Absolute photometry in single pointing mode: PHT05 and PHT25

The primary use of the AOTs PHT05 and PHT25 is for absolute determination of sky surface brightness from 3.3 to 240 micron of sources like the zodiacal light, the galactic, the extragalactic background. Only one filter and one aperture (in case of PHT-P) can be selected. Observers who wish to observe in more filterbands should repeatedly execute PHT05 or PHT25.

Both AOTs have undergone a major re-design as the tests of the pre-launch design version showed several deficiencies. It was found that chopping against the cold or heated FCS yields signal drifts which do not allow to stabilize the signal sufficiently within one chopper plateau. In addition, when measuring against the FCS both radiation from the sky entering the instrument as well as straylight inside the focal plane falsifies the FCS signal.

For these reasons the two AOTs have been re-designed in the following way:

1. All measurements are in staring mode to allow the best signal stabilization.
2. A dark measurement, by blocking the optical path to the detector, can be optionally included to measure the dark signal due to dark current and glitches at the time of the measurement. This measurement is used to determine the photometric zero point. The measurement time is set to 256 seconds.
3. A measurement against the cold FCS can be optionally included. This determines the straylight level induced by the celestial source when measuring against the heated FCS and provides a correction for it. The measurement time is set to 128 sec.
4. The measurement against the heated FCS is always included. The FCS flux level is set to the same value as the expected sky flux level. The FCS measurement has the same integration time as the sky measurement.

PHT05 or PHT25 observations in raster mode are not possible. To absolutely calibrate extended sky emission it is recommended to combine PHT05 with PHT03 raster mode or PHT25 with PHT22 raster mode. PHT05 and PHT25 can now be applied for Solar System Objects. Detection of faint point sources on top of a brighter sky background should be done in the chopped mode of PHT03 and PHT22, respectively.