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Spectral Templates - Observations and Data Reduction

This page contains an abbreviated description of the observations and data reduction. More details can be found in the Winge et al. 2009 publication indicated before. Users are particularly referred to that paper for examples in the usage of the templates.


The GNIRS sample:

The observations were done using the Integral Field Unit (IFU) in the Gemini Near-Infrared Spectrograph (GNIRS) at Gemini South, 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. All GNIRS data were obtained as part of programme GS-2006B-DD-3.

Note that the IFU mode is no longer available with GNIRS at Gemini North.


Star
"Blue" setting
"Red" Setting
UT date
Exp time (s) Tell. std UT date Exp time (s) Tell. std
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

Each group of observations included a science target, one star (two standards were provided, giving better airmass match if observed before or after the target, or if observed before or after the target transited, but only one observed), 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 a 4" offset between the A and B positions. This was set as an 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 is 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 retrieved 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, release 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 were partly vignetted) and slice 13 (which was 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 PHU 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.
  9. added back the header information lost in step 6, corresponding to the content of the PHU of the corresponding MEF frame obtained from step 5. (new on Version 1.5)

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.




The NIFS Sample:


The data were obtained with NIFS on the Gemini North telescope with the K_G5605 grating and HK_G0603 filter, resulting in a FWHM for the arc lamp lines of ~3.2Å. Each observation consisted of five individual exposures, with the star centred on the array then offset to each corner. The table below lists the observations, where for each object we present the date(s), associated programme ID, total exposure time on-source and the hot star used to correct for the telluric lines.



Star
UT date ProgID
(GN-20)
Exp time
(s)
Telluric
std
NIFS Sample 1 (V1.5)
HD210885 
 20071015
06B-Q-107
150
 HIP109911
HD107467
 20060118
06A-SV-123
225
HIP56147
HD105028
20070430
07A-Q-25
30
HIP55564
HD10598
20061230
06A-SV-123 225
HIP8535
BD+44337
20061230 06A-SV-123 150
HIP7291
HD109655
20060201
06A-SV-123 50
HIP6141
HD3989
20071015
06A-SV-123 150
HIP4129
HD30354
20070130
06A-SV-123 675
HIP22348
HD236791
20070102
06A-SV-123 75
HIP7291
HD27796
20061230
06A-SV-123 75
HIP20674
HD235774
20071015
06A-SV-123 300
HIP109911
NIFS Sample 2 (V2.0)
HD109053
20070624
07A-Q-45 48
HIP68120
HD139195
20060722
06A-C-11
40
HR5685
HD108164
20060213
06A-C-11
540
HIP59394
HD124440
20070504
07A-Q-45 50
HIP68120
HD162211
20060722
06A-C-11
160
HIP93194
HD166229
20060722
06A-C-11
40
HIP93194
HD339034
20070505
07A-Q-62 160
HIP96153
HD129975
20070508
07A-Q-62 480
HIP72220
HD121447
20060213
06A-C-11
360
HIP59394
HD613
20071005
07A-Q-62 480
HD221491
HD181596
20060722
06A-C-11
120
HIP95853
BD+44 337
20071006
07A-Q-62 900
HD14212
HD201065
20060722
06A-C-11
80
HIP111169
Ves145
20070624
07A-Q-62 48
HD186440
BD+03 2954
20070603
07A-Q-45
660
HIP50459
HD118290
20070503
07A-Q-62 40
HIP65198
BD+09 4750
20071004
07A-Q-62 40
HIP196544
BD+59 274
20071004
07A-Q-62 340
HIP5361
BD-01 3097
20070508
07A-Q-62 200
HIP74689
BD+39 4208
20070502
07A-Q-62 40
HIP99359

The data reduction was accomplished using tasks in the gemini.nifs IRAF package. The reduction procedure included trimming of the images, flat-fielding, sky subtraction, wavelength and s-distortion calibrations. We have also removed the telluric bands and flux calibrated the frames by interpolating a black body function to the spectrum of the telluric standard star. Finally, the continuum shape was removed from the spectruum of each star (using the IRAF task continuum), normalizing all fluxes to unity.

Similar to the GNIRS spectra, the extraction procedure results in loss of the primary header content. This was added back to the spectra presented here in order to propagate the relevant instrument/telescope information.

Last update 2012 June 23, Cláudia Winge