[Shiraz (Ray)]

At Rick's suggestion, have finally completed and validated the model for predicting the effects of flat noise when dithering. The attached graphs show how much difference dithering makes - to the extent that one would have to have a very good reason not to use it.

However, these results are for a perfect world where the only noise is shot noise and the dithering does not allow any noise correlation between frames. Some real world test data, for an image sequence that was imperfectly dithered in one axis only, are also graphed. These probably indicate that real world results are sensitive to dither efficiency. In addition, I found during the validation process that my master calibration frames have some excess (read and dark?) noise that has got into the flats, even though the camera is inherently quiet and 20 darks and 40 bias frames were used in the calibration. So the real world results were somewhere between the "model" dither and non-dither predictions.

The second image is a startling example of how effective dithering is. This shows one of the model validation images resulting from stacking 15 evenly illuminated subs after calibration with a fairly noisy master flat. The central zones in the test subs were clone stamped by hand to slightly offset the data and thereby simulate dithering in the central region. The outer region was not dithered. After stacking, the noise is vastly reduced in the central zone, even though the cloned regions cannot be distinguished at all in the individual sub images.

to sum up:
- Use as much flat data as you can - more is always better.
- Use dither if you possibly can.
- The rule of thumb that you should take one flat for every light will be fairly conservative if you dither, but it is probably still worthwhile in the real world where dithering may be imperfect and there can be residual read and dark noise embedded in the flats.
- if you have a CCD with low FPN and you do not need to correct for vignetting or dust, then consider not using flats at all - just dither.

Now to try to work out where that excess flat noise came from....