Glossary¶

The terms this documentation uses without stopping to explain them. Roughly in the order you meet them.

Plate solving¶: Working out, from the pattern of stars in an image, exactly where the telescope was pointing and at what scale and rotation. ARGOS delegates it to ASTAP. Without a solve, a click on a star is just a pixel; with one, it is a position on the sky and therefore an identity.
WCS¶: World Coordinate System. The result of a plate solve: the transformation between pixel coordinates and \((\alpha, \delta)\) on the sky. ARGOS computes one per solved frame and does not write it into the FITS file — see FITS headers reference.
CFA · Bayer pattern¶: Colour Filter Array. A colour sensor does not measure colour per pixel; it has a mosaic of red, green and blue filters, one per pixel. The Seestar’s arrangement is GRBG. A raw frame is this mosaic, undebayered — which is why it looks grey and grainy at 1:1.
Green plane¶: ARGOS’s measurement grid: the average of the two green samples in each 2×2 Bayer tile, giving an image half the width and half the height of the raw frame. Green is used because it is the densest sampling of the mosaic. All photometry and all astrometry happen here, so a pixel in the solution is the same pixel that is measured. Defined in Differential photometry — method and equations §1.
ADU¶: Analogue-to-Digital Unit. The raw number stored per pixel — what the converter reported, not a physical quantity. Multiply by the electron gain to get electrons, which is what noise calculations need.
TG¶: The AAVSO code for an untransformed green magnitude. It is what ARGOS measures and reports. It is not Johnson \(V\), and reporting it as \(V\) is wrong.
Instrumental magnitude¶: \(-2.5\log_{10}(\text{flux})\), straight from the pixels. It is specific to your telescope, your filter and your night, and means nothing on its own. It becomes a real magnitude only by comparison with stars of known brightness.
Comparison star¶: A star of known, published magnitude in the same field, used to convert your instrumental magnitudes into real ones. Several of them together are an ensemble.
Ensemble¶: The set of comparison stars, taken together. Their average offset between catalogue and instrumental magnitude is the zero point for that frame. More stars means the accident of any one being wrong matters less.
Zero point¶: The number you add to an instrumental magnitude to get a real one, derived fresh on every frame from the ensemble. When the sky dims, every star dims, the zero point moves, and the target’s magnitude stays put — which is the whole trick of differential photometry.
Check star¶: A star you have every reason to think is constant, measured exactly like the target. It is a control: if the check star moves, the problem is your ensemble or your night, not the target. It does not check the focus.
AUID¶: AAVSO Unique Identifier. A stable name for a star in AAVSO’s system, independent of which catalogue you found it in. Comparison stars from a VSP sequence carry one; a star you clicked on your own does not, which is why it cannot contribute a zero point.
VSX · VSP¶: Two AAVSO services. VSX is the variable-star index — identity, type, period, range. VSP is the Variable Star Plotter, which publishes sequences: comparison stars with calibrated magnitudes and AUIDs, chosen for a specific field.
Aperture · annulus¶: The circle within which a star’s light is summed, and the ring around it from which the sky background is measured. Their radii and what they cost you are in Differential photometry — method and equations §3.1.
FWHM¶: Full Width at Half Maximum. How wide a star image is — the working measure of seeing and focus. In ARGOS it is in green-plane pixels, twice the size of raw pixels.
HFD¶: Half-Flux Diameter. The diameter enclosing half a star’s light. It behaves better than FWHM when badly out of focus, which is why autofocus uses it.
MAD¶: Median Absolute Deviation. A measure of scatter that a single wild value cannot inflate, unlike a standard deviation. ARGOS uses it wherever one bad frame or one bad star must not be allowed to dominate.
Airmass¶: How much atmosphere you are looking through, relative to straight up. 1.0 at the zenith, 2.0 at about 30° altitude. Everything gets dimmer, redder and noisier as it rises.
BJD_TDB¶: A timestamp corrected to the solar system’s centre of mass and on a uniform time scale — the standard for anything where seconds matter, such as transit timing. AAVSO submissions use plain JD_UTC instead; both are exported.
Sub · sub-exposure¶: One individual frame of a series, as opposed to a stack. ARGOS measures subs, never stacks.