PHT03 - Multi-filter PHT-P photometry
PHT22 - Multi-filter PHT-C photometry
PHT05 - Single filter absolute PHT-P photometry
PHT25 - Single filter absolute PHT-C photometry
These AOTs perform photometric observations in one or more filters, either in staring or chopping mode, with the PHT-P or PHT-C subinstruments, respectively. Absolute measurements of the fluxes are possible by chopping against the internal fine calibration sources. In the latter case no measurement of the celestial background is provided. If a background determination is necessary, it has to be measured separately.
All 4 AOTs can also be used in raster mode (see Sect. 3.3 for details). In that case only one detector subsystem can be selected at a time reducing the number of filters available. The AOTs PHT03 and PHT22 are constrained to staring mode, whereas for PHT05 and PHT25 the chopper is used to obtain the absolute flux reference from the FCS. Similar to the PHT-C mapping mode PHT32 the complete sequence of celestial positions is measured in one filter before switching to the next filter and repeating the entire raster. For PHT03 and PHT22 every such single raster is embraced by two FCS calibrations. For PHT-C mapping observations a better baseline drift correction can be achieved using the dedicated mapping AOT PHT32. Therefore, this AOT is recommended for precision photometry of extended sources with PHT-C.
PHT03
This mode uses PHT-P, where the user can select any combination of
filters (
to 
).
The number of selected filters can range between one and all 14 (see Tab. 1).
An individual aperture can be selected for each filter (see Tab. 2).
In the case of raster observations, the available filters
are limited to those for a particular detector subsystem; i. e. for P1,
P2 or P3 (see Tab. 1).
This mode provides the standard photometry of compact and point-like sources like comets, stars, and galaxies. The possibility to perform chopped measurements with one or more reference positions permits distinguishing between very faint sources against a high and structured background such as the infrared Cirrus. Diffraction limited as well as wide beam photometry can be performed with this AOT.
For the selection of filters, the user is referred to Tab. 1, to Fig. 7, and to the filter bandpasses in the appendix.
Due to the read-out procedure (Sect. 2.6 ), a series of measurements can be extracted which allows for analysis of short term variations of the source flux in the range of several seconds. The measurements at each read-out interval will be preserved.
PHT22
This mode for PHT-C is the long wavelength analogue of PHT03 (
to 
). As such, it requires no
selection of apertures. The number of filters can be between one and all 11
(see Tab. 3).
In the case of raster observations, the available filters are those
for one detector subsystem, i.e. either for C100 or for C200 (see Tab. 3).
The applicable science cases are similar to those for PHT03. The same comments on variability monitoring as for PHT03 apply.
In multi-pointing staring raster mode the sides of the arrays are generally tilted relative to the scan orientations. Thus complete sky coverage may not be achieved, which is illustrated in Fig. 15. (Note that the orientation of the scan on the sky can be selected arbitrarily, but the orientation of the arrays is fixed to the spacecraft coordinates). Therefore, for fully consistent mapping the dedicated mapping AOT PHT32 should be used. (see Fig. 16)
PHT05 and PHT25
These are the absolute photometry modes for PHT-P ( to
, PHT05) and for PHT-C ( to 
,
PHT25), in which photometric calibration is achieved by chopping against
the internal fine calibration source.
Only one filter and one aperture (in case of PHT-P) can be
selected. The observer can adjust the reference flux of the FCS between
1/10 and 10 times the sum of the source flux and background flux level.
This is to ensure that both signals (FCS and sky signal) are within the
dynamic range of the detector. Raster observations are permitted in this mode.
For low fluxes (
) chopping can also be done against the
switched off FCS, which will have a temperature level of about 4 K.
This is the temperature of the optical support structure on which the
instrument is mounted. Such a measurement may serve for defining
a zero-point.
These modes are especially well suited for observations aiming at accurate determination of the absolute brightness of sources like the Zodiacal Light, the galactic background and the extragalactic background. The possibility of selecting a large aperture for this observation makes it suitable for the detection of low surface brightness emission.
This AOT does not provide information about the sky background at the position of the source; if this information is necessary it must be measured separately with the normal multifilter mode.
The FCS flux is adjusted to the expected sky brightness, unless the FCS is chosen to be off.
Important:
For all observing modes one must carefully check or derive the source flux and background levels using data bases for sky flux maps. For chopping observations it is necessary to check the extent of the source and the neighbouring sky to establish avoidance regions (see illustrations in Fig. 10).
