# Calculate MIRAC Sensitivity

The sensitivity calculation returns the signal-to-noise ratio (S/N) one can expect for a range of total observation times using the telescope, filter, source type and flux, and observing mode selected below. It is assumed that the data are taken in the standard chop-nod mode (chop mode for the 2-5 micron filters). If the source is compact (<10 arcsec) it can be put in 2 or 4 of the chop-nod beams, increasing the sensitivity. The noise observed is produced by shot noise from the telescope and sky background, detector read and system (A/D toggle) noise, detector low frequency excess noise, and sky noise. Typically the total is 1.04 to 1.1 times shot-only noise. All calculations assume the data are taken in chop-nod mode.

If Point Source is selected, the FWHM of the point-spread-function (PSF) is used to calculate the point source sensitivity. The point source sensitivity is calculated from the surface brightness noise in a diameter given by the root-square-sum of the Airy disk and a seeing disk, assuming half the source flux is within this diameter. The observed image FWHM is determined by diffraction plus the following three effects: telescope aberration, chopper distortion, and seeing (these latter three are combined in a single user-specified parameter below called 'seeing'). At the IRTF, the three combined effects are typically .5 arcsec. Alternatively, you canuse the 'User-entered PSF' option to directly specify the FWHM of the PSF. If Extended is selected instead of point source, the sensitivity for an extended source is calculated, in Jy/arcsec^2.

Note that the results present the total on-source time required; standard star and gain calibration observations must also be done.

Change the entries below as needed, then click on Submit.

Which telescope? Source in how many nod beams:

Flux: (Jansky for point source, Jy/arcsec^2 for extended)

Which filter? Extended or point source?

The following seeing and FWHM values are used only if Point source has been selected.

Seeing (arcsec):

If 'User-Entered' selected, enter the PSF FWHM: