Note that at L’ and M’ the exposure times for the full array are close to the readout time and so efficiency is low; at L’ it is 60% and at M’ 50%. These overheads are not included in the figures below. The point source figures are based on actual measurements of faint point sources and not extrapolated from the surface brightness figures. The L’ and M sensitivities are from recent measurements on dry nights with CSO Tau 0.04-0.06, and seeing 0.3-0.7 arcseconds. Good seeing and dry conditions help significantly at M and observers may want to ask for their time to be flexed accordingly.
|Filter||per pixel||per sq.arcsec.|
|Filter||Exposure Time, seconds|
For point sources the sensitivities assume that the source is jittered on the array and separate blank sky frames are not necessary. If sky frames are necessary, for example for extended sources, then only half the time is spent on source and the sensitivities are lower by 0.4 mag than those given in all the Table. For point sources if image quality improves and smaller s/w apertures can be used the sensitivity goes up, e.g. if a 1″ s/w aperture can be used, the signal stays the same, the noise decreases by a factor of two, and the sensitivities improve by 0.7 mag. For surface brightness with other apertures increase the signal by the increase in area, increase the noise by the square root of the increase in area, and hence increase the S/N by the square root of the increase in area. To calculate S/N for a different magnitude after the same length of time, multiply the S/N by the ratio of the fluxes. To calculate S/N for the same magnitude but different exposure, multiply the S/N by the square root of the ratio of the exposure times. For narrowband filters the signal is reduced by the reduced passband of the filter and the noise is reduced by the square root of this, hence the S/N is reduced by the square root of the flux reduction factor. The narrowband K and L filters are typically 1.5% wide compared to the broadband width of 15%, hence the S/N for these filters should be reduced by a factor of 3, or the magnitudes made brighter by 1.3mag.