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3.7 Dark signal subtraction

synopsis:  (1) residual systematic uncertainty in the sky measurement which depends on orbital position during measurement and space weather conditions. (2) residual systematic uncertainty in the FCS measurement causing an additional systematic photometric uncertainty for the sky measurements.

limitations and applicability:
The dark correction is highly uncertain at the end of a revolution. Not applicable to chopped or differential measurements where the sky background subtraction includes implicitly the dark signal subtraction. The background must be observed together with the target observation to ensure that the dark signals in source and background pointing are identical. The correction is not important for bright sources (cf. Table 1).

description:
The detector/CRE chain as well as the ionising radiation introduce an instrumental signal which causes a zero level offset (``dark signal''). The dark signal depends on the properties of the detector and the time of the observation in the science window. A time dependent dark signal is removed.

purpose correction:
Removal of a spurious (instrumental) signal.

uncertainty/noise introduced:
Observations in which a differential flux is measured are not affected by uncertainties in the dark signal correction. Examples are: chopped observations, absolute photometry observations in which the dark signal measurement is included, maps covering discrete sources, etc. Also, for observations for which the sky signal is much higher than the dark signal, the uncertainties are negligible.

Residual uncertainties in the dark signal correction becomes important for faint targets whose signals are close to the dark signal.

Systematic uncertainties in the dark signal subtraction of the FCS measurement affects the responsivity determination for all AOTs except for the absolute photometry AOTs (PHT05, PHT25) where a dark or FCS straylight measurement was performed. The signal uncertainties of the dark and straylight measurements contribute then to the statistical uncertainty.

To be consistent with the reset interval correction (Sect. 3.6, all dark signals except PHT-S are transformed to the equivalent signal for a reset interval of $t_{reset}=\frac{1}{4}$s. This introduces an additional uncertainty due to the residual uncertainties in the transformation. For some detectors the dark signal after transformation has become negative, this a consequence of the CRE properties and has no deeper physical meaning.

In Table 1 we give the equivalent fluxes for dark signals for each filter. Since the dark signal is time dependent, the average minimum value (column 2), average maximum value (column 3) along the revolution and the $1\,\sigma$ statistical scatter due to space weather effects from revolution to revolution (column 4) are listed for each filter. The dark signals increase along the revolution and the highest values are found for the end of the science window. A typical responsivity is assumed for each detector. For C100 and C200 the dark signals are available for each pixel. Since the dark signal values are similar for all C100 pixels except for pixel 6 we list pixel 6 and the average of the remaining 8 pixels. For C200 we list the average of the 4 pixels.

The table can be used to estimate the typical dark signal uncertainties.


Table 1: Dark signal equivalent fluxes, see text for explanation of columns
  ${\bf F_{\nu, min}}$ ${\bf F_{\nu, max}}$ ${\bf {\Delta}F_{\nu}}$
filter ID mJy mJy mJy
P1_3.29 468 544 323
P1_3.6 93 109 64
P1_4.85 92 107 64
P1_7.3 69 80 47
P1_7.7 352 410 243
P1_10 262 304 181
P1_11.3 929 1080 641
P1_11.5 168 196 116
P1_12.8 385 447 265
P1_16 709 824 489
P2_20 126 254 23
P2_25 121 245 22
P3_60 -196 1144 52
P3_100 -142 827 37
C_50 -36 561 111
C_60 -26 404 80
C_60pix6 573 1245 156
C_70 -27 416 82
C_90 -11 175 35
C_90pix6 248 539 68
C_100 -19 301 60
C_105 -37 567 112
C_120 -97 -64 7
C_135 -63 -42 5
C_160 -50 -33 4
C_180 -105 -69 8
C_200 -318 -209 23

auxillary data:
Cal-G files: PPDARK, PSDARK, PC1DARK, and PC2DARK for the different subsystems and include estimates of the statistical uncertainty.

keyword(s):
None


next up previous contents
Next: 3.8 Signal deglitching Up: 3. Derive SPD level Previous: 3.6 Reset interval correction
ISOPHOT Error Budgets: Derive_SPD Processing Steps, Version 1.0, SAI/98-091/Dc