This might be in the FITS Header, but I couldn't find it.
I am looking at the FITS files for SDSS 1D spectro images from DR7 using the SDSS Data Archive Server at das.sdss.org (there might be an easier way to do this now with DR12).
http://das.sdss.org/spectro/ holds all FITS files generated by the spectroscopic pipelines.
The URL for a typical FITS file is
to access FITS file
mmmmm is the mjd,
pppp is the plate ID number and
fff is the fiber ID number.
It is explained here: http://classic.sdss.org/dr5/dm/flatFiles/spSpec.html
Primary HDU image: spectrum, continuum-subtracted spectrum, noise in spectrum, mask array. HDU 1 and 2: Line detections and measurements. Under most circumstances, the line measurements in HDU 2 should be used. HDU 3: Emission-line redshift measurements HDU 4: Cross-correlation redshift measurement HDU 5: Line index measurements HDU 6: Mask and per-pixel resolution
That is, for primary HDU image holds the spectrum. The first row is the spectrum, the second row is the continuum subtracted spectrum, the third row is the noise in the spectrum (standard deviation, in the same units as the spectrum), the forth row is the mask array. The spectra are binned log-linear. Units are 10^(-17) erg/cm/s^2/Ang.
Question: I see FIVE rows in the primary HDU, not FOUR. What do these values mean?
The header information states:
ARRAY1 = 'SPECTRUM' / units of (10^-17 erg/s/cm^2/A ARRAY2 = 'CONTINUUM-SUBTRACTED SPECTRUM' / units of (10^-17 erg/s/cm^2/A ARRAY3 = 'ERROR ' / units of (10^-17 erg/s/cm^2/A ARRAY4 = 'MASK ' / mask bit array
But what is the fifth array?
EDIT: Solely to prevent link rot, the URL linked below for plate spectra http://data.sdss3.org/datamodel/files/BOSS_SPECTRO_REDUX/RUN2D/PLATE4/spPlate.html presents:
File Format Number EXTNAME Type Contents HDU0 NPIX x NFIBER float image Flux in units of 10^-17^ erg/s/cm^2^/Ang HDU1 IVAR NPIX x NFIBER float image Inverse variance (1/sigma^2^) for HDU 0 HDU2 ANDMASK NPIX x NFIBER 32-bit int image AND mask HDU3 ORMASK NPIX x NFIBER 32-bit int image OR mask HDU4 WAVEDISP NPIX x NFIBER float image Wavelength dispersion in pixels HDU5 fields PLUGMAP binary table Plug-map structure from plPlugMapM file HDU6 SKY NPIX x NFIBER float image Average sky flux in units of 10^-17^ erg/s/cm^2^/Ang
The data I see for
HDU0: 127.76799774 127.74199677 127.71600342 127.68900299 129.7460022 HDU1: 3.67299652e+00 3.88988495e+00 4.13349915e+00 4.10649872e+00 HDU2: 7.47320271 7.04110146 6.8415041 6.68683195 6.52122736 6.4465971 HDU3: 8.38860800e+07 1.67772160e+07 1.67772160e+07 0.00000000e+00 HDU4: 5.96072006 5.84291983 5.67241001 5.57223988 5.43919992