Exoplanet transit preparation

ARGOS can prepare an uninterrupted acquisition sequence for a confirmed exoplanet transit. It is a capture and live-preview tool: it does not perform dark, flat or bias calibration, nor transit fitting. Reduce the raw FITS in Siril, then produce and model the final relative-flux light curve in the appropriate post-processing workflow — AstroImageJ is the usual free choice, and its paper (Collins et al. 2017) is worth reading before your first attempt.

What depth is realistic

A 30 mm aperture collects very little light. The target class that works is a deep transit on a bright host — the hot Jupiters whose published depth runs around one to two per cent. Before committing a night, compare the archive’s depth column against the scatter you can actually reach on the transit timescale — not the point-to-point scatter.

That distinction is the whole game. ARGOS’s run-level scatter (§5) is a high-frequency number and will flatter you: it does not contain the correlated drifts that dominate anything measured over two or three hours. The honest test is to bin your out-of-transit residuals into bins the length of the ingress and see where the RMS stops falling as \(1/\sqrt{N}\) (Pont, Zucker & Queloz 2006). That floor is what your depth must beat, several times over.

For orientation: atmospheric scintillation alone puts about 15 mmag on a 20-second sub through a 30 mm aperture at airmass 1.5 (§9). A 1 % transit is 10 mmag deep. You are binning many frames to see it, and the red noise is what decides whether the binning helps.

Before the night

  1. In Settings, search and save the observing site. Coordinates and elevation are needed for the BJD_TDB prediction used by the planner.

  2. In Sequencer → Exoplanet transit, enter a planet designation such as HD 189733 b and choose Find planet.

  3. ARGOS queries the NASA Exoplanet Archive’s PSCompPars table once and caches the successful result locally. It displays the host-star name, period, duration, depth and published transit-midpoint reference.

  4. Confirm that the reported epoch is BJD_TDB and verify the ephemeris close to the observing night. ARGOS refuses an epoch whose time standard is not explicitly BJD_TDB rather than applying an implicit conversion — a silent HJD-to-BJD confusion is worth seconds of timing error (Eastman, Siverd & Gaudi 2010).

  5. The selected telescope target is the host star, not the planet. Review it in Telescope and use the existing explicit GoTo action when ready. The Target visibility plot switches to the transit night and shades the requested coverage interval; the transit card also shows its local midpoint.

Prepare the acquisition

Choose the observing settings in the transit panel:

  • Baseline is out-of-transit time before ingress and after egress. The default is 60 minutes on each side; increase it when the event and night allow it.

  • Exposure and cadence are start-to-start values. Cadence must be at least as long as exposure. During the sequence ARGOS deducts acquisition and download time from the cadence budget rather than adding an idle delay after every frame; if the hardware cannot keep up, it takes the next frame as soon as possible and does not pretend the requested cadence was achieved.

  • Use one filter throughout the series. The generated plan has one Light step, no dithering, no autofocus and no filter changes so the cadence and flux time series remain stable.

Choose Prepare transit sequence. It replaces the editable sequence table with the calculated number of Light frames covering the selected baseline and published duration. This does not schedule or automatically start the mount: review the event, target altitude, focus, framing, exposure level and actual start time before pressing Start sequence.

Scientific limits

The panel predicts timing from the archive ephemeris; it is not a replacement for an up-to-date transit-and-observatory visibility calculation — cross-check against the Exoplanet Transit Database or the Swarthmore transit finder. The displayed coverage is in BJD_TDB, and the observer remains responsible for whether the event is observable at sufficient altitude and for checking uncertainty growth in an old ephemeris.

ARGOS’s Photometry window remains a raw-sub preview. Its measurements are useful to see that the target, comparison stars and cadence behave sensibly in the field, but they are not a claim of a publishable transit light curve. The Relative flux switch shows F_target / sum(F_comparisons) normalised to the displayed series median. This path does not require catalogue magnitudes, so it remains available for a manual transit comparison ensemble.

Warning

That median normalisation is a display convenience, not an out-of-transit baseline, and the difference bites exactly here: if most of your run is inside the transit, the median sits inside the event, the baseline is dragged down and the depth on screen looks shallower than it is. This is one more reason to hold a real baseline on each side. Measure depth from the exported, un-normalised relative_flux column against your own out-of-transit segment.

When the sequence starts, ARGOS stores observation.json beside session.json and the raw FITS. For a transit it preserves the planet and host names, archive source, period, BJD_TDB epoch, predicted contact/coverage times, duration and published depth. It is a post-processing hand-off record, not a measurement.

Data and privacy

NASA is contacted only after the observer requests a planet lookup. Results are cached locally under ~/Argos/cache/exoplanets.json; ARGOS sends no telemetry and never uploads session data.

The source fields ignore case and common missing spaces: hd189733b, HD 189733 B and hd 189733 b identify the same planet. Cached source names appear in the completion menu without a network connection. A new planet or a partial first search still needs the NASA archive; repeat an online search to refresh a cached ephemeris. Settings → Catalogues · data lists the local cache locations and the online/offline behaviour of CDS, NASA and AAVSO catalogues.

Target naming and Stellarium

The FITS OBJECT value identifies the imaged target, not the observing session. Selecting a Scientific source supplies its canonical catalogue name to Capture and the acquisition plan. For a planet, it is the host star (HD 189733), while the planet designation (HD 189733 b) and ephemeris are kept in observation.json.

Stellarium’s telescope protocol sends only J2000 RA/Dec. After a GoTo, ARGOS may offer nearby locally cached catalogue names, but it never changes OBJECT automatically: the observer must choose the intended candidate. In the Stellarium connection card, Look up target names online after a GoTo explicitly permits a CDS/SIMBAD query only when no local match exists; it is off by default and no session data are uploaded.

See also