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 m), array imaging (40 - 240 m), polarimetry (25, 170 m) and spectrophotometry (2.5 - 12 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: , on-source integration time, 1 reference background position. The numbers in parentheses in the `Pre-flight' column give an estimate of the 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]	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)			21	10	15	detector/background noise
S2(SS)			11	26	0.5	detector noise

Key features of the instrument were:

• The filter set allowed nearly simultaneous and therefore consistent measurements (with regards to variable sources) from the NIR to the FIR.
• A range of apertures (5 - 180) could be selected to match the point spread function of the selected filter and thus to optimise the source-to-background contrast.
• Because the cold space telescope contributed negligible emission, relatively wide beams could be used to detect faint extended emission and measure absolute surface brightness.
• A chopping mirror in the beam path permitted differential measurements to detect faint sources superposed on a much higher background level.
• The chopper mirror also allowed for oversampled scans on bright celestial objects at far infrared wavelengths.
• Internal reference sources provided a homogenous calibration relative to the best available sky standards.
• The PHT spectrometer sub-instrument was optimised in wavelength coverage and spectral resolution for the investigation of dust features.
• The high sensitivity and therefore relatively short observation times permitted differential measurements through different polariser settings.