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The considerations given in "Introduction and purpose of PHT05 and PHT25" have led to the following design of the absolute photometry AOTs: * All measurements are in staring mode to allow the best signal stabilization. * Only one filter and one aperture (in case of PHT-P) is selected per observation. * A dark signal measurement obtained by blocking the optical path to the detector can be included. The integration time is set to 256 sec. * A measurement of the FCS with no heating power applied (cold FCS) can be included. This measurement provides a correction for the straylight component. The integration time is set to 256 sec for the P1, P2, P3, and C100 detectors. For C200 the time is set to 128 sec. * 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. The measurement sequence is as follows: - (optional) Dark 256 sec - (optional) Cold FCS 256 sec for P1, P2, P3, and C100 128 sec for C200 - Sky N sec - FCS N Sec where N is an integer power of 2 with a minimum of 32 sec. Recommendations on the Sky (and consequently FCS) times are given on page "What is the optimum integration time".
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