Spectroscopic Mode Image Quality of STIS I. First Order Modes

ABSTRACT This ISR reports the spectroscopic mode image quality of STIS first-order modes as determined from the on-orbit observations as part of the cycle-7 calibration program. Calibration spectra with an internal lamp were obtained for 7 first-order gratings using a narrow (2-pixel wide) slit, and were used to extract instrumental line profiles at different positions of the CCD and the MAMA detectors. The line-widths, measured at different positions of the detectors for different gratings, are presented graphically, and are found to be close to 2 pixels for all modes. The line profiles (shown in normal and log scales) are fairly well represented by a single Gaussian. The encircled energies are plotted as a function of the number of pixels, which show that most of the power is contained within +/2 pixels of the line center.