References and further reading¶

Everything ARGOS relies on, and everything it hands off to. Each entry says what ARGOS actually uses it for, so you can check a claim against its source rather than take this documentation’s word for it.

Note

Where a method is cited below, ARGOS implements the published formula, not the published software. The implementation and its constants are stated in Differential photometry — method and equations; the citation tells you where the formula comes from.

Companion software¶

ASTAP

Plate solving. ARGOS shells out to the ASTAP executable and one of its star databases (two separate downloads, both required) to recover a WCS from the green plane.

Han Kleijn — https://www.hnsky.org/astap.htm

Siril

Calibration, registration and stacking of the raw frames ARGOS preserves. This is where a quick-look curve becomes a reducible dataset.

Free-Astro team, GPLv3 — https://siril.org

star_var_script

The companion reduction script: a Siril script for automatic variable-star study with the Seestar S30 Pro. ARGOS’s run-level uncertainty floor is deliberately the same model, so the live preview and the final reduction quote comparable error bars.

Jules Perret — https://github.com/jperret21/star_var_script

Stellarium

Optional pointing client. ARGOS exposes the Stellarium Telescope Protocol on TCP 10001, so you can slew by clicking in a planetarium.

Stellarium team, GPLv2 — https://stellarium.org

Standards and protocols¶

ASCOM Alpaca — the REST/JSON device interface ARGOS speaks to the Seestar (port 32323) and to the simulator. Telescope, Camera, FilterWheel and Focuser are the four device types used. ASCOM Initiative — https://ascom-standards.org/api/ · developer documentation

FITS 3.0 — the file format ARGOS writes: 16-bit unsigned, linear, still in its Bayer mosaic. The keyword conventions used are catalogued in FITS headers reference. Pence, W. D., Chiappetti, L., Page, C. G., Shaw, R. A., & Stobie, E. 2010, A&A, 524, A42 — doi:10.1051/0004-6361/201015362

AAVSO Extended File Format — the submission format ARGOS’s target export produces. The observer code goes in Settings; left unset, exports are stamped XXX. AAVSO — https://www.aavso.org/aavso-extended-file-format

TG — the untransformed green band. ARGOS measures the green plane of the CFA mosaic and reports it as TG, the AAVSO code for an untransformed green channel. It is not Johnson \(V\), and no colour transformation is applied. AAVSO, DSLR Observing Manual — https://www.aavso.org/dslr-observing-manual

Catalogues and services¶

Queried live when there is internet, then cached locally so the field still works offline. Cache locations are set in Settings → Catalogues · data.

Source	Used for	Endpoint / citation
AAVSO VSX	Variable-star identity, type, period, magnitude range	https://vsx.aavso.org/index.php · Watson, Henden & Price 2006, SASS, 25, 47
AAVSO VSP	Comparison-star sequences with AUIDs and calibrated magnitudes	https://app.aavso.org/vsp/
Gaia DR3	Field stars for overlays and identification	ESA TAP service · Gaia Collaboration, Vallenari et al. 2023, A&A, 674, A1 — doi:10.1051/0004-6361/202243940
SIMBAD	Deep-sky and stellar identity, cross-identifications	https://simbad.cds.unistra.fr/ · Wenger et al. 2000, A&AS, 143, 9
CDS Sesame	Name → coordinates resolution for arbitrary designations	https://cds.unistra.fr/cgi-bin/nph-sesame/
NASA Exoplanet Archive	Transit ephemerides, depth and duration	TAP service · Akeson et al. 2013, PASP, 125, 989 — doi:10.1086/672273

Acknowledging the catalogues

If an ARGOS session leads to a published result, the catalogues carry their own acknowledgement requirements — VSX and VSP through the AAVSO, Gaia through the ESA/DPAC statement, SIMBAD and Sesame through CDS, and the Exoplanet Archive through NASA/IPAC. ARGOS does not do this for you.

Methods and formulae¶

Each of these is implemented in argos.core.photometry or argos.core.imaging and is written out, with its constants, in Differential photometry — method and equations.

The CCD equation. The uncertainty budget of a single aperture measurement: source photon noise, sky photon noise and read noise, each summed over the aperture pixels. Implemented in argos.core.photometry.aperture; see §3.3. Merline, W. J. & Howell, S. B. 1995, Experimental Astronomy, 6, 163 — doi:10.1007/BF00421131 · Howell, S. B. 2006, Handbook of CCD Astronomy, 2nd ed., Cambridge University Press.

Aperture size and growth curves. Why a 2.5×FWHM radius is not the SNR-optimal choice, and what it buys instead. Howell, S. B. 1989, PASP, 101, 616 — doi:10.1086/132477 · Naylor, T. 1998, MNRAS, 296, 339 — doi:10.1046/j.1365-8711.1998.01314.x

MAD as a robust scale estimator. The factor 1.4826 that converts a median absolute deviation into a Gaussian-equivalent standard deviation. It appears three times in ARGOS: rejecting a bad comparison star, estimating the run-level scatter, and setting the star-detection threshold. The consistency constant is standard robust-statistics material (Hampel 1974; Huber 1981); Rousseeuw & Croux is cited here for the properties of the estimator, and argues for Sₙ/Qₙ instead of MAD. Rousseeuw, P. J. & Croux, C. 1993, JASA, 88, 1273 — doi:10.1080/01621459.1993.10476408

Important

The MAD has ~37 % Gaussian efficiency. With the 10–30 points a short run provides, any scatter ARGOS estimates through a MAD — including the systematic floor of Differential photometry — method and equations §5 — is itself uncertain at the ±20–30 % level. Treat it as an order-of-magnitude floor, not a calibration.

Successive-difference variance estimation. The observed scatter of a light curve is estimated from second differences, whose variance is \(6\sigma^2\) for independent errors. Differencing removes any locally linear trend, which is what makes the estimate usable on a genuinely variable star. von Neumann, J. 1941, Annals of Mathematical Statistics, 12, 367 — doi:10.1214/aoms/1177731677

Correlated (“red”) noise in time-series photometry. Why the point-to-point scatter of a light curve understates the uncertainty on anything measured over a longer timescale — a transit depth above all — and the binned-RMS / β-factor diagnostic that exposes it. Pont, F., Zucker, S., & Queloz, D. 2006, MNRAS, 373, 231 — doi:10.1111/j.1365-2966.2006.11012.x

Scintillation. The atmospheric noise floor that no amount of exposure on a small aperture removes, and which dominates bright-star photometry through a 30 mm objective. Young, A. T. 1967, AJ, 72, 747 — doi:10.1086/110303 · Osborn, J., Föhring, D., Dhillon, V. S., & Wilson, R. W. 2015, MNRAS, 452, 1707 — doi:10.1093/mnras/stv1400

Ensemble differential photometry. The idea of building a zero point from many comparison stars rather than one. Honeycutt, R. K. 1992, PASP, 104, 435 — doi:10.1086/133015

Warning

ARGOS does not implement Honeycutt’s method. Honeycutt solves a global least-squares system over a whole inhomogeneous set of exposures; ARGOS forms a clipped mean zero point independently on each frame, which is what a live preview can do. The citation is context, not attribution.

Airmass. Pickering’s formula, accurate to better than 0.01 airmass down to 1° altitude and finite at the horizon, unlike a naive \(\sec z\). The same function fills the FITS AIRMASS header and the light-curve metric, so the two cannot disagree — see argos.core.imaging.sky_geometry. Pickering, K. A. 2002, DIO, 12, 20 — https://www.dioi.org/vols/wc0.pdf

Julian date. The Fliegel–Van Flandern integer day-number algorithm, used for the fast per-frame JD; BJD_TDB is delegated to astropy. Fliegel, H. F. & Van Flandern, T. C. 1968, Communications of the ACM, 11, 657 — doi:10.1145/364096.364097

Barycentric Julian Date in TDB. Why BJD_TDB — and not HJD, and not JD_UTC — is the time standard a transit or a period study should be reported in. Eastman, J., Siverd, R., & Gaudi, B. S. 2010, PASP, 122, 935 — doi:10.1086/655938

Half-flux diameter (HFD). The focus metric ARGOS samples during an autofocus sweep and writes to the HFD header: the diameter enclosing half the star’s flux. It is better behaved than FWHM far from focus, which is exactly where an autofocus routine has to work. Weber, L. & Brady, S. 2001, Fast Auto-Focus Method and Software for CCD-based Telescopes — the whitepaper behind FocusMax; the publisher’s copy is gone, so this links to the Internet Archive: archived PDF

What ARGOS does not implement

Star detection is a MAD-based threshold followed by 3×3 local maxima — it is not DAOFIND (Stetson 1987, PASP, 99, 191) and does not fit a PSF. It exists to count stars, measure focus and let you click on one; it is not a source-extraction pipeline. Photometry is aperture photometry only: no PSF fitting, no deblending, no crowded-field solution.

Scientific context¶

Background reading for the observing side rather than the code.

AAVSO Guide to CCD/CMOS Photometry — the practical reference for comparison star choice, ensemble construction and what makes an observation submittable. https://www.aavso.org/ccd-photometry-guide

AAVSO Manual for Visual Observing / VSX target selection — how to find a variable worth an evening. https://www.aavso.org/observing

Exoplanet Transit Database (ETD), now part of the VarAstro portal — amateur transit timing, and a realistic picture of what depth is detectable with what aperture. https://var2.astro.cz/ETD

AstroImageJ — the reference free tool for transit photometry and fitting. Useful as a cross-check on any ARGOS transit. Collins, K. A., Kielkopf, J. F., Stassun, K. G., & Hessman, F. V. 2017, AJ, 153, 77 — doi:10.3847/1538-3881/153/2/77

Python libraries¶

ARGOS’s runtime dependencies, in the order they matter scientifically.

Astropy — FITS I/O, WCS, coordinate frames, time scales and the BJD_TDB correction. Astropy Collaboration 2013, A&A, 558, A33 · 2018, AJ, 156, 123 · 2022, ApJ, 935, 167 — https://www.astropy.org

NumPy — every array operation in the imaging and photometry paths. Harris, C. R. et al. 2020, Nature, 585, 357 — doi:10.1038/s41586-020-2649-2

Alpyca — the Python ASCOM Alpaca client library. https://github.com/ASCOMInitiative/alpyca

PyQt6 / Qt 6 — the desktop interface. Confined to argos/ui and argos/workers; see ARGOS — Architecture. https://www.riverbankcomputing.com/software/pyqt/

pyqtgraph — the live plots: light curve, focus V-curve, quality trends. https://www.pyqtgraph.org

Citing ARGOS¶

ARGOS is GPLv3+ software by Jules Perret. There is no paper. If it contributed to a result, cite it as software with its version and repository:

Perret, J. 2026, ARGOS: differential photometry for smart telescopes,
v0.4.1, https://github.com/jperret21/argos

Important

An ARGOS live curve is a quick-look on uncalibrated sub-exposures. A result worth citing anything for came out of a reduction downstream — cite that reduction’s tools too.