We describe here the performance of the instrument together with the
calibration processes conducted to convert the
raw digitised signal into wavelength and absolute flux units, as well
as those designed to derive any instrument characteristics needed to
assess the quality of the data and the performance of the instrument.
Some of the calibration processes described here are meant to derive
the calibration files which are used in
the pipeline processing; others are used to characterise the instrument
behaviour.
The calibration of the data comprises different steps:
- The conversion of the raw digitised signal from the detectors and
mechanisms into photocurrent at each mechanism position is termed
Derive-SPD (Standard Processed Data)
(see Section 4.3); it was designed during
ground testing and verified in the early phase of the ISO mission.
The first stage of processing therefore requires no
astronomical calibration and will in general not be discussed here.
- The conversions of mechanism position into wavelength on one hand,
and of photocurrent into absolute flux units on the other hand, do
require astronomical calibration. These conversions are performed as
far as possible in Auto-Analysis (AAL - see Section 4.4).
However, the pipeline processing only
derives the true flux for point like sources at the centre of the
LWS field of view because it makes no attempt to
correct for the angular response of the instrument, (i.e. the beam
shape, see Section 5.9), or for any
anomalous instrument behaviour such as the
channel fringing seen on the spectra of extended or
off-axis sources. The fringes and other unwanted effects
are described in Chapter 6 and
can be corrected for by using dedicated
interactive software in LIA (LWS Interactive Analysis - see
Section 8).
The accuracy achieved with the calibration and the derived instrument characteristics are given in the tables included in this chapter. Typical accuracy numbers are summarized in Table 5.1.
| Mode | Wavelength accuracy | Photometric accuracy |
| Grating | 0.25 resolution elem. | point sources: 10%-20% depending on flux and detector |
| (L01 & L02) | 0.07  m for SW1-SW5 |
extended sources: 50% |
| 0.15 m for LW1-LW5 |
||
| Fabry-Pérot | FPS: 6 km s |
line intensity: within 20% |
| (L03 & L04) | FPL: 13 km s |
continuum: to be scaled to the grating flux |
For a list and a description of the calibration files, we refer to Section 7.3.