Data Retrieval

Data retrieval is possible only from the HTML-format output document. Spectra may be retrieved individually or in groups, and multiple retrieval of concatenated, high resolution spectra can be restricted to a specific wavelength interval. Spectra are delivered as FITS files, and are easily imported into standard image processing packages such as IDL, IRAF (via the external TABLES package) or MIDAS.

Single spectrum retrieval

The concatenated high resolution spectra and low resolution SILO images, grey-shaded columns of the output table, are stored at the Principal Centre in Vilspa and a mirror site, CADC in Canada, and the low resolution spectra and rebinned high resolution spectra are stored at your National Host. To fetch a single spectrum, click on the link (Low, Rebin, High or SILO) in the output table Fetch column corresponding to that spectrum. Single spectra are retrieved uncompressed and, depending on the characteristics of your browser, you will be prompted to give the local filename either before or after download. It is recommended that you use the same naming convention as that under which the data are stored on the servers:

where Camera is SWP, LWP or LWR, Image is the 5-digit image number, Dispersion is L,H or R (rebinned) and Aperture is L or S.

The sizes of these data products are:

Multiple spectrum retrieval

Spectrum selection

You may include/exclude all files of a particular type for multiple retrieval, e.g. Mark: High, will mark/unmark all high resolution spectra, and to fine-tune your selection further you may include/exclude individual spectra by checking/unchecking them in the Mark column corresponding to their location (National Host or Principal Centre/Mirror [shaded grey]).

Wavelength selection

For faster retrieval from the Principal Centre/Mirror, you may restrict multiple download of high resolution spectra to a specific wavelength interval. By default, the full camera range is assumed (and displayed for reference), but if you request a wavelength cut (in Ångstroms) for a camera, that interval rounded to the nearest Ångstrom, is applied to all selected high resolution spectra for that camera. In this case, the naming convention adopted for the retrieved spectra is CameraImageDispersionAperture_lambda1_lambda2.FITS, where lambda1_lambda2 is the wavelength cut in Ångstroms, e.g. SWP45467_1400_1750.FITS. For a given camera, if one or both input wavelengths is blank, non-numeric or lies out of camera range, the full camera range is assumed. Clicking Reset Wavelengths resets the input wavelengths to the default full-camera values.

Note: Compared with retrieval of the full camera range, wavelength selection imposes a server processing overhead of ~0.3sec/spectrum, but reduces download time by ~100(1-r)%, where r is the fraction of the camera range selected. For current network bandwidths, this results in a considerable net saving of retrieval time for all but the largest wavelength cuts.

Retrieval format

Multiple spectrum retrieval generates a packed file in either tar or ZIP format. Compression of packed files is recommended for network efficiency, in particular when downloading large data sets. ZIP files are compressed automatically, and for tar files, three compression schemes are available: bzip2, gzip, and compress. Ensure that the corresponding unpacking and decompression programs are available on your system before selecting the retrieval scheme from the pull-down menu (default tar.gz).

The following tables may be useful for helping you decide on which packing and compression scheme to use. The National Host table refers to packed files containing only low resolution spectra (Low), only rebinned spectra (Rebin) and equal numbers of low resolution and rebinned spectra (L+R), with the packed file sizes normalised to unit spectrum content, and quoted in KBytes. The Principal Centre table refers similarly to concatenated high resolution and SILO files.
National Host
Principal Centre/Mirror
The most efficient compression is achieved by bzip2, but you should be aware that its compression/decompression cycle is more than ~5 times longer than for the other schemes. This is a small overhead compared to the savings in bandwidth for downloads of SILO images from the Principal Centre/Mirror, but is a consideration for downloads of high resolution, low resolution and rebinned spectra, where the bandwidth savings of bzip2 compared to gzip are marginal. Especially, bzip2 is not recommended when retrieving HiRes wavelength cuts to avoid combining both sets of server overheads.

Retrieval of the selected spectra from the National Host or Principal Centre/Mirror servers is effected by clicking the corresponding Retrieve Marked Data buttons. You will be prompted to assign local filenames for the packed, compressed files from each server either before or after download, depending on the browser you use. Ensure that you choose different filenames for the downloads from each server; it is recommended that you name them according to the standard naming convention for the packing and compression scheme chosen (that is, file.tar.gz, file.tar.bz2, file.tar.Z, file.tar or file.zip).

Note: the compress algorithm produces zero compression for a packed file consisting solely of concatenated, high resolution spectra.

Home | HelpDesk | Overview | LAEFF

Last update: Jun 30, 1999.