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.
| 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 |
| CREATOR | VO creator ID |
| 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 |