From kaf@cygnus Sun Jan 12 12:27:36 1997 Received: from cygnus.mit.edu by wiwaxia AA21116; Sun, 12 Jan 97 12:27:35 EST Received: by cygnus AA22537; Sun, 12 Jan 97 12:27:34 EST Date: Sun, 12 Jan 97 12:27:34 EST From: Kathryn A. Flanagan To: dd@cygnus Status: R We have analysed the Al Ka HSI images of all three gratings with respect to spectral resolution, image-contrast, and other things which might limit the instrument's ability to detect spectral features. We have NOT yet determined efficiencies; the HSI alone is not the appropriate instrument for this. It turns out that the spectral resolving power is dominated by the angular resolution of the mirror system: taking only images with top or bottom shutter open reduces the HRMA focus size in the dispersion direction. Even in these cases, the width of the diffraction orders remains approximately the same for all observed orders (0, up to +/- 3). Only a very slight broadening of the higher orders could be observed in higher orders. If this is due to grating period variations, an upper limit for these would be approximately 3e-4 (HEG) and 5e-4 (MEG). These numbers still contain a broadening contribution due to the tilt of the dispersion direction with respect to the nominal y-axis. The LETG, whose dispersion axis is along y, does not show any broadening in higher orders. The measured resolving powers l/dl (top/bottom shutters only) at 8.3 Angstroem (Al Ka) are 830 and 1100 for HETG 1st and 2nd order, respectively. For the MEG we derived 520 and 1130 (1st, 3rd), and for the LETG 180 and 550 (also 1st and 3rd). The conclusion of this exercise is that the spectral resolving power of all three instruments will be given by HRMA alone (as expected or, at least, hoped)! In addition we looked for 'grating scattering' effects, i.e. effects causing some kind of wings. Although hard to analyse because source continuum affects the result and we haven't yet analysed HRMA focus images (instead, we have used the 0th order data), one can roughly estimate that the contrast of the diffraction orders (peak/background) must be better than several hundred, in some cases even more than 1000. Sidenote: when analysing these data, a hotspot at the approximate HSI position 2430, 590 was found. It shows up in almost every image. The countrate varies (as usual for hotspots; long persistant low brightness is interrupted occasionely by bursts). We have found a countrate during our test of the order of 1 ctss.