Some peculiarities of the C100 detector

Greet Decin ¹, Ronny Blomme ², & Mark C. Runacres ²

¹ Instituut voor Sterrenkunde, KULeuven, Celestijnenlaan 200B, B-3001 Leuven, Belgium

² Royal Observatory of Belgium, Ringlaan 3, B-1180 Brussel, Belgium

We have used the ISOPHOT instrument to obtain the following observations:

• 22 oversampled P32 maps of hot stars (60, 90 and 160 m)
• 31 rectangular chopped P22 observations of K giants (90 and 160 m)
• 60 P22 raster maps of G dwarfs (60 m)

Generally speaking, we are observing faint sources on a rather strong, often structured, background. Extracting fluxes requires considerable care in the data reduction.

We have noticed certain peculiarities in the behaviour of the C100 detector, of which we present some examples. Most notably, the same point on the sky, observed with a different combination of spacecraft pointing, chopper position and pixel, does not always produce the same flux. Also, some observations show an abnormal similarity between on-source and off- source measurements. This similarity is much closer than the pixel-to-pixel variations in a given observation. Furthermore, we have noticed glitch-like events followed by an extremely long episode of chaotic behaviour of the signal.

We discuss different possible explanations and suggest a number of solutions.