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Exposure Time Calculator

The exposure time calculator presented below is a Java 1.6 applet. If you have any problems, please contact the LT Support Astronomer stating your browser, operating system and any error messages you encounter.


To use the calculator, simply enter the apparent magnitude of your target, along with the desired signal-to-noise ratio. Then select the instrument, filter and binning you require and press the "Do Calc" button. In general you should use 2x2 binning with IO:O.

The resulting table shows exposure times for different sky brightnesses above the darkest sky and for different seeing (FWHM) for point sources. It is assumed that the object and sky apertures used are each a factor of 1.5 times the FWHM. For extended sources a per square arcsecond value is also provided.

Exposure Calculator java applet disabled for print version

IMPORTANT: Exposure times as low as 1 second may be quoted in the above table. However due to shutter effects a minimum exposure time of at least 10 seconds is strongly recommended. Shorter exposures are unlikely to produce photometric accuracies of 1% or less.

Please note: Currently only RATCam and IO:O have been implemented in the above calculator. The RISE and IO:THOR entries are not enabled. RATCam was decommissioned in February 2014, but as its exposure characteristics under 2x2 binning in SDSS-R band are very similar to those of RISE, we recommend selecting RATCam with this configuration for exposure calculations for RISE for the time being. Note that although IO:THOR was also decommissioned in February 2014 it still appears in the calculator's list of instruments. This will be corrected in a future instalment of the calculator. For details of other instruments' sensitivity, see their respective instrument pages.


An exposure time calculator for spectroscopy with FRODOspec is under development. For now please use the zero-point estimates presented on the FRODOspec web pages as a guide. Please feel free to contact us if you require any assistance or guidance.