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The Effect of Subframe Number on Flat Fields

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Background

 

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.

 

Method

 

  • A series of 40 flat field images were taken with sky flats at dusk to an ADU value of 25,000.
  • From the 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.
  • Then 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 frames.
  • The master 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 column.

 

Results

Number of
No. Frames Combined
Master Flat
Click on Image to Enlarge
Calibrated Image
Click on Image to Enlarge
5


10


15


20


25


30


35


40



Observations

 

  • It is apparent to me that there is marginal improvement, if any, beyond 25 frames.
  • Most of the improvement seems to be achieved by as few as 15-20 frames.
  • My 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.

Chris Abissi

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