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9.3 Detector Jumps

Detector jumps are another feature seen in the detector responsivities apparently similar to glitches. For unknown reasons the signal level seems to jump, mostly up and then decaying, but also sometimes down. Jumps can be as high as 10 $ \mu $V/s. They affect the zero level of the detector(s) and therewith dark current subtraction. They occur about once per 5 hours per detector. For low flux observations at high gain they are a big nuisance. In band 1 and 2 about half of the jumps are associated with glitches. However, no correlation has been found between the characteristics of these jumps and those of the glitches.

Part of the jumps are due to sudden changes in the pulse-shape correction. The change can be either in amplitude or in decay time or in both. Why the pulse-shape is suddenly changing is unclear. Since the introduction of pulse-shape correction (Section 7.2.6) in OLP 10, the jumps have lost some of their acuity although they are still present.

Detector jumps come in two different kinds. Either a single detector jumps or the complete detector block jumps.

9.3.1 Single detector jumps

The document by Heras 1997, [13] summarizes a study made on single detector signal jumps. These effects occur in only one detector at a time and can be generally described as a sudden increase or decrease of the signal, which then remains constant or has a long recovering time ($ \gg$ 10 s) after the jump. These jumps are normally seen at low signal levels. Examples of such jumps in SPD data are shown in Figure 9.8

Figure 9.8: Four examples of single detector signal jumps. In all cases the signal from one detector suddenly jumps to a different value and remains there for a log time. These are pre-OLP 10 plots.
\rotatebox {90}{\resizebox{!}{15cm}{\includegraphics{fig1_jump.eps}}}

The signal jumps observed for each detector vary significantly between different cases, and may be negative or positive.

Jumps in band 3 are of different nature from jumps in bands 1 and 2. In band 3 it sometimes has the character op popcorn-noise, jumping up and down between two more or less fixed levels. This popcorn-noise is associated with `popping' pulse-shape corrections.

Users are recommended to examine the SPD from their observations to look for such jumps. If they are present the user can either throw the affected portions of the data away or try to adjust the baseline of the affected portion to the pre- and post-jump baseline.

9.3.2 Scan jumps

The symptoms are that each detector in a block suffers a sudden rise in sensitivity, which slowly decays with time. They usually only affect one of an up-down scan pair, and the only solution is to throw the affected scan portion away. As they affect an entire detector block they are given the name `scan jumps'.

next up previous contents index
Next: 9.4 Aperture Size Up: 9. Caveats Previous: 9.2 Memory Effects
ISO Handbook Volume V (SWS), Version 2.0.1, SAI/2000-008/Dc