SDSS spectra FITS format: what do the rows mean?

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 http://das.sdss.org/spectro/1d_26/0283/1d/spSpec-mmmmm-pppp-fff.fit

to access FITS file spSpec-mmmmm-pppp-fff.fit, where 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

that

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?

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


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
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 spSpec-53166-1615-513.fit are:

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

• I'm voting to close this question as off-topic because it is not about physics.
– Danu
Sep 10, 2015 at 20:57
– Danu
Sep 10, 2015 at 21:19
• crossposted: stackoverflow.com/q/32511536 Sep 10, 2015 at 21:34
• I feel like this is on-topic since it is the official astronomy data, not someone's random files. It would be the same if the LHC published raw counts on their website.
– user10851
Sep 10, 2015 at 21:41
• In keeping with my opinion on earlier meta questions, I'd be in favor of this being on-topic because of the specificity of format questions asked: it asks about a single astrophysical data set and nothing else. That said, my opinion is not universally held, and I'm not going to take preemptive, moderator-powered action without a clear consensus. Sep 11, 2015 at 0:30