7.4 Flux calibration accuracy

7.4.1 General

Four main effects limit the flux calibration accuracy:

1: The Spectral Energy Distributions (SED's) of the astronomical sources used for flux calibration are only known to between 4 and 10%.
2: Pointing, see section 5.10.
3: Memory effects, see section 5.8, ``Memory Effects''.
4: Dark current subtraction. See section 5.4, ``Detector Noise & Dark Current''.

7.4.2 Grating

The errors expected on the flux calibration of the grating are shown in Table 7.4.2. It lists the requirements on ISO, the accuracy achieved in ground tests and the flight objective.

**Table 7.1:** Grating Flux calibration errors.
	Absolute	Cross-	RSRF²		Beam	Pointing	Non-	Overall
		talk¹	fast	slow	diff	jitter	linearity	w/o memory
Requirements	$~(\lambda/2)\%$	<2%	1%	10%	10%	-	<5%	10-30%
Ground tests	<30%	<2%	1%	10%	-	-	<10%	34%
Flight objective	$~(\lambda/2)\%$	1-2%	<1%	<5%	10%	<5%	<5%	-

Notes

1: Crosstalk refers to the amount of electrical crosstalk between detectors in each band. Refer to Section 8.2.6.
2: RSRF is the Relative Spectral Response Function. The errors vary from band to band, and chapter 6 should be read for a complete description of the RSRF.

What has been achieved in flight at the time of the OLP V6 delivery (April 97) is shown in table 7.4.2.

**Table 7.2:** Grating Flux calibration Accuracy as of OLP V6 delivery
Band	Key¹	Bandpass	Key $\lambda$	Worst	Location
	Wavelength		1 $\sigma$	accuracy	of worst
	$\mu m$	$\mu m$	%	1 $\sigma$ %
1A	2.48	0.05	5	7	edges
1B	2.87	0.07	5	7	edges
1D	3.08	0.07	5	7	edges
1E	3.80	0.10	5	7	edges
2A	4.50	0.10	7	12	4.0 $\mu m$
2B	5.90	0.20	7	15	7.0 $\mu m$
2C	7.70	0.20	11	25	12.0 $\mu m$
3A	14.0	0.30	11	25	12.0 $\mu m$
3C	17.0	0.30	11	20	17.0 $\mu m$
3D	24.0	0.60	12	20	19.5 $\mu m$
3E	28.5	0.60	20	30	29.5 $\mu m$
4	32.0	0.60	30	35	edges
5A	11.8	0.01	FP tied to grating LW sections
5B	14.0	0.01	FP tied to grating LW sections
5C	17.0	0.01	FP tied to grating LW sections
5D	24.0	0.01	FP tied to grating LW sections
6	27.0	0.01	FP tied to grating LW sections

Notes

1: Key wavelength/bandpass combinations are chosen where the response of the detectors is at it's maximum and where the spectra of astronomical calibration sources are relatively featureless. There is one per band.

Flux reproducibility was checked by observing the standard HR6705 twelve times during various orbits. Fluxes between the various measurements agreed to within 6% for band 1, and 12 to 15 % for bands 2 to 4.

7.4.3 FP

The accuracy expected on the flux calibration of the FP are the same as that on the grating, but add an extra 1 to 20% due to wavelength dependant leakage.

Next: 7.5 Wavelength calibration Up: 7. SWS Calibration Previous: 7.3 Flux calibration

SWS Instrument & Data Manual, Issue 1.0, SAI/98-095/Dc