7.4 Estimation of Exposure Times

7.4.1 Estimation of Exposure Times for Imaging Observations

The exposure times for FORCAST imaging observations should be estimated using the on-line exposure time calculator, SITE. SITE can be used to calculate the signal-to-noise ratio (S/N) for a given "total integration time", or to calculate the total integration time required to achieve a specified S/N. The total integration time used by SITE corresponds to the time actually spent integrating on-source without overheads. These integration times are used as input for SPT and for SSPOT, both of which will automatically calculate the necessary overheads. The format of the S/N values output by SITE depends on the source type. For Point Sources, the reported S/N is per resolution element, but for Extended Sources, it is the S/N per pixel.

For mosaic observations the total integration time required for a single field should be multiplied by the number of fields in the mosaic to obtain the total time, which is to be entered in SPT.

An important consideration in planning observations is whether FORCAST should be used in single channel mode, or in dual channel mode, since one gains the extra filter observation at the cost of lower system throughput in the individual bands. On the SITE form, the single channel mode is specified by selecting the filter of interest for one channel and "Ignore" on the other channel in the "Instrument properties" section.

7.4.2 Estimation of Exposure Times for Spectroscopic Observations

The exposure times for FORCAST grism mode spectroscopic observations should be estimated using the FORCAST on-line grism exposure time calculator. This calculator can be used to calculate the signal-to-noise ratio (S/N) for a given "total integration time", to calculate the total integration time required to achieve a specified S/N, or to estimate the limiting flux for a desired S/N.

In either case, overheads should not be included, as SPT calculates them independently.