Next: 5.7 Calibration files Up: 5 Calibrations Previous: 5.5 PHT-S wavelength calibration
Next: 5.7 Calibration files
Up: 5 Calibrations
Previous: 5.5 PHT-S wavelength calibration
All raw PHT measurements undergo the same processing steps to obtain the mean signal per chopper plateau or mean signal per raster point. These steps are described in detail in Chapter 7. Here we only list the relevant correction steps:
Single pointing photometry AOTs include one FCS measurement for each detector used. The FCS measurement is taken after completion of the filter sequence for a given detector (see section 3.7).
Assuming that the derived responsivity applies to all filters of the same detector, the signal from each filter is converted to a in-band power. Depending on the filter and aperture selected, the powers are converted to an a flux density in Jy or MJy/sr using equations in section 5.2.3.
For AOTs in mapping mode each raster measurement is bracketed by two FCS
measurements with identical FCS heating power. For the calibration
it is assumed that the responsivity varies proportionally with time between
the two FCS measurements. A linear interpolation in time between the
responsivities of the two FCS measurements is performed to determine
the responsivity at the time of the integration on a raster point.
Since the map plus FCS measurements are obtained for one filter at a
time, no uncertainties due to 
are involved.
During a sparse map concatenated chain (P17/18/19 for PHT-P or P37/38/39 for PHT-C) two FCS measurements are obtained, the first measurement after the sky measurement(s) in PHT17/PHT37 and the second after the sky measurement(s) in PHT19/PHT39. The FCS in-band powers are adapted to the flux levels as specified in the start and end AOTs and can therefore be different FCS heating power settings.
For the sparse map AOTs P17/18/19 and P37/38/39, the multi-filter option can be used. In such case a linear interpolation in time between the responsivities of the two FCS measurements is performed as well as a transfer of the calibration to the filters for which no FCS calibration are obtained.
In this mode one FCS measurement is collected for the last (largest) aperture in the sequence. An important source of photometric uncertainty is the accuracy in the aperture scaling (see 5.4). Uncertainties in the beamprofile can be minimized by comparison with a similar observation on a point source.
For P05 and P25 only one filter band can be selected and the FCS measurement has the same integration time as the source measurement. The observer can therefore minimize statistical and transient uncertainties. Uncertainties due to dark signal and FCS straylight can be minimized by including dedicated dark and cold FCS measurements in the observation. Since the absolute photometry AOTs are single filter observations, the photometric accuracy only depends on the uncertainties in the FCS power calibration tables and either in the illumination matrix for PHT-C or the aperture scaling for PHT-P (both for FCS and sky).
The instrument set-up of a P40 observation is always the same. After dark
signal subtraction, the signal of each pixel is directly converted to a
flux density in units of 
. The PHT-S spectral
response function was determined from observations of calibration stars
(section 5.2.4). In-orbit beam profile measurements were used to
determine the conversion from a point source flux to an extended source flux.
Each target measurement with PHT40 is preceded by a ``pseudo'' measurement of 32 seconds in dark instrument configuration. This measurement offers a qualitative assessment whether the responsivity of the PHT-S detectors is affected by a transient introduced by a preceding PHT40 observation.
For example, in case the preceding PHT40 observation involved a source with bright spectral features, the pixels that detected the features can still be in a transient while the pseudo dark is being taken. It is also found that different pixels in the PHT-S array show different transient behaviour. This can cause artifacts in the spectrum, especially after observing a strong continuum source.
Possible features in the dark spectrum at a certain wavelength should caution the observer that features in the same pixels of the actual source spectrum could have an instrumental cause. The dark measurement in PHT40 is not intended for subtraction from the source spectrum.
ISOPHOT Data Users Manual, Version 4.1, SAI/95-220/Dc