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Hubble Space Telescope
FOC Instrument Science Report #097

FOC Instrument Science Report #097       June 9, 1997

"FOC f/48 Spectrograph Rectification"
by M. Voit

Abstract excerpted from the report:

The f/48 wavelength calibration procedure (ISR OSG-FOC-096) accurately
corrects geometric distortion in the dispersion direction of an FOC
long-slit spectrographic image. This report describes how we correct the
distortion of spectrographic images in the spatial direction, using data
from the f/48 spectrophotometric calibration program. We find that applying
a shear transformation to the wavelength-calibrated data shifts the spectra
so that they run parallel to the dispersion direction to within ~0.1 degrees.
The shear distortion turns out to be time-dependent on sub-orbital timescales,
making it difficult to align the spectra more closely. Residual displacements
of the spectrum centroids from a linear fit are <0.3 pixels and are probably
an artifact of the geometric correction procedure. Because of the distortion's
time dependence, observers are advised to calibrate and rectify spectrographic
images using an INTFLAT taken within the same orbit. We describe a new
procedure that rectifies and calibrates the wavelength scale of FOC f/48
spectrographic images in a single step. We also call attention to a revision
in the standard wavelength scale.