Flatfielding

The sensitivity of a CCD to incident photon flux is not uniform across the whole of its surface and before data can be said to be properly relatively flux calibrated this needs to be corrected for. The variations in CCD response can be on the large scale (one end of the CCD to the other) and pixel-to-pixel. The relative flux levels on different parts of the CCD are also vignetted by of the optics of the instrument and telescope, this variation also needs correcting for and is performed together with the CCD sensitivity corrections ⁴.

Flatfield calibration frames are usually taken of a photometrically flat source using the same optical setup as that used to take the object frames. In the past images of the interior of the telescope dome have been used for this purpose, however, it now generally thought that images of the twilight/dawn sky are more representative of a true flatfield, having the same global illumination as the data and having a good signal level (remember that calibration frames will be applied to the object data at some stage and hence will introduce a noise contribution to the final data values, it is therefore essential to get a good set of calibration frames with lots of signal if this process is to introduce the absolute minimum of noise, CCDPACK provides calibration frame combination routines to produce `best bet' calibration frames with very low noise levels), but these frames have a colour response which may be not representative of the colour of the night time sky. If this factor is important then specially taken night sky flatfields must be produced. These can be taken of star free parts of the sky or produced from many object frames whose (contaminating) objects are removed, before median stacking to remove more spurious data values. Note in this final case that the noise levels required to correct for small scale variations are very time consuming to meet.