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Optical throughput

Throughput / Photometric Calibration

Photometric standards have been used to measure the throughput of the atmosphere+telescope+instrument system based on comparison of GPI datacubes with IRTF Spex spectra. Observations of HD 51956 (=HIP 33657) from the first run in November 2013 were reduced (dark subtraction, bad pixel correction, destriping, basic datacube extraction). The total intensity in each spectral channel of the cube is calculated and compared to the corresponding spectral channel of the low resolution Spex spectrum, using same physical units. The gain of the detector is taken taken into account. The following figure presents throughputs obtained with and without the coronagraphic Lyot stop and apodizers for Y,J,H,K1 and K2-bands.

Throughput plot

The dashed lines show the throughput for the coronagraph modes (including apodizers and Lyots) while the solid lines above show the throughput for direct imaging with no apodizer or Lyot stops in the beam.

Independent calculations for verification has been performed on additional targets and the results are very similar.

Throughput plot

Line styles the same as the previous plot. The colours indicate the different targets, blue - HD 51956, red - HD 20619, green - HD 8049b. HD 51956 was observed in both unblocked and direct modes, whilst HD 20619 and HD 8049b were only observed in one of the two configurations each. The flux for HD 51956 and HD 20619 was calculated by summing the counts in each of the spectral channels (with the result not significantly changing when a large aperture was used instead). For HD 8049b, an aperture of 3x FWHM was used, with the FWHM measured in each slice. The measurements where corrected by dividing by a Cerro Pachon airmass=1.0, water vapour column=2.3mm atmosphere.