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Soft X-ray Camera and Boresight Camera


The SXC + boresight camera:  click to magnify!


The soft X-ray cameras on HETE-2 are a set of CCD-based one-dimensional coded-aperture X-ray imagers, similar in function to the WXM. There are two SXCs on HETE-2: one localizes X-rays parallel to the spacecraft X-axis, the other parallel to the Y-axis. Because of the small CCD pixel size, the angular size of a SXC pixel is only 33 arc-seconds.

The SXCs complement the WXM on board HETE-2. The effective area of the WXM is significantly larger than that of the SXC, and its energy range extends further into the gamma-ray band. On the other hand, the SXC has better angular resolution, and it is sensitive to photons with energies of 0.5-2 keV, a range in which a significant fraction of a GRB's energy may be radiated. All in all, we expect the SXC to see roughly one third of the GRBs that the WXM sees.

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The Soft X-ray Camera along:  click to magnify!

SXC Construction

A single SXC unit consists of a pair of 2048x4096 CCDs roughly 100 mm behind a finely-ruled mask. The CCDs are oriented so that their columns are parallel to the mask elements. The CCDs are read out in continuous-clocking, row-summing mode, so they operate essentially as one-dimensional imagers.

Specifications for the SXCs are:

  • Built by:     MIT CSR
  • SXC dimensions:     10cm x 10cm x 17.5cm
  • Effective Area:     7.4 cm2 per SXC
  • CCDs:     Two MIT/LL CCID-20 CCDs per SXC: 2048x4096 pixels, 15 micron pixels
  • Energy Range:     500 eV to 14 keV
  • Spectral Resolution:     46 eV @ 525 eV, 129 eV @ 5.9 keV
  • Detector Quantum Efficiency:     93% @ 5 keV, > 20% (0.5-14 keV)
  • Timing Resolution:     1.2 s
  • Field of View:     0.91 sr
  • Focal Plane scale:     33" per CCD pixel
  • Burst Sensitivity:     (4 sigma) 0.47 cts cm-2 s-1
  • Steady source Sensitivity:     (4 sigma) ~700 mCrab t -1/2
  • Localization Precision:     Faint Burst, 5 sigma - 15", Bright burst, 22 sigma, 3" (1 Crab; 10s)
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    A boresight camera:  click for more information!

    SXC Aspect

    With their fine CCD and mask features, the SXCs are especially sensitive to thermal shifts over the course of an orbit, shifts that could compromise the astrometric precision of SXC localizations. To minimize this effect, we have constructed two "boresight" aspect cameras which are mounted to the same baseplate the SXCs are mounted on. The boresight cameras will provide a link of the SXC's X-ray sky to the optical sky, one which should be stable to a few arc-seconds over the temperature extremes HETE will encounter.

    Boresight camera specifications:

  • Lens:     Custom-modified Zeiss 80mm, f/2.8
  • CCD:     MIT/LL CCID-22: 512x512 frame-store device, 15 micron pixels
  • Operating procedure:     Frame-store shift followed by 2x2 pixel summing
  • Field of view:     5.5 degrees square
  • Pixel angular size:     74 arc-seconds (in 2x2 pixel summing mode)
  • Readout time:     1.0 seconds
  • Bandpass:     560 - 670 nm
  • Limiting sensitivity:     R = 7-8
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