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Data Flow: Telescope to Catalogs


The SDSS's primary data products are the survey photometry and the spectroscopic measurements for the quasar, galaxy and star samples. They can be accessed as binary (.fits) tables and images through the Data Archive Server (see the archive access page). The Data Archive Server also contains intermediate data products.

The storage structure of the Data Archive Server reflects the way in which the data are acquired and processed. To help understand the archive structure and the logical relation between the various data products, this section gives a high-level description of the data flow from the telescopes through our pipelines into the archives. The details of our algorithms are described elsewhere.


The figure gives a simplified description of the data flow. Data  flow

All observations are done at Apache Point Observatory. There is a drift-scan camera for 2.5m Imaging. Based on the imaging survey, we select targets for the 2.5m Spectroscopy survey carried out with a 640-fiber spectrograph. For photometric calibration, there is a conventional CCD camera on the 0.5m Photometric or Monitor Telescope (interchangeably called PT or MT). The various pipelines produce nearly 100 different types of data files (see the Data Model for a description of all of them), both working files and scientific outputs. Note that the names of files in the Data Archive Server reflect their contents as well as the name of the pipeline which has created them.

Photometric / Monitor Telescope (PT/MT)

Associated pipelines Function Scientific output DAS filename
mtpipe Nightly extinction measurements;
Photometric calibration of secondary standard patches
Extinction and photometric calibration information tsField

2.5m Drift-scan camera

Associated pipelines Function Scientific output DAS filename
astrom astrometric calibration astrometric coefficients tsField
WCS information infpC
Postage-stamp pipeline psp PSF characterization for each pixel as a function of time PSF images and parameters PSF
photo image flat field, bias subtract, cosmic ray recognization
and object extraction
photometric coefficients tsObj

2.5m Spectrographs

Associated pipelines Function Scientific output DAS filename
target Target selection Completed Tile file tiBoundaries
spectro2d Extraction of spectra, wavelength and flux calibration, sky subtraction, combination of multiple exposures Calibrated 1-d spectra grouped into FITS files by plate (640 spectra/file) spPlate
spectro1d Object classification, redshifts determination, measurement of line strengths and line indices Measurements grouped into a fits file with the 1d spectrum (spSpec*.fits) spSpec

Last modified: Tue Aug 26 17:19:38 CDT 2003