comparison star
Definition: A non-variable field star measured alongside a target to track transparency and instrumental changes common to both.
Why it matters here: It provides the reference needed to decide whether a target’s apparent change is intrinsic or caused by the observing system.
Relationships: The target-to-comparison flux ratio becomes a differential magnitude or normalized light curve.
Boundary / common confusion: A comparison star must be checked for stability and suitable brightness/color; sharing the field does not guarantee suitability.
differential magnitude
Definition: The logarithmic brightness difference between a target and a comparison source measured in the same frame.
Why it matters here: It suppresses common atmospheric and instrumental variations and exposes relative variability.
Relationships: \(\Delta m=-2.5\log_{10}(F_t/F_c)\); a deeper target dip increases the differential magnitude when the comparison stays stable.
Boundary / common confusion: It is a relative measurement and does not by itself supply an absolute calibrated magnitude.
common-mode noise
Definition: Variation that affects the target and reference stars similarly, such as transparency or throughput changes.
Why it matters here: Differential photometry works by canceling this shared component before interpreting target variability.
Relationships: Flux ratios or ensemble normalization remove variations correlated across many stars.
Boundary / common confusion: Color-dependent extinction, detector-position effects, or a variable comparison may not be common mode and will not cancel cleanly.
light curve
Definition: A sequence of brightness measurements plotted or modeled as a function of time.
Why it matters here: It is the primary observable used to infer variability, transit depth, event timing, and periodicity.
Relationships: Each point combines photometric measurement, timestamp, uncertainty, and often comparison-star normalization.
Boundary / common confusion: A smooth-looking curve can still be undersampled or systematically biased; cadence and error bars are part of the result.
transit depth
Definition: The fractional decrease in observed stellar flux during an occultation or transit.
Why it matters here: It quantifies the amplitude of the lecture’s dip and is the signal that must exceed photometric noise.
Relationships: For an ideal dark planet and uniform stellar disk, \(\delta\approx(R_p/R_\star)^2\), while differential magnitude gives the observed flux ratio.
Boundary / common confusion: Limb darkening, blending, starspots, and systematics make the simple radius relation approximate.
baseline
Definition: Out-of-event measurements defining the normal brightness level against which a variation is compared.
Why it matters here: A stable baseline is required to measure the depth and significance of a dip.
Relationships: Normalization often divides the light curve by a fitted baseline constructed from pre- and post-event data.
Boundary / common confusion: A short or drifting baseline can absorb real variability or create an artificial trend.
ensemble photometry
Definition: Differential photometry that combines several comparison stars into a weighted reference.
Why it matters here: It reduces reliance on any single comparison and can improve precision and robustness.
Relationships: The ensemble reference is commonly a weighted mean flux, with weights based on uncertainty and stability.
Boundary / common confusion: Adding poor, variable, saturated, or color-mismatched stars can make the ensemble worse rather than better.