CXO Project Science: ACIS Shielding

Spacecraft & SIM ACIS Shielding Studies(Last update: 06 Oct 1999)
In an effort to find the best position for ACIS during radiation belt passages, Kurt Dietz (SD22, MSFC) has extensively reworked the Chandra geometry model to better predict ACIS shielding. In particular, the Focused Structure Bench (which lies between the SIM Translation Table and the SIM-to-OBA interface), the Translation Table, the external source enclosure and nearby electronics boxes, and the SIM Thermal Skirt/OBA Aft Bulkhead interface Density of the graphite/epoxy composite material is 1.66 grams/cc.

The optimally shielded position for the ACIS detector lies between positions Z = 31.45 and Z = 42.8 cm, relative to the ACIS-S aimpoint, where Z = 0 when ACIS-S is on the optical axis. In motor steps (column 3TSCPOS in S. Wolk's sim_data_summary) these coordinates are 31.45 cm = -48185 and 42.80 cm = -92900. For reference, the ACIS-S positions when HRC-I and HRC-S are located on the optical axis are Z=32.04 and 44.51 cm, respectively. Thus, ACIS is within the optimally shielded range when HRC-I is on the optical axis.

For values of Z < 31.45, the outer edge of the ACIS focal plane is exposed in a direct path (not passing through the ACIS Collimator) into the SIM stovepipe baffle but not directly to the telescope assembly and the HRMA. For values of Z > 42.8 cm, a straight line path (not passing through the ACIS Collimator) extends from ACIS through the OBA Aft Bulkhead/SIM Thermal Skirt gap (the location of the reported light leak). Due to intervening structure, however, there is no important decrease in column density toward this gap and the upper Z value can be extended to Z=44.51 cm, corresponding to HRC-S at the optical axis, without loss of shielding performance. Within these bounds, the minimum column density above the ACIS collimator is 1.7 g/cm2 and the column density to (not through) the HRMA is 2.55 gm/cm2.

HRC is in or near the focus position for SIM positions 31.45 < Z < 42.80. Therefore, in order to utilize these positions for ACIS protection, it is recommended that HRC take appropriate protective measures such as closing the HRC door.

The purpose of the following drawings (postscript format) is to outline the key components and to provide a sense of their size and relative placement:

Isometric View of the entire SIM assembly showing the Thermal Skirt, Hat Brim, and cavity containing the ACIS External Source

Cut-away view through the Y-Z plane showing key components. ACIS is off-axis (HRC-S is at Focus)

Cut-away view through the Y-Z plane showing key dimensions. ACIS is off-axis (HRC-S is at Focus)

Fraction of 2 pi steradian sky (towards HRMA), at a point on ACIS-S, seeing column density less than some value for various SIM positions.

The resulting column densities (g/cm2) along y,z direction cosines as seen from the ACIS-I and ACIS-S aim points for different SIM positions are:

Sample image with features identified.

Image as seen by ACIS-I with HRC-I at focus (ACIS-I offset Z=36.426 cm from focus)

Image as seen by ACIS-I with HRC-S at focus (offset Z=48.899 cm)

Image as seen by ACIS-S with HRC-I at focus (ACIS-S offset Z=32.036 cm from focus)

Image as seen by ACIS-S with HRC-S at focus (offset Z=44.509 cm)

Image as seen by ACIS-S with ACIS-S at focus (offset Z=0 cm)

Tantulum-only column density, as seen by ACIS-S with HRC-I at focus (offset Z=36.426 cm), mainly due to OBA Stovepipe baffle extending from OBA Aft Bulkhead.

Titanium-only column density, as seen by ACIS-S with HRC-I at focus (offset Z=36.426 cm), entirely due to ACIS Collimator. Note collimator footprint differs slightly from reality.

Graphite-only column density, as seen by ACIS-S with HRC-I at focus (offset Z=36.426 cm), showing complete coverage.

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