In calculating observing times with IRIS some overheads must be taken into account, in particular a dead time of about 6-15 seconds between exposures. Since many exposures are made up of numerous short ones, with shifts between, this can be costly. Observers should allow for a duty cycle of around 60% for most observations, though for long spectroscopic observations of faint objects it may be possible to spend 90% or more of the time taking data.
Only approximate sensitivities are available for IRIS. The actual performance that will be achieved depends on time of year and telescope and outside air temperature. The table below gives the values for direct imaging and spectroscopy. For spectroscopy, relatively long exposure times are required at J with the IJ echelle to become background limited (it is not possible to be background limited below 1.2um because of the rapidly decreasing QE of the array). It is not safe therefore to extrapolate the quoted J sensitivity to shorter wavelengths. The current performance of the array in general shortwards of 1.5um is deteriorating because of an effective increase in the read noise. We do not recommend IJ spectroscopy of faint objects.
|Imaging||18.5||18.3||16.8||17.4||5 sigma 60 sec, in a 2" x 2" aperture|
|Echelle spectroscopy||17||16||15||3 sigma 1000 sec, per pixel|
|Grism spectroscopy||16.5||15.5||3 sigma 1000 sec, per pixel|
The imaging sensitivity figures assume use of f/15 intermediate or f/36 wide modes. f/36 intermediate may be more sensitive in good seeing for a point source (ie smaller measurement aperture). f/15 wide is approximately 0.7 mags worse than these values.
For polarimetry, the required exposure times can be calculated assuming the need to achieve a certain signal to noise in each of the Q and U Stokes parameters (linear polarimetry), or in V (circular polarimetry). Each Stokes parameter is calculated from two separate exposures with the waveplate shifted between them, so the error in Q, U or V is simply the error in a single exposure divided by 1.414.
This Page Last updated: 26 March 1996 by Stuart Lumsden