extinction

Applies an extinction correction to the flatfielded data. If more than one sub-instrument (a sub-instrument is defined as one of the arrays or one of the photometric pixels) was used for the observation then extinction will prompt for one of them. For the arrays, the choice is LONG or SHORT and for the photometric pixels (which will be looking at different parts of the sky) the choice is P2000, P1350 and P1100. Each observation will have to be reduced separately from this stage on. For long, coadded integrations it is likely that the transparency of the sky will change during the observation. The actual values of the extinction coefficients will usually be determined by skydipping before, after, and depending on the sky conditions, possibly in between the group of integrations that are to be coadded (note that it is standard practice to split a long integration into smaller chunks). If the first opacity differs from the second then the extinction is linearly interpolated between the relevant times. Note that extinction requires the sidereal time at which each extinction coefficient was determined but 0 can be given in each case if the extinction remained constant over the integration.