HETG, LETG - ACIS-S Gaps Tables, Nov.'99

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The IDL procedure hetgs_gaps.pro was updated and new HETGS and LETG-ACIS-S gap tables created. Note that the offsets in these tables have a sign such that positive corresponds to a motion of the zeroth-order image in the from-S2-towards-S3 direction.

These gap values are based on an effective gap size of 0.502 mm, corresponding to 10 arc seconds on the sky. It is ``effective'' in the sense that the gap includes columns 1 and 1024 of the devices from which no events are reported. This single value is approximate and accurate to about 2 pixels. The actual gap sizes vary slightly; the precise values of the ACIS-S chip geometry are given in the CXCDS file pix_corner_lsi.par and will be incorporated in MARX version 2.50.

All HETGS observations are nominally dithered with an amplitude of +/- 8 arc seconds in the spacecraft Y (dispersion) direction. There will thus be reduced coverage in the spectral regions within one gap-width on either side of the gaps.

Finally, because the target may be acquired up to +/- 20 arc seconds from the nominal aim point, it is best to try to avoid placing known features of interest within three gap widths of the tabulated gap edges.

Please send any comments to Dan Dewey at dd@space.mit.edu.

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