18  Planetary and Lucky Imaging

Deep-sky imaging fights noise with long exposures; planetary imaging fights the atmosphere with short ones. Lucky imaging (Law et al. 2006) records thousands of very short frames, keeps only those captured in the fleeting instants of steady seeing (seeing is the astronomer’s word for how much the atmosphere blurs and shimmers the view), and stacks the survivors into an image sharper than any single long exposure could be. The VIDEO tab implements this workflow in two panes: Capture records continuous high-frame-rate video to SER files (The SER Astronomical Video File Format, n.d.) (SER is a simple raw video format shared by planetary imaging tools), and Process runs them through the built-in lucky-imaging stack pipeline.

18.1 The Capture sub-tab

A live preview canvas fills the available area, and the controls alongside it configure the stream:

  • Exposure (s), typically 5-50 ms for planetary work;
  • Gain (the camera’s amplification setting), camera-dependent; planetary imaging favors high gain;
  • Bin, usually 1x1, or 2x2 if you are undersampled. Binning merges adjacent pixels into one larger pixel; undersampled means each pixel covers more sky than the finest detail your telescope delivers;
  • Target name, which becomes the folder name (Jupiter, Saturn, Moon, and so on);
  • Max duration (s), an auto-stop after N seconds, where 0 means no cap;
  • White balance R / B sliders, visible only for one-shot-colour (OSC) cameras whose INDI driver (INDI is the open-source framework Polaris uses to talk to equipment) exposes the WB_R and WB_B properties (ZWO, QHY); mono cameras hide the row.

Two buttons drive the capture. Start Stream opens the camera stream, using the driver’s built-in video mode (CCD_VIDEO_STREAM) if the driver supports it; otherwise Polaris falls back to taking rapid single exposures in a loop on the server. The button label shows the live frame rate. Record is enabled only while streaming and begins writing every frame to a SER file at {ImageOutputDir}/{Rig}/planetary/{target}/{ISO-timestamp}.ser. A counter shows frames and bytes written, with a dropped-frame warning if the writer cannot keep up with the stream.

Figure 18.1: The Capture sub-tab streaming the Moon: the live preview, scroller controls for exposure, gain, and more, the record shutter, the live capture and transmit frame rates on the Stop Stream button, and the mount jog pad for nudging the framing.

Suggested capture settings

Capture settings are saved per rig. As starting points, for Jupiter or Saturn on an 8-inch SCT (Schmidt-Cassegrain telescope): exposure 8-12 ms, gain around 350 (ZWO IMX462 or IMX664 sensors), and a max duration of 120 s in good seeing or 90 s to stay under the planet’s rotation cap (record longer and Jupiter’s fast spin starts to smear the detail). For the Moon at high magnification: exposure 1-2 ms, low gain (0-100), and 30-60 s per region, assembling a mosaic later.

18.2 The Process sub-tab

The Process sub-tab drives the built-in stacker (the PlanetaryStackerService):

  1. The SER file dropdown is populated recursively from {ImageOutputDir}/planetary/, with a Refresh button to rescan.
  2. Keep top X% is the quality cutoff, default 50%. The stacker picks the sharpest X% of frames; smaller values give a sharper but noisier result.
  3. Output name sets the base filename for the stacked result.
  4. Stack kicks off the job, and Abort cancels it mid-run.

The seven-phase pipeline

A status bar shows the phase live as the job progresses:

  1. Reading: opens the SER file and lists its frames.
  2. Analyzing: scores every frame’s sharpness by computing its Laplacian variance.1 This phase parallelizes well; a typical Raspberry Pi 4 takes about 30 s for 1000 frames of 800x600 16-bit video.
  3. Ranking: sorts frames by quality, descending, and takes the top slice set by Keep top X%.
  4. Aligning: finds the centroid (the brightness-weighted center point) of the brightest region in each kept frame and lines the frames up on it, refined to sub-pixel accuracy with a parabolic fit.
  5. Stacking: a mean stack with a per-pixel count, producing uint16 output (16-bit unsigned integers, the same precision as the camera’s raw data).
  6. Writing: saves the result under planetary/{target}/stacked/ as a FITS file (FITS is the standard astronomy image format).
  7. Ok / Fail: the terminal state.

18.3 The quality metric

Frame quality is the variance of a 3x3 Laplacian filter applied to the centred region of interest (ROI) of each frame, a standard sharpness metric (Pertuz et al. 2013). Higher variance means a sharper frame. The cutoff slider lets you discard atmospheric-blur frames: pick an aggressive cut (top 20%) for the best detail, or a lenient one (top 80%) for lower noise at the cost of a slightly softer result.

18.4 Alignment

Alignment uses the brightest-pixel centroid, refined parabolically on a 5x5 neighborhood for sub-pixel accuracy. This works well for bright targets with a single brightness peak: the Moon, Jupiter, Mars, and the body of Saturn.

The limitation is extended sources without a single peak, such as Saturn’s rings, where alignment shifts can drift. The workaround is to stack in smaller batches per processing pass; future versions will add a thresholded centroid and better alignment for extended objects.

18.5 Output

The stacked result lands at {ImageOutputDir}/{Rig}/planetary/{target}/stacked/{outputName}_{ts}.fits as a 16-bit mono FITS. Open it in PixInsight, the free Siril (The Siril Team, n.d.), or Photoshop and apply wavelet sharpening (the multi-scale sharpening technique planetary imagers favor) and saturation in your favorite tool; the Polaris EDITOR (Chapter 25) targets deep-sky post-processing, so planetary finishing is best done externally.

18.6 Common pitfalls

The frame rate caps at about 5 fps even though the camera supports a native stream. Check that CCD_VIDEO_STREAM is actually exposed by your driver. Some INDI drivers default it to off, and it needs to be enabled in the indiserver parameters.

Dropped-frames warning. The writer cannot keep up with the stream cadence. Reduce the ROI, the region of interest, by capturing a smaller sub-frame (smaller frames write faster), lower the binning, or lower the target frame rate via the driver’s STREAMING_DELAY property.

The stacked output is dark. The frames were mostly cropped to a small bright region. Try a wider ROI, or increase the keep percentage to include more frames.

Storage explodes. Sixty seconds at 30 fps of 800x600 uint16 video is roughly 1.7 GB. Watch your disk; planetary captures fill SD cards fast. Chapter 19 covers where captures live and how to move them off the host.

  1. A Laplacian filter responds strongly to fine edges; the variance of its output is high for crisp frames and low for blurry ones.↩︎