Glossary

The terms these pages use without stopping to define them. Each entry links to the page that goes into detail.

ADC ceiling: The largest value the analog-to-digital converter can report (adc_max, 65535 for a 16-bit sensor), expressed in electrons as adc_max * gain. A pixel saturates at the smaller of the full well and the ADC ceiling; that per-frame level is written to the SATLEVEL header card and the per-pixel result to the SATMASK extension.
ADU: Analog-to-digital unit, the integer a camera reports for each pixel. It is the electron count divided by the gain, plus the bias level, clipped to the range of the ADC. image_adu is in these units; see Noise and saturation.
Airy: The diffraction pattern of a circular aperture: a bright core surrounded by faint rings. It is the in-focus PSF model, computed at each star’s effective wavelength through the band.
asinh stretch: A display scaling that is linear for faint pixels and logarithmic for bright ones, so a sky-level star and a saturated one can be seen in the same image. field.plot() uses it. See Plotting.
bias: A constant offset added to every pixel before digitization, in ADU. Both WCC sensors carry a bias of zero in the ETC configuration, so the clip at zero ADU removes negative read-noise excursions.
CLEAN image: The noiseless expectation image in electrons (image_clean), the sources plus sky and dark with no noise drawn, kept for photometric closure tests and written as the optional CLEAN FITS extension.
count rate: The number of electrons per second a star produces in the detector through the full telescope, filter and detector throughput. It comes from the ETC, once per spectral type at G = 15, and is scaled per star by its G magnitude. See From Gaia photometry to count rates.
dark current: Electrons generated in a pixel without any light, at a rate in e-/s per pixel that comes from the ETC configuration.
encircled energy: The fraction of a star’s total light that falls within a given radius of its centre, as a function of that radius. The default photometry aperture is the 95 per cent encircled-energy radius.
focus: The amount of defocus in waves: 0 is the in-focus Airy PSF, 1 and 2 use the Zemax Huygens PSF data. The WCC defocuses on purpose to spread bright stars over more pixels before they saturate. See PSFs and defocus.
full well: The largest number of electrons a pixel can hold before it saturates. The Sony IMX455 well is about 16 ke-. Together with the ADC ceiling it sets the per-frame saturation level.
Gaia DR3: The third data release of the Gaia mission, the source catalogue wcc-sim queries. Each star carries a G magnitude and the BP and RP magnitudes whose difference, the BP-RP colour, selects its spectral template. See Gaia catalogs.
gain: The conversion between electrons and ADU. In wcc-sim the gain parameter and the GAIN header card are in electrons per ADU, so a delivered frame converts back to electrons as (ADU - bias) * gain.
Huygens PSF: A physical-optics PSF computed by Zemax and bundled with wcc-etc, used for the +1 and +2 wave defocused modes. Both are annular, with a centrally depressed core.
jitter: Pointing motion during the exposure, modelled as a Gaussian blur of the PSF with a width jitter_sigma_mas in milliarcseconds.
mas: Milliarcsecond, one thousandth of an arcsecond. Plate scales and jitter are quoted in mas.
n_reads: The number of equal frames a total exposure is split into and summed. Saturation is evaluated per frame, so more reads keep bright stars unsaturated at the cost of more read noise. See Noise and saturation.
oversampling: The PSF is rendered on a grid finer than the detector pixels (11 times finer by default), shifted to the star’s sub-pixel position, and then binned down to detector pixels. See PSFs and defocus.
Pickles template: A library of observed stellar spectra sorted by spectral type. wcc-sim uses the dwarf templates to turn a Gaia colour into a spectrum. See From Gaia photometry to count rates.
plate scale: The angle on the sky covered by one pixel: 16.87 mas per pixel for the Sony IMX455 and 20.64 mas per pixel for the Hamamatsu qCMOS.
PSF: Point-spread function, the image a point source such as a star leaves on the detector. wcc-sim renders each star by placing a PSF at its position.
read noise: The Gaussian noise added each time a frame is read out, in electrons. With n_reads coadded frames the read-noise variance grows as n_reads.
scattered-light halo: The measured stray-light profile of the telescope, added around every star as a term that reaches across the whole array. Its normalisation is recorded in the SCATNORM header card. See Scattered light.
sensorfilter: The "kind:band" string that names a detector and filter in wcc-etc, such as zwo:r or qcmos:bb. A suffix such as zwo:r+1 or zwo:bb2 names a defocused configuration. See Concepts: the pipeline.
sky background: The zodiacal-light surface brightness through the filter, converted by the ETC to a rate in e-/s per pixel and added uniformly to the image.
SNR: Signal-to-noise ratio of a measurement, here the flux of a star in an aperture divided by the noise in that aperture.
stamp: The finite square image of the PSF drawn for each star, stamp_npix detector pixels on a side. Light outside the stamp is handled by the wings and the scattered-light halo.
TAN: The gnomonic (tangent-plane) projection used by the WCS, the standard choice for a flat detector imaging a small patch of sky.
WCS: World coordinate system, the mapping between pixel coordinates and sky coordinates stored in the FITS header. wcc-sim writes a TAN WCS.
wings: The analytic power-law extension of the PSF beyond the stamp edge, fitted to the stamp’s outer annulus and drawn out to where it falls below a fraction of the background noise. See PSFs and defocus.