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Astra Data Products

Data products that are produced by Astra can be categorized in the following ways:

  • A summary of all observed targets is stored in the `mwmTargets` file, which includes source-level information (e.g., identifiers, cartons, astrometry, and photometry). The `mwmTargets` file lists all stellar sources observed thus far in Milky Way Mapper, regardless whether they were observed with BOSS, APOGEE, or were part of APOGEE in Data Release 17. The `mwmTargets` file has very little information about the spectra we have collected for those targets, just the number of spectra that we have from APOGEE and BOSS. This file is more lightweight than mwmAllStar and mwmAllVisit, discussed below.
  • Summary of all observed spectra. There are two kinds of summaries here:
    • the `mwmAllVisit` file describes the visit-level spectra (nightly stacked exposures of a given source); and 
    • the `mwmAllStar` file describes at the star-level (stacked visit spectra for a given star and instrument).

We do not coadd spectra across instruments that were observed at from both observatories (e.g., we do not co-add APOGEE spectra from LCO with APOGEE spectra from APO).

  • Spectrum data products. These are value-added data products constructed from reduced products from the data reduction pipelines. There are two files created for each `sdss_id`. These are:
    • `mwmVisit`: this contains all rest-frame resampled visit spectra from all telescopes and all instruments for a given `sdss_id`; and
    • `mwmStar` this contains all co-added rest-frame resampled spectra from all telescopes and all instruments for a given `sdss_id`.
  • Pipeline summary tables include results from a pipeline, but they do not include data arrays (e.g., best-fit model spectra are not included in pipeline summary tables). There are two kinds of summary tables here:
    • `astraAllVisit<PIPELINE>`: this contains all `<PIPELINE>` results for all visit-level spectra.
    • `astraAllStar<PIPELINE>`: this contains all `<PIPELINE>` results for all stacked spectra.
  • Pipeline spectrum data products include pipeline results for a single `sdss_id`. For pipelines which do not produce data arrays (e.g. best-fit spectra), these do not contain any information that is not already found in the pipeline summary tables, `astraAllVisit` and `astraAllStar`. Consistent with other data products, the pipeline spectrum-level products are distinguished by whether the input spectra were visit- or star-level products:
    • `astraVisit<PIPELINE>`: this contains the `<PIPELINE>` results for visit-level spectra for a given `sdss_id`. Everything in this file will also exist in the corresponding pipeline summary table, except the `astraVisit<PIPELINE>` file will also contain the best-fitting model spectrum, the best-fit continuum, and any other pixel arrays.
    • `astraStar<PIPELINE>`: this contains the `<PIPELINE>` results for star-level spectra for a given `sdss_id`. 

The pipeline spectrum data products all share a reasonably consistent data model and design, but might vary in detail about the fields that exist. For example, only the `astraStarASPCAP` file contains fields like `model_flux_al_h`, which is the best–fit model spectrum when we fit [Al/H], since no other pipeline produces this output.

Spectrum data product details

Astra creates `mwmVisit` and `mwmStar` spectrum data products before any pipelines are analyzed, for convenience to the collaboration and the public. These data products can then be analyzed by various pipelines within Astra.

`mwmVisit` files

The Milky Way Mapper visit (`mwmVisit`) files include all APOGEE and BOSS visit spectra taken from Apache Point Observatory and Las Campanas Observatory, and taken any time between SDSS-IV and SDSS-V. Here a ‘visit’ refers to something like a ‘nightly visit’: a single or set of exposures taken in a night from a particular telescope.

The `mwmVIsit` FITS header data units are organized in the following order, which mimics the `mwmStar` files:

  1. Source-level metadata (headers only).
  2. BOSS visit spectra acquired from Apache Point Observatory
  3. BOSS visit spectra acquired from Las Campanas Observatory
  4. APOGEE visit spectra acquired from Apache Point Observatory
  5. APOGEE visit spectra acquired from Las Campanas Observatory

The `mwmVisit` files are value-added products created from APOGEE `apVisit` files and BOSS `specFull` files. However, unlike the `apVisit` and `specFull` files, the `mwmVisit` spectra are stored in vacuum wavelengths in the stellar rest-frame. This implies that we need a reliable radial velocity to shift the spectrum to the rest-frame. If no reliable radial velocity exists, then the spectrum will still be stored in the `mwmVisit` file, but it will be stored in the observed frame.  

`mwmStar` files

The Milky Way Mapper star (`mwmStar`) files include co-added APOGEE and BOSS spectra taken from both Apache Point Observatory and Las Campanas Observatory. This file represents our best combined spectrum of a star, as observed by a given telescope and instrument combination.

The `mwmStar` FITS header data units are organized in the following order, which mimics the `mwmVisit` files:

  1. Source-level metadata (headers only).
  2. BOSS coadded spectrum from all Apache Point Observatory observations
  3. BOSS coadded spectrum from all Las Campanas Observatory observations
  4. APOGEE coadded spectrum from all Apache Point Observatory observations
  5. APOGEE coadded spectrum from all Las Campanas Observatory observations

The SDSS-V APOGEE Data Reduction Pipeline presently cannot reduce APOGEE spectra that were taken during SDSS-IV and released as part of Data Release 17. This is a result of numerous hardware and software improvements between SDSS-IV and SDSS-V (e.g., from plates to robots; installation of the Fabry Perot Interferometer), which naturally lead to different data characteristics. The SDSS-V APOGEE Data Reduction Pipeline team naturally prioritized the processing of SDSS-V data. 

When Astra creates a mwmStar or mwmVisit file from SDSS-IV APOGEE DR17 spectra, the data arrays (flux, inverse variance) are copied exactly from the SDSS-IV apStar file. No re-sampling or interpolation is done on the data.

If a star was observed with APOGEE in SDSS-IV and with APOGEE in SDSS-V, then this is a special case. In these situations astronomers will find the mwmVisit file includes spectra from both SDSS-IV and SDSS-V, and those spectra can be differentiated by the `apred` column. However, in the mwmStar file we only use the SDSS-V visits to produce a co-added spectrum: the SDSS-IV visits are not used in the co-add. This can be checked by the `in_stack` field in the mwmVisit file: if the star was observed with APOGEE in SDSS-IV and SDSS-V then the `in_stack` column will be `False` for all SDSS-IV observations. The reason that we do not currently co-add spectra between SDSS-IV and SDSS-V is because they were reduced differently, and we have not fully quantified potential differences in the reduced spectra. This is in line with the decision not to coadd APOGEE spectra from different telescopes. Expert users should know that these decisions will be reconsidered for Data Release 20: analyses should not hard-code in this assumption.

Keywords from the SDSS-IV APOGEE DR17 files are homogenised to the SDSS-V keywords and the Astra glossary. For example, what was called V_HELIO in DR17 will appear as V_RAD in the SDSS-V files. There are a very small number of these translations that have been done to include the SDSS-IV APOGEE DR17 spectra in Milky Way Mapper as seamlessly as possible.

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