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Space Telescope Imaging Spectrograph
Improvements to the Flux Calibration of the Echelle Spectroscopic Modes

New throughput curves for all MAMA medium- and high-resolution echelle spectroscopic modes (E*M and E*H) were obtained based on calibration observations of the fundamental WD standard star G191B2B. For the prime modes, the revised on-orbit sensitivity curves were derived using data taken in the time period 1998-2001 (programs 7657, 8067, 8421, and 8915) which involved zero as well as non-zero monthly offsets of the Mode Select Mechanism (MSM; these offsets are recorded in science header keywords MOFFSET1 and MOFFSET2 for the two detector axes). For the secondary echelle modes, new on-orbit sensitivity curves were derived from data taken in 2001 at non-zero MSM offsets (program 8915). Throughputs have generally increased and the differences between the old and new values are of the order of 3% or less, apart from the E140H prime modes where such differences can be as high as 5-10%.

The blaze shift correction for the echelle prime modes was slightly revised. This correction is necessary in order to properly take into account how the echelle blaze function shifts as a function of time and position on the detector. For the spatial component of the blaze shift, the new correction still makes use of the same coefficients derived by Lindler in March 2002. However, new coefficients and zero points as functions of order and side of STIS operations (Side 1 or Side 2) were inferred for the temporal component of the blaze shift (for a description on how these parameters were derived, please refer to the forthcoming STIS ISR by Aloisi: "Blaze Shift Correction for STIS echelle Prime and Secondary Modes"). The new blaze shift correction reduces the flux offset between adjacent orders from about as much as 10-15% to less than 1-2%.

The blaze shift correction is now applied for the first time to the echelle secondary modes in the same way it is applied to the prime modes. This has been possible thanks to the availability of the new on-orbit sensitivities for such modes. The improvements in the flux calibration of the echelle secondary modes reduce the flux uncertainties between adjacent orders from as much as about 30% to less than 1-2%.

On 2006-Nov-03, a number of new photometric throughput (pht) reference files were delivered which include all the above mentioned improvements to the flux calibration of the STIS medium- and high-resolution echelle modes. Echelle observations using prime or secondary wavelength settings and the 0.2X0.2 aperture should now have typical flux accuracies of the order of ~5%, although isolated individual observations may occasionally be sufficiently miscentered in the 0.2X0.2 aperture to have a more noticeable flux loss.