ADU (analog-to-digital unit)
Definition: The numerical count reported by a detector electronics chain after accumulated charge has been digitized.
Why it matters here: ADU is the raw observable in the calibration example; it is the starting point, not yet a physical flux.
Relationships: Detector gain links counts to collected electrons, while bias, dark, and flat corrections determine which part of the count is attributable to incident light.
Boundary / common confusion: An ADU is not a photon and does not have the same physical scale across cameras or gain settings.
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.
gain
Definition: The conversion factor between collected photoelectrons and digitized detector counts, commonly expressed as electrons per ADU.
Why it matters here: Gain is needed when a count measurement must enter a photon-statistics noise model in electron units.
Relationships: With gain \(g\), an idealized conversion is \(N_e=gN_{ADU}\); read noise and saturation specifications are also interpreted in this electron scale.
Boundary / common confusion: Changing gain does not create more photons or necessarily improve dynamic range; it changes the electronic conversion and noise tradeoff.
calibration frame
Definition: An exposure made to measure an instrumental contribution rather than the astronomical target itself.
Why it matters here: Bias, dark, and flat frames make the science image interpretable by separating detector behavior from sky signal.
Relationships: Calibration frames are combined into master products and applied through the reduction pipeline with their metadata and uncertainties.
Boundary / common confusion: A calibration frame is not automatically suitable merely because it has the right label; exposure, temperature, filter, and optical configuration must match.
provenance
Definition: The documented origin and processing history of a datum, including instrument settings, calibration inputs, software steps, and versions.
Why it matters here: Provenance lets another observer determine how a displayed number was produced and whether it can be reproduced.
Relationships: It connects raw files to calibrated products, uncertainty estimates, archive queries, and the final scientific claim.
Boundary / common confusion: A filename or citation alone is not complete provenance when decisive processing choices remain undocumented.