Spectrometer Prototype Design Notes

• Heilmann; Facility Science Team (FST) Meeting, August 20-22, 2008, GSFC
• Heilmann et al; AAS Jan 2009

Here we list only basic items or assumptions for a model IXO/CAT prototype spectrometer.

Inputs:

• Grating efficiency files are from Ralf Heilmann/MIT.
• The grating period is 1000 Angstroms, and the blaze angle is 3 degrees.
• Mirror area files from Paul Reid/CfA.
• The imaging PSF is 5 arsec (Half Power Diameter)
• The grating readout detector QE is for a CCD with 100A Al filter.
• The microcalorimeter resolution used was 2 eV FWHM. The calorimeter QE is from the 2006 FST responses.

Configuration:

Resolving power of 3000 is what we estimate from a 45-degree (half-angle) sector, so two of these completely cover the outer annuli. This implies two readout arrays (for simplicity, we only provide one set of responses for the combined area). Here are schematics of the mirror aperture:

This is a schematic of a sub-apertured grating array and beam (left), and an idealized image from the sub-aperture (left grayscale image, hourglass shape) and full-aperture image (right, circular image).

We speculate that with "astigmatization" of the gratings (a gradient in diffraction direction with azimuth across a sector), we can change the image from an "hourglass" into a "C-shape" and gain in resolution, up to 5000. Another way to obtain 5000 would be with a smaller sub-aperture, but at the expense of effective area since there would not be room in the focal plane for ~4 readout arrays.

The CAT gratings contribute to effective area for the microcalorimeter in zeroth order transmission. For higher energies the CAT gratings become practically transparent (transmission > 99%), such that only thick support structures lead to small reductions in effective area. The innermost of the three sets of mirror annuli is not covered by gratings and contributes fully. We have computed responses for this zeroth order plus direct beam, since it is simultaneously obtained with the dispersed spectrum.