Measure a changing sky.

1. Launch CAst.
2. Open Mode.
3. Choose Differential Photometry.

The window title changes to Citizen Astronomy - Differential Photometry. The main action at the top left is Open.

Use .fit, .fits, .xisf, .tif, .tiff, or a supported camera RAW format such as .cr2, .cr3, .nef, .arw, or .dng. JPG and PNG files are reported and ignored.

• Keep a usable observation time, exposure, and filter in the metadata.
• Use valid celestial WCS, or provide enough pointing and scale information for plate solving.
• Use calibrated light frames, or enable Calibration in the workflow.
• Keep each channel/filter internally consistent.

1. Click Open.
2. In Differential Photometry Workflow, find 1. Folder.
3. Click Folder and choose the image directory.
4. Wait for the scan.
5. If Loaded Results appears, select an object and click Use Selected Object.

Calibration

Apply bias, dark, and flat calibration before measurement.

Star Alignment

Use the alignment path for a sequence that should share pixel geometry.

Plate-solving

Required and kept on. CAst accepts, repairs, or solves WCS.

Pull Periods

Request catalog or literature periods.

Calculate Periods

Infer periods from the measured data.

Create Measurements Table

Build the detailed table now. Leave off for a faster first run and load it later.

Use designation filters to choose catalog families. Brightest Variables limits a dense field by percentage. Analyze Best is an alternative field-screening path for strong measurable targets; it is not another percentage filter.

If the image timestamps omit a timezone, set Image Timestamp Timezone before Generate.

Generate scans metadata, validates WCS, queries field catalogs, builds target-specific comparison pools, estimates seeing, measures the images, creates magnitude contexts, applies quality analysis, and builds light curves.

• Watch 3. Progress in the workflow.
• Read Work Log for WCS, catalog, calibration, and measurement details.
• While a run is active, Close becomes Terminate.

If Select Sources To Process appears

Filter variables, checks, exoplanets, or references. Use Process Default for the configured limit, Process All Visible after filtering, or Process Selected for highlighted rows.

After Generate, CAst normally selects Filter: Overview and Y: Standard Magnitude.

The gear beside Source Results opens Filter Source Results. There is no permanent name/type search row in the current layout.

1. Click a target in Source Results.
2. Confirm that the image centers or annotates the same source.
3. Confirm that the light curve changes to that source.
4. Move through frames or use Blink.
5. Inspect a suspicious curve point beside its frame and measurement detail.
6. Use CDS, SIMBAD, Gaia, or VSX from the selected row when available.

Use Display, Curves, or Invert to make the field easier to see. These are preview controls and do not change measurement pixels.

Check saturation, SNR, centroid shift, comparison scatter, background, flux error, and the original frame. Use Overview to see whether comparisons and the check star contain the same trend.

Click Filter for Filtering Settings. Filtering changes the analysis/view; it does not delete the original measurement. Export Report preserves accepted and rejected rows separately.

CAst starts with a target-specific ensemble ranked mainly by magnitude similarity and then separation.

1. Select a target and click Scan Comps.
2. Use 1. Limits only when the field needs hard magnitude, color, or distance constraints.
3. Mark promising stars in 2. Favourites.
4. Inspect ensembles in 3. Combinations.
5. Click Use Selected.

1. Turn on Aperture Editor.
2. Double-click the target.
3. Shift-click a comparison.
4. Ctrl-click a check star.
5. Drag to move an aperture; drag handles to resize aperture and annulus.
6. Right-click for Delete or Change Name.
7. Click Apply.

The editor starts independently of the selected automatic target. Enable Keep Comparison Stars in Settings → Light Curve only when you want to seed the manual set from the automatic ensemble.

Click Increase SNR to combine adjacent measurements. CAst limits bins by variability type and period: 1.5% for sharp variables/transits, 5% for smooth variables, and 3% otherwise, with a default 600-second absolute cap.

The button changes to Reset SNR. Use it to restore the original cached series. In-bin outlier rejection is optional and off by default in Advanced settings.

• Pull Period retrieves catalog/literature information where available.
• Calculate Period infers from the measured series.
• Harmonic Fit is the general default Infer method.
• Box Least Squares is intended for transit- and eclipse-like boxes.
• Fit modes include Polynomial, Periodic, Spline, and Auto.

Click Fold to compare repeated cycles at phase φ = ((t − t₀) / P) mod 1. A plausible period should align the event without creating the same structure in comparison/check stars.

1. Click O-C.
2. Enter T₀ (HJD), period in days, and Maximum or Minimum.
3. Use Mark Extrema, Import..., or Pull AAVSO....
4. Review the extrema log and export it when ready.

A slope suggests the adopted epoch/period is offset. Curvature can motivate a period-change analysis, but it is not by itself proof of a physical mechanism.

1. Complete an ordinary Generate run.
2. Click Discover.
3. Use the full field or Shift-drag an ROI.
4. Enable Try multiple comparison-star pools only when the extra search is useful.
5. Click Start Discover.

Candidate score is triage, not a discovery claim. Check artifacts, catalogs, surveys, and additional nights. Label and Train Model build an optional local detectability classifier from your own labeled examples.

1. Complete the one-time Science Export settings and observing site.
2. Generate, then inspect Overview and Standard Magnitude.
3. Select the target rows to export.
4. Choose File → Export Report.
5. Read AAVSO Preflight Review.
6. Correct missing observer/chart metadata, filter O, missing airmass, unexpected DIF rows, or skipped points.
7. Export, inspect the Extended Format file, revise when needed, then upload through AAVSO.

MTYPE=STD requires a valid standard-magnitude context; otherwise the row is DIF. TRANS=YES is written only for STD rows when you explicitly mark the data transformed. CAst does not calculate transformation coefficients.

• Theme-based keeps the current visual presentation.
• Scientific produces a publication-oriented plot.
• Animation animates the light curve.
• Target Field Animation combines an aligned/cropped star field with the curve and moving time marker.
• File → Export Report writes tables, manifests, AAVSO files, plots, and annotated images.

Settings are grouped here so the operating steps remain continuous.