CXO Project Science: Stray-Light Requirements

1998 Jun 16 22:00 GMT

In preparation for the AXAF stray-light test, conducted during thermal-Vacuum testing at TRW, AXAF Project Science re-evaluated the focal-plane stray-light requirement based on the as-built performance of the AXAF Science Instrument (SI) detectors. The as-built requirements are comparable to or less demanding than the as-designed ones, and retain some practical margin against those in the Observatory CEI Specification.

Summary

The following table summarizes the stray-light requirements at the top of the respective focal plane, for the as-built High-Resolution Camera (HRC) and AXAF CCD Imaging Spectrometer (ACIS). A more detailed table appears later in this report and an active Excel spreadsheet (stray-uvis.xls) is available. For reference, the AXAF Observatory Contract End Item (CEI) Specification is approximately log(PST) < -12 for solar XUV, < -10 for solar UV, < -8 for solar visible, and < -7 for bright-earth visible radiation, at the aperture of the detector.

Sources of stray XUV-visible radiation

The plots show the spectral flux for direct illumination by the following sources:

• Sun at 1 astronomical unit (magenta): Data (flux_sun.dat) are synthesized from Ref 1 below 3200 Angstrom, interpolated to a 5800-K blackbody above 4000 Angstrom.
• 100-W lamp at 30 cm (blue): Data (flux_lamp.dat) are synthesized for a 3372-K blackbody.
• Bright earth at 10-Mm altitude (green) and at 100-Mm altitude (red): Data (flux_earth10.dat and flux_earth100.dat) are synthesized from Ref 2 below 1100 Angstrom, from Ref 3 between 1100 and 3200 Angstrom, interpolated to a 0.4-albedo reflected solar spectrum above 3500 Angstrom.

XUV-visible efficiency of AXAF detectors

The plots show the XUV-visible net efficiency of the AXAF Science Instrument (SI) detectors:

• HRC-I and HRC-S: Data (eff_hrc-i.dat and eff_hrc-s.dat) are from Ref 4, extrapolated as a constant longward of 6000 Angstrom.
• ACIS-I and ACIS-S: Data (eff_acis-i.dat and eff_acis-s.dat) are synthesized using the transmission of the optical blocking filters (OBF) from Ref 5 and using a CCD quantum efficiency of 50% at 7000 Angstrom with a FWHM of 4000 Angstrom, per Ref 6.

HRC net efficiency

ACIS net efficiency

Response of AXAF detectors to stray XUV-visible radiation

The plots show the spectral count rate for direct illumination of AXAF Science Instrument (SI) detectors by the following sources:

• Sun at 1 astronomical unit (magenta).
• 100-W lamp at 30 cm (blue).
• Bright earth at 10-Mm altitude (green) and at 100-Mm altitude (red).

HRC rate for direct illumination

ACIS rate for direct illumination

Requirements on point-source transmittance (PST)

The table shows the requirement on the maximum count rate from stray XUV-visible radiation, the band-integrated count rate for direct illumination by the sun and bright earth, and the consequent maximum PST consistent with each SI detector's stray-light requirement. The spectral bands are 100-500 Angstrom for XUV, 500-3000 Angstrom for UV, and 3000-11000 Angtrom for visible. The requirement on the PST is at the top of the focal plane, i.e., just above the optical filter of the respective SI detector.

Detector		HRC-I	HRC-S	HRC-I	HRC-S	HRC-I	HRC-S	ACIS-I	ACIS-S
Spectral band		xuv	xuv	uv	uv	vis	vis	vis	vis
Max rate	Log(ct s^-1 cm^-2)	-2.0	-2.0	-2.0	-2.0	-2.0	-2.0	+4.7	+4.7
Direct solar rate	Log(ct s^-1 cm^-2)	+7.0	+8.4	+0.1	+7.0	+2.5	+5.5	+9.3	+10.7
Max solar PST	Log(PST_sun)	-9.0	-10.4	-2.1	-9.0	-4.5	-7.5	-4.6	-6.0
Direct earth (10-Mm) rate	Log(ct s^-1 cm^-2)	+2.7	+4.0	-2.4	+4.5	+0.9	+4.0	+7.8	+9.2
Max earth (10-Mm) PST	Log(PST_earth10)	-4.7	-6.0	+0.4	-6.5	-2.9	-6.0	-3.1	-4.5
Direct earth (100-Mm) rate	Log(ct s^-1 cm^-2)	+1.1	+2.4	-4.0	+2.9	-0.7	+2.4	+6.2	+7.6
Max earth (100-Mm) PST	Log(PST_earth100)	-3.1	-4.4	+2.0	-4.9	-1.3	-4.4	-1.5	-2.9

For the HRC, the as-built requirements on the XUV and UV PST are approximately the same as the as-designed ones. The HRC requirements on the visible PST, although not identified originally, are less stringent than those on the UV PST. For the ACIS, the as-built requirements on the visible PST are a couple orders of magnitude less demanding than the as-designed ones, primarily because of better performance of the ACIS optical blocking filters.

The most stringent SI-detector stray-light requirement is for solar illumination of the HRC-S: log(PST) < -10.4 for XUV or < -9.0 for UV, depending upon the uncertain decrease of reflectance into the XUV. For reference, the AXAF Observatory Contract End Item (CEI) Specification is approximately log(PST) < -12 for solar XUV, < -10 for solar UV, < -8 for solar visible, and < -7 for bright-earth visible radiation, at the aperture of the detector.

Note that the as-built HRC-S requirement has an order-of-magnitude negative margin against the Observatory CEI Specification for stray-visible solar radiation. However, satisfying the CEI stray-UV specification ensures satisfying the visible one, and the CEI specifies the value at the detector's aperture rather than at its focal plane. Furthermore, the TRW design provides several orders of magnitude margin agains

Calculating the visible PST from the lamp test

The table compares the count rate of each detector for direct solar illumination and for illumination by a test lamp (100 W at 30 cm). A measured count rate (or upper limit) during the lamp test then provides a calculated value (or upper limit) for the point-source transmittance (PST).

Detector		HRC-I	HRC-S	ACIS-I	ACIS-S
Spectral band		vis	vis	vis	vis
Pixel density	Log(pixels cm^-2)	+6.4	+6.4	+5.2	+5.2
Direct solar rate	Log(ct s^-1 cm^-2)	+2.5	+5.5	+9.3	+10.7
Direct lamp rate	Log(ct s^-1 cm^-2)	+2.6	+5.6	+9.5	+10.9

The following then gives the point-source transmittance (PST) for visible radiation:

log(PST) = log(measured rate) - log(direct rate).

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Acknowledgements

References

1. L.J. Heroux and H.E. Hinteregger (1985) Handbook of Geophysics and the Space Environment, ed. A.S. Jura, Chapter 2.

2. R.R. Meier (1991) Space Science Reviews 58, 1-185.

3. R.E. Huffman (1985) Handbook of Geophysics and the Space Environment, ed. A.S. Jura, Chapter 22.

4. M.V. Zombeck (1998) S.A.O. HRC team, HRC Calibration page.

5. G. Chartas (1998) Penn State ACIS team, private communication.

6. M.W. Bautz (1998) M.I.T. ACIS team, private communication.

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