Reading individual spectrum files

Every SDSS spectrum is stored in a so-called spSpec file. This file contains the spectrum along with derived quantities such as redshift, classification, tables of line fits and line indices, etc.. The spSpec naming convention is spSpec-mmmmm-pppp-fff.fit, where mmmmm refers to the mjd of observation, pppp the plate number and fff the fiber number. There are 1987 different plate/mjd combinations in the DR7 data, while the fiber numbers runs from from 1 to 640. Note that the same object is usually on the same plate in all data releases, but the MJD and fiber number may change from release to release (if better data become available).

The spSpec file is a fits image with binary table extensions. Here we provide a overview of the contents of an spSpec file; please consult the spSpec data model page for complete details.

The spSpec file's header contains several parameters such as object identification and coordinates, observing information, spectral classification, redshift, etc.. The fits image contains a 4x(roughly 4000) image whose first row is the spectrum, second the continuum subtracted spectrum, third the error, and forth a bitmask. Notice that the wavelength vector is not contained in the image, but must be generated from parameters in the header. SDSS spectra are binned in constant Log( ) and the wavelength can be obtained from the header parameters COEFF0 and COEFF1 (or alternatively CRVAL1 and CD1_1) as follows:

= 10^{(COEFF0 + COEFF1*i)}, where i denotes the (zero indexed) pixel number.

Remember that these are vacuum wavelengths. Six binary tables are included, most importantly HDU 2 which contains a table of gaussian fits to line positions, and HDU 3 which contains a table on line index parameters. Also included, but of less general interest, are: HDU 1 which contains line parameters used in the emission-line redshift determination; HDU 3 which contains information on emission-line redshifts; HDU 4 which contains a table of cross-correlation redshifts with all the templates, and HDU 6 which has additional mask information as well as the spectral resolution as a function of wavelength.

One can read them using standard FITS IO libraries. Alternatively, one can use routines in IDL, SM, MIDAS, and IRAF to read them. You can also use Pat Halls' IRAF routines for displaying SDSS spectra.

We provide example SM code which plots a spectrum and lists some information contained in the spSpec file (see the SM homepage if you don't have SM). Here is a page on how to display SDSS spectra with IRAF.