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Next: 6. Data processing Up: ISOLWS DATA USERS MANUAL Previous: 4. Instrument Characteristics

Subsections

   
5. Calibration and Accuracy

   
5.1 Calibration philosophy

The calibration of the LWS instrument can be split into two main areas: the instrumental characteristics and the astronomical calibration. In this context, the instrumental characteristics calibration are those properties of the instrument which need to be known in order to extract the astronomical information from the data stream. These are mainly used in Derive-SPD (cf. section 6.3).

Astronomical calibration, on the other hand, is needed to convert from the internal "engineering" units of the instrument to astronomical units. This is mainly used in Auto-Analysis (cf. section 6.4).

The instrumental characteristics calibration can be divided into:

The astronomical calibration includes, for both grating and Fabry-Perots:

   
5.2 Instrument characterisation procedure

   
5.2.1 Detector characteristics

Debiasing parameters
These are measured with the internal illuminators, using different compensation voltages at the start of the ramps. The compensation levels induce different effective bias levels for the ramps. From the second order polynomial fits to the data the debiasing parameter a (see section 4.2) can be determined.
Detector Dark Current
The dark currents can only be measured together with any current induced by background or stray radiation. To minimize this background radiation the measurement is done by placing a Fabry-Perot - deliberately made non-parallel - in the beam. Tests have shown that this gives the lowest background on the detectors, the FP acting as a highly efficient blocking filter. ISO is then pointed to a relatively clear part of the sky to get minimal straylight from off-axis sources. The observation is done with several bias levels. The grating is scanned to look for spectral signatures in the dark current, and the illuminators are used to check for responsivity changes during the dark current measurement.

   
5.2.2 Electronics characteristics

No need for in-orbit characterisation could be identified. All characteristics are determined in the laboratory and while building the instrument. The important characteristics are:

During the Spacecraft Commissioning Phase (SCP) several tests were done to detect possible large changes in these values that would point to problems with the instrument.

   
5.2.3 Fabry Perot characteristics

The optimum offsets for the Fabry-Perot etalons for FPS and FPL are determined by observing an unresolved line at several offset values around the expected optimum values (offset mapping). This gives a set of line spectra. For each of these spectra the FWHM of the line and the peak heights are determined. From the contour maps of the FWHM of the line versus the two FP offset values (for the coils of one FP) the position of the minimum FWHM is determined. The offset values corresponding to this minimum are optimum for parallelism.

   
5.2.4 Optical characteristics

The beam pattern of the instrument is determined by taking full grating spectra over a raster map of a strong source. The result is a map of the beam-pattern of the instrument as a function of wavelength.

   
5.3 Astronomical Calibration procedure

   
5.3.1 Wavelength calibration

5.3.1.1 Grating calibration

Grating mode calibration
This is done using first a full grating scan on a source with known lines to identify the lines suitable for wavelength calibration, those that are strong enough to give good signal-to-noise and that are narrow enough to determine an accurate line centre. Individual scans of these lines are then made to provide accurate line positions. These line positions are fitted to the known wavelengths with a third order polynomial to give a grating calibration that is adequate for the low resolution mode of the instrument.
Spectral element
The size of the grating spectral element is determined using the mixed mode observations. Detailed FP scans are made with the grating at a fixed position, giving a very detailed profile of the grating response function: from this, the size of the spectral element (Full Width Half Maximum) is determined. The observation is done for several detectors and grating positions to get the wavelength dependence of the profile and spectral element.

5.3.1.2 Fabry-Perot calibration

Wavelength Calibration
The wavelength calibration of the Fabry-Perots is carried out by observing strong, narrow lines with known wavelengths. The centres of these lines are determined as a function of the commanded position of the Fabry-Perot. The corresponding order of interference and etalon gap are calculated abd the gap is fitted as a third order polynomial in commanded position.
Spectral element
The size of the Fabry-Perot spectral element should be determined using the wavelength calibration observations. The Fabry-Perots are scanned over lines known to be narrow, on sources with a reasonably well known velocity dispersion. The Fabry-Perot profile is determined by deconvolving the observed lines shape with the expected line shape given the source structure and velocity dispersion. From this the size of the spectral element (Full Width Half Maximum) is determined. The observation is done for several lines to get the wavelength dependence of the profile and spectral element. Due to the lack of suitable sources for this calibration. Even the source G0.6-0.6, which has very narrow lines has proven to give difficulties for the determination of the FP spectral element, since the velocity dispersion of the source is close to the resolution of the FPs. Investigations are still ongoing to improve the methods for obtaining the FP resolution from data that has been taken in orbit (instead of in the lab with laser lines).

   
5.3.2 Relative Response and Flux calibration

5.3.2.1 Grating relative response

The absolute flux calibration is done by directly comparing the detector photo current of a standard source with the known (model) flux (corrected for the optical characteristics of the telescope and the LWS instrument).

The LWS has been calibrated by observing Uranus, a source for which a good model exists and which is point-like in the LWS beam. At each wavelength the expected flux density (power per unit area) from Uranus, within the spectral resolution element, is calculated from the model and the corresponding photocurrent is measured. This calibration therefore provides a direct relationship - for a point source - between the photocurrent and the flux density at the entrance pupil of the ISO telescope. This relationship is then used to assign a flux density to the observed photocurrent from any other source.

The observations for the flux calibration are repeated for several bias levels. The grating relative response needs to be known before the FP relative response calibration and the FP transmission calibration can be performed.

For the photometric calibration and the relative response calibration several sources were used during PV phase and during the routine calibration observations. Table 5.1 gives the sources for the different calibrations.


 
Table 5.1: Sources used for the photometric calibration and the relative response calibration of the LWS grating and Fabry Perot subsystems. The primary source for the grating flux calibration is Uranus, the other sources are observed regularly for monitoring purposes.
Source Type Observation
Absolute Flux Calibration and monitoring
Uranus planet Fixed grating position On and Off source and full scan
Neptune planet Fixed grating position On and Off source and full scan
Ceres astroid Fixed grating position On and Off source
Pallas astroid Fixed grating position On and Off source
Vesta astroid Fixed grating position On and Off source
Arcturus star Fixed grating position On and Off source
Aldebaran star Fixed grating position On and Off source
$\gamma$ Dra star Fixed grating position On and Off source
S106 HII region fixed grating and full scan
G298.288 HII region fixed grating and full scan
NGC6543 PN full grating scan
NGC7027 PN full grating scan
Relative response calibration
Uranus planet End to end grating scans (extended range)
FP Relative Response and transmission Calibration
Jupiter planet FP scans with fixed grating (mixed mode)
 

5.3.2.2 Fabry Perot relative response

For the Fabry Perot response the FP is scanned with a high oversampling with the grating at a fixed position (mixed mode observations). Again here the source spectrum is removed (by means of a model or a grating scan). Finally the profile is normalized to the absolute transmission at a given wavelength (the FP transmission is determined in a separate procedure, see below).

5.3.2.3 Fabry Perot transmission

The FP transmission is determined by using scans of strong lines with both the grating and the Fabry Perot. Comparing the output of the detectors for these scans of the same line for grating and Fabry Perot gives the the Fabry Perot transmission at the wavelengths of the lines.

   
5.4 Calibration sources and achieved accuracy

   
5.4.1 Spectroscopic calibration

5.4.1.1 Grating calibration

The wavelength calibration of the LWS grating consists of the set of detector angles and the coefficients of the third order polynomial fit of the LVDT value versus grating angle. Since version 6 of the OLP pipeline, the detector angles given in Table 5.2 are used. The shifts in this table indicate the change of the angles with respect to the ones determined on the ground. The angles have not changed in version 7 of the OLP pipline.


 
Table 5.2: The detector angles derived from the grating wavelength calibration, and the difference between the in orbit and ground based angles for each of the ten detectors.
det. SW1 SW2 SW3 SW4 SW5 LW1 LW2 LW3 LW4 LW5
angle 67.80 58.74 49.71 40.73 31.72 63.26 54.29 45.27 36.275 27.32
shift -0.1 -0.01 0.0 0.0 +0.02 +0.02 +0.01 +0.01 +0.04 +0.04
 

During the routine checks of the grating wavelength calibration it was found that a change in the calibration occurred in revolution 346. Therefore it was decided to use a timedependent wavelength calibration consisting of two sets of parameters, one valid for observations performed before revolution 346 and one for observations performed in revolution 346 and after. The calibration was derived using 7 fine structure lines observed with at least two detectors each in a number of different sources (mainly planetary nebulae and HII regions). Table 5.3 gives an overview of the lines and sources that were used for the calibration.


 
Table 5.3: The sources and emission lines that are used for the wavelength calibration of the grating and of both FPs. For the FPS calibration all lines were observed in three FP orders. For the FPL the lines were observed in two orders. Type indicates the source type (PPN: Proto Planetary Nebula; PN: Planetary Nebula).
Source Type Lines
Grating wavelength calibration
NGC6543 PN 52, 57, 63, 88 $\mu$m
NGC6826 PN 52, 57, 63, 88 $\mu$m
NGC7027 PN 52, 63, 88, 145, 158 $\mu$m
G298.228 HII region 52, 57, 63, 88, 145, 158 $\mu$m
IRAS15408-5356 HII region 52, 57, 63, 88, 145, 158 $\mu$m
NGC6302 HII region 52, 57, 63, 88, 122, 145, 158 $\mu$m
IRAS23133 HII region 63, 145, 158 $\mu$m
FP wavelength calibration
NGC7027 PN 63.184 [OI], 157.741 [CII]
NGC3603 P1 HII region 51.814 [OIII], 57.317 [NIII], 157.741 [CII]
NGC6826 PN 51.814 [OIII], 88.356 [OIII], 157.741 [CII]
G0.6-0.6 HII region 51.814 [OIII], 57.317 [NIII], 63.184 [OI], 121.898 [NII]
    145.525 [OI], 157.741 [CII]
 

The accuracy of the grating wavelength calibration has been checked by measuring the central wavelengths of the lines observed in a large number of auto analysis results from observations of NGC7027, NGC6543, S106 and W Hya. This check has shown that the wavelength calibration is better than 1/4 of a resolution element of the grating (i.e. 0.068 $\mu$m or about 330 km/s at 60 $\mu$m, and 0.145 $\mu$m or about 290 km/s at 150 $\mu$m). The width of the resolution element was also determined from these observations. For the SW detectors it was determined to be 0.27 $\mu$m and for the LW detectors 0.58 $\mu$m. (The nominal values derived from ground based tests were 0.29 $\mu$m and 0.60 $\mu$m respectively.)

The accuracy that is achieved for the wavelength with the grating is given in Table 5.4.


   
Table 5.4: Accuracy of the wavelengths for an LWS spectrum. These accuracies are based on actual measurements. 85% of all measurements were within these limits for the accuracy. Note that the accuracy for the FPs can be improved using the new calibration coefficients given in section 7.7.
mode Accuracy
grating calibration $\sim$25% of a resol. element
  0.068 $\mu$m for SW detectors
  0.145 $\mu$m for LW detectors
FPS calibration $9 \cdot 10^{-4}$ $\mu$m RMS
  scatter in the fitted calibration
FPL calibration $2.7 \cdot 10^{-3}$ $\mu$m RMS
  scatter in the fitted calibration
FP absolute accuracy $\sim$10 km/s or 1/3 of
  a resolution element

5.4.1.2 Fabry-Perot calibration

The wavelength calibration of the LWs Fabry-Perots was derived using observations of 4 sources with known velocities which show strong lines that span a sufficient range in FP gap and wavelength values to adequately sample the parameter space. The cetroid of the measured response is taken to corerspond to the line center (since lines are clearly not gaussian in shape, a gaussian fit to the lines can not be used). The sources and lines that were used are given in Table 5.3.

The internal accuracy of the calibration is given by the RMS noise on the polynomial fits done to obtain the calibration coefficients . This was determined to be $9\cdot10^{-4} \mu$m for FPS and $2.7\cdot10^{-3} \mu$m for FPL.

Another check for the accuracy of the FP wavelength calibration was done with the weekly calibration monitoring observations. A total of 7 lines (3 on FPS and 4 on FPL) in 7 sources was observed regularly with an L04 AOT during the calibration time. The sources and lines that were used are listed in Table 5.5.


 
Table 5.5: The sources and lines in this table were used for the monitoring of the FP wavelength calibration.
Source Lines
G0.6-0.6 51 ([OIII]), 57 ([NIII]), 88 ([OIII]), 122 ([NII]), 157 $\mu$m ([CII])
G36.3-0.7 51 ([OIII]), 88 $\mu$m ([OIII])
NGC3603 57 ([NIII]), 88 $\mu$m ([OIII])
NGC6302 57 ([NIII]), 157 $\mu$m ([CII])
NGC7023 63 ([OI]), 145 ([OI]), 157 $\mu$m ([CII])
NGC7027 51 ([OIII]), 63 ([OI]), 145 ([OI]), 157 $\mu$m ([CII])
NGC7538 51 ([OIII]), 63 ([OI]), 88 ([OIII]), 157 $\mu$m ([CII])
S106 63 ([OI]), 157 $\mu$m ([CII])
 

These monitoring observations have been used to determine RMS errors for the determination of the wavelengths with the FPS and FPL. These errors are given in Table 5.6. The errors are for almost all lines smaller than 10 km/sec. It should be noted however, that for the FPL, these errors tend to be systematic the error gets bigger as the gap between the FP etalons gets bigger (the 157 micron line is at the smallest gap value, the 145 micron line at the largest). This points to a small error in the FPL calibration, especially in the higher order coefficients, that are not well determined.


 
Table 5.6: The RMS errors in the wavelengths determined from the FP monitoring observations.
Line RMS error
  $10^{-3}~\mu$m km/sec
FPS
[OIII] 51.8 0.95 5.5
[NIII] 57.3 0.72 3.8
[OI] 63.2 0.87 4.1
FPL
[OIII] 88.4 1.5 5.1
[NII] 121.9 2.2 5.4
[OI] 145.5 6.6 13.6
[CII] 157.7 3.0 5.6
 

Independently the accuracy with which the wavelength of a line can be determined was checked using a set of observations of a number of objects. For NGC7023 the central wavelengths of three lines (63, 145, 157 $\mu$m) were determined for a number of repeated observations. The velocities derived from repeated observations of the same line are equal to within 10 km/sec, showing that the accuracy of the calibration with respect to repeated observations of the same source is better than 10 km/sec.

Further tests, using observations of sources with known velocities, show that the velocities derived from the LWS FP observations are within 10 km/sec from the values quoted in the literature. Also the difference between velocities determined with FPS and FPL is smaller than 10 km/sec. We thus conclude that the accuracy of the LWS Fabry Perot wavelength calibration is 10 km/sec, or one third of the LWs spectral resolution element.

The accuracy that is achieved for the wavelength with the FPs is given in Table 5.4.

   
5.4.2 Photometric calibration

5.4.2.1 Grating calibration

The accuracy of the photometric calibration is determined by a number of factors:

All these factors together lead to a photometric accuracy for LWS grating mode spectra of 10% between repeated scans on the same detector (this is mainly due to the effect of responsivity changes), and 30% between adjacent detectors (mainly due dark background removal problems for faint point sources and to source extend for extended sources).

5.4.2.2 Fabry-Perot calibration

For the Fabry-Perot mode, the photometric accuracy was determined by comparing the integrated line fluxes observed with the FP with the fluxes observed with the grating or line fluxes published in the literature. The sources and lines are given in Table 5.7. It was found that for strong lines accuracy is typically better than 30%. For faint lines however, the FP fluxes can be off by almost a factor 2. This is mainly due to the removal of the darkcurrent which is known to be problematic for low signal levels (see also section 7.13 and 7.15).


 
Table 5.7: The sources and lines in this table were used for the determination of the photometric accuracy of the Fabry-Perot data.
Source Type Lines
NGC6543 PN 57, 88 $\mu$m
NGC7027 PN 51, 63, 145, 157 $\mu$m
NGC6357I HII region 52, 57, 63, 88, 145, 157 $\mu$m
M82 Galaxy 63, 88, 122, 157 $\mu$m
 

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Next: 6. Data processing Up: ISOLWS DATA USERS MANUAL Previous: 4. Instrument Characteristics

ISOLWS Data Users Manual, Issue 5.0, SAI/95-219/Dc