After determining the optimal exposure for my flat field the
next question that I had to answer was how many frames were necessary to get a
good result. I had read according to Berry and Burnell on page 182 of the
Handbook of Astronomical Image Processing that “at least 16” flat frames should
be combined. But what if you weren’t looking for the minimum number but rather
the optimum number? I undertook an empiric determination of the effect of
adding different numbers of frames together to make the master Flat and then
using them to calibrate the same light frames.
series of 40 flat field images were taken with sky flats at dusk to an ADU
value of 25,000.
original 40 Flat Field frames, a Master Flat Field was created using 1-5,
1-10, 1-15, 1-20, 1-25, 1-30, 1-35, and 1-40 of the images for combines of
5, 10, 15, 20, 25, 30, 35, and 40 individual frames respectively.
each of these master flats was used to calibrate the same light frames. I
thought that it would be helpful, for comparison, to use the same light
flat field image that was created is displayed in the 2nd column
and the resulting image using that master flat is displayed in the right
Number of No. Frames Combined
Master Flat Click on Image to Enlarge
Calibrated Image Click on Image to Enlarge
It is apparent
to me that there is marginal improvement, if any, beyond 25 frames.
of the improvement seems to be achieved by as few as 15-20 frames.
original practice of combine 40 frames seemed to be unnecessary so I
currently take 25 to a maximum of 30 flat field images in most cases. If I
am doing sky flats I may cut that to 20 frames to allow more flat exposures
for different filters to be completed.