Glossary ======== The terms these pages use without stopping to define them. Each entry links to the page that goes into detail. .. glossary:: :sorted: ADU Analog-to-digital unit, the integer a camera reports for each pixel. It is the electron count divided by the :term:`gain`, plus the bias level, clipped to the range of the ADC. ``image_adu`` is in these units; see :doc:`user_guide/noise`. 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``. 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. 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 :term:`ADC ceiling` it sets the per-frame saturation level. 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. read noise The Gaussian noise added each time a frame is read out, in electrons. With :term:`n_reads` coadded frames the read-noise variance grows as ``n_reads``. dark current Electrons generated in a pixel without any light, at a rate in e-/s per pixel that comes from the ETC configuration. 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. 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 :doc:`user_guide/noise`. 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 :doc:`user_guide/fluxes`. 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 :doc:`user_guide/concepts`. focus The amount of defocus in waves: 0 is the in-focus :term:`Airy` PSF, 1 and 2 use the Zemax :term:`Huygens PSF` data. The WCC defocuses on purpose to spread bright stars over more pixels before they saturate. See :doc:`user_guide/psf`. 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. 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. 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. 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. 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 :doc:`user_guide/psf`. 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 :term:`wings` and the :term:`scattered-light halo`. 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 :doc:`user_guide/psf`. 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 :doc:`user_guide/scatter`. WCS World coordinate system, the mapping between pixel coordinates and sky coordinates stored in the FITS header. wcc-sim writes a TAN WCS. TAN The gnomonic (tangent-plane) projection used by the WCS, the standard choice for a flat detector imaging a small patch of sky. 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 :doc:`user_guide/catalog`. 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 :doc:`user_guide/fluxes`. 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. 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 :doc:`user_guide/plotting`. SNR Signal-to-noise ratio of a measurement, here the flux of a star in an aperture divided by the noise in that aperture. 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.