Usage

Running the pipeline

  1. Download and unpack the archive containing your SALT data. This pipeline will operate on the contents of the product directory.

  2. Run the code, e.g.:

    rss_mos_reducer product reducedDir maskName

    maskName is the combination of OBJECT_MASKID from the corresponding FITS header keywords. So, for example if your data for an object named J0058.1+0031 were taken with a mask called P001788N07 and live in a directory called product then you would run the pipeline with:

    rss_mos_reducer product reduced J0058.1+0031\_P001788N07

    If you want to just list the masks found under a directory called product (without doing anything with the data) you can use:

    rss_mos_reducer product reduced list

    And if you’re super confident that nothing can go wrong, you can ask the pipeline to process data for all the masks it can find (in this case, under the product directory):

    rss_mos_reducer product reduced all

    The processed data from each mask will be found in its own subdirectory under reduced/ in the above example.

    You can run the code with the -h switch to see a help message with more options. The most important are -t, for setting the threshold used for identifying MOS slitlets in the flat field frames (only needed if you find slits are missing - use the DS9 .reg files to check), and -i, which switches on the iterative sky subtraction method for the spectral extraction. This currently seems to work better than the default extraction method, but is slower.

  3. After the pipeline has finished, you will find data at various stages of reduction under the reducedData dir (reduced for the above example). At the moment, this is not cleaned up.

    Three methods are used for extracting spectra:

    1. Individual 1D spectra are extracted from each science frame and then stacked. This might be desirable if object traces shift from frame-to-frame. These are placed under:

      `reducedDir/OBJECT_MASKID/1DSpec_extractAndStack/`

    2. The 2D spectra are stacked and then a 1d spectrum is extracted. These are placed under:

      `reducedDir/OBJECT_MASKID/1DSpec_2DSpec_stackAndExtract/`.

      The stacked 2D spectra are also placed in this directory.

    3. An alternative extraction method, that runs on the 2D stacked spectra produced by method 2. This uses the iterative sky subtraction, and fits for the object trace running along the slit. These are placed under:

      `reducedDir/OBJECT_MASKID/1DSpec_altExtract/`.

    Currently method 2 seems to work best, but output from all methods are made when running the pipeline, so you can choose between them afterwards.

    If the iterative sky subtraction method is enabled (using the -i switch), then _iterative is appended to the name of the output directory (e.g., 1DSpec_2DSpec_stackAndExtract_iterative).

    File names include the slit number (e.g., *_SLIT10.fits). You can check which slit (identified from the flat frame) corresponds with which extracted spectrum by examining the masterFlat_?.fits file(s) and the corresponding DS9 region file(s) (ending .reg) in the reduced/OBJECT_MASKID directory.

    The 1D spectra are in FITS table format (stored in the extension 1D_SPECTRUM) with the following columns:

    • SPEC - the object spectrum

    • SKYSPEC - the spectrum of the sky

    • LAMBDA - the wavelength scale corresponding to both of the above (in Angstroms)

    The 2D spectra are FITS images, which can be examined with DS9. They include the wavelength solution as the WCS stored in the header.

  4. The rss_mos_visual_inspector tool can be used to look at the 1D spectra, compare to template spectra from SDSS, and measure redshifts. This needs the tkinter module to be installed, and for the matplotlib` backend to be set to ``TkAgg.

    Run this using, e.g.:

    rss_mos_visual_inspector 1DSpec_2DSpec_stackAndExtract/1D_*.fits results

    In this example, each spectrum will be displayed in turn, and a text file containing the results of redshift measurements (and any comments by the user) will be written in the results/ directory, together with a plot of each spectrum. These are recorded from the state when the user clicks the Done - show next button in the top right of the graphical user interface. Hopefully it should be fairly obvious how to use most of the plotting controls. Note that you must click Redraw plot after making changes to either the template redshift or spectral smoothing.

    Note

    The XC Galaxies, XC LRGs and XC QSOs buttons will not work unless you have IRAF and PyRAF installed (and even then the canned settings may not work well on all setups).

Wavelength calibration

For wavelength calibration, a reference model needs to be made. So far this has been done for the following grating/lamp/detector binning combinations:

  • pg0900 Ar (2x2 binning)

  • pg0900 Ne (1x2 binning)

  • pg0900 Ne (2x2 binning)

  • pg0900 Xe (2x2 binning)

  • pg1300 Ar (2x2 binning)

  • pg1300 Ar (1x2 binning)

  • pg1300 Ne (1x2 binning)

  • pg1300 Xe (1x2 binning)

  • pg1300 CuAr (2x2 binning)

  • pg1800 Ne (2x2 binning)

More can be added relatively easily (see the rss_mos_create_arc_model script, which will print instructions when you run it).

The quickest way to check the wavelength calibration is to inspect the arcTransformTest*.png and skyCheck*SLIT*.png files, found under reducedDir/OBJECT_MASKID/diagnostics/. The former show the transformed arc spectrum compared to the reference model, while the latter plot the sky signal extracted from each science frame for each slit, with the reference wavelengths of prominent sky lines shown as vertical dashed lines for comparison. If these don’t line up, then an additional (or revised) reference model is needed.

The size in Angstroms of any offset from prominent sky lines in each final stacked spectrum is logged in reducedDir/OBJECT_MASKID/diagnostics/skyWavelengthCalibCheck.csv. As well as the median offset of identified sky lines with respect to their reference wavelengths, the results averaged across all slits are listed at the bottom of this file (look for “all slits median” and “all slits RMS”). This test shows that wavelength calibration using the pg0900 Ne (2x2 binning) reference model is good to < 1 Angstrom. Other reference models have not been tested extensively yet.

Longslit mode

The pipeline can also run on longslit data. The code detects object traces in each science frame, and creates “pseudo-slitlets” around each detected object. The processing steps are otherwise identical to those for MOS data. If necessary, you can use the --longslit-threshold option to change the detection threshold used.

Manually specifying slit (or object spectrum) locations

If you know the position of the slits in the FITS file, and/or the automated object/slit finding is not working well on your data for some reason, you can specify them in a plain-text file. To do this, you can do this using the -F argument, e.g.:

rss_mos_reducer -F slit_loc.txt product reduced all

This bypasses the automated slit/object finding routine and uses the input locations given in the text file. Be sure to structure the slit location files with three columns: slitno (i.e., slit number), ystart (i.e., y-coord coinciding with beginning of slit), and yend (i.e., y-coord coinciding with end of slit).

Below is a very simple example of a manual slit location file, for just one object, found in rows 988–1026 of some SALT RSS image:

slitno   ystart   yend
10       988      1026

No flats? No problem!

If you don’t have flat fields for your observations, you can skip flat fielding altogether with the -n argument. If no flat fields are found for your observations, the pipeline will attempt to drop into this mode automatically.

Note that if you do not have flats for MOS data, you must also specify the location of your slits using the -F argument (see above).

Things which can/should be improved/added

  • Extraction (not optimal at the moment)

  • Spectrophotometric calibration using a standard (not implemented)