Data examples

One part of the commissioning of the absolute photometry AOTs is the
measurement of the calibration star HR6705 (gamma Dra) together with a
background position in several filter bands. The background position was
observed first in order to begin with a low signal level, the subsequent
observation of the star was concatenated.

In Fig 1a and 1b we present the measured signals for filterband P_25 as a
function of time for the two concatenated PHT05 observations. The measurement
order is as follows:

measurement	duration (sec)	type
1 background	256		dark signal
2		128		Cold FCS
3		64		sky: background
4		64		FCS measurement
1 on star	128		Cold FCS
2		64		sky: star+background
3		64		FCS measurement

Since the AOT logic has not been finalized for these commissioning experiment,
the FCS straylight measurements lasted only 128 sec and not 256 sec.

>From Figs 1a and 1b the following points can be noted:

* all signals are drifting over timescales longer than 64 sec
* the two straylight measurements show a strong downward drift. This
  drift is caused by the illumination of the detector by the sky just
  before the Cold FCS measurement starts. This shows that 128 sec is
  not adequate for an accurate straylight measurement.
* the sky measurement on the star as well as the corresponding FCS measurement
  shows a large transient drift which is not sufficiently settled after 64
  sec.
* although the drift is substantial in the star's sky measurement, the
  signal level is less than 4% of the FCS signal (last measurement in Figs 1a
  and 1b).

Using the most recent ISOPHOT calibration tables we derive from the
measurements:

I(background, P_25)	= 20.2 +/- 1.1 MJy/sr
F(HR6705, P_25)		= 32.4 +/- 0.2 Jy

Where the fluxes are derived assuming a constant power spectrum. COBE
and ground calibration data yield:

I(background, 25)	= 21.5 MJy/sr
F(HR6705, 25)		= 30.2 Jy

The stellar spectral energy distribution has been integrated over the
filter transmission curve to derive the predicted flux.

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