Interactive Dark Current Subtraction 

This tool can be used indifferently with all LWS AOTs. Its use is critical to assess and improve the quality of the data. Fresh users must ask advise from instrument experts at RAL or IPAC.

Invoking the Interactive Tool 

The command to be given to start this tool and a description of optional keywords are given below:
; Example      ia_dark,dir='/user/iso/data/',tdt='44700574',/update
; Arguments    Name     I/O  Type     Description
;                       ------------------------------------------------------------
;                        tdt            I   string      the tdt number associated with the observation. For example if the tdt
;                                                             number is 44700574 then the routine will look for data files l*44700574.fits
;                                                             If not specified the routine assumes that files to be read are called
;                                                             l*.fit (viz the SOC processed files from the CD_ROM).
;                        dir             I  string       the directory from which the input files are to be read (and where output
;                                                              files will be written). It can be a relative or absolute path. e.g. dir='data/'.
;                                                              If not specified then the data are read from the user's curent working
;                                                              directory.
;                       /mc            I   keyword   If set, the routine reads the MC-FITS format data files from the
;                                                              ARC_DAT directory. Only expected to be used by the instrument team.
;                      /update       I   keyword  If set, it is assumed that the user will update a set of previously
;                                                              determined dark currents, and resubtract them from the original SPD data. It is
;                                                              no longer mandatory (starting from version 2.1 of this routine, delivered with LIA
;                                                              7.3) to re-analyse all 10 detectors.

The following Widget will pop-up (the plot area will be empty; button 'LW3' has been hit in the example below):


A Description of the User Interface 

The black area is the main plot window, with its control buttons (Replot, Unzoom, Hardcopy) on its right side. In the example above the 'SW3' button has been hit (see below) and the data (not yet calibrated either in flux or wavelength) collected in the AOT for detector LW3 are shown as a function of time (ITK) in Log-Log scale; the user can zoom in by clicking and dragging using the left button of the mouse. The data are plotted as a function of time and the Y-axis is linear, and they refer to both internal calibration and scientific data; in particular:
  1. two sets of red points: this is the dark current measurement  done at the beginning of the AOT; remember that the second set of red points IS NOT GOING TO BE USED in the processing as the detectors at this stage are still recovering (check that the signal is decreasing with time) due to the strong signal received from the Illuminators.
  2. another set of red points may appear if the Illuminator Flashes have been done in the OLD scheme. This set is not used in the processing (see previous point).
  3. a set of white points: these are the actual scientific data. In the case shown above they come from an L01 AOT ; the time series of scans accumulated is clearly discernible. Remember that at this stage the data is not yet calibrated so each scan resembles the instrument transmission profile (referred to as Relative Spectral Response Function - RSRF) for that detector.
  4. another two sets of red points: this is the DC measurement done at the end of the AOT (see point 1).
  5. a final set of red points (see point 2).
A smaller plot area is located on the top-right of the main window. This area is not interactive, and presents the full (calibration + scientific) data set plotted with Y-axis in logarithmic scale and as a function of time in seconds (so you can have an idea of the duration of your observation). While the appearance of the main plot area will change as you go along with the processing, this smaller plot will always stay there. In addition to the data types shown in the main plot area (and described above), the smaller plot also show as green points the data taken while the Illuminators were switched-on.

Below the plot area the following groups of buttons are visible from left to right:

At the bottom of the Widget there is a text area where messages are prompted to the user about actions to be performed and displaying information. Information about the properties of each data point is also reported in this message window; just click on the main plot area with central button of the mouse, and the characteristics of the nearest data point will be reported: time, photocurrent, raster point ID, detector number, line number and scan number.

The DC estimate & subtraction Cookbook 

The following is the list of the steps that should be followed for the DC estimate & subtraction.
Document by: Sergio Molinari, IPAC/Caltech
Last Update: Jan 19, 1999