Data and software: 'Measuring Fiber Positioning Accuracy and Throughput with Fiber Dithering for the Dark Energy Spectroscopic Instrument'

Supplementary material to the DESI publication "Survey Operations for the Dark Energy Spectroscopic Instrument".

The main "figures.py" script generates the figures in the paper from the included data files.  The software relies on the DESI software stack available at github.com/desihub, and conveniently available at NERSC in the default DESI environment.  The main fiber dither analysis code is currently housed in the 'desicmx' product on desihub (github.com/desihub/desicmx).

Contents:

A .tar.gz file, containing the following:

Software files

• figures.py: Main dither analysis plotting routines.  Uses the github.com/desihub/desicmx/analysis/dither package for some bits of the analysis.  The "make_all_figures" routine generates all of the figures in the paper. 
• util_efs.py: plotting routines

Data files

• dither202*.fits: data & analysis results for each dither sequence.  Each sequence is identified by the night it was taken on, and a number of files are produced for it.  We describe one example here:
  • dither20210916.fits
    
    • All information entering into the fit from the sequence.  Contains three extensions, one for each of the BRZ bands.  Each extension contains the following:
      • expid - exposure number
      • exptime - shutter open length
      • mjd_obs - MJD of exposure
      • targetid - DESI target ID of target for each target
      • camera - camera used for computing flux
      • target_ra, target_dec - ra/dec of target
      • fiber - fiber number
      • objtype - morphological type of target from imaging; PSF for dither targets
      • flux_g,r,z - imaging fluxes from Legacy Survey in grz bands
      • spectroflux - spectroscopic flux from integrating over the DESI spectrum
      • spectroflux_ivar - inverse variance corresponding to spectroflux
      • delta_{x,y}_arcsec - intended fiber dither
      • {x,y}focal - position of fiber in focal plane
  • dither20210916-{B/R/Z}.fits - dither analysis for each of the BRZ filters.  Two extensions.
    • First extension: dither analysis results.  Contains:
      • {x,y}tel: fit telescope offsets (arcsec)
      • d{x,y}tel: uncertainty in {x,y}tel
      • targetid: DESI target id
      • transparency - fit transparency for each exposure
      • dtransparency - uncertainty in transparency
      • psfparam - fit PSF parameters; array of [FWHM, Moffat index, ellipticity in the x direction, ellipticity in the y direction]
      • dpsfparam - uncertainties in psf parameters
      • starflux - predicted total star fluxes before transparency & fiber aperture losses
      • dstarflux - uncertainties in starflux
      • {x,y}fiboff - predicted systematic fiber offset (the ~chief result of this work)
      • d{x,y}fiboff - the uncertainty in {x,y}fiboff
      • chi2fib - the chi^2 of the fit for this fiber
      • chi2fibnull - the chi^2 of the fit for this fiber for an extremely dumb null hypothesis where the true flux entering the fiber is always equal to zero.
      • guessflux - a guess for the starflux based on the imaging magnitude; basically the imaging flux times an appropriate constant depending on the size of the DESI mirror, the width of the spectral bandpass, and the DESI system throughput
      • fiber - the fiber number
      • expid - the exposure number
      • fiber_ditherfit_{ra,dec} - the predicted ra & dec of each fiber, incorporating the intended location, the telescope offsets, the intentional dither offsets, and the systematic positioning offset
      • modflux - the model flux entering the fiber for each target & exposure.
    • Second extension: the data on which the dither results are based.  This is just copied from the ditherYYYYMMDD.fits files for the appropriate filter, and selected down to just the fibers for which there was enough information to do the full fiber dither fit.
• gfa-matched-coadds-ditheronly.fits: information recorded by the DESI guide-focus-array cameras for each dither exposure.  Fields used by the dither analysis include:
  • expid - the exposure id
  • night - the night the exposure was taken on
  • fwhm_asec - the FWHM measured by the guider in arcsec
  • fiber_fracflux - the fraction of the flux the guider estimates would enter the fiber for a point source perfectly centered in the aperture
  • transparency - the transparency of the exposure estimated from the guider
  • airmass - the airmass of the exposure
• simulation-results.pkl: results from the simulation.  This is a nasty python dictionary.  The top level of the dictionary contains the name of the scheme that was tested, with names like 'triangles 0.8' saying that it corresponds to the 'triangles' scheme of dithers with a scale of 0.8 arcseconds.  Each entry contains a list of two items.  The first item is the simulation truth values, and the second item is the fit results.
  • truth items:
    • data: subset of information in data files needed for dither analysis
    • xtel, ytel, fwhm, transparency, xfiboff, yfiboff, starflux: true parameters for each exposure and target in the simulation
  • fit items:
    • xtel, dxtel, ytel, dytel, transparency, dtransparency, psfparam, dpsfparam, starflux, dstarflux, xfiboff, dxfiboff, yfiboff, dyfiboff, chi2fib, chi2fibnull, guessflux, fiber, expid, fiber_ditherfit_ra, fiber_ditherfit_dec, modflux
    • These are all documented above in the dither analysis results.
• data/wave*.dat: optical ray tracing results.  The dither figure uses the 'xave' and 'yave' values, which correspond to the locations in the focal plane that the ray tracing predicts each ray will land.  The different files correspond to different wavelengths, positions on the sky, and dates.

Dependencies: The included software uses the 'desimeter' and 'desimodel' packages, available on github through the desihub organization.  Otherwise it depends on the usual astronomy software stack: numpy scipy matplotlib astropy.  Alternatively, people with access to NERSC can load the default DESI environment and pull in all needed dependencies.