2.9 Current Caveats on SWS Data Products

 

Excellent results have been obtained from SWS, with very low noise measured, especially in band 1 . The basic Off-Line Processing (OLP ) products can be used in the analysis of your data, with special processing only needed for exceptional cases.

There are, however, some points to be aware of concerning the data products from OLP Version 6.1, primarily that the AOT testcases do not cover the entire parameter space of SWS observations and there are sets of input data for which the code gives non-optimal results. Many AOT testcases have been selected to test OLP V6, and all have been reduced and checked. However, these do not cover the entire range of possible SWS observations. Thus while there is no reason to believe the pipeline does not work correctly, it has not been proven correct for all input data.

Cases for which the pipeline is not optimised, and is therefore known to fail include:

1

Memory Effects 

Memory effects are not accounted for in OLP V6 and so may affect the data. See section 5.5, ``Memory Effects'', for a discussion of this.

2

Low flux cases.

Cases where the continuum is weak can cause problems for dark current subtraction, especially when memory effects  are included. When the amount of flux falling on the detector  is small, there may be cases where (flux plus dark current  plus noise ) is less than the (dark current plus noise). Dark current subtraction then results in negative fluxes in the AAR product. The limiting flux for this can be quite high, e.g. of the order of a few Jy for band 2 , and the actual level might be determined by the scan rate and/or number of scans at a required S/N. See section 5.2, ``Detector Noise & Dark Current'' for a discussion of dark currents and noise.

Such problems can also manifest themselves as distortions in the spectral energy distributions of low flux sources, especially noticeable in AOTs 1 & 6  .

3

Fast SWS01 AOTs 

There are two possible problems with fast AOT 1's.

-

Datapoints near unresolved lines marked as glitched . This is because the grating is moving during one reset interval, which is not the case for the other AOTs. If the grating scans over an unresolved line the slope of the detector output for one reset interval will deviate from a straight line, leading the OLP software to conclude a glitch is present. See section 8.5, ``Glitch removal effects on spectral lines in fast speed AOT1 observations'', for more information on this.

-

The calculated flux for a line depending on actual wavelength of the peak of the line. This is because the observed fringe pattern is smeared version of the Relative Spectral Response Function (RSRF), due to the grating moving.

The result of both these effects is that the net line flux and the line profiles can be appreciably affected.

4

Flux/aperture points

SWS has three apertures (four if the virtual Fabry-Pérot aperture is counted), and observers of extended sources  who switch between these apertures must be aware that their reported fluxes will jump as the aperture changes.

5

RPID  passed to AAR

SWS has no raster AOTs and therefore does not use the Raster Point ID passed in the headers of the various data products. The only exception to this is observations of Solar System Objects (SSO's), which are tracked using raster observations and therefore have varying RPIDs in the data.

A problem exists in the passing of RPIDs in the pipeline. The RPID is set to 1 (1..n for SSOs) in the SPD data products, but this is not carried over to the AAR. The AAR products contain 0 in the RPID field.

6

RSRF files

While the RSRF files have been greatly improved sections 6.5.1, ``Present quality of RSRFs'' and 6.5.6, ``Spurious Spectral Features seen in Band 2C'' should be read.

7

OLP V6 AAR data products for AOT 6 might not contain all wavelength scans

One of the changes made to the SWS pipeline for OLP V6 was the deletion of all additional data (e.g. reference scans, dark currents, FP data for grating AOTs) from the AAR products. This was achieved by not passing such data from the SPD to the AAR data products.

However, it has been recently found that, for a few AOT 6 (wavelength range scans) not all the wavelength scans are passed into the AAR from the SPD. This means that the AAR contains all the wavelength range requested, but not all the scans to reach the required S/N ratio.

Observers can determine if they suffer from this problem by examining the observation by time keys. If a plots of wavelength against time for SPD and AAR shows that some requested wavelength ranges are present in the SPD at a certain time (ITK) but no such wavelength is present in the AAR for that time then data is probably missing. Observers should then contact helpdesk@iso.vilspa.esa.es for further guidance.