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7.6 Wavelength calibration accuracy

The goal was to achieve an accuracy of between 10 and 20% of a resolution element for the two gratings. An accuracy of 10% was hoped for the LW grating as this is used by the FP wavelength calibration.

The errors expected on the wavelength calibration of the grating are shown in Table 7.6. It lists the requirements on ISO, what has been achieved in ground tests and during operations.

**Table 7.3:** Grating Wavelength calibration errors.
	SW	LW
		$<40\mu$	$>40\mu$
Requirements	$<\lambda/10000$	$<\lambda/10000$	$<\lambda/10000$
Obtained on	<1 LVDT	<0.5 - 1.0 LVDT	2.0 LVDT
ground	$\lambda/10000$	$\lambda/12000-\lambda/20000$	$\lambda/ 6000$
Flight Achieved	1 LVDT	0.5 - 1 LVDT
	$\lambda/5000 - \lambda/12000$	$\lambda/8000 - \lambda/16000$

The limitation on the grating wavelength calibration accuracy appear to come from pointing, see section 5.10.

The wavelength calibration stabilisation was very good. A slight shift was seen in the wavelength calibration between PV and rev 450 of about 1 LVDT which was corrected by an update of the calibration files. Overall, the wavelength calibration of the SWS was kept accurate to within 1 LVDT. Further information on the wavelength calibration of the grating during the in-orbit lifetime or ISO can be found in the document `Wavelength Calibration' by Do Kester, Douwe Beintema & Edwin Valentijn, 2 July 1998.

For the two FPs relevant information concerning wavelength calibration accuracies is shown is shown in Table 7.6. Throughout the mission no wavelength shifts of more than 1 FP scan step were found, within the requirements listed in the table. More information on this can be found in the document `Status of SWS Fabry-Pérot wavelength calibration and instrumental profile' by H. Feuchtgruber, 5 May 1998.

**Table 7.4:** FP Wavelength calibration errors.
	SW-FP1	LW-FP1
Requirements	$<\lambda/100000$	$<\lambda/100000$
Obtained on ground	$\lambda/125000$	$<\lambda/125000-\lambda/200000$
Flight Achieved	$\lambda/10^5$	$\lambda/10^5$

Next: 8. The Pipeline and Up: 7. SWS Calibration Previous: 7.5 Wavelength calibration

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