Reduction of IFU Spectral Images with IRAF

This tutorial will reduce observations from programs GS-2012B-Q-26 and GN-2012B-Q-226 (PI: M. Schirmer), IFU spectroscopy of the “green bean” galaxy, J224024.1–092748. The observations were obtained on the nights of 2012 Oct 18-19 (GMOS-N) and 2012 Aug 27+29 (GMOS-S), and included g-band direct (acquisition) imaging of the field of interest, IFU 2-slit spectroscopy, and supporting calibration exposures. The spectra were obtained with the following configurations:

Configurations and Spectral Coverage

Configuration	Passband	Rest Passband
B600/499 + g	429–542 nm	324–409 nm
B600/625 + r	559–688 nm	422–519 nm
R831/853 + z + CaT	847–905 nm	639–683 nm

The original science goal was to analyze the 3-D kinematics, ionization state, and physical parameters of the gas in this Type-2 Seyfert galaxy at \(z=0.326\) (Davies, et al. 2015: [DST]).

IFU spectra are especially complex to reduce, and demanding science goals require careful attention to detail. For an advanced discussion and a detailed recipe for deriving high quality science products from this program’s data, see the posting by James Turner on the Gemini Data Reduction User Forum (download the reduction script: allred.cl) . For a high-level description of IFU reductions, see IFU Workflow. Here is an outline for reduction of this program’s IFU data:

Contents

• Reduction of IFU Spectral Images with IRAF

  • Retrieve & Organize Data

  • Processing Preparation

    • Software

    • Reference File Checklist

    • Create File Lists

  • Calibration Processing

    • Processing with gfreduce

    • Spectrum Trace

    • Flat Normalization

    • Arcs

  • Basic Processing

    • Reject Artifacts

  • Standard Star Processing

    • Aperture Sum

    • Sensitivity Function

  • Science Processing

    • Calibration

    • Wavelength Alignment

    • Data Cube

For other tutorials, see the following links:

• Reduction of Images with IRAF

• Reduction of Long-Slit Spectra with IRAF

• Reduction of Multi-Object Spectra with IRAF

Retrieve & Organize Data

The first step is to retrieve the data from the Gemini Observatory Archive (see Archive Searches). You may search the GOA yourself, or instead just cut-and-paste the direct URLs in your browser.

# IFU data of Green Bean galaxy:
https://archive.gemini.edu/searchform/cols=CTOWEQ/GS-2012B-Q-26/notengineering/GMOS/NotFail
https://archive.gemini.edu/searchform/cols=CTOWEQ/GN-2012B-Q-226/notengineering/GMOS/NotFail

After retrieving the science data, click the Load Associated Calibrations tab on the search results page and download the associated bias and flat-field exposures. See Retrieving Data for details. Unpack all of them in a subdirectory of your working directory named /raw. Be sure to uncompress the files.

It is highly recommended to create an observing log (see Creating an Observing Log), and review the exposures to understand the observing program and the nature of the science exposures. A review of the log for this program reveals the following:

• GCAL flat exposures were obtained contemporaneously with the science and standard star exposures, which will enable an accurate tracing for the fiber locations on the detector format.

• The Arc exposures were obtained as dayCal observations, well separated in time from the science exposures and other Arcs. Thus, it is only useful to process one Arc for each configuration, since they cannot be combined and instrument flexure will introduce zero-point errors. A refinement to the wavelength calibration for science exposures will be necessary.

• Exposures of standard stars were obtained, but not on the same nights as the science exposures, and no single star was observed in all configurations.

• Multiple science exposures were obtained in each configuration, all with long durations. The emission features are weak, the cosmic rays are plentiful, and (not surprisingly) the spectral images will have to be aligned post-extraction, prior to combining.

• Some twilight flat dayCal exposures were obtained, but are not needed or used for this IFU program.

Science & Standard Star Observations

DateObs	Configuration	Target	Used?
2012-08-27	GS-B600/625	J2240-0927	Yes
2012-08-29	GS-B600/499	EG131	No
	GS-B600/625	EG131	Yes
2012-10-05	GN-B600/499	BD+28_4211	Yes
2012-10-08	GN-B600/499	J2240-0927	Yes
2012-10-18	GN-R831/853	J2240-0927	Yes
2012-10-31	GS-B600/625	LTT9239	Yes
2013-04-03	GS-R831/853	Wolf1346	No

Reductions for the observations highlighted in bold, above, will be described in the following sections.

Processing Preparation

Software

Example Reduction Script

You can perform all of the processing steps for this tutorial by downloading the IFU Tutorial CL script.

• Download: gmos_ifu_proc.cl

Place the script in the work directory. Within an IRAF session load the rv, gemini, gemtools, and gmos packages, then execute it in command-mode. It will take awhile to run (the better part of a day on a 2010-era Mac Pro).

#...then:
cd /path/to/work_directory
cl < gmos_ifu_proc.cl

You may find it more useful to download the script just to follow this tutorial in detail, and use it as the basis for reducing other IFU observations. The processing steps for this tutorial are described below, but only a subset of the commands for G600/625 are given in the interest of brevity.

Suppressing Cosmic Rays

To suppress the cosmic rays effectively, particularly for the long-duration science exposures, install the L.A.Cosmic script (see [VD])

• Download: download: lacos_spec.cl

if you have not already done so. You may define this task in your login.cl file to avoid this step each time you start IRAF. See: setting up the Add-on Task for CR Rejection. This task, which is called by gemtools.gemcrspec, works much better than the gmos.gscrrej task, which is called when the fl_gscrrej is set in gfreduce processing.

Reference File Checklist

The Table below lists the basic calibration files that will be constructed for the science data processing steps, apart from the wavelength solutions (which are written to the /database subdirectory). Files that include an asterisk (*) wildcard character refer to any names with the specified suffix.

Required MasterCals

Type	GMOS-N	GMOS-S
Static BPM	[N/A]	bpm_gmos-s_EEV_1x1_3amp_full
Bias Residual	MCbiasN	MCbiasS1 and MCbiasS2
Flat-field	eprgN20121018S0070_flat	eprgS20120829S0062_flat
	eprgN20121018S0073_flat	eprgS20120827S0069_flat
	eprgN20121005S0296_flat	eprgS20121031S0025_flat
Sensitivity	Sens_B6-499	Sens_B6-625
	Sens_R8-853

Fetch and uncompress the Static BPM MasterCal

• Download: bpm_gmos-s_EEV_v2_1x1_spec_MEF.fits.bz2

for full-frame read-outs for the GMOS-S EEV CCDs. The GMOS-N CCDs have very few cosmetic defects, so a Static BPM MasterCal is not necessary.

You will need to make local copies (in your work directory) of the monochromatic magnitudes for the standard stars (see: Standards List). These standards will be used to make a common sensitivity function for GMOS-S B600/625, but the reference files reside in different directories in the IRAF system.

• EG131: gmos$calib/eg13.dat

• LTT9238: onedstds$ctionewcal/l9239.dat

Now take a deep breath; this is going to be a bit of a slog.

Create File Lists

The next steps will create lists of calibrations and science files for use in processing. This is tedious, but will make the processing much easier to follow. The selection is achieved by matching a specific combination of header keyword-value pairs (see GMOS Processing Keywords). Examples of file selection with the hselect task are given below. Note the use of gemtools.gemextn to remove the FITS kernel syntax from the resulting file names.

Build the lists of calibration files.

cd /path/to/work_directory/raw

string obsSelect
string ccdSelect

# All bias & flat files match a single CCD RoI and binning.
ccdSelect = "&& detrO1ys>1024 && ccdsum?='1 1' "

## Biases must match the instrument, obs. type & class, and CCD RoI & binning:
obsSelect = "instrume?='GMOS-S' && obstype?='BIAS' && obsclass?='dayCal' "

# Select bias exposures within ~10 days of the target observations.
s2 = " && @'date-obs' > '2012-08-20' && @'date-obs' < '2012-09-01'"
s1 = obsSelect // ccdSelect // s2

# Select biases using info. from both PHDU & HDU-1.
hselect ("S*.fits[1,inherit=yes]", "$I", s1, > "bias_tmp.txt")

# Trim IRAF kernel notation.
string omit = "exten,index,kernel"
gemextn ("@bias_tmp.txt", omit=omit, outfile="biasFilesS1.txt")
# ...and similarly for the Oct. GMOS-S and the GMOS-N biases.

## Flats must also match the aperture, grating, and central wavelength.
string bandSelect

# Start with the GCAL flats for B600/625:
obsSelect = "instrume?='GMOS-S' && obstype?='FLAT' "
bandSelect = "&& maskname?='IFU-2' && grating?='B600+_G5323' && grwlen=625."
s1 = obsSelect // ccdSelect // bandSelect
hselect ("S*.fits[1,inherit=yes]", "$I", s1, > "flt_tmp.txt")
gemextn ("@flt_tmp.txt", omit=omit, outfile="fltGcal_B6-625.txt")
#...and similarly for the GMOS-N settings: R831/83 and B600/499

Continue with building the list of arc, standard star, and science exposures. Ultimately the standard star selection criteria below match only one file per configuration, but the exercise is more generally applicable. Note the S prefix in selecting filenames below. This is not strictly necessary since INSTRUMENT is constrained to be GMOS-S, but it does reduce by about half the number of file headers to be opened.

## Arcs
# Only one arc is useful for each configuration,
# so choose filenames from the log.
print ("N20121005S0902.fits", > "arc_B6-499.txt")
print ("S20120828S0005.fits", > "arc_B6-625.txt")
print ("N20121018S0197.fits", > "arc_R8-853.txt")

bandSelect = "&& maskname?='IFU-2' && grating?='B600+_G5323' && grwlen=625."

## Science exposures
# Match the instrument, object name, observation class, mask, grating & cenWave:
obsSelect = "instrume?='GMOS-S' && i_title?='J2240-0927' && obsclass?='science' "
s1 = obsSelect // bandSelect
hselect ("S*.fits[1,inherit=yes]", "$I", s1, > "sci_tmp.txt")
gemextn ("@sci_tmp.txt", omit=omit, outfile="sci_B6-625.txt")

## Standard stars (two epochs)
# EG131:
obsSelect = "instrume?='GMOS-S' && obstype?='OBJECT' && i_title?='EG131' "
s1 = obsSelect // bandSelect
hselect ("S*.fits[1,inherit=yes]", "$I", s1, > "std_tmp.txt"
gemextn ("@std_tmp.txt", omit=omit, outfile="EG131_B6-625.txt")

# LTT9239:
obsSelect = "instrume?='GMOS-S' && obstype?='OBJECT' && i_title?='LTT9239' "
s1 = obsSelect // bandSelect
hselect ("S*.fits[1,inherit=yes]", "$I", s1, > "std_tmp.txt"
gemextn ("@std_tmp.txt", omit=omit, outfile="LTT9239_B6-625.txt")
# ...and similarly for the other settings: B6-499 and R8-853

Move the file lists to the parent work directory. Most other processing lists may be built from the above lists and the IRAF sections task, as shown in the following sections.

Calibration Processing

Generating calibration reference files beyond the Bias Residual MasterCal is sufficiently involved that the topic will be covered here. The following section describes the preparation of calibration reference files necessary for the removal of the instrumental signature.

Processing with gfreduce

Many reduction steps are performed by the gfreduce task. This task has more than 75 parameters; the table below lists the defaults for the “flag” keywords—i.e., keywords with logical values to indicate whether to include a specific step in the processing.

gfreduce Processing Flag defaults

Flag	Default	Description
fl_addmdf	Yes	Append MDF extension?
fl_bias	Yes	Subtract Bias Residual?
fl_fixgaps	No	Interpolate over chip gaps after extraction?
fl_fixnc	No	Auto-correct for nod count-mismatch in N&S observations
fl_fulldq	No	Decompose DQ into constituent bits before transforming them?
fl_extract	Yes	Extract the spectra?
fl_fluxcal	Yes	Apply flux calibration?
fl_nodsuffle	No	Using one of the N&S slitmasks?
fl_gsappwave	Yes	Insert approximate dispersion solution into header?
fl_gnsskysub	No	Subtract sky from N&S images?
fl_gscrrej	Yes	Clean images of cosmic rays?
fl_inter	Yes	Perform operations interactively?
fl_novlap	Yes	Extract only non-overlapping sections of 2-slit spectra?
fl_over	Yes	Perform overscan correction?
fl_skysub	Yes	Subtract mean sky spectrum?
fl_trim	Yes	Trim overscan region?
fl_vardq	No	Propagate VAR and DQ?
fl_wavtran	Yes	Apply wavelength calibration?

Processing IFU data is more complicated than for other instrument configurations, for a variety of reasons including:

• The gfreduce task does not fully integrate some options and operations (e.g., an input parameter for the Static BPM), and frankly, it is a little cantankerous.

• The narrow spatial extent of the IFU fibers, and the close packing of the output fiber spectra on the FPA means that effects such as instrument flexure and cosmic-ray hits have an outsized impact on the data processing, and must be addressed with care.

• Some operations such as flat-field generation have not been broken out into a separate task, and other tasks do not fully support file lists.

The workflow for IFU processing is, therefore, somewhat iterative and requires using the IRAF cl to iterate over file lists.

Spectrum Trace

Ideally GCAL flats were obtained contemporaneously with the science exposures so that they may be used as templates for tracing the spectrum from each fiber on the science image array; that is the case for this program. A representative shift in location of the fiber traces for two different flat-field exposures is shown in the figure below, which illustrates the need for a contemporaneous GCAL flat with science exposures.

A shift of about 1 pixel in the horizontal position of fiber spectra in two GCAL flat-fields, taken weeks apart. Each fiber spectrum subtends only about 6 spatial pixels, corresponding to 0.2 arcsec in the focal plane. Wavelength increases from right to left.

The first reduction step is to process those GCAL exposures though bias correction and extraction, so that the extraction parameters will be written to the /database directory for downstream use. Recall that there are two epochs for the B600/625 configuration.

unlearn gfreduce
unlearn gireduce
gfreduce.logfile="gfreduceLog.txt"

# Note the trailing slash, which is for some reason required for gfreduce:
gfreduce.rawpath="./raw/"

# Extract GCAL flat-fields (two epochs for GMOS-S)
gfreduce ("@flatGcal_B6-625-1.txt", bias="MCbiasS1", fl_extract+, \
   fl_gscrrej-, fl_gsappwave-, fl_wavtran-, fl_skysub-, fl_fluxcal-, \
   fl_vardq+, fl_inter-)

gfreduce ("@flatGcal_B6-625-2.txt", bias="MCbiasS2", fl_extract+, \
   fl_gscrrej-, fl_gsappwave-, fl_wavtran-, fl_skysub-, fl_fluxcal-, \
   fl_vardq+, fl_inter-)
# ...and so on for the other configurations.

Recall also that the intermediate files from each major step in the data reduction process are tagged with a prefix: see File Nomenclature for a list. The erg* extracted flat-field files generated above are used for two purposes: the fiber extraction parameters, and as input to the flat-field normalization.

The gfreduce task does not have an option for inserting a Static BPM MasterCal into the DQ extensions of the processed flats, so this must be done explicitly. Alas, the applicable task, addbpm, appears not to work properly. So, insert the DQ extensions in-place, then interpolate over the bad columns.

string bpm = "bpm_gmos-s_EEV_1x1_3amp_full.fits"
string extn = "[DQ,"
# Build a list of files with the processing prefixes attached.
sections ("rg//@flatGcal_B6-625.txt", option="root", >"rgFlat_B6-625.txt")

# Use an IRAF list-structure to loop over the file names.
list = "rgFlat_B6-625.txt"
while (fscan (list, s1) != EOF) {
   for (i=1; i<=3; i+=1) {
      s2=bpm//extn//i//"]"
      imcopy (s2, s1//extn//i//",overwrite+]")
   }
}

The flat-field exposures are of such short duration that cosmic-rays (CRs) are not a concern. Now interpolate over bad columns in the science array, so as not to corrupt down-stream extractions.

unlearn gemfix
gemfix.logfile="gemfixLog.txt"
gemfix ("@rgFlat_B6-625.txt", "p@rgFlat_B6-625.txt", \
   method="fit1d", bitmask=1, order=32)

Caution

It would be best at this point to model and subtract scattered light from the flat-fields (or any well exposed source). Unfortunately the gfscatsub task is unreliable, sometimes producing wildly incorrect results, so that the scattered light correction will be skipped in this tutorial.

For some detectors on GMOS-S (the EEV and the Hamamatsu CCDs) the relative QE correction between sensors should be applied. This step is skipped in this tutorial, but if implemented then the correction must be applied to both the flats and the astrophysical targets.

If the gfscatsub task is fixed in some future release, and the QE correction is performed (in v1.13.1+), bear in mind that these steps prepend a b and a q, respectively, to the output filenames. Therefore the filename lists used below would all need to be updated.

Flat Normalization

Now extract the flats from the exposures, and normalize by fitting a 1-D curve in the dispersion direction to remove the color term. Note that gfresponse must be called in a loop to process a list of exposures, and that a very high order is required to fit the variations in the spectral response.

# Fit the response to the extracted flat-field fibers.
unlearn gfresponse
gfresponse.logfile="gfresponseLog.txt"

sections ("ep//@rgFlat_B6-625.txt", option="root", >"eprgFlats_B6-625.txt")
list = "eprgFlats_B6-625.txt"
while (fscan (list, s1) != EOF) {
    gfresponse (s1, s1//'_flat', skyimage='', function='spline3',
        order=47, sample="1:1,30:2855,2879:2879")
}
# ...and so on for the other configurations.

Arcs

The wavelength calibration can be performed once the order trace has been defined, so the arc exposures will be processed next. The traces (specified with the reference parameter) do not match the dayCal arcs perfectly, but they are adequate for wavelength calibration. The following basic reduction performs overscan correction and trimming, but does not apply the Bias Residual MasterCal because dayCal Arc exposures are normally obtained in fast read-out mode.

gfreduce ("@arc_B6-625.txt", reference="eprgS20120829S0062.fits", \
   fl_bias-, fl_extract+, trace-, recen-, fl_gsappwave+, fl_wavtran-, \
   fl_gscrrej-, fl_skysub-, fl_fluxcal-, fl_vardq-, fl_inter-)

The default CuAr line list (gmos$calib/CuAr_GMOS.dat) has been pretty heavily groomed to produce good fits at low S/N, and with 1500 fibers one either has to trust the default or perform only the first few wavelength solutions interactively.

unlearn gswavelength
gswavelength.logfile="gswaveLog.txt"
gswavelength ("erg@arc_B6-625.txt", fl_inter-, fwidth=8, minsep=2.5)

Basic Processing

With the internal calibrations in place, proceed with processing the science and standard star exposures through Bias correction. Start by assembling the list of all exposures, but segregate them by date so that the appropriate Bias Mastercal may be used.

concatenate ("EG131_B6-625.txt,s*_B6-625.txt", "allSfiles.txt")

gfreduce ("@allSfiles.txt", bias="MCbiasS1", reference="eprgS20120829S0062", \
   fl_extract-, fl_gscrrej-, fl_gsappwave-, fl_wavtran-, fl_skysub-, \
   fl_fluxcal-, fl_vardq+, fl_inter-)

gfreduce ("@LTT9239_B6-625.txt", bias="MCbiasS2", reference="eprgS20121031S0025.fits", \
   fl_extract-, fl_gscrrej-, fl_gsappwave-, fl_wavtran-, fl_skysub-, \
   fl_fluxcal-, fl_vardq+, fl_inter-)

# Insert the MasterCal BPM into the [DQ] extensions.
concatenate ("allSfiles.txt,LTT9239_B6-625.txt", "allFiles.txt")
sections ("rg//@allFiles.txt", option="root", >"rgAllfiles.txt")
list = "rgStd_B6-625.txt"
while (fscan (list, s1) != EOF) {
   for (i=1; i<=3; i+=1) {
      s2=bpm//extn//i//"]"
      imcopy (s2, s1//extn//i//",overwrite+]")
   }
}

Reject Artifacts

Cleaning the exposures of cosmic rays is important for the standard stars, but essential for the science spectrograms. Perhaps 4 or 5 iterations will be necessary, at least for the science exposures. Note that the gemcrspec task requires that the lacos_spec.cl task be installed (see Add-on Task for CR Rejection). This task will run for quite awhile, perhaps overnight on a desktop machine….

unlearn gemcrspec
gemcrspec.logfile="gemcrspecLog.txt"
gemcrspec ("@rgAllfiles.txt", "x@rgAllfiles.txt", sigfrac=0.32, \
   niter=4, fl_vardq+)

# Replace bad pixels (CRs & bad collumns) with interpolated values.
unlearn gemfix
gemfix.logfile="gemfixLog.txt"
gemfix ("x@rgAllfiles.txt", "px@rgAllfiles.txt", method="fixpix")

Note that lacosmic does not completely clean the CRs, although they are marked in the BPM, and it does not deal well with bad columns. Thus the final interpolation above over bad pixels with gemfix is necessary, and vital for downstream processing.

Caution

Again, this is the place where the scattered light and QE corrections would appear in the processing workflow.

Standard Star Processing

Resume basic processing of the Standard Stars with gfreduce to apply the flat-field and wavelength calibrations, then sky subtraction. The wavelength transformation parameters below will select image columns in the dispersion direction that are in common to all the fibers in the undistorted frame. (The specific parameters were adopted from inspecting a prior reduction run.) Recall that there are different trace references for the standard stars, so they are processed separately.

gfreduce.rawpath="./"
sections ("pxrg@EG131_B6-625.txt", >"cleanEG131_B6-625.txt")
sections ("pxrg@LTT9239_B6-625.txt", >"cleanLTT9239_B6-625.txt")

gfreduce ("@cleanEG131_B6-625.txt", fl_addmdf-, fl_over-, fl_trim-, \
   fl_bias-, fl_extract+, reference="eprgS20120829S0062", trace-, recenter-, \
   response="eprgS20120829S0062_flat", \
   fl_gsappwave+, fl_wavtran+, wavtraname="ergS20120828S0005", \
   w1=5618., w2=INDEF, dw=0.4622, nw=2822, \
   fl_skysub+, sepslits+, fl_fluxcal-, fl_gscrrej-, fl_vardq+, fl_inter-)

gfreduce ("@cleanLTT9239_B6-625.txt", fl_addmdf-, fl_over-, fl_trim-, \
   fl_bias-, fl_extract+, reference="eprgS20121031S0025", trace-, recenter-, \
   response="eprgS20121031S0025_flat", \
   fl_gsappwave+, fl_wavtran+, wavtraname="ergS20120828S0005", \
   w1=5618., w2=INDEF, dw=0.4622, nw=2822, \
   fl_skysub+, sepslits+, fl_fluxcal-, fl_gscrrej-, fl_vardq+, fl_inter-)

Aperture Sum

All the fibers from the object field are combined with gfapsum into a 1-D spectrum, using the MDF extension (which was inserted into each file) to select the fibers.

unlearn gfapsum
gfapsum.logfile="gfapsumLog.txt"
sections ("ste@cleanEG131_B6-625.txt", option="root", >"steEG131_B6-625.txt")
list = "steEG131_B6-625.txt"
while (fscan (list, s1) != EOF) {
   gfapsum (s1, lthreshold=0., fl_inter-)
}
# Make a more informative file name for EG131.
copy astepxrgS20120829S0061.fits EG131sum_B6-625.fits

sections ("ste@cleanLTT9239_B6-625.txt", option="root", >"steLTT9239_B6-625.txt")
list = "steLTT9239_B6-625.txt"
while (fscan (list, s1) != EOF) {
   gfapsum (s1, lthreshold=0., fl_inter-)
}

The three sequential exposures for LTT9239 can be combined.

unlearn gemcombine
gemcombine.logfile="gemcombineLog.txt"
sections ("a@steLTT9239_B6-625.txt", option="root", >"asteLTT9239_B6-625.txt")
gemcombine ("@asteLTT9239_B6-625.txt", "LTT9239sum_B6-625", \
   reject="none", scale="exposure")

Sensitivity Function

The next step is to derive the sensitivity function from the standard star spectra. To use the Mauna Kea extinction function for GMOS-N data, download: mk_extinct.txt and place it in your work directory. If you have not already done so, make a local copy of the monochromatic magnitudes reference files for these standards (see Reference File Checklist).

copy onedstds$ctionewcal/l9239.dat ./
copy gmos$calib/eg131.dat ./

Now create the sensitivity function MasterCal, setting caldir to the current directory.

unlearn gsstandard
gsstandard.logfile="gsstandardLog.txt"
gsstandard.observatory="Gemini-South"

gsstandard ("EG131sum_B6-625.fits,LTT9239sum_B6-625.fits", "std_B6-625", \
   "Sens_B6-625", starname="eg131,l9239", caldir="./", \
   extinction="onedstds$ctioextinct.dat", order=11, fl_inter+)

The sensitivity curves for the separate standards agree very well, once a grey shift is applied.

Fit to the sensitivity function with B600/625 after greyshift of the two contributing standard stars EG131 and LTT9239. One artificial point (blue) was added beyond the end of the reddest passband to avoid an otherwise unphysical decline in the derived sensitivity. Click image to enlarge.

Science Processing

Almost done. With all the calibrations in place, resume processing with gfreduce to extract the spectra, apply flat-field and wavelength calibrations, subtract sky, then apply extinction correction and flux calibration.

Calibration

Since GCAL flats were taken contemporaneously with the science exposures, the flat obtained closest in time will be used for the trace reference. Note that the wavelength range has been trimmed slightly (using the w1, w2, dw, and nw parameters), in order to exclude ends of the spectrum that are not covered in some fibers once the distortion correction has been applied. This truncation is not strictly necessary, but can be convenient for display and downstream processing.

gfreduce.rawpath="./"
sections ("pxrg@sci_B6-625.txt", >"cleanSci_B6-625.txt")

gfreduce ("@cleanSci_B6-625.txt", fl_addmdf-, fl_over-, fl_trim-, fl_bias-,
   fl_extract+, reference="eprgS20120827S0069", trace-, recenter-, \
   response="eprgS20120827S0069_flat", \
   fl_gsappwave+, fl_wavtran+, wavtraname="ergS20120828S0005", \
   w1=5618., w2=INDEF, dw=0.4622, nw=2822, \
   fl_skysub+, sepslits+, fl_fluxcal+, sfunction="Sens_B6-625", \
   extinction="onedstds$ctioextinct.dat", \
   fl_gscrrej-, fl_vardq+, fl_inter-)
# ...and similarly for other grating settings.

This concludes basic processing and calibration, though some refinements and advance data products are discussed below.

Wavelength Alignment

Since a dayCal Arc exposure was used to determine the wavelength scale, the separate science exposures will in general not have identical wavelength zero-points owing to instrument flexure and the time-dependent barycentric correction. To determine the wavelength offsets, load the rv package and run the rvidlines task on the background aperture (i.e., the [SKY] extension). Reference night sky emission lines will be adopted from Osterbrock et al. (1996, PASP, 108, 277). Download: skylines.txt and place it in your work directory.

unlearn rvidlines
rv.observatory="gemini-south"
rvidlines.coordlist="./skylines.txt"
rvidlines.ftype="emission"
rvidlines.logfile="rvLog.txt"

rvidlines ("stepxrgS20120827S0066.fits[SKY]", threshold=7., \
   nsum=1, maxfeatures=10, fwidth=10., cradius=10., minsep=5.)
# ...and so on for the other target exposures.

The output will include a mean shift in velocity and redshift z. Compute a wavelength difference and add the WCS reference wavelength:

\[\Delta\lambda = \lambda_0 + (-z * \bar{\lambda})\]

where \(\lambda_0\) is taken from the file header (\(\mathtt{CRVAL1} = 5618\), the zero-point specified during the gfreduce calibration processing), and \(\bar{\lambda}\) is the central wavelength of the configuration (6250). The output from this same analysis by James Turner produced the shifted wavelengths used below to update the file headers.

unlearn hedit
hedit.update=yes
hedit.verify=no

hedit ("cstepxrgS20120827S0066.fits[sci]", "CRVAL1", 5618.164)
hedit ("cstepxrgS20120827S0067.fits[sci]", "CRVAL1", 5618.268)
hedit ("cstepxrgS20120827S0068.fits[sci]", "CRVAL1", 5618.387)
hedit ("cstepxrgS20120827S0070.fits[sci]", "CRVAL1", 5618.560)
hedit ("cstepxrgS20120827S0071.fits[sci]", "CRVAL1", 5618.667)
hedit ("cstepxrgS20120827S0072.fits[sci]", "CRVAL1", 5618.787)

The calibrated spectral images are now on the same wavelength scale, but are not yet perfectly aligned spatially.

Data Cube

One optional but, for this program, important output product is a datacube of the science exposures which is a 3-dimensional representation of the IFU observations. The datacube is an \((x,y,\lambda)\) 3-space that can be used to visualize the observations, or to facilitate the extraction of spatially summed spectra or monochromatic, 2-dimensional images. The gfcube task resamples the image when the atmospheric dispersion correction is enabled, and can optionally rejects pixel flagged in the BPM (but recall that they were interpolated over in prior processing). We also elect to convert the brightness units to flux per square arcsec.

unlearn gfcube
gfcube.logfile="gfcubeLog.txt"

# Unfortunately, gfcube does not accept file lists.
sections ("cste@cleanSci_B6-625.txt", >"calibSci_B6-625.txt")
list = "calibSci_B6-625.txt"
while (fscan (list, s1) != EOF) {
   gfcube (s1, fl_atmdisp+, fl_flux+, fl_var+, fl_dq+)
}

Spatial Alignment

The separate exposures should be combined maximize the S/N for analysis, but they must first be aligned spatially. This can be accomplished with the PyFU datacube mosaicing packge, posted by James Turner to the Gemini Data Reduction User Forum. Use of this package is beyond the scope of this tutorial, but the results show that spatial shifts affect the final 3 science exposures in the sequence, obtained just after the GCAL flat. Edit the WCS keywords in the headers of these calibrated spectra:

hedit dcstepxrgS20120827S0070.fits[sci] CRVAL1 65.48042

hedit dcstepxrgS20120827S0071.fits[sci] CRVAL1 65.47042
hedit dcstepxrgS20120827S0071.fits[sci] CRVAL2 0.045000

hedit dcstepxrgS20120827S0072.fits[sci] CRVAL1 65.50042
hedit dcstepxrgS20120827S0072.fits[sci] CRVAL2 0.035000

Combine Cubes

Now the datacubes may be combined, again with the PyFU package. Here we use the imcombine task, which will suffice for a quick-look; no rejection is enabled, for simplicity.

unlearn imcombine
imcombine ("dcstepxrgS20120827S00*.fits[SCI]", "j2240-097.fits")

The datacube may be visualized in an appropriate tool, such as SAOImage DS9.

Combined, false color image of the galaxy J224024.1–092748 obtained with with grating R600/625 in the rest frame of [O_III] \(\lambda5007\). This narrow-band (about 28 km/s) image is one spatial frame of a combined data cube, with north up and east to the left. Click image to enlarge.

Extracting spectra from the datacube is straightforward using apsum.

Extracted spectra in the vicinity of [O_III] \(\lambda5007\) from the nucleus (left) and the nearby ionized region (right). These spectra correspond to the left- and right-hand side, respectively, of the color image above. Click image to enlarge.

A great deal of science analysis is possible with these data products, as the [DST] paper demonstrated.