Description of Focal Plane Geometry PIA Software Modules and Tools

16.1 Introduction

The software for the analysis of the focal plane geometry data for the ISOPHOT instrument splits into three parts corresponding to the following tasks:

  1. Analysis and consistency check of Focal Plane Geometry Calibration file
  2. Analysis of data from FPG map
  3. Analysis of data from FPG cross scans

The purpose of this little document is to describe the usage and architecture of the PIA modules available for these tasks. As these s/w tools are for expert use only and are in general not useful for the astronomical user, the contents of this chapter might differ in style and contents from the rest of the PIA User Manual.

16.2 The Focal Plane Geometry Calibration File

The details of the contents and layout of the Focal Plane Geometry Calibration (FPGC) File is given in "ISO OAD/SOC Auxiliary Data ICD" [4], on pages 31-33. The software described here is based on the descriptions given there and the reader is referred to this document for any detailed questions related to the definition of the FPGC file.

The PIA software for the analysis and consistency check of the FPGC file comprises the following modules: fpgc_manager.pro, read_fpgc.pro and anal_fpgc.pro.

16.2.1 fpgc_manager.pro

The fpgc_manager programme provides a user interface for the analysis of Focal Plane Geometry Calibration Files. After entering the IDL as for a PIA session, i.e. executing the envi shell script and the startup routine, but without actually entering the PIA, the fpgc_manager programme can be started from the IDL prompt:

IDL> fpgc_manager

This brings up a menu called "FPGC Data Selection" allowing the user to select for which sub-instrument data shall be analysed. The File pull-down menu offers the options to Select FPGC file, Analyse FPGC file and Quit. The Help button enters the PIA Help menu (not yet implemented).

The Select FPGC file menu point open a standard file selection menu, filtered for the selected FPGC file type, in order to allow the user to select the file he/ she wants to analyse.

The Quit menu point ends the fpgc_manager programme and returns to the IDL prompt.

The Analyse FPGC file menu point starts the actual FPGC analysis process after the selection of a FPGC file, i.e. it calls the module anal_fpgc and displays the information it returns. This results in three window to appear on the screen, two PIA_XPLOT (cf. "The PIA_XPLOT Menu" on page 53) windows and one text output widget. The first plot window contains the raster data from the FPGC file overlaid with the ideal raster as would be expected from the FPGC header. The second plot window shows the euclidic distances between the ideal raster positions as derived from the FPGC header and the actual positions from the FPGC data records. The distances are plotted for every raster leg individually, allowing the user to judge the quality of the performed raster and the influence of the deviations form the ideal raster on the FPG data. This plot also allows to find out whether any systematic increase of deviations can be found in the data as the raster progresses. The text widget gives a print-out of all information extracted from the FPGC file relevant for the PHT data evaluation.

16.2.2 read_fpgc.pro

The module read_fpgc.pro is an IDL routine that reads a FPGC file and returns the information necessary for the analysis of FPG data for PHT from this file. The read_fpgc.pro module is called by the anal_fpgc.pro module in order to provide PIA with the necessary information from the FPGC file. It can, however, also be called from the command line in order to manually investigate the FPGC file using standard IDL routines. The module checks whether the FPGC file is for the PHT instrument by reading the Header record #2, offset 32, and returns with an error status if the instrument ID is not PHT.

Comment: The module read_fpgc.pro uses direct access to read a FPGC file. If the files give to the module via infile does not obey the FPGC file standard the software will crash or return incorrect data. No internal consistency check on the correctness of the FPGC format is performed.

16.2.3 anal_fpgc.pro

The anal_fpgc programme is used to analyse a FPGC file. It is usually called by the fpgc_manager programme, it can, however also be called from the IDL command line interface. It should be noted, that this leads to a slightly different behaviour, i.e. the PIA_XPLOT windows do not show up simultaneously on the screen as the necessary environment, as given by fpgc_manager, is missing. Anal_fpgc reads the Focal Plane Geometry Calibration File using the routine read_fpgc and produces two plot to allow the analysis of the fpgc file. The first plot window contains the raster data from the FPGC file overlaid with the ideal raster as would be expected from the FPGC header. The second plot window shows the euclidic distances between the ideal raster positions as derived from the FPGC header and the actual positions from the FPGC data records. The distances are plotted for every raster leg individually, allowing the user to judge the quality of the performed raster and the influence of the deviations form the ideal raster on the FPG data. This plot also allows to find out whether any systematic increase of deviations can be found in the data as the raster progresses.

anal_fpgc returns a text string array "text" containing the essential fpgc data as returned by read_fpgc and in addition containing the data for the ideal raster and the calculated distances.

16.3 The Focal Plane Geometry Map

The Focal Plane Geometry Map is analysed using modules which are integrated into the PIA software package as they comprise data analysis methods useful not only for the special application of focal plane geometry, but also for general astronomical data analysis. The main goal of the focal plane geometry map is to derive a corrected set of coordinates for the s/c pointing from a raster map of the PHT focal plane. This map is the first step in pinpointing the exact off-sets needed to accurately point the PHT sub-instruments. It is used in order to give a rough first set of offsets which will afterwards be confined using the Focal Plane Geometry Cross Scans (cf. "The Focal Plane Geometry Cross Scans" on page 91).

The method to achieve the necessary goal as described above is to make use of an adapted version of the IDL Astronomy Library find routine. A detailed descrip tion on how this routine works internally can be found in the documentation of the library. Thus, only a short introduction into the necessary input parameters and their influence on the processing will be given here.

The routine that will be used is the pia_find programme, which is embedded into the PHT Interactive Analysis, Astronomical Applications mapping routines.

The pia_find routine launches a user input window allowing the user to specify the input parameters needed for the find routine in order to work properly. These input parameters are:

Source Threshold: This threshold specifies how high a point source must be above the background to be detected. The value is given as absolute brightness in image units. The default is set to 1.5 times the median of the entire image.

Full Width Half Minimum of Point Source: The full width half minimum that shall be used for the search for a point source, given in raster coordinates. 1.0 is given as the (not very useful) default value.

Sharpness Statistics, lower and upper limit: Lower and upper limit of sharpness statistic. For details refer to the manual of the astronomical library. The default values are 0.2 and 1.0 respectively.

Roundness Statistic, lower and upper limit: Lower and upper limit of sharpness statistic. For details refer to the manual of the astronomical library. The default values are -1.0 and 1.0 respectively.

The button "Find Stars" starts the find process and launches a window with the results of the find procedure, i.e. x,y-coordinates (in the original raster parameters of the image), sharpness and roundness statistic values for all point sources found, fulfilling the criteria given by the user input parameters. The button "Get original parameters" resets the user input to the default parameters of the programme. "Exit" return the control to the PIA main programme.

Note: The original find routine was written for large CCD images and consequently does not work properly for small images (smaller 7x7). In order to overcome this problem the given image is extended by 3 row on each side. The additional data points are filled with the median of the entire image. As this procedure might lead to problems, the user is warned when using this routine with small images.

16.3.1 interpol_fpgc.pro

As the routine find returns the centre position of the star used for the raster measurements of the Focal Plane Geometry in raster coordinates (i.e. m,n or x,y) rather than in FPGC space craft coordinates, it is necessary to transform this coordinate set. This is performed via the routine interpol_fpgc.pro. It will perform a linear interpolation from the raster coordinates into the FPGC coordinates. This method is not very accurate, but given the spacings of the performed raster of 10 arcsec compared to the expected ISO s/c relative pointing errors, the accuracy achieved is sufficient for the purpose of determining the first course determination of s/c pointing off-sets for the PHT-P aperture. The final, more accurate determination is performed using the FPG cross scans (cf. "The Focal Plane Geometry Cross Scans" on page 91). It is assumed, that the routine interpol_fpgc.pro is called from the IDL prompt after the startup script was performed. It simply takes the name of the related FPGC file and a set of two raster coordinates and, after a consistency check, it will return a set of interpolated FPGC coordinates (Y , Z).

INTERPOL_FPGC

16.4 The Focal Plane Geometry Cross Scans

The Focal Plane Geometry Cross Scans are supposed to confine the positional data for the PHT sub-instruments such that a final set of s/c offset parameters can be derived. In order to achieve this the data from the PHT instrument must be combined with the data from the FPGC file. The PIA routine that was written for this purpose is called fpg_cross.pro.

The routine fpg_cross can be used in two different modes "automatic" and "interactive" depending on whether the parameter INTER is set to "N" or "Y", the default is "N".

16.4.1 Automatic Mode

In automatic mode fpg_cross only requires two parameters, the name of the ISCP file, containing the pre-processed data ISOPHOT data from the FPG scan, and the related FPGC file. It will analyse the two files, report any inconsistencies and return the calculated the centre positions for either of the two scans in the centre parameter. The method for determining the scan centre is "centre moment". This evaluation is quick and fairly reliably and it allows to run the fpg_cross procedure from within other programmes. It is, however, recommended if the time available is sufficient to use the "Interactive Mode" to verify and to improve the findings of the "Automatic Mode".

In case PHT-S data are analysed the returned centre array will contain the calculated centre coordinates for all 128 PHT-S pixels.

The parameter SCAN can be used in order to distinguish between scans along the spacecraft "Y"- and "Z"-coordinate axis.

The PIXEL keyword is ignored in "automatic" mode.

16.4.2 Interactive Mode

The interactive mode of fpg_cross is different from the automatic mode, not only w.r.t. the user interacting with the system and additional graphical information that is given, but it also allows the to access a more sophisticated fitting algorithm than is applied in the automatic mode. This distinction is made because complex fitting algorithms require a good set of start parameters in order to converge. These start parameters can only be defined automatically for simple fitting algorithms or with a high computing effort. It is usually better to allow the user to select a set of "sensible" start parameter and if necessary to allow him/her to repeat the fit. The results of the simple automatic processing are, however, also made available to the interactive user for comparison.

In case of PHT-S data analysis in interactive mode, it is required to specify in the keyword PIXEL the number of the PHT-S pixel for which the interactive fitting shall be performed.

After the s/w has performed the necessary consistency checks on the input files, the user will be offered the possibility to perform interactively a fit using the pia_multi_fit routine. This routine was initially written by the SWS IDT and is used within the PIA s/w with their kind permission. It was adapted to the PIA and additional fitting algorithms were added in order to make it applicable to the procedures out-lined here. A description of this fitting s/w can be found in "Mutiple Fitting" on page 63. The pia_multi_fit routine is called twice for the two scans per formed in opposite direction allowing the user to find the best possible fit for either of them individually. The fitting function "double fermi" was included into the fitting s/w because it is very well adapted to the problem of fitting the centre of the distribution as given by the FPGC cross scan profile. The s/w, however, also allows to fit a gauss profile. Both fits can be combined with a polynomial for adaptation to the background.