05-May-1998

Status of SWS Fabry-Perot wavelength calibration and instrumental profile
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Author: H. Feuchtgruber


0) Introduction and Purpose

This report represents an update of the respective part of the PV report on
the same topics. Results of all dedicated internal measurements are included.
For external calibrations, a set of observations is included, demonstrating the
validity of the internal calibration throughout the mission. The calibration
is within specification, however of course, much more data on external sources 
exists which could be used to confirm or improve the achieved accuracy, but 
time did not yet allow to go for that. 


1) FP parallelisation and instrumental profile

The parallelisation of both SWS FP sections is considered to be at an optimum
for the specified scanranges throughout the whole mission. It was done once in
PV and then only checked via the regular CAP6332 measurements. The instrumental 
profile determination CAP, which is also based on internal calibrator lines, 
gives same spectral resolution values as specified in the observers manual.

	FP-SW: R=41900 +- 7000 at 24.61 micron
	FP-LW: R=29900 +- 3600 at 24.78 micron

The instrumental profile on external sources appeared to be difficult to 
characterise, because there are hardly any strong sufficiently narrow lines 
available. The spectral lines observed on the planets were either saturated or
broadened by their rotation of ~10 km/s.
However lower limits for the FP resolution could be determined by observations
of targets with intrinsic line widths of less or equal 10-15 km/s:
(Examples from selected H2O, CO2 and H2 lines)

Lambda		  13.89 	17.03	 	29.83		 40.69
Lambda/DLambda    >22000	>25000		>30000		>30000

There might be a couple of more targets in the ISO data base which would be 
suited to get even better estimates for the FP resolution, but there is no
simple way to find those observations from scratch. Given the above 
restrictions, no indications have been found so far that the FP instrumental 
profile deviates from an Airy (= Lorentian) shaped profile. 


2) Internal wavelength calibrations

The measurements of the internal wavelength calibrations of the SWS FP-SW and 
FP-LW are completed and the data is reduced. The resulting accuracy was as 
expected of the order of 1 FP scan step at 24 micron. 

The measurements of the internal calibration with respect to stability indicate
no change of the wavelength scale until the end of the mission within a 
fraction of a step of the FP drive. Checks were done during PV and in routine 
phase. A comparison of the later ones and the ones during PV show no 
significant deviation. The results of two representative measurements are 
plotted in Figure 2. The vertical bars indicate the borders of the valid
scan ranges for the two FP sections. It is difficult to interpret the small 
(<= 0.5 steps) deviations throughout the mission (Figure 1), as the grating 
wavelength calibration has shifted a bit in time. This means that the relative
intensities of the various peaks of the FP calibration source have changed as 
well (see Figure 2), being transmitted through a slightly different position 
within the grating profile, causing possibly little shifts in the peak 
determination. However all shifts found are well within the specified 1 FP 
step accuracy.    


Figure 1.



Figure 2.


3) External wavelength calibrations  

The external wavelength calibration suffers from the lack of accurate wavelength
standards needed by the FP. However the few lines where rest wavelength and
radial velocity is known accurate enough, verified the internal calibration 
and the resulting accuracy is, as specified in the observers manual, better 
than 1/3 of a resolution element. 

External standards observed (rest wavelengths in micron):

FP-SW				FP-LW

Line	 Lambda    Source	Line	 Lambda    Source

NeII     12.81355  NGC7027     	H2 S(0)  28.21883  Saturn
MgV      13.5212   NGC7027	H2 S(0)  28.21883  Jupiter
CO2      13.89 (Q) W-Hya     	H2O      29.83671  W-Hya
CO2	 14.98 (Q) Saturn	H2O      31.77205  W-Hya
H2 S(1)  17.03484  Jupiter     	HD R(2)  37.70155  Jupiter
H2 S(1)  17.03484  Saturn      	H2O      39.37496  Jupiter
H2O      20.66180  NML-Cyg      H2O      40.69087  W-Hya
				H2O      43.89349  Jupiter
				H2O      44.19473  Jupiter

Of course, much more lines with reasonably good knowledge of rest wavelength 
and radial velocity have been observed during the mission, but so far no 
attempt has been made to reduce and combine all these observations into a
single consistent set of data describing the SWS FP wavelength calibration in
a more statistical way. Given the fact that the calibration is already well
within specification, this task has low priority.  


4) Further documentation on that subject

	PV report
	Special A&A issue, A&A, 315, L60