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 5.7.

3

Memory effects, see section 5.5, ``Memory Effects''.

4

Dark current subtraction. See section 5.2, ``Detector Noise & Dark Current''.

7.4.2 Grating

The errors expected on the flux calibration of the grating  are shown in Table 7.1. It lists the requirements on ISO, the accuracy achieved in ground tests and the flight objective.

 

Absolute Cross- Beam Pointing Non- Overall

fast slow diff jitter linearity w/o memory

Requirements -

Ground tests - -

Flight objective -

 

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 7.2.

 

Band Bandpass Worst Location

Wavelength accuracy of worst %

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

2B 5.90 0.20 7 15

2C 7.70 0.20 11 25

3A 14.0 0.30 11 25

3C 17.0 0.30 11 20

3D 24.0 0.60 12 20

3E 28.5 0.60 20 30

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 .