The LWS Interactive Analysis (LIA)

The LWS Interactive Analysis (LIA, latest version found here) is a software package designed to allow the user to inspect, reprocess and recalibrate their LWS data with the possibility of interactively customizing the various data reduction stages to their particular set of data. The LWS data shipped by ESA are produced by an automatic reduction pipeline (Off-Line Processing or OLP); the calibration accuracy of these products is generally good within 20% on average.  LIA allows the user to check the quality of his/her scientific and calibration data; besides, there are cases where the quality of the data may be  worse than normal for a series of reasons, and this is where LIA also enters into play.

LIA is a joint developement of the ISO-LWS Instrument Team at Rutherford Appleton Laboratories (RAL, UK - the PI Institute) and the Infrared Processing and Analysis Center (IPAC/Caltech, USA).  Version 8.0 of LIA,for ISAP 1.6 and later versions users only, has been released on May 4, 2000 and is available for you to download.

Three classes of routines are available under LIA:
 

• INSPECT_SPD:Displays SPD results before and after the application of the standard deglitching algorithm. A two panel plot with GUI appears on the screen. The user can display the data in LWS instrument time key (ITK) units, or in seconds from the start of the observation. The display can be zoomed to look in more detail at the effects of the glitches. Various glitch statistics are reported.

• INSPECT_IPD:Displays illuminator flash sequence data for closed flashes. The dark currents as measured at the start of each closed flash are also displayed. The user can get an impression here whether the calculated dark currents are in line with the latest best estimates from the PV phase/Handover flashes. Dark currents in flux units are also listed.

• INSPECT_DRIFT:Displays the SPD results before and after the detector responsivity drift correction is applied. Only suitable for displaying grating LWS01 data.

• INSPECT_ABS:Displays the absolute responsivity correction factors (the ratio of closed illuminator flash data referenced to the illuminator flash data at the time of flux calibration observation).

• INSPECT_RSRF:Displays the SPD data (LSPD file), the Relative Spectral Response Function (RSRFs in the LCGR file) and the AAR data (LSAN - the LWS Auto-Analysis results file) in wavelength units. A two panel plot with GUI appears on the screen. The upper panel shows the LSPD and the RSRF (normalised to the LSPD data at the grating rest position). Only suitable for grating AOTs, i.e. LWS01 and LWS02. This routine is expected to be useful for verifying the existence, or otherwise, of some of the more interesting features present in the LSAN file.

• INSPECT_RASTER:Displays the pointing directions for raster observations, as well as for single pointings.

• CALC_ERRORS (for Grating data only): As the errors from the SPD stage to the AAR stage are not being propagated, this routine provides a means for estimating the continuum/line flux uncertainties by combining the systematic instrument errors with the statistical errors computed for the target by ISAP. Only suitable for grating AOTs, i.e. LWS01 and LWS02.

• IA_DARK: Dark Current (DC) subtraction. The estimate and subtraction of the DC is a critical point particularly for faint sources and for FP observations in general; in these cases signal received is at the detector noise level and hence an incorrect DC subtraction may lead to negative photocurrents; as these are later divided by the instrumental transmission profiles (RSRF) the resulting calibrated spectrum will be potentially difficult to interpret.

• IA_DRIFT: Responsivity Drift (RD) correction. It is well known that the LWS detectors present a temporal drift in responsivity . Its removal is based on the evaluation of the time series of averages of  spectral scans; the obvious requirement is that the scan range  and  the source's intrinsic signal does not vary. The sole case in which this requirement is met is with single pointing L01 AOTs (fortunately the majority of LWS observations).

• IA_ABSCORR: Absolute Responsivity Correction (ARC) factors. ARCs are the values used to scale the detector responsivities to the values measured when the  LWS primary calibrator (Uranus) was observed, and on which the instrumental transmission profiles of the instrument have been derived.

• FP_PROC (for FP data only): An interactive tool which produces calibrated LSAN files for FP observations. It can handle both L03 and L04 AOTs and allows important functions such as i) realignment of scans in an L03 AOT, ii) correction for incorrect grating positioning in an L04 AOT, or iii) reprocessing using either a default or a user supplied set of dark currents (for this reason FP_PROC also belongs to the third routine class - see below).

 
• GUI_FPMF (for FP data only): This tool is written for Interactive FP Multi Gaussian Fitting for a single "blended" line. The input data should be averaged LWS spectra written out a single- line data produced by ISAP-Average.

 
• DEFRINGING_GUI : This tool is written for interactive defringing of LWS data (L01, L03 and L04).

• SHORT_AAL (for Grating data only): utilitity to recalibrate an L01 or L02 observation. It essentially performs wavelength and flux calibration using interactively reprocessed data (see above) as input. It can also perform a general reprocessing (without using the interactve routines) using default settings which are different from those used by the OLP. There is a number of keywords which need to be used when calling SHORT_AAL depending on which of the interactive routines have used. The following table summarizes all the possible combinations:

 

Interactive Dark Current  subtraction ?  (IA_DARK)	Interactive  Responsivity Drift division ? (IA_DRIFT)	Interactive Absolute Responsivity Correction estimated ? (IA_ABSCORR)	KEYWORDS to SHORT_AAL
N	N	N	no keywords
N	N	Y	/newabs
N	Y	N	/newdrift
N	Y	Y	/newdrift,  /newabs
Y	N	N	/newdarks
Y	N	Y	/newdarks,  /newabs
Y	Y	N	/newdarks,  /newdrift
Y	Y	Y	/newdarks,  /newdrift,  /newabs


 
• FP_PROC (for FP data only, same routine as in second class): utilitity to recalibrate an L03 or L04 observation. It essentially performs wavelength and flux calibration using interactively reprocessed data (see above) as input. It can also perform a general reprocessing (without using the interactve routines) using default settings which are different from those used by the OLP. It also allows interactive manipulation of the data (for this reason FP_PROC also belongs to the second routine class, see above).

A number of keywords should be used when calling FP_PROC, depending on which of the interactive routines have previously been used. The above table, valid for SHORT_AAL, also applies for FP_PROC with the exception of the keyword relative to the DRIFT correction (which only applies to grating LWS01 AOT).

Data reduction & analysis recipes are available at http://www.ipac.caltech.edu/iso/lws/lia/lws_recipes.html.

LIA is a relatively new package which is proving extremely helpful to understand and improve the quality of the calibrated LWS spectra. Due to its novelty, though, bugs are being found, especially thanks to the input from the user community. Some of these may be serious in the sense that for certain AOTs and revolution ranges, the files produced through the LIA interactive routines may be incorrect. Identified problems are being fixed. More details about  the problems and corrected versions of the routines are available here
 

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Last modified - 15 September 1999