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2.1 Introduction

ISOPHOT (PHT) was the imaging photo-polarimeter on board the Infrared Space Observatory ISO. Its four principal modes were single detector element aperture photometry (3 - 120 $\mu $m), array imaging (40 - 240 $\mu $m), polarimetry (25, 170 $\mu $m) and spectrophotometry (2.5 - 12 $\mu $m). The wavelength range that was covered by IRAS was extended by a factor of two towards longer wavelengths. This wavelength regime, which is dominated by the emission of very cool objects (15 - 30 K), is not accessible from the ground and only with considerably less sensitivity with airborne instruments.

Since ISO was a satellite observatory designed for pointed observations, the much longer dwell times allowed to measure with ISOPHOT objects significantly fainter (by a factor 10 to 100) than the IRAS survey detection limit. However, in the FIR the gain in sensitivity did not increase with the square root in integration time, but was limited either by detector noise or sky confusion. See Table 2.1 for typical sensitivity from faint source programmes. The table with further details is given in Klaas et al. 1997, [22].


Table 2.1: ISOPHOT sensitivity limits: $S/N=1$, $256\,s$ on-source integration time, 1 reference background position. The numbers in parentheses in the `Pre-flight' column give an estimate of the $1\,\sigma$ confusion noise at the corresponding background brightness. The confusion noise has to be added to the pure detector noise limit. The in-flight values include all possible noise sources.
    Sensitivity    
Det. Filt. Aper. Pre-flight In-flight Bckgr. Limiting
    [arcsec] [mJy] [mJy] [MJy sr$^{-1}$] Param.
P1 P_4.85 52 circ. 2 2.5 0.1 detector noise
P1 P_11.5 52 circ. 2 5 15 background noise (ZL)
P2 P_25 79 circ. 7 13 40 background noise (ZL)
P3 P_60 180 circ. 3.5(+5) 20 10 detector/cirrus noise
          dark current 100 x higher
C100 C_90 45 (1 pix) 1(+5) 7.5 10 detector/cirrus noise
C200 C_160 90 (1 pix) 3(+21) 40 5 cirrus noise
S1(SL)   $24\times24$ 21 10 15 detector/background noise
S2(SS)   $24\times24$ 11 26 0.5 detector noise

Key features of the instrument were:


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Next: 2.2 Overview Up: 2. Instrument Description Previous: 2. Instrument Description
ISO Handbook Volume IV (PHT), Version 2.0.1, SAI/1999-069/Dc