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?
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 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
No comments:
Post a Comment