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Sensitivity and Overheads

The NIRI Integration Time Calculator (ITC) can be used to determine limiting magnitudes, exposure times, S/N ratios, background levels, etc. for a wide range of source properties, observing conditions, and NIRI configurations. 

The ITC has been used to generate tables of sensitivity estimates, which usually agree with values measured on the telescope to 20%. However, because some conditions vary unpredictably (e.g., the OH sky emission, which dominates the 1-2.4μm background, can vary by a factor of two from hour to hour), one should consider the ITC predictions to be accurate only to within a few tens of percent.

The ITC is not available for AO imaging at wavelengths beyond the K band. However, it is possible to use ALTAIR for imaging in the 3-4μm region. For AO imaging at L', throughput is reduced by a factor of about 7 because the Altair dichroic was not originally designed for thermal observations. Experiments show that with ALTAIR the background per pixel at L' increases by 10X (3.2X increase in noise). Thus, signal to noise (S/N) is reduced by a factor of ~20 with ALTAIR due to the dichroic and warm optics. ALTAIR does improve a 0.3" FWHM image of a point source at L' to ~0.1" FWHM, implying a S/N increase of 9. Thus, overall, there is about a factor 2 reduction in S/N with the use of ALTAIR in L'. However, there is an overall gain in angular resolution, which may be useful for some programs involving bright targets which require improved angular resolution.

  • Sensitivity Tables (no AO)
  • Sensitivity Tables (AO)
  • Overheads