (18C)

The following images were calibrated with the two above generated flats.

PK036-0.01 (16C), calibrated with clouds flat            calibrated with milky glass flat

NGC 7479 (15C), calibrated with clouds flat                    calibrated with milky glass flat

Again it seems that there is a temperature dependence between flat field and raw images. Raw images that were taken at a temperature close to the temperature of the flat seem to calibrate better. It seems to be necessary that the CCD has very accurate temperature control.

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