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HLA NICMOS G141 data

NICMOS G141 spectra

The HLA NICMOS grism archive includes NICMOS G141 grism data up to July 2007. The grism images and related undispersed filter images were combined into more than 800 associations. Data Release 1 includes spectra with little of no contamination from neighbouring objects, no or insignificant image defects on spectra. Spectra from about 80% of all associations were extracted. Examples of some high quality spectra are shown in Figure 1.

The sources are identified on the combined undispersed filter images. These direct images are used both to identify the locations of the objects in order to extract in the dispersed images and to derive their relevant morphological parameters and to identify nearby objects that could possibly contaminate the various spectra. In this release, we also use external 2MASS sources to better evaluate the contamination on sources close to the boundaries of the direct images.

The astrometry of the original HST frames has been improved whenever possible by cross-correlating the direct images with external reference catalogs (for that purpose, we used UCAC2, 2MASS, SDSS-DR5, USNO-B1.0, and GSC2.3.2). The use of different catalogs and of a specific matching algorithm allowed us to correct the astrometry for almost all datasets, and as a results 96% of the presented spectra have corrected coordinates; in addition, in 87% of cases we were able to use more than a single object to refine the astrometry (see Figure 2). The final estimated median astrometric accuracy is approximately 0.2 arcsec.

The photometric accuracy of the undispersed image was verified using the 2MASS Point Source Catalog. The observed scatter is approximately 0.15 mag in the H band (F160W filter), but occasionally larger error are observed (Figure 3). Within our precision, there are no sign of non-linearity in the whole range of magnitudes.

The accuracy of the flux calibration of the spectra is better than 10%. The flux calibration was independently verified by comparing the total integrated flux within the F160W wavelength window with the magnitude as derived from the undispersed images in the same filter; the results obtained are shown in Fig. 4. For further details, we refer to the draft of the HLA data paper.

The NICMOS HLA archive returns for each target a cut-out of the undispersed image, a 2-dimensional spectrum (position versus wavelength), and a 1-dimensional integrated spectrum of the whole target. These products are provided as VO-compatible FITS files and as static preview images. The FITS files contain metadata in the form of FITS keywords. A description of the keywords for the 1d spectra is listed in Table 1.

Figure 1: Examples of NICMOS G141 slitless spectra reduced for and available via the NICMOS HLA

Figure 2: The histogram shows the fraction of released spectra with coordinates corrected using a given number of external reference objects

Figure 3: Direct image F160W magnitudes versus the 2MASS H-band magnitudes. The red line shows the ideal locus where the two magnidues are identical.

Figure 4: The "spectroscopic" magnitude, i.e. the magnitude obtained from the integration of the spectral flux within the total filter throughput, as a function of the direct image F160W magnitude. Darker crosses corresponds to objects with increasing contamination from nearby sources in their spectra.

Table 1:The metadata of the 1-dimensional integrated fits spectra

Keyword Description
TFIELDS number of table fields
TTYPE1 column 1 name
TFORM1 column 1 data format
TUNIT1 column 1 unit
TUCD1 column 1 UCD
TUTYP1 column 1 UTYPE
TTYPE2 column 2 name
TFORM2 column 2 data format
TUNIT2 column 2 unit
TUCD2 column 2 UCD
TTYPE3 column 3 name
TFORM3 column 3 data format
TUNIT3 column 3 unit
TUCD3 column 3 UCD
TTYPE4 column 4 name
TFORM4 column 4 data format
TUNIT4 column 4 unit
TUCD4 column 4 UCD
TUTYP4 column 4 UTYPE
TTYPE5 column 5 name
TFORM5 column 5 data format
TUNIT5 column 5 unit
TUCD5 column 5 UCD
TUTYP5 column 5 UTYPE
TTYPE6 column 6 name
TFORM6 column 6 data format
TUNIT6 column 6 unit
TUCD6 column 6 UCD
VOCLASS data model name and version
VOSEGT dataset or segment type
VOPUB the VO publisher
TITLE dataset title
VOLOGO URL for creator logo
VERSION data release version
RADECSYS astrometric reference system
REFCAT astrometric reference catalogue
TIMESYS timescale
MJDREF zeropoint of timescale in MJD
SPECSYS spectral frame origin
DATE file creation date (YYYY-MM-DDThh:mm:ss UT)
TELESCOP the observing facility
INSTRUME the instrument used
PHOTMODE sequence of optical elements in the light path
FILTER filter of the associated direct image
TIMESDIM SI factor and dimensions
SPECSDIM SI factor and dimensions
FLUXSDIM SI factor and dimensions
FLUX_UCD flux axis ucd
SPEC_UCD spectral axis ucd
SPEC_RES spectral resolution
TIMEUNIT unit for time
CR_IDENT software version used to generate data
SPEC_FIL sampling filling factor
SPECBAND bandpass description
SKY_RES the pixel scale of the instrument [arcsec/pix]
CRETYPE dataset xcreation type
ASN_ID name of HST association
APERNUM internal spectrum identification
OBJECT the target name
RA_TARG the target right ascension [deg]
DEC_TARG the target declination [deg]
RA location of spectral axis RA [deg]
DEC location of spectral axis DEC [deg]
EQUINOX equinox
GAL_LONG target galactic longitude [deg]
GAL_LAT target galactic latitude [deg]
ECL_LONG target ecliptic longitude [deg]
ECL_LAT target ecliptic latitude [deg]
OBJSHAPA major axis object extend [arcsec]
OBJSHAPB minor axis object extend [arcsec]
OBJPA orientation of major axis in NESW [deg]
APERLEN the slit length in ["]
APERWID the "slit width" in [arcsec]
OBJTYPE object treated as poinlike (1/0 = y/n)
PHOTPLAM pivot wavelength of direct image filter [AA]
PHOTBW width of direct image filter [AA]
MAG SExtractor AB-magnitude in direct image
EMAG SExtractor AB-magnitude error in direct image
DER_SNR derived SNR
APERPA slit orientation NESW [deg]
DISPPA trace angle on the sky NESW [deg]
SPEC_BW width of the spectral axis [AA]
SPEC_VAL mean of the spectral axis [AA]
TSTART start of first exposure [MJD]
TSTOP end of last exposure [MJD]
TMID date of observation
DATALEN number of spectral points
WCSOFFRA RA offset applied to the original WCS ["]
WCSOFFDE DEC offset applied to the original WCS ["]
WCSNOBJ Number of sources used for offset determination
WCSNOBJ1 Number of UCAC2 sources used for offset determination
WCSNOBJ2 Number of 2MASS-PSC sources used for offset determination
WCSNOBJ3 Number of SDSS-DR5 sources used for offset determination
WCSNOBJ4 Number of USNO-B1.0 sources used for offset determination
WCSNOBJ5 Number of GSC2.3.2 sources used for offset determination
MAGBAND1 filter name for spectral magnitude
SMAG1 spectral AB magnitude value
SMAGERR1 spectral AB magnitude error
APERRA1 RA of first slit corner
APERDE1 DEC of first slit corner
APERRA2 RA of second slit corner
APERDE2 DEC of second slit corner
APERRA3 RA of third slit corner
APERDE3 DEC of third slit corner
APERRA4 RA of fourth slit corner
APERDE4 DEC of fourth slit corner

Maintained by Martin Kümmel <>