HETG Capella Focus Analysis

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The best-focus location and uncertainty-range is shown for the zero-order (diamond) and a variety of HEG ("x") and MEG ("[]") lines in the spectra. The HEG lines do not seem to focus at +250 microns forward from the MEG's as simulations had suggested. Rather, the overall best HETG focus is in the range of +50um to +100um w.r.t. the "-1.002 mm" location. The formal mean and errors for the HEG and MEG best-focus locations are shown on the plot.

9/1/99, 9/4/99 (slight rev.s), 9/7/99 (mean values, conclusion added)

Conclusion: Best overall HETG focus is in range of +50um to +100um w.r.t. the "-1.002 mm" focus value (-505 steps). It is reasonable to adopt the current (9/7/99) ACIS-S best focus value of "-0.948 mm" (= +54um w.r.t. ...), as determined by CXC Cal group ( note on internal site ) for HETG operation. That is set HETG focus at -468 motor steps.


  For HETG observations in the P13 load a good focus to use
is what I believe the new ACIS-S focus value is as derived
by Cal team.  Specifically the old standby of "-1.002 mm"
or -505 motor steps for ACIS-S now gets changed to:

   "-0.948mm"  or -468 motor steps


     Dan D and Herman M

Analysis The Capella focus observations were analyzed using the "foc_do_it_all" routine in the HAK s/w. To take advantage of the CXCDS aspect corrections, the analysis was done using "Sky" coordinates, X, Y, rotated (and flipped) so that Sky X is along TDETX and Sky Y along TDETY. Various plots were created from this analysis and are available here:

Zero-order The (unpiled) zero-order "streak events" show minimum FWHM values of 32 to 33 microns - this is very good! (Non-piledup simulations predicted ~ 37 um; previous no-grating measurements indicated best FWHM of ~40 um.) The measurements yield a best focus for these zero-order events at +0.091 +/-0.024 mm w.r.t. the "-1.002 mm" location which differs somewhat from the expected values of +0.020 to +0.040 mm (FWHM analysis) or +0.054 mm (CXC Cal analysis.)

HEG focus There does not seem to be a clear signature that the HEG is better focussed in the +0.2 mm exposure as SAOSAC simulations suggested.

Resolving Power E/dE plots for all of the defocus values fit the "optimistic/conservative" E/dE expected curves rather well - see links in the "E/dE" cells on observation summary links at http://space.mit.edu/HETG/flight/obs/. This confirms the conclusion that we have the focus we want!

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