[AstroPy] Issues in DR14/DR15 headers read using astropy.io.fits

[Benjamin Alan Weaver]

Hello Peter,

The files you are using come from two separate reductions of the SDSS
spectroscopic data, separated by roughly 10 years.  Plate 7512 comes from
the BOSS/eBOSS survey.  The 3rd HDU contains data from the BOSS/eBOSS spAll
file (
https://data.sdss.org/datamodel/files/BOSS_SPECTRO_REDUX/RUN2D/spAll.html).
However, plate 0425 comes from the SDSS-I/II surveys, so the 3rd HDU
contains data from the specObj file instead (
https://data.sdss.org/datamodel/files/SPECTRO_REDUX/specObj.html) .  See
also the full description of spec files:
https://data.sdss.org/datamodel/files/BOSS_SPECTRO_REDUX/RUN2D/spectra/PLATE4/spec.html
.

This is a copy of a response from the SkyServer helpdesk that was sent on
17 July.

Kia ora koe,
Benjamin Alan Weaver

On Fri, Jul 26, 2019 at 3:27 PM Peter Dzwig <pdzwig at summaventures.com>
wrote:

> I am looking for some general help with FITS Headers
>
>
>
> I have two (SDSS) files:
>
> (1) remote_filename =
> '
> https://dr15.sdss.org/optical/spectrum/view/data/format=fits/spec=lite?plateid=0425&mjd=51898&fiberid=0440
> '
>
> (2)remote_filename =
> '
> https://dr15.sdss.org/optical/spectrum/view/data/format=fits/spec=lite?plateid=7512&mjd=56777&fiberid=0396
> '
>
> I wrote some Python code using astropy which extracted the spectrum of
> an object from the second HDU of the appropriate .fits file. In
> particular I wanted to extract certain fields
>
> For example:
>
> f = fits.open(remote_filename)
>
> specdata2 = f[2].data
>
> RA = specdata2[0][59]
> Dec = specdata2[0][60]
>
> # Classification
>
> Class = specdata2[0][61]
>
> # Redshift and error
>
> redshift = specdata2[0][63]
> redshift_error = specdata2[0][64]
>
> used to work fine. In the case of (1) above, it does. In the case of (2)
> it just produces junk. I used the formulation for a large number of
> files from DR12
>
> FWIW I am running Python 3.7 over Ubuntu 18.04. I think that is probably
> irrelevant as a colleague who is running in another configuration has
> the same sort of problem
>
> Any ideas why?
>
> It seems that there might be the following reasons:
>
> (i) there is a change in the formats (somewhere) in DR15
>
> (ii) something has changed in the way io.fits treats some HDUs that I
> have missed.
>
> Is there perhaps a cleaner way of getting this info?
>
> I have posted this to the Skyserver Helpdesk without any response. Am
> happy to take this offline if people want.
>
> Thanks for any help,
>
> Peter
>
>
>
> --
>
> Dr. Peter Dzwig
> _______________________________________________
> AstroPy mailing list
> AstroPy at python.org
> https://mail.python.org/mailman/listinfo/astropy
>


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