SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
Figure 4-29 shows the measured efficiency of a prototype beam splitter with the SPIRE wavelength range
indicated by the shaded region.
Efficiency
0.6
0.5
0.4
0.3
0.2
0.1
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70
Wavenumber [cm-1]
4.7 Internal calibrators
4.7.1 Photometer calibrator (PCAL)
Figure 4-29 - Measured efficiency of FTS beam splitter prototype
The purpose of the photometer calibrator is to provide a repeatable signal for monitoring of detector health
and responsivity for ground testing and in-flight operation. It is not an absolute calibrator, but may be useful
as part of the overall calibration scheme. The baseline design consists of a thermal source inside an
integrating cavity, the body of which is at 4 K. The cavity has a light pipe output with a 1-mm diameter
aperture. PCAL has a mass of < 30 gm and is located within the Beam Steering Mirror housing behind the
beam steering mirror itself (M4), at an image of the system pupil (telescope secondary mirror). The fraction
of M4 area obscured is 0.2%, and does not result in any loss of signal as the obscured central area is within
the region of the pupil obscured by the hole in the primary mirror. PCAL shares the BSM wiring harness.
Because all of the detectors in the photometer arrays view the pupil with near equal efficiency, PCAL
produces a very uniform illumination over the arrays (this is an advantage but not a requirement). It is
envisaged that the calibrator will be operated in flight at regular but not frequent intervals (once per hour or
more). Operation of PCAL requires the BSM to be switched off and the telescope pointing to be fixed, so
that there are no sources of detector power modulation except the calibrator itself. A predetermined current
excitation sequence will be applied over a period of ~ 10 seconds, and the corresponding detector signal
measured.
The requirement for the brightness of PCAL is that it provide the equivalent of a unit emissivity black body
temperature of 40 K or more over its 1-mm emitting area. PCAL occupies (1/30) 2 of the 30-mm diameter
pupil area. The power level that it produces at the detectors can be compared to that due to the telescope (80-
K 4% emissivity) by the following simple calculation. With a temperature of 40 K and unit emissivity, it
87
2RT
R^2 + T^2