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SpeXter - On-the-fly Extraction of Spectra
Motivation
The usefulness of current large astronomical archives are limited by the fact that users have to visually inspect and pre-select spectroscopic data. This makes it difficult for archive researchers to identify specific observations which might be of interest to them. Within the context of the AVO, this fundamental limitation should be remedied.
The aXe spectral extraction tool will be a starting point for an AVO spectral pre-viewer and data selection tool. This method was already successfully applied to both HST's ACS slitless spectroscopy data and to VLT's FORS2 multi-object slit spectroscopy.
It provides non-optimal but quick and robust bulk extraction and full calibration of spectra which can be used to browse the spectral content of a dataset stored in an archive. Such a capability will be a first, yet fundamental step towards successfully using spectroscopic data within the AVO.
Florian Kerber (ESO/ST-ECF), Norbert Pirzkal (ESO/ST-ECF)
Mar. 20, 2003
Processing Steps
ACS
- archive request: _FLT images were used
- IRAF: split mutli-extension fits into two files (science extensions 1 & 4)
- sExtractor: get target list from direct images (configurations files .sex, .par)
- aXe: extract set of 1-D spectra (configurations files a4vo_FORS.0, a4vo_FORS.1)
- return results (e.g. plot with Specview)
FORS2
- archive request: raw, master bias (once), fims (once)
- python script: patch FITS header (where needed)
- aXe: extract set of 1-D spectra (configurations files a4vo_ACS1, a4vo_ACS4)
- return results (e.g. plot with Specview)
Solved & Open Issues
ACS
| name | description | status |
|---|
| direct-grism pair files | automatically pick the right files from set of archive products for aXe processing | open |
| sextractor conf. | detection sensitivity needs to be set for each frame | open |
FORS2
| name | description | status |
|---|
| new chipset | prescan, overscan, binning | solved |
| slit positions | patch in missing positions | solved |
| out of range errors | need to divide global slit list by chip to avoid them | solved |
| reference slits | exclude targets named 'refslit' | solved |
| background subtraction | automatic? possible? necessary?1 | open |
1 use reference slits to get sky spectrum, then subtract it from image spectrum?
Sample Spectra
ACS
Some images (.pdf) of the spectra
Sc. ext. 1 - 14, 33, 40, 41, 45
Sc. ext. 4 - 21, 25, 27, 31, 34
FORS2
2 Usually master bias files are used (M.FORS2.yyyy-mm-ddThh:mm:ss.mmm.fits). If no master are present, normal biases are chosen.
Spectrum examples:
- Fig. 1 shows an example of result spectrum obtained with aXe on FORS2 image ( data set ID = FORS2.2002-06-08T01:56:08.677, slit ID = FORS2.2002-06-08T01:51:03.195, bias ID = M.FORS2.2002-10-23T08:45:16.541). In Fig. 2 there are two spectra of the same object, showing difference using/not using aXe_BE task to remove background (see a zoom in Fig. 3)
- Fig. 4 shows spectrum from image FORS2.2002-10-04T04:05:23.476 (OBJ=155; RA=53.1221583333; DEC=-27.8470055556)
Requirements
Hardware
| | CPU cycles | data volume
input - scratch - output | memory | other |
|---|
| ACS | | | | |
| FORS2 | | | | |
Software
- OS
- Unix (Solaris 8) / Linux (RedHat 7.3, SuSe 8.1)
- tools
- external libraries
- Python Scientific & Statistics libraries
- ...
Status
A number of FORS2 and ACS spectra were extracted at ESO and results have been reviewed by scientists. The goal of producing previews which are sufficient for selecting interesting observations for consequent archive retrieval has been achieved. No further efforts are scheduled at present (Jan. 2004).
A summary report (.pdf, .ppt) from M. Leoni is available.
References
Tech. Contact
MarcoLeoni, MarkusDolensky
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