gnirsinfoifu -- Description of reduction scripts for integral field GNIRS spectroscopic data
gnirsinfoifu
The GNIRS package contains tasks for processing GNIRS long-slit, cross-dispersed and integral field spectroscopy data. Detailed descriptions of the individual tasks can be found in their help files. This document summarizes the tasks and describes how they can be used to reduce integral field unit (IFU) data.
The tasks produce logfiles of the performed processing steps. The name of the logfile may be set in each individual task, or at the package level by setting gnirs.logfile.
The tasks add header keywords to the output images. These header keywords contain valuable information about the performed processing steps and the values of the critical parameters that were used.
All GNIRS images are written as multi-extension FITS (MEF) files. Raw data have two unnamed extensions. Most of the header information is written to the primary header unit [0] (PHU). The detector array data is in extension [1]. After being processed with NSPREPARE, GNIRS data extensions will be named as described in GNIRSINFO. Processing with NSCUT will generate extensions for each cross-dispersed order.
It is recommended to use imtype="fits". This is set automatically when loading the GEMINI package.
- NFQUICK - View IFU acquisition images
This task reconstructs a two-dimensional image of the IFU field from the raw acquisition exposure (without using NSPREPARE) and allows an offset to be measured for a central star.
- NSHEADERS - Prepare the GNIRS package for a particular instrument
This task is described in more detail in GNIRSINFO. It should be called before the other scripts, to configure the package for processing data from a particular instrument.
- NSPREPARE - Prepare GNIRS data for reduction
This task is described in more detail in GNIRSINFO. It must be called before the other tasks listed below.
- NSREDUCE - Process GNIRS data
NSREDUCE is used for basic reduction of raw data - it provides a single, unified interface to most of the tasks below and also allows for the subtraction of dark frames and zero level corrections.
Note that NSREDUCE will not call NSPREPARE.
- NSFLAT - Produce a normalized spectral flatfield
NSFLAT is used to produce a normalized spectral flatfield image. For IFU data it is usually used after calling NSREDUCE or NSCUT. To preserve a consistent normalization between slices, IFU flats must also be processed subsequently with nsslitfunction.
- NSEDGE - Update the BPM to flag aperture edges
NSEDGE takes the bad pixel map (BPM) produced by NSFLAT and adds information about the edge of apertures. If the BPM is then used by NSPREPARE to process the observations, aperture edges that appear will be flagged as bad (edges are included in the aperture if the MDF defines an aperture that is slightly too large, or if the dispersion direction isn't perfectly aligned with the Y axis of the detector).
- NSSLITFUNCTION - Include sky illumination information in the flat
NSSLITFUNCTION extends the flatfield produced by NSFLAT to include a low-order flatness correction, based on the sky illumination, as well as correcting the NSFLAT normalization for inter-slice variations. The output from this task is used as the flatfield image for further reduction.
- NFFLT2PIN - Generate a synthetic pinhole trace from a flat field
NFFLT2PIN takes an uncut IFU flat-field spectrum, processed with NSPREPARE, and uses it to generate an artificial pinhole spectrum that can be traced by NSSDIST.
- NSAPPWAVE - Determines a simple wavelength calibration
NSAPPWAVE provides a quick, approximate wavelength solution for GNIRS spectroscopic images. The wavelength solution is based on the header information and (by default) information in the table gnirs$data/nsappwave.fits (use the ttools package to examine this - ttools.tprint will display the contents - but if it is necessary to add/delete entries or create a new table, this can be done with the dedicated NSWEDIT task).
The task is for quick reduction purposes and to provide a starting point for the wavelength calibration derived by NSWAVELENGTH.
For IFU (or long slit) data, the central wavelength will normally be calculated from the values of nsheaders.key_wave and nsheaders.key_waveorder, so the entry in nsappwave.fits will be INDEF (although the dispersion will be defined in the table and used). To allows this behaviour, the specorder column in the MDF should have a value of -1 (to indicate that, unlike for XD data, it is not to be used).
- NSWAVELENGTH - Establish an accurate wavelength calibration
NSWAVELENGTH (automatically, if possible) determines the wavelength solution for GNIRS calibration lamp images, starting with the initial value supplied by NSAPPWAVE (this is done using the underlying IRAF tasks autoidentify and reidentify).
This task does not calibrate the data directly, but provides a database entry that can be used by NSFITCOORDS (described below).
- NSSDIST - Establish S-distortion calibration
NSSDIST traces the location of spectral orders or IFU image slices on the detector. For the long-slit or cross-dispersed modes, this is done using standard star or pinhole data. For the IFU, the processed (cut) output of NFFLT2PIN is used in place of a pinhole spectrum, to trace the image slices based on a flat-field spectrum.
- NSFITCOORDS - Compute 2D dispersion and distortion maps
NSFITCOORDS uses the wavelength solution found by NSWAVELENGTH and/or the spatial correction found by NSSDIST to create coordinate fitting maps and adds them to the database directory and the output headers of each science extension.
- NSTRANSFORM - Rectify spectra
NSTRANSFORM applies the coordinate fitting maps calculated with NSFITCOORDS to images. The data are rectified and interpolated.
- NSEXTRACT - Extract GNIRS spectra
This could be used to extract spectra from IFU image slices in a similar way to long slit spectra, but the normal IFU reduction process proceeds via NFCUBE.
- NSSTACK
Combine exposures at the same nod/dither position.
- NFCUBE - Generate a data cube
NFCUBE stacks the extensions (image slices) in processed IFU data to produce a three-dimensional 'cube'. This can be imported into the Euro3D analysis tools, for example (untested), or examined using standard IRAF tasks by specifying appropriate one- or two-dimensional image sections as part of the filename (e.g., display datacube.fits[sci][*,500,*] 1).
For typical reductions the user will need appropriate flat fields, arc calibration images, and science images.
0. Use GEMTOOLS.GEMLIST to make separate lists of the files associated with different exposure types. Typically these will be called obj.lis, arc.lis, star.lis. dark.lis, lamps.lis etc. Tasks can then be invoked on all the data of one type using iraf's "@" syntax. For example, NSPREPARE might be called with inimages=@obj.lis,@star.lis and then NSREDUCE with inimages=n@obj.lis (which prepends the letter "n" - nsprepare.outprefix - to each filename).
1. Use NSPREPARE to update the raw data headers and attach the mask definition file (MDF) as a binary table on all files. Note that dark frames will not have an MDF attached by default. Instead, the appropriate MDF is added in NSREDUCE or NSFLAT to match the data being reduced (the darks may be taken with a different telescope configuration, so the MDF from NSPREPARE, based on header information, would be incorrect).
2. Use NSREDUCE or NSCUT to cut the calibration (flat/arc) spectra to the size specified by the MDF, placing different IFU slices in separate image extensions.
3. Use NSFLAT to generate a normalized flat field (for each IFU slice or cross-dispersed order) from lamp flats. A mask (BPM) will also be generated by thresholding - this can be used to flag bad pixels in other data.
4. Process twilight sky flats similarly, and use NSSLITFUNCTION to produce a "super flat".
5. Optionally, use NFFLT2PIN to create an image from an uncut flat-field spectrum that can later be traced in place of a pinhole spectrum (required for accurate creation of a datacube at the final step).
6. Use NSREDUCE to process the object data. The skyrange parameter is typically 50% larger than the time between exposures, so that only immediately neighbouring frames are used for sky.
7. Group exposures by spatial offset and combine object observations using NSSTACK.
8. Use NSWAVELENGTH to calibrate arc data (after cutting and optionally applying a flatfield with NSREDUCE).
9. Optionally, measure spatial curvature for the image slices with NSSDIST, using the output from step 5 after cutting with NSCUT or NSREDUCE (this step is required for accurate alignment of the slices if a datacube is to be created).
10. Use NSFITCOORDS to calculate coordinate maps using the wavelength calibration from NSWAVELENGTH and/or the spatial distortion from NSSDIST.
11. Use NSTRANSFORM to resample the combined object data to a uniform (linear or logarithmic) wavelength scale, with consistent spatial alignment between different slices and wavelengths (assuming NSSDIST was used).
12. Use NFCUBE to construct a datacube from the final data.
An example reduction script is available, see GNIRSEXAMPLES.
Support for variance and data quality handling is somewhat limited (and has not been tested very extensively). This is partly due to lack of built-in support in IRAF. If error handling is required, please read the documentation for individual help tasks carefully and check the results for accuracy.
The tasks in the GNIRS package are designed to operate on MEF FITS images that have been processed using NSPREPARE (for GNIRS data) or NPREPARE (NIRI data).
It is possible that the reduction procedure for IFU data will change non-trivially in a future release of the Gemini IRAF package, allowing for replacement of some of the current tasks with improved, more generic versions.
For more information regarding the GNIRS data structure and for generic GNIRS package information, see GNIRSINFO. For more information about reducing cross-dispersed data, GNIRSINFOXD. For more information about the longslit tasks, see GNIRSINFOLS.
gnirsinfo, gnirsinfols, gnirsinfoxd, gnirsexamples, nsflat, nsreduce, nsappwave, nswavelength, nstransform, nsextract, nprepare, nsprepare, nssdist, gemlist, nfflt2pin.