Components to the throughput calculation

Introduction

The DESI throughput model comes from the systems engineering throughput budget spreadsheet DESI-0347, augmented with higher resolution throughput data for the spectrographs + CCDs from DESI-0334. These are combined with the KPNO extinction model ZenithExtinction-KPNO.fits and pre-calculated fiber input geometric loss in $DESIMODEL/data/throughput/fiberloss*.dat .

The various contributions to the throughput are stored in a binary table following the throughput format used by Specter:

https://github.com/sbailey/specter/blob/dev/doc/datamodel/throughput.md

It is basically a binary table with columns:

  • wavelength : in Angstroms
  • extinction : atmospheric extinction in magnitudes per airmass
  • fiberinput : geometrical loss at fiber input
  • throughput : all other throughput terms, e.g. mirrors, spectrograph, CCDs

Different types of sources are affected by different combinations of throughput terms.

Term OBJECT SKY CALIB
EXTINCTION yes yes no
FIBERINPUT yes no no
THROUGHPUT yes yes yes

Source types:

  • OBJECT: astronomical objects, affected by all sources of throughput loss
  • SKY: sky spectra do not have a geometrical loss term for the fiber input. Positioner misalignments and changes to the atmospheric PSF still get the same amount of sky light down the fiber.
  • CALIB: calibration lamps internal to the dome do not see atmospheric extinction or fiber geometric loss terms.

The sections below detail the input data used to generate this table.

Atmospheric Extinction

Affects astronomical objects and sky spectra, but not calibration exposures.

Depends upon airmass; extinction curve from ZenithExtinction-KPNO.fits included in this product.

Fiber Input

Affects astronomical objects, but not sky or calibration spectra.

Includes both PSF/aperture losses and pointing/guiding mis-alignment.

From:

  • DESI-0347v2
    • row 16 “PSF and Aperture efficiency”
    • row 27 “Lateral errors”
    • row 52 “Fiber Defocus overall budget”
    • Multiplied then interpolated with a cubic spline

All other telescope, fiber, and instrument throughputs

Affects all object types.

From DESI-0347v11 row 112 (total throughput) divided by the low resolution spectrograph throughtput row 93, then multiplied by the high resolution spectrograph+CCD throughputs in DESI-0334 -thru.txt files.

TO DO

The spectrograph throughput numbers in DESI-0334 have been superseded by as-built measurements from a variety of DocDB entries for each component of the spectrographs, as listed in DESI-0347 rows 97-110. These newer numbers are not yet included in desimodel.