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Spectral Templates - Observations and Data Reduction


The observations were done using the Integral Field Unit (IFU) in the Gemini Near-Infrared Spectrograph (GNIRS), with the grating 110.5 l/mm, yielding a resolving power of R~6000 (FWHM=3.4Å at 2.293μm).  The list of observations is given below, where for each spectral setting we present the date(s), the total exposure time on source and the hot star used to correct for the telluric lines.  Observations were dominated by the instrumental overheads: for example, in the "blue" observation of  HD39425, the 36sec of on-source time required 4.85 minutes of clock time (begining to end of observing sequence, not including slew and target acquisition).


Star
"Blue" setting
"Red" Setting
UT date
Exp time (s)
Telluric std(s)
UT date Exp time(s)
Telluric std(s)
HD20038 20061013
20061018
1440
1920
HR607 20060912
20061007
20061013
1440
1440
720
HR607
HD209750


20061020
108
HR7950
HD6461 20061010
560
HR100 20060905
20061007
840
840
HR100
HR100
HD173764 20061012
180
HR7136
20060904
20061021
180
180
HR7950
HR7316
HD36079 20070104
72
HR2020
20060914
72
HR2020
HD1737 20061008
180
HR100 20060914
180
HR100
HD213789


20060904
360
HR8959
HD212320 20061006
480
HR8959 20060904
480
HR8959
HD213009


20060903
90
HR8959
HD35369 20061015
180
HR2020 20061005
180
HR2020
HD64606 20070104
960
HR3314
20070102
960
HR3314
HD224533 20070107
120
HR8959
20060914
120
HR8959
HD4188 20061012
160
HR100 20061005
120
HR100
HD206067 20061006
180
HR8959
20060831
180
HR7950
HD34642 20061017
180
HR2020 20060912
180
HR2020
HD198700


20060904
20061021
120
120
HR7950
HD218594 20061020
120
HR8959
20060903
36
HR8959
HD26965 20061212
180
HR2020
20060913
20061001
180
120
HR2020
HR2020
HD39425 20061212
36
HR2020
20061001
36
HR2020
HD38392 20061017
600
HR2020 20061001
360
HR2020
HD4730 20061008
240
HR100 20060903
96
HR100
HD191408


20060905
20061001
180
180
HR7254
HR7950
HD9138 20061019
144
HR607 20060910
90
HR607
HD720 20061012
144
HR100 20060910
144
HR100
HD32440
20070106
320
HR705
20070104
320
HR705
HD63425B 20061212
240
HR2672
20061212
240
HR2672
HD113538
20070104
1000
HR4933
20070106 1000
HR4933
HD2490 20061015
144
HR100 20060911
144
HR100
HD112300


20070116
30
HR5107


The standard group of observations included a science target, one or two telluric stars (giving better airmass match if observed before or after the target, or if observed before or after the target transited), a set of calibrations comprising three arcs and a set of 10 GCAL flats. Calibrations (arcs and flats) were usually observed right after the science target, or after a set of targets was observed, but before the grating was moved to another configuration.

Observing sequences were defined as several (2-5) repeats of ABBA sequences, with an p=4" offset between A and B positions. This translates to a offset perpendicular to the  long axis of the IFU field-of-view, large enough to move from a centred object completelly off to sky (although in some of the cases where the seeing was really poor it was still possible to detect the wings of the PSF in the B position).  On-target efficiency with this setup gets reduced by 50%, but it avoids the problem of overlapping PSF wings due to the small size of the IFU if trying to dither on source.

Exposure times were calculated using the GNIRS ITC for two cases: (a) the maximum exposure time that would not saturate a single exposure (1 coadd) under IQ=70%, CC=50% conditions; and (b) the integration time per exposure required to obtain the desired signal to noise under IQ=Any, CC=90% conditions. A large number of coadds (rather than a longer integration time per exposure) was used to go from (a) to (b), thus avoiding the risk of saturation if observations were carried out under variable conditions (for example, clear patches between clouds).  The same procedure was used to define  the telluric standard observations.

The GNIRS data frames as delivered from the Gemini Science Archive are in the standard Gemini MEF (Multi-Extension Format), where the primary header unit (PHU, extension [0]) includes all header information from telescope, environmental monitoring system and instrument; and the data extension [1] contains the pixel values.

Data reduction was performed using the tasks in the gemini.gnirs IRAF package, the released Version 1.9, of July 28, 2006, and comprised the following steps:

Calibrations - Flats and arcs
  1. nsprepare: reformats the files to add the IFU Mask Definition File and applies the linearity correction to the data. The resulting file contains the PHU, one binary table extension with the MDF and one data extension [SCI,1] with the actual pixel values.
  2. nsreduce: cuts each of the 21 IFU slices according to the MDF inserted above to a separate SCI extension. No dark correction is applied to either flats or arcs.
  3. nsflat: combine the ten frames by extension, using ccdclipping for rejection and normalizing by the median of the illuminated area in each slice (as defined by the MDF). In average, the processing resulted in a S/N ~200-300 for each extension, with exception of slices 1 and 21 (which are partly vignetted) and slice 13 (which is damaged).
  4. used gemcombine to average the three processed arc frames to improve visibility of faint lines.
  5. nswavelength: obtain wavelength solution from combined arc. Using the Ar lamp, there are four lines in the "red" setting, and six in the "blue" setting. A low order polynomial (legendre order=3) was used, with residuals of the order of 0.15Å or smaller.
Science data and telluric stars
  1. nsprepare: reformats the files to add the IFU Mask Definition File and applies the linearity correction to the data. The resulting file contains the PHU, one binary table extension with the MDF and one data extension [SCI,1]
  2. nsreduce: cuts each of the 21 IFU slices according to the MDF inserted above to a separate SCI extension. Subtract adjacent pairs of object-sky frames and divide by the flatfield.
  3. nsstack: since we had only one position with actual data (the B position was blank sky), and the target objects were bright point sources observed under poor seeing conditions, we simply stacked all A  positions without any effort to improve alignment of the individual object frames by shifting according to the offsets registered in the headers. In most cases all frames were within 0.3arcsec tolerance (according to the offsets registered in the headers), but there were a few observations where drifts of up to 0.8arcsec were seen (usually due to clouds or very poor seeing affecting guiding performance).
  4. nstransform: applied the wavelength transformation to the stacked frame
  5. nsextract: interactively extracted the spectrum from each slice, in order to exclude those with very low signal (the targets was not always well centred), the two edge slices and the damaged slice when the spectrum happened to fall within the damaged region. The output from this task is still a MEF file, with each SCI extension containing a 1D spectrum.
  6. used a simple cl script wrapped around specred.scombine to combine the valid spectra obtained in step 5. With this step, a single 1D standard FITS spectrum is created, but most of the information contained in the header of the MEF files is lost (scombine propagates the header of the first extension included in the combining list).
  7. finally, for the science data, applied the telluric correction using the standard specred.telluric task.
  8. combined the telluric-corrected spectra for those targets observed more than once.
One additional step was applied to the data presented here, which was to remove the continnum shape by fitting a low order polynomial to the final telluric corrected spectrum.


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Last update 2007 April 09 Claudia Winge