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Comparison of Digital Flat Fielder vs. Sky Flats

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Introduction

This was not so much a study, but rather a comparison of the difference between light box flats and sky flats when the exposures were equal and the number of flat fields used to create the master flat was equal. For the two images below 40 flat field images all taken to 25,000 ADU were used.

Methods

I chose to compare the flat field through the red and green filters taken with the Adirondack Video Digital Flat Fielder to those made by imaging the twilight sky. The red was selected because the digital flat fielder is relatively deficient in the red frequencies. Therefore it took about 3 times as long to create these flats at 25,000 ADU as it did to create the Green flat. Additionally, the flat fielder is placed immediately in front of the OTA. This might possibly have some effect on the dispersion of light entering the telescope and thus how it shadows the various dust motes.

Results

The center column is the Adirondack Video Digital Flat Fielder and the right column is the sky flat. The first row of each pair is the master flats taken for that filter and the second row of each pair is the resulting calibrated image using the same light frames.



Digital Flat Fielder images
Sky Flat Images
Master Luminance
Flat


Calibrated
Luminance Image


Master Red
Flat


Calibrated
Red Image


Master Green
Flat


Calibrated
Green Image



Summary

 

After close inspection of the two calibrated images I believe that there is more complete elimination of artifacts in the images calibrated with the Twilight SkyFlats. This is most noticeably in the red exposure, but was also apparent in the lower corners of the green where the vignetting is better corrected in the sky flat version.

 

Conclusion

 

  • My conclusion is that while the flat fielder flats have great convenience, whenever possible I am going to use sky flats.
  • I have also concluded that I really need to clean the dust off these filters and CCD cover slip!

 

Chris Abissi

23 January 2007


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