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2 Photometric Accuracies

Three different kinds of photometric accuracies are provided in this document:

• absolute accuracy is a measure of the uncertainty of the absolute flux level of any source measured with ISOPHOT w.r.t. well known independently and absolutely calibrated flux standards, either black body standards or standard stars (Vega, Sirius) directly calibrated against black body standards. An absolute flux measurement with ISOPHOT has to involve a measurement against the internal reference source (FCS) in the same filter, because the latter one was calibrated against celestial standards.
• relative multi-filter accuracy is a measure of the uncertainty of a spectral energy distribution derived from an ISOPHOT multi-filter measurement with a measurement against the internal reference source in only 1 filter, i.e. the measurements in the other filters have to be tied to the absolute measurement by relative scaling factors. When establishing the relative multi-filter accuracies the systematic offset due to the absolute accuracy term had been subtracted. Therefore, absolute and relative multi-filter accuracy have to be added up for the appropriate measurements in order to derive the total photometric accuracy.
  For PHT-S the term multi-filter accuracy gives the accuracy of the relative spectral shape between different wavelength ranges (covered by groups of 20 pixels).
• relative flux reproducibility is a measure how well a many times repeated observation of the same non-variable source gives the same flux.

Table 2.1 gives a summary overview of the currently achieved photometric accuracies with ISOPHOT. These values have been established as global values from an end-to-end processing involving all likely uncertainty sources. They have been derived during the scientitifc validation of the OLP V7.0 (cf. Report on the PHT Scientific Validation for OLP Version 7.0).

The reproducibility accuracies have been derived from special calibration monitoring programs repeating the measurements 15 to 25 times during a period of more than 1 year and with time intervals between one and six weeks.

Tables 2.2,  2.3 and  2.4 in the subsequent sections give more details on the calibration accuracy assessment, providing the AOT submode, a more detailed flux range specification, and the names of the flux standards used for the assessment.

 

Table 2.1:   ISOPHOT Photometric Calibration Accuracy Overview. For the wavelength range of the detector system, please refer to Table 1.1.

	detector	source	source	typical	absolute	reproducibility	multi-filter
		type	brightness	on-source	accuracy	1 of	accuracy
		point/		integration		20 samples
		extended		[s]	[%]	[%]	[%]
	P1	p	50 - 6000 Jy	32	10	5	20
	P1	p	0.1 - 50 Jy	256	30	10	TBD
	P2	p	10 - 200 Jy	32	< 15	2	10
	P2	p	0.5 - 10 Jy	256	< 20	12	TBD
	P3	p	0.5 - 25 Jy	32	15	7	20
	P3	p	< 0.5 Jy	256	30	7	TBD
	C100	p	0.5 - 25 Jy	32	10	3	10
	C100	p	< 0.5 Jy	360	30	20	TBD
	C100	e	5 - 15 MJysr-1	32	20	10	< 30
	C200	p	1 - 900 Jy	128	20	10	TBD
	C200	p	0.2 - 1 Jy	240	30	10	TBD
	C200	e	4 - 10 MJysr-1	64	20	6	< 30
	SS	p	2 - 5000 Jy	256	< 30	< 30	< 30
	SS	e	50 - 105 MJysr-1	256	< 30	< 30	< 30
	SL	p	1 - 5000 Jy	256	< 30	< 30	< 30
	SL	e	50 - 105 MJysr-1	256	< 30	< 30	< 30