When to include a dark measurement

The next table lists the equivalent surface brightness of the dark
signal using typical values for the detector responsivity and dark signal.

------------------------------------------------------------------------------
				Equivalent Brightness	Aperture
Det/filter	Dark signal	in aperture		diameter
		mV/s		MJy/sr			arcsec
------------------------------------------------------------------------------
P1/11.5		2		0.02			180
P2/25		10		0.4			180
P3/60		8		0.06			180
C100/60		30		1.2			per pixel(*)
C200/160	3		0.1			per pixel
------------------------------------------------------------------------------
(*) C100 pixel 6 has an exceptionally high dark signal which is about 4-5
    times higher.

For smaller apertures the equivalent surface brightness is proportional to
the inverse of the aperture area. For different filters of the same detector
the equivalent brightness must be multiplied by C1(1)/C1(2), where C1(1) is
the value of C1 for the filter in the table above and C1(2) is the C1 value of
the other filter (see Table 2 of the Addendum to ISOPHOT Observers
Manual).

PHT05 and PHT25 commissioning observations have indicated that the dark
signal is quite stable and does not deviate by more than 50% from the
default dark signals collected with dedicated dark observations. However,
exceptional deviations due to bad "space weather" or during observations
at the end of a revolution are not ruled out. Under those circumstances a
higher ionizing radiation flux may cause a significant dark signal increase.

In case you would like to achieve an uncertainty in your total flux which is
less than 50% of the dar signal equivalent brightness values we strongly
recommend to include a dark measurement in your observation. Else the
default values for the dark signals provided by the PHT Instrument Team are
adequate. The default values are weekly checked throughout the mission.

In case several PHT05 or PHT25 observations with the same detector and
different filters are performed consecutively, one dark measurement at the
beginning of the sequence is sufficient to determine the dark signal for
several filters.

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