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
Star	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

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.
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.
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).
used gemcombine to average the three processed arc frames to improve visibility of faint lines.
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

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]
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.
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).
nstransform: applied the wavelength transformation to the stacked frame.
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.
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).
finally, for the science data, applied the telluric correction using the standard specred.telluric task.
combined the telluric-corrected spectra for those targets observed more than once.
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.