Q 1.1: What is the Hubble Legacy Archive?
Q 1.2: The archive already distributes pipeline-reduced data, why is there a separate HLA?
Q 1.3: What data are available in the HLA?
Q 2.1: What slitless spectra are available from the HLA?
Q 2.2: I cannot find a spectrum for ..., even though I know it was observed with the ACS/WFC or NICMOS grisms?
Q 2.3: Where can I find a list of slitless spectra in the HLA?
Q 3.1: I found a line / feature in my spectrum. Is it real?
Q 3.2:. What is the slit width of the ACS/WFC or NICMOS spectrograph?
Q 3.3:. I know of lines in the spectrum of my target, but I cannot see it in the slitless spectrum.
Q 3.4:. When I press "Search ESO/HST Archives", "Search HLA Images (at STScI)" or "Search SIMBAD Archive", I get an empty list back or the form hangs.
The Hubble Legacy Archive (HLA) is an effort by STScI, ST-ECF and CADC to make high-quality calibrated HST data available to the community. Currently, HLA provides enhanced image products and catalogs, footprints and slitless spectra from HST.
The STScI pipeline remove the instrument signature from HST data. However, the pipeline calibrated data are in most cases not usable for science. Creating catalogs, measuring magnitudes or coordinates, or extracting spectra is left to the user of the data. HLA provides such high-level products. These high-level products allow to mine HST data with much greater ease. In some cases, HLA products can directly be used for science. In addition, HLA provides tool and VO serves for browsing of the data.
With data release 3 (DR3) the HLA contains co-added ACS and WFPC2 images, STIS, NICMOS and ACS/WFC spectra, and associated data products including catalogs, foot prints and calibrated spectra. More details are available from the STScI HLA portal. In DR3, the ST-ECF has added 1235 individually validated ACS/WFC slitless spectra.
For NICMOS, spectra for about 80% of all in-focus NICMOS G141 pointings are available, a total of 2470 spectra with H-band AB magntiudes brighter than 23.5. For ACS/WFC, a sample of 1235 extracted spectra taken at two pointings close to the GOODS field south are available. Spectra which are significantly affected by bad pixels, contamination by neighboring objects or other problems were not extracted or excluded from the data release.
Currently, the HLA slitless data contains NICMOS G141 high quality spectra in uncrowded fields from in-focus datasets and a small sample of the available ACS/WFC grism data. Your target might be on a grism images with is not included in the current release, or might be on one of the images but has not yet been extracted because of contamination, low s/n or other issues.
On the HLA slitless data form, simply select an instrument and press search to get a full list of spectra. Optionally, this list can be sorted by various parameters.
The best way to judge the reality of any spectra feature is to inspect the 2d spectrum for bad pixels or other irregularities. The preview of the 2d spectrum can be used for the purpose.
ACS/WFC and NICMOS are slitless spectrographs. The size of the target determines the wavelength resolution of the 1d spectrum. The effective "pseudo-slit" can be seen as the footprint from the VirGO tool or STScI HLA interface.
The ACS/WFC and NICMOS grism spectra are very low resolution. It is possible that lines easily visible in higher resolution spectra are not visible with the ACS/WFC and NICMOS spectrographs. These spectrographs is best suited to find bright individual lines in uncrowded part of the spectrum, or broad spectral feature such as bands.
The search radius in external archives is fixed to 2 arcmin. If the an empty list is returned, it probably means that the external archive does not contain any data within the search region. If the search hangs, it probably means that the archive is temporarily down.