Input Spectra Format

Input spectra in FITS format are supported as either image or binary table HDUs. The wavelength grid, object type, and flux units are specified by additional columns, images, and keywords. Details below.

Input Spectra: Binary Table

HDU 0 : ignored

HDU 1 : Binary table of spectra

Required:

  • flux[nwave] or flux[nspec, nwave] column

  • Wavelength grid: - header keywords CRVAL1 and CDELT1, or - wave, wavelength or loglam column with dimensions matching flux

Optional:

  • objtype[nspec] column, or header keyword OBJTYPE. Default ‘STAR’. See below for details on OBJTYPE.

  • FLUXUNIT header keyword (see below)

Other columns and HDUs may be present and will be ignored.

Input Spectra: Image HDU

HDU 0 image

  • flux[nspec, nflux]

  • FLUXUNIT header keyword (see below)

Wavelength grid defined by one of these:

  • HDU 0 keywords CRVAL1 and CDELT1 with optional LOGLAM flag (see below)

  • HDU EXTNAME='WAVELENGTH' image with wavelength grid in Angstroms - wavelength[nflux] or wavelength[nspec, nflux]

  • HDU EXTNAME='LOGLAM' image with wavelength grid in log10(Angstroms) - loglam[nflux] or loglam[nspec, nflux]

Object type defined by one of these:

  • HDU 0 keyword OBJTYPE (if all objects are the same)

  • HDU EXTNAME=’TARGETINFO’ binary table - OBJTYPE column required - other columns are user-specific and will be ignored

Flux Units

Flux unts are specified by one of the following:

  • TUNITnn keyword for binary table flux column

  • FLUXUNIT keyword in same HDU as image/table with flux

Options are:

  • Treated as function values to be multipled by bin width: - erg/s/cm^2/A (default) - erg/s/cm^2/A/arcsec^2 - photon/A - photon/A/arcsec^2

  • Treated as delta functions at each given wavelength: - photons - erg/s/cm^2 - erg/s/cm^2/arcsec^2

For example, an astromical object is typically in units “erg/s/cm^2/A” and will be converted to photons using all throughput terms of the throughput model. A sky spectrum may be in “erg/s/cm^2/A/arcsec^2” and will be multiplied by the area of the fiber instead of having a fiber input geometric loss applied.

Wavelength Grid

If the wavelength grid is not specified by a binary table column or an image HDU, it may be specified by header keywords CRVAL1 and CDELT1 and optionally LOGLAM (0/1 for linear/log10, default=linear=0).

e.g. to specify wavelengths [3600, 3601, 3602, …]:

CRVAL1  =                 3600 / Reference wavelength in Angstroms
CDELT1  =                    1 / delta wavelength

or to specify log10(wavelength) spacing [3.5500, 3.5501, 3.5502, …]:

CRVAL1  =               3.5500 / Reference log10(Angstroms)
CDELT1  =               0.0001 / delta log10(Angstroms)
LOGLAM  =                    1 / Log10 spaced wavelengths

The FITS standard also requires CRPIX1 and CTYPE1 but these are ignored.

Object Type

The default object type is “STAR” (astronomical point source), but other options may be given by either

  • OBJTYPE header keyword, or

  • objtype[nspec] column of binary table format

Options:

  • SKY : Geometric losses of input fiber size are not applied

  • CALIB : Calibration sources such as arc or flat lamps, or tuneable laser systems. Atmospheric extinction is not applied, nor are geometric losses of input fiber size.

  • STAR or anything else: treated as an astromical object with all sources of throughput loss applied.