PHT05
The absolute sky surface brightness level at 25 
m shall be measured
towards a single position in the galactic plane. Therefore, PHT05 is used
with an aperture of 99'' and the setting
POINT_OR_EXTENDED SOURCE = E. In order
to eliminate straylight from the galactic plane onto the FCS the option
COLD_FCS= Y is chosen. This option will include an additional measurement
of the cold FCS.
The fluxes are:
EXPECTED SOURCE FLUX : 40 MJy/sr
UNCERTAINTY IN SOURCE FLUX : 4.0 MJy/sr
MAXIMUM BACKGROUND FLUX : 0.0 MJy/sr
For this observation a MAXIMUM BACKGROUND FLUX has not been specified since the background itself is the target of the observation. The Zodiacal Light contribution is contained in the EXPECTED SOURCE FLUX.
The power on the detector and the noise terms are calculated according to the relationships given in Section 4 for a 99'' aperture and an extended source. Since the aim is to measure the background level very accurately, a S/N ratio of more than 100 should be achieved. Applying Eqns. 4, 8, and 9 we obtain:
The total noise is:
For the integration time Eq. 4 yields 
. However, for
reasons mentioned in Section 5 concerning detector drifts
the minimum of 32 sec integration should be used, in particular since the
accuracy of the flux level has to be rather high. With 32 sec the S/N ratio
is:
.
The amount of time spent measuring the flux level of both the cold (130 sec) as well as the heated FCS (32 sec) adds to a total exposure time of 194 sec. Furthermore, the following overheads have to be added:
time for target acquisition and slewing : 180 sec
time for instrument set-up (cf Table 8): 100 sec
time for stabilization of heated detector : 90 sec
The total observing time for this observation is approximately 564 sec.