bias frame
Definition: A nominally zero-exposure image that maps the electronic offset added during detector readout.
Why it matters here: It reveals why an unilluminated pixel can have a large positive value and establishes the additive calibration floor.
Relationships: A master bias estimates the offset and read-noise pattern; a matched dark may already contain this bias pedestal.
Boundary / common confusion: A bias frame does not measure thermal charge, and subtracting both a bias and a dark that already includes bias would double-correct the data.
dark current
Definition: Thermally generated charge accumulated in a detector pixel during an exposure even when no photons arrive.
Why it matters here: It can imitate faint source signal or produce hot pixels unless a matched dark exposure or detector model removes it.
Relationships: Dark signal generally depends on exposure time and temperature and enters the additive part of the calibration and SNR models.
Boundary / common confusion: Dark current is a signal with shot noise; a dark subtraction removes its mean pattern but cannot erase the noise already accumulated.
flat field
Definition: A normalized image of relative pixel and optical throughput measured under spatially uniform illumination.
Why it matters here: Dividing by the flat removes vignetting, dust shadows, and pixel-response variations that would otherwise bias flux comparisons.
Relationships: The flat is the multiplicative term \(F\) in \(I_{cal}=(I_{raw}-D)/F\), so values above and below one rescale counts in opposite directions.
Boundary / common confusion: A flat is valid only for the optical configuration, filter, focus, and detector state in which it was obtained.
quantum efficiency
Definition: The fraction of incident photons at a given wavelength that produce collected detector electrons.
Why it matters here: It sets wavelength-dependent sensitivity and therefore affects exposure planning and count-to-flux interpretation.
Relationships: Expected electrons scale approximately as incident photons times quantum efficiency and total optical throughput.
Boundary / common confusion: Quantum efficiency is not constant with wavelength and is not the same as the gain from electrons to ADU.
vignetting
Definition: A position-dependent loss of illumination caused by the optical system, usually strongest toward the field edges.
Why it matters here: It explains the radial flat-field pattern and why equal sources can have unequal raw counts at different detector positions.
Relationships: A normalized flat maps vignetting so division restores a common photometric response across the field.
Boundary / common confusion: Flat correction compensates stable vignetting but cannot recover photons that were never detected with the same SNR as the field center.
read noise
Definition: Random electronic uncertainty introduced when the detector charge is measured and digitized.
Why it matters here: It limits faint-source measurements and makes many very short exposures less efficient than fewer longer exposures.
Relationships: For an aperture of \(n_{pix}\) pixels, the variance contribution is \(n_{pix}R^2\), added in quadrature with photon-noise terms.
Boundary / common confusion: Read noise is not removed by bias subtraction; the mean bias pattern can be removed, but stochastic read noise remains.
linearity
Definition: The regime in which recorded counts are proportional to the number of collected electrons or incident photons.
Why it matters here: Linearity is what permits calibrated counts to be compared, averaged, and converted into relative flux.
Relationships: A linear detector obeys \(C=aN_e+b\) over its usable range, allowing additive and multiplicative calibration models.
Boundary / common confusion: Near saturation or at very low signal, detector response can depart from linearity and invalidate simple scaling.