This calculator computes the signal to noise of a MMIRS spectrum pixel-by-pixel. It requires that you download a tar file containing a Unix shell script, "snmmirs", and a set of data files for different filter and grism combinations. These data files contain observed sky background and spectra of an A0 star. The script then will compute signal to noise based on your requested parameters. The results are written as a text file to the standard output, which you can plot with your favorite plotting program. The output SNR is per spectral pixel spatially integrating over the profile using an aperture with size 1.4 * FWHM. It also assumes a S/N penalty factor of 1.3 for faint objects due to sky subtraction of adjacent frames assuming an A B A' B' dither pattern.
The script is invoked as follows:
snmmirs grismfilter mag slitwidth seeing exptime
grism -- the name of the grism + filter
mag -- Vega mag of the source -- the computed s/n is for an A0 type sepctrum
slitwidth -- width of slit in pixels -- must be 2 4 6 8 10 12 (note that MMIRS has a slightly different set of slits)
seeing -- fwhm in arcsec
exptime -- exposure time in seconds
Example:
snmmirs HK_HK_short 17 2 0.5 3600
The available choices of GRISM_FILTER for the calculator are
H_H_short HK_HK_short HK_H_short HK_K_short HK_zJ_short H_Y_short H_zJ_short J_J_short J_zJ_short H_H_long HK_HK_long HK_H_long HK_K_long HK_zJ_long H_Y_long H_zJ_long J_J_long J_zJ_long HK_HK_lowbackLong refers the 7 arcmin long slit. Short refers the the same slits, but masked off to only 10 arcsec long using the dekker wheel. The intent of having the short slit is to minimize scattered light in the instrument. HK_HK_lowback refers to the performance hoped for in 2011A after the baffle problem is corrected.
Some caveats:
A grid of precomputed conditions has been generated assuming a short slit with a width of 0.4 arcsec (2 pixels) and seeing of 0.5 arcsec. Postscript plots are available here.