Can you image effectively without using flats? Mike S. does so very effectively and some camera makers suggest that it can be a useful strategy. While trying to sort out a calibration software problem, I did some modelling that seems to indicate that imaging without flats can be a good idea, but not for everyone - like many things in this hobby, “it depends”….
As I see it, flats have 2 purposes:
1. To remove large-scale illumination defects such as vignetting and dust bunnies
2. To remove noise generated by sky radiation interacting with pixel-to-pixel CCD sensitivity non-uniformity (fixed pattern noise or FPN).
If you have clean fast optics and a smallish CCD, you may not have major illumination defects and can get by without using flats for purpose 1.
Efficient imaging is also possible under some circumstances without using flats to remove FPN (purpose 2). To put some numbers on the effects of FPN, the attached graphs show how the SNR varies with total integration time for a variety of CCD Non-Uniformity (NU) values (using a 10inch high-QE system system with about 1 arcsec sampling as an example). The solid lines show how the SNR increases with integration time if you use flats to get rid of FPN. The dashed lines show the SNR performance without using flats and for three different CCDs, with NU values of 0.2%, 0.5% and 1%. Non-uniformity data is a little hard to come by, but I think (hope) that these values represent the types of CCDs that we use.
With dark sky (left hand panel) and a CCD with low NU (0.2%), flats do not make much difference to the SNR, which remains fairly close to the values that would be obtained using flats. However, flats definitely do help with the other two CCDs under the dark sky, particularly for longer integration times.
With a relatively bright sky (right hand panel), flats are worthwhile for all of the CCDs and SNR is significantly impacted if they are not used. The situation is particularly bad for the CCD with 1% NU, where the FPN dominates (even for short exposures) and no amount of additional integration can improve the SNR.
It should also be noted that an NU of 1% is not really an indication of a poor quality CCD – FPN can be removed with appropriate flat fielding and then all of the CCDs will achieve the identical shot-noise dominated performance shown as a solid line in the graphs.
In summary, it seems that, if you have a fast scope, a smallish CCD with low NU and dark skies, then you can image efficiently without using flats. Otherwise, you need to use flats or you will be losing out on SNR, particularly over long integrations and under bright skies. Indeed, without flats, some CCDs will be practically unusable under a bright sky. Thanks for looking - would be grateful for any feedback/experience. Regards ray
Ref:
http://www.narrowbandimaging.com/inc...3_expanded.pdf