I downloaded spectra from Sloan Digital Sky Survey (SDSS), which has the following column headers flux, loglam, ivar, and_mask, or_mask, wdisp, sky and model (determined by tdump and tprint in pyraf.stsdas).

I know about flux, loglam, sky and model but I wanted to know what are the other headings are and where are they used. ['loglam' is the log10(wavelength)]

Also, is there any way from which I can determine redshift using the SDSS spectra, by not looking on the quasar page.


The answers can be found on the Documentation page:

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 in pixel=dloglam units
sky       float32   subtracted sky flux [10-17 ergs/s/cm2/Å]
model     float32   pipeline best model fit used for classification and redshift

AND and OR masks denotes rows where data is flagged in all or one columns, respectively.

As for redshift determination from SDSS spectra; you can identify emission line features like e.g. the H$\alpha$ - [N II] triplet around 6700 Å restframe, of the O II 3727+3729 doublet, or the H$\beta$ + [O III] 4959+5007 complex, and dividing their observed wavelengths by their restframe wavelengths.

  • $\begingroup$ Can you explain 'Flaged' $\endgroup$ – Syed Ali Jun 21 '16 at 4:28
  • $\begingroup$ Sorry, meant to say "flagged". You could also say "masked" - marked that for some reason there is something wrong with the data in that column. I do not know which masking system they use, so I would probably use the "or" mask to discard data where this is set. $\endgroup$ – Thriveth Jun 21 '16 at 8:19

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