FLAMINGOS I Spectroscopic Sensitivity Estimates

Brightnesses of point and extended sources giving signal-to-noise ratios (S/N) of 5 in a one-hour on-source integration are provided below for FLAMINGOS I and H+K grism. While the J+H grism is not currently available, it is possible to use the H+K grism along with the J+H filter to cover the J+H window with J in 2nd order. Sensitivies for J given in the table assume this configuration. Like other visiting instruments, FLAMINGOS is not integrated within the Gemini Integration Time Calculator (ITC), however similar methods were used to make these calculations.

Important caveats: These sensitivities have not yet been confirmed experimentally. These numbers are estimated using average sensitivities over the specified wavelength intervals. Within each IR band (and indeed within each of these narrow intervals) the sensitivity can be a very strong and rapid function of wavelength. On-sky commissioning tests in 2001B will be needed to verify sensitivities. Additionally, with the current H+K bandpass filter used for grism spectroscopy there is a high instrumental background believed to originate within the MOS dewar. With this current filter, the sensitivities given in the table would be reduced by approximately 0.75 magnitudes. A replacement filter (which cuts off the thermal component of the K-band) is on order and expected to be available for use in 2002A.

These sensitivities are for a median image quality of 0.5 arcsec seeing. Clear conditions, average background, and average airmasses less than 1.2 are assumed. In these conditions use of the wider, 6-pixel (0.47 arcsec) slit is assumed. Sensitivities can be scaled to narrower slits using the NIRI slit throughput as a guide. The 0.47 arcsec slit results in an effective resolution of R~350 over the H+K bandpass. J in 2nd order gives a resolution of R~470 with this slit. The meaning of the observing conditions criteria are explained in detail in the observing condition constraints.

For point sources, it is assumed that the one hour integration is made up of two 30-minute exposures nodded along the slit. Added noise due to the background subtraction is included. The S/N is calculated from an optimized length along the slit (which gives the highest S/N (i.e. ~1.4 times the EED).

For extended sources, sensitivities are for a 1 square arcsec aperture. The exposure is broken up into two 30 minute exposures, one nodded completely off to sky. If the source is sufficiently compact to allow nodding along the slit, the sensitivity will improve by sqrt(2). If S/N=5 is desired for a smaller aperture than 1 square arcsec, the exposure time would need to be increased by the sqrt(ratio of the areas).

Last update Aug 30, 2001; Michael Ledlow