Detection
Stamp: 63×63 px
EVERY BLOB GETS A NUMBER.
Detection turns the difference image into a catalog — one row per blob above the noise, each with a position, a brightness, a shape, and a saved cutout. This tutorial covers what gets measured, and why the threshold is deliberately loose. Nine tutorials, read in order — this is the fourth.
Unlike the Overview page, nothing on this page is invented. This describes the real pipeline — the same one the viewer, survey and cell browsers actually run.
Finding and measuring the changes
Every blob above the noise gets a position, a brightness, and a shape.
Detection runs directly on the difference image built in the previous stage — the residual is the search space now. A segmentation algorithm scans it at a deliberately inclusive 3σ threshold: a missed faint source is permanent, while a false detection only costs a later classifier a few milliseconds. It runs on both signs of the residual — positive for something that brightened or appeared, negative for something that faded — because both are real signal worth keeping.
Position where it sits on the sky.
Right ascension and declination, read straight off the frame's WCS solution.
Flux how much light changed.
The summed brightness of the residual itself.
SNR how confident the call is.
Signal measured against the local noise floor — the same number the 3σ threshold is set against.
Shape elongation and ellipticity.
Near 1.0 for a point source, higher for something smeared into a streak — the tell a later stage uses to separate a star from a mover.
Every detection is saved as a 63×63 pixel image cutout.
That size was landed on independently, then checked against ZTF's own official alert-stamp convention — and matched it exactly. A small, unplanned confirmation that a from-scratch approach converged on the professional standard.
Go watch it run.
Everything above is the real pipeline, explained in the order it runs. The browsers below are that same pipeline's actual output — a working ZTF difference-imaging system, not a mockup of one.
Or read what this project actually is