Read-out signal; bias

Usually the amplifier which boosts the signal prior to its digitisation by the ADC will also generate an offset, false signal or bias, which is imposed in addition to the real signal generated by the illuminating light (there are sound reasons for doing this). This bias varies slightly with position on the chip, can vary slowly with time (though this is minimised if the chip is kept at a constant temperature) and inevitably has noise associated with it. There are two techniques for estimating and correcting the bias.

Bias strips

Here the CCD controller software is written in such a way that the images generated contain regions (usually two narrow strips on either side of the chip) that are created by reading out the CCD without sampling any of its stored charge (see Figure ). These regions are called bias strips or overscan pixels. The values of pixels within these strips consist only of the bias and its noise. Usually for each row in the image the pixels in the corresponding row of the bias strips are averaged and the resulting value is subtracted from all the pixels in the row. The bias strips serve no further purpose and can then be discarded, thus reducing the size of the images.

Bias frames

Here the entire CCD array is read-out without sampling any stored charge (that is, no light is incident on the detector) so that any small scale structure in the noise is detected and can subsequently be corrected for. Such frames are called bias frames. In practice bias frames are acquired by taking short exposures with the shutter closed before or after each night of observing. Typically in order to reduce read-out noise several frames are taken and averaged. The resulting `master' bias frame is then simply subtracted from the genuine image frames.

Figure: Typical CCD geometries. In the figure on the left the readout direction is `Y', the bias strips are located with bounds I,J,K,L and the useful CCD area is M,J$+1$,N,K$-1$ (approximately; you should probably allow a gap of more than $\pm 1$ pixel between the bias and light-sensitive regions). In the figure on the right the readout direction is `X', the bias strips are located with bounds I,J,K,L and the useful CCD area is N,J$+1$,K$-1$,M$-1$. (Note that some observatories recommend that you only use the left-hand strip; if you use the right-hand one too, check that it is not contaminated by residual charge)

Which method is preferable depends on the quality and stability of the chip. If the chip and amplifier are stable during the observing session them observing separate bias frames is straightforward and gives satisfactory results. Conversely, using bias strips can be more convenient because you do not have to acquire, store and process separate bias frames. Of course, if the CCD controller software does not generate bias strips then you must use separate bias frames.

However you make the bias correction, you need to apply it to all the other frames acquired: target objects, flat fields (see below) etc. Often making the bias correction is the first stage of CCD data reduction.