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Image reconstruction without input catalogue

When og_create has finished without any errors, move from the top-level directory ($REP_BASE_PROD) into the analysis directory (``./obs/spi_analysis/'') and launch the spi_science_analysis script

cd obs/spi_analysis

After a few seconds the GUI will appear, provided that the DISPLAY environment variable is set properly, otherwise you will be prompted for the individual parameters on the command-line. In general, but especially for learning SPI data analysis, we strongly recommend using the GUI.

Figure: spi_science_analysis main GUI

Image ssa_main

The top section holds the most basic parameters of the pipeline, the list of detectors which are to be used in the analysis and the coordinate system. Do not change the default values for the current example analysis.

The middle section concerns the catalogue extraction/specification part. For most of the bright sources, the available catalogue source positions[*] are more accurate than the positions which can be derived from the SPI data (due to the limited spatial resolution). The best results are therefore obtained by fixing the known source positions in the image reconstruction process. However, as some sources are highly variable and the catalogue fluxes not always reliable, it is sometimes difficult to predict which sources will be detected in a given observation. In order to avoid specifying wrong positions (where no significant sources can be detected), it is a good idea to first explore your data set and see which sources can be detected without using any prior information. Un-select the ``catalogue extraction'' and leave the ``spiros input catalogue'' entry empty (we explain how to use the catalogue in next Sect. [*]).

Select the pointing definition, event binning, background modeling, and image analysis tasks, and click on the corresponding buttons to specify task specific parameters.

  1. For the pointing step, you can use a pre-defined list of bad pointings to automatically exclude from your analysis pointings with known problems such as high background level or out-of-nominal instrument characteristics. This option is selected by default and we strongly recommend not to un-select it (or only for testing purposes).

  2. In the ``energy definition'' (Fig. [*]), you have three parameters. In the first field, you can define a number of energy regions. In the second one, you specify the actual limits in keV of these regions, e.g., 20,40,60 is for two regions: 20-40keV and 40-60keV. In the third field, you provide the number of bins for each energy region (positive numbers provide linear scaling, negative numbers logarithmic scaling). This allows for a more elaborate definition of the binning than a simple logarithmic or linear binning over the whole range. If you want gaps (e.g., 20-40keV and 80-100keV) in your binning, you can simply set the number of bins in the region in-between to 0.

    Figure: Energy bin definition GUI

    Image ssa_energy

    For our example, we use a 20-40keV energy region, with one bin.

  3. The histogram option allows to run phase-resolved analyses, or alternatively to add a simulated source in the mono-energetic analysis. The bottom section allows you to select/un-select single events (SE) not analysed by the PSD electronics. For this first example, do not change the default values: leave both options of top section (Histogram options) un-selected and the option of bottom section (SE+PE or only PE) selected. They will be described in dedicated sections. The user should read the details on using the PSD events in Section [*].

  4. The background can be modeled using several options:

    1. flat fields
    2. templates
    3. tracers

    Figure: Main background GUI

    Image ssa_main_back

    Figure: Options for the flat field background

    Image ssa_back_flatfield_II

  5. The last step of the SPI pipeline is the image deconvolution by spiros. Click on the spiros button to get the spiros GUI. Spiros performs image deconvolution in order to create images, spectra, or light curves. Since we want to create an image first, we set the spiros mode to IMAGING.

    Figure: spiros main GUI

    Image ssa_spiros

    In the ``Selection Parameters'' field of the spiros GUI, you can specify three subsets of energy bins, pointings and detectors. If you leave the fields empty, spiros will use all data, otherwise only the specified energy bins, pointings, and detectors will be taken into account in the analysis. You can automatically ignore obviously bad data points by entering ``AUTO'' in the pointing subset. You can also use the AUTO filter in addition to specifying the pointings to use. For example, ``AUTO,5-25,30-50'' will use pointings 5-25 and 30-50 and apply the AUTO filter to these pointings. All other pointings will not be used.

    The bottom section allows you to choose the ``Background method'', the ``Optimization statistic'', and the ``Bins for src location''.

    For the background method, you must use method 3 if you selected the ``flat-field'' model. If you are using the GEDSAT model use method 2 (you can also use background method 5, also known as ``mean count modulation model'' (MCM), but be aware that this method may produce wrong results if you have bright variable sources in the FoV).

    With spiros background method 3, one background scaling coefficient is derived for each time interval (specified as a number of pointings in the background model step) assuming that detector to detector background variations follow those from the flat-field model.

    With spiros background method 2, one background scaling coefficient is derived for each detector in the image reconstruction process. In this way, we assume that the background variations in all detectors follow that of the background model (see Sect. [*] or the spiros User Manual for more explanations).

    For the optimization statistic, you can choose between maximum likelihood or . The latter is faster and more robust but can be problematic if you have lots of energy bins with zero counts. For imaging in broad energy bands, is definitely better, while you might want to compare results obtained with and likelihood options when extracting spectra.

    The bins for source location determines in which energy bins spiros locates the sources. The typical setting will be ``FIRST'' where spiros uses just the first energy bin. But you may also use ``ALL'', where spiros will search for sources in each bin independently, or you may give a range of bins to be used.

    For our example, we use background method 3, as optimization statistic, and ``FIRST'' for the source location bins.

    The options specific for imaging are available by clicking on the button ``imaging'' in the spiros GUI.

    Figure: spiros imaging mode GUI
    Image ssa_spiros_ima

    The top section allows you to select the maximum number of sources you want spiros to look for (in addition to those that might be provided through a catalogue, see Sect. [*]) and a lower sigma threshold. spiros stops its iterative source search process either when it has found the maximum number of sources or when the sigma threshold has been reached. In our case, we ask spiros to search for up to 3 sources with a detection significance larger than 3 sigma (enter 3 instead of the default 6).

    We use the CAR (Cartesian) projection type for (almost) all cases. With the CAR projection the derived images have angular coordinates, with pixels regularly spaced by a constant angular step.

    Since SPI is a coded mask instrument, you have to define the image FoV, i.e., the sky region that spiros should ``reconstruct''. An easy way to specify the image FoV is to use the POINTING option. The output image will then range from the minimum to the maximum of the central latitude/longitude values of all pointings included in your observation. Using ``POINTING+FCFOV'' or ``POINTING+ZCFOV'' will add half of the SPI fully-coded or zero-coded FoV, respectively, on each side of the image defined by the pointing centers. ``POINTING'' or ``POINTING+FCFOV'' produces the nicest intensity images, but be aware that strong sources just outside your image FoV can completely bias your analysis (see Sect. [*]) and is therefore only recommended when no further sources are close to the current FoV. Otherwise, if you suspect there exist sources close to the FoV which may affect the reconstruction process, the ``POINTING+ZCFOV'' is a good choice for most observations. However, in this case the resulting intensity images need careful tuning in ds9.

When the setup is done, click twice ``Ok'' (once on ``imaging'' and once on ``spiros''), then ``Run'' the pipeline. Depending on the number of science windows, the histogram building and image deconvolution can take quite some time.

Once the pipeline is finished (no errors should be apparent), you will find in your directory, among other output files, the intensity, significance, and error image files in FITS format:


Before examining these images, convert the output catalogue produced by spiros into a format usable by ds9:

cat2ds9 "source_res.fits" source_res.reg

ds9 spiros_image_intensity_result.fits -region source_res.reg

ds9 spiros_image_sigma_result.fits -region source_res.reg

In our analysis example, and after adjusting appropriately the images with ds9, the resulting intensity and significance images should look like this:

Image crab_ima_osa11 Image crab_ima_sig_osa11

There are three sources in these images. In addition to the obvious, strong Crab detection, spiros has found two spurious sources with very low significance. In most cases, spiros does find spurious sources with detection significance in the range 3 to 5 sigma. Great care should be taken to make sure that sources with such detection significance are indeed real, and if you only want to see sources detected ``beyond any doubt'' raise the spiros lower sigma threshold to higher values (6 or 7 is usually a good choice).

The output file


provides quantitative results formatted as a FITS table. You can open this file with, e.g., the ftool ``fv'':

fv source_res.fits

The resulting spiros Crab position is RA=83.617 DEC=22.019, while the catalogue position is RA=83.612 DEC=21.994. The difference is of the order of 1.7arcmin. As expected for very high S/N sources, this value is much smaller than the SPI spatial resolution. The Crab flux in this 20-40 keV energy band is ph/sec/cm , corresponding to a detection significance of 154. You can also see that the detection significance of the spurious sources is 6.4 and 5.8.

All these results can also be viewed, perhaps more conveniently, at the end of the log file (called spi_sa_``UT date''.log), below the following title

=                                                                              =
=        Summary of results from SPIROS-9.3.6 in IROS analysis mode            =
=                                                                              =

where you find the most important information about the analysis, such as the sources found at each iteration and information about the resulting images.

next up previous contents
Next: Data screening Up: Cookbook Previous: Observation Group (OG) creation   Contents