Data model: spec

General Description

The spec-PLATE-MJD-FIBER.fits files contain a repackaging of all spectral data for a given plugging (plate, mjd, fiber), including both the coadded spectrum and optionally the individual exposure frames. This groups the information from spFrame, spCFrame, spFlat, spPlate, spZbest, spZline, and spAll so that for each object one only needs to read a single file.

    HDU 0  : Header info from spPlate
    HDU 1  : Coadded spectrum from spPlate
    HDU 2  : Summary metadata copied from spAll
    HDU 3  : Line fitting metadata from spZline
    HDU 4+ : [Optional] Individual spCFrame spectra [B, R for each exposure]

A "lite" version of these files also exists in the "lite" subdirectory tree. These contain HDUS 0-3, but do not have the individual frame data and thus are much smaller.

Naming Convention
spec-[0-9]{4}-[0-9]{5}-[0-9]{4}\.fits, where [0-9]{4} is a zero-padded, four-digit number containing the plate number, [0-9]{5} is a five-digit number containing the MJD number and [0-9]{4} is a four-digit number containing the fiber id.
Approximate Size
1.5 Mbytes
File Type
FITS
Written by Products
idlspec2d/bin/reformat_spectra.py and reformat_sdss_spectra.py

HDU 0: Header keywords only

Copied from spPlate with the following additions/modifications:

HDU 1 (extname COADD): Coadded Spectrum from spPlate

Binary table with columns:

Required Columns
Name Type Comment
flux float32 coadded calibrated flux [10-17 ergs/s/cm2/Å]
loglam float32 log10(wavelength [Å])
ivar float32 inverse variance of flux
and_mask int32 AND mask
or_mask int32 OR mask
wdisp float32 Wavelength dispersion (sigma of fitted Gaussian) in units of number of pixel
sky float32 subtracted sky flux [10-17 ergs/s/cm2/Å]
model float32 pipeline best model fit used for classification and redshift

HDU 2 (extname SPALL)

BOSS spectra: copy of row from spAll for this (plate, mjd, fiber)

SDSS-I/-II spectra: copy of row from specObj for this (plate, mjd, fiber)

HDU 3 (extname SPZLINE)

copy of row from spZLine for this (plate, mjd, fiber)

HDU 4 (extname EXPIDxxx): individual frames

For each exposure there is one HDU for the red camera and one for the blue. These are in the order of the EXPIDnn keywords in the HDU 0 header, and their EXTNAME keywords match the EXPIDnn keywords from HDU 0.

Required Header Keywords: Taken from spCFrame files. BOSS spectra have the addition of YPIX0 defining the y-pixel location of the first flux bin.

Binary table with columns:

Required Columns
Name Type Comment
flux float32 coadded calibrated flux [10-17 ergs/s/cm2/Å]
loglam float32 log10(wavelength [Å])
ivar float32 inverse variance of flux
mask int32 mask
wdisp float32 wavelength dispersion in dloglam units
sky float32 subtracted sky flux [10-17 ergs/s/cm2/Å]
calib float32 conversion between flux and electrons: flux = electrons*calib
x float32 x-pixel location of trace on CCD (BOSS spectra only)

Conversion between flux, sky, and electrons

How to convert flux, sky, and ivar back to extracted photons (electrons):

    obj_photons  = flux / calib
    sky_photons  = sky / calib
    ivar_photons = ivar * calib^2