Identify a solved field¶

ARGOS separates plate solving from catalogue identification. This is important: ASTAP first establishes a mapping between image pixels and sky coordinates. Only then can ARGOS query, cache and project catalogue objects onto the frame.

Field identification is an observing aid. It makes the contents of a frame inspectable and lets the observer retain a target, comparison ensemble and check star with the session. It does not turn a live frame into a calibrated measurement or prove that every visible source has a catalogue identity.

1. Solve before assigning a role¶

With a representative full-resolution light frame open, use Field → Identify field. ARGOS invokes the configured ASTAP executable and reads the returned WCS. Verify the solved centre, plate scale, rotation and overlays against the image before selecting a target or comparison star.

If markers do not line up with stellar centroids, stop and solve that problem first. Changing a display stretch, CFA colour mode or zoom must not be used to “correct” a mismatch: those are display operations, while a solution error is a coordinate error.

2. Control what is drawn¶

The overlay controls deliberately distinguish layers from selected roles:

The bar has two rows: the object layers, then the display controls.

Object layer	What it draws
Stars	Gaia DR3 stars, enriched with conventional SIMBAD identities when available.
Variables	Variable stars from the AAVSO VSX catalogue.
Galaxies	Galaxies identified by SIMBAD or the bundled Messier/NGC/IC catalogue.
Nebulae + clusters	Nebulae and stellar clusters from SIMBAD or the bundled Messier/NGC/IC catalogue.
Exoplanets	Confirmed exoplanet hosts returned by the NASA Exoplanet Archive, cache first. The marker identifies the host, not a resolved planet.
Other objects	Other physically classified SIMBAD sources, such as radio or X-ray sources.

Display control	What it does
Coordinate grid	RA/Dec grid derived from the solved WCS.
VSP references	AAVSO VSP reference candidates — not necessarily the ensemble you selected.
Selected stars	The target, selected comparison stars and check stars for this observation.
Labels	Compact non-overlapping labels. Keep this off in crowded fields and hover or click a marker for its full identity.

The faint-limit control is a catalogue/display budget, not a detection threshold. A higher magnitude limit can return many more objects, increase the time needed to identify a field, and make labels unreadable. The maximum object limit in Settings → Catalogues · data constrains the query/rendering budget for this reason. Start with a useful depth, then increase it only when a source of interest is still missing.

Filtering one layer does not remove data from the session or alter the FITS. It only changes what is displayed over the current image.

Not every magnitude on screen is a photometric magnitude

The Stars layer shows Gaia \(G\) — a very broad white-light band, and not \(V\). It is there so you can tell a 12th-magnitude star from a 16th, and it is excellent for that. It is not what builds the zero point.

Only the VSP references carry sequence magnitudes calibrated by the AAVSO in a standard band, with an AUID. That is why a manually clicked star can be kept as a comparison and still contribute nothing to the ensemble: it has a position and a flux, but no calibrated magnitude to difference against. See §4 of the method page.

3. Catalogue coverage and identity¶

ARGOS combines an embedded essential object catalogue with local caches and optional network requests. The exact result depends on the solved footprint, selected layers, faint limit, object budget and whether the requested service has cached data for that field.

An uncircled star is not necessarily a failed detection. It may be fainter than the selected limit, absent from the chosen catalogue, excluded by the object budget, not returned by an optional service, or too close to another source for a reliable projected match. Conversely, an annotated name is a catalogue cross-match, not an assertion about photometric suitability.

Click a marker for the information card. When available it shows a concise catalogue identity, coordinates, catalogue magnitude(s) and image measurements such as FWHM/HFD, SNR or saturation. Long identifiers are shortened in the overlay; the card is the authoritative place to inspect the full identity.

Catalogue requests are explicit. ARGOS can use the network for optional lookups, but the resulting cache is local and can be inspected or refreshed in Settings. An offline session uses the embedded catalogue and whatever matching entries have already been cached; it does not silently upload frames or telemetry.

4. Select the photometry roles¶

Once the WCS and markers are credible, click an object and assign one role:

Role	Purpose
Target	Science source whose differential signal is followed.
Comparison star	Reference star contributing to the ensemble zero point.
Check star	A source expected to remain constant, measured against the ensemble as a diagnostic.

ARGOS may propose comparison candidates after a target has been selected. Treat this as a ranked starting point, not as a scientific endorsement. Inspect saturation, isolation, brightness, colour and the future post-processing requirements; keep the exact selected ensemble recorded with the session.

The same target or comparison selection is visible in the photometry tables and in the Selected stars overlay. Selecting or removing a row updates the image overlay so the table and the field do not become two conflicting views of the session.

5. During a sequence¶

The sequence continues to retain field and measurement context on incoming frames. A live curve or new overlay should be interpreted as an operational check: common shifts can be caused by clouds, focus, tracking, saturation or a changing sky background. Use Review to follow a suspicious curve point back to its source FITS frame.

ARGOS does not calibrate raw lights with darks, flats or bias frames during this quick-look identification/measurement path. Keep all raw calibration frames and perform the final calibrated reduction independently.

6. Troubleshooting¶

Symptom	First check
Marker is not centred on a star	Confirm ASTAP’s centre, scale and rotation on a full-resolution frame.
Needed source is absent	Check the relevant layer, faint limit and object budget; then check cache/network availability.
Labels fill the frame	Hide labels, reduce the faint limit or narrow the layers; inspect individual markers instead.
Too few comparison candidates	Confirm the solve and catalogue filters, then select candidates manually and record the rationale for final reduction.
Same object has several names	Use the information card’s full identity and coordinates; overlay labels are intentionally abbreviated.