Appendix B — Glossary
The astrophotography terms used throughout this book and the Polaris UI, in about two lines each, alphabetically. Chapters gloss each term briefly on first use; this is the fuller reference.
Alpaca. The modern, network-based protocol for astronomy devices, ASCOM’s HTTP successor (The ASCOM Initiative, n.d.). Polaris speaks Alpaca natively; on Windows any classic ASCOM driver can be bridged through the ASCOM Remote Server.
ASCOM. The long-standing Windows standard for astronomy device drivers (The ASCOM Initiative, n.d.). Classic ASCOM drivers use Windows-only COM plumbing; Alpaca is the same standard reworked to run over the network from any machine.
ASTAP. The Astrometric Stacking Program (Kleijn, n.d.), the fast offline plate solver bundled with Polaris. Needs a star database alongside the program (Chapter 2).
Astrometry.net. The blind plate-solving engine that can identify a star field with no pointing hint at all (Lang et al. 2010). Polaris uses it, locally or via the nova.astrometry.net web service, as a fallback solver.
Autoguiding. Using a second camera to watch a star and send tiny corrections to the mount, canceling tracking errors during long exposures. Polaris guides through PHD2 or its built-in guider (Chapter 12).
AUTORUN. Polaris’s name for the simple list-style sequence runner (Chapter 14), as opposed to the tree-based Advanced Sequencer (Chapter 15).
Back-focus (backspacing). The mechanical distance from the telescope’s rear flange to the camera sensor that the optical design expects; reducers and flatteners publish their required value (typically 55 mm or 56 mm). Polaris’s Main Telescope card shows the required value once you pick an optical tube and accessory from the catalog.
Background extraction (BGE). Removing the gradient (light pollution, moon glow) from an image. Polaris runs GraXpert’s AI model for this, in the browser or on the host (The GraXpert Team, n.d.).
Backlash. The slack in a gear train: when a focuser or mount reverses direction, the first part of the motion takes up the slack before anything moves. Auto-focus offers backlash compensation for focusers with this hysteresis.
Bahtinov mask. A slotted mask placed over the telescope aperture that turns a bright star into a diffraction spike pattern; the central spike sits exactly between the others only at perfect focus. Polaris overlays a digital assistant on the pattern (Chapter 11).
Bayer matrix / debayering. The checkerboard of red, green, and blue filters printed over a color camera’s pixels, and the reconstruction (debayering, or demosaicing) that turns the raw checkerboard into a full-color image.
Bias frame. A zero-length exposure that captures only the sensor’s read noise pattern. Subtracted from lights and flats during calibration.
Binning. Combining adjacent pixels into one (2×2 reads four pixels as one). Reduces resolution but boosts signal-to-noise ratio and frame rate.
Calibration (guiding). Teaching the guider how the mount responds to pulse commands, done once per pointing region and redone after a meridian flip if the camera orientation changed. Polaris’s Smart Calibrate button automates the step calculation (Chapter 12).
Calibration frames. The bias, dark, and flat frames used to correct sensor artifacts in light frames. STUDIO generates the master versions (Chapter 20).
CCD_VIDEO_STREAM. An INDI property some cameras (ZWO, QHY, gphoto DSLRs) expose to switch into continuous streaming at 10-30 frames per second without per-frame exposure round-trips. Polaris’s camera stream detects and uses it automatically.
Collimation. The alignment of a telescope’s mirrors or lenses on a common optical axis. Miscollimation shows up as asymmetric, comet-shaped stars that no amount of focusing fixes.
Cosmetic correction. Removing hot and cold pixels from a stack by statistical rejection, so isolated defective pixels do not survive into the final image.
Dark frame. A long exposure with the shutter closed or the lens covered, capturing thermal noise. Subtracted from lights to remove hot pixels and dark current.
Declination (Dec). The celestial equivalent of latitude, measured in degrees from −90° (south celestial pole) to +90° (north). Half of an (RA, Dec) coordinate pair.
Deconvolution. Mathematically reversing the blur that optics and atmosphere impose, classically with the Richardson-Lucy algorithm (Richardson 1972; Lucy 1974). Polaris offers an AI version through GraXpert’s model (Chapter 23).
Denoising. Reducing random noise while preserving real detail. One of the three AI cleanup operations Polaris runs in the browser (Chapter 23).
Dithering. Slightly shifting the mount between exposures so hot pixels and fixed-pattern noise never land on the same sky twice and average out in the stack. Polaris triggers it through PHD2, configurable per sequence.
Drift. The slow movement of a target across the sensor over time, from residual tracking error and atmospheric refraction. Live stacking can auto-recenter on a schedule to correct it (Chapter 17).
Drizzle. A stacking technique that reconstructs finer detail than any single frame holds, by placing many dithered, undersampled frames onto a finer output grid (Fruchter and Hook 2002).
DSO (deep-sky object). Anything beyond the solar system worth photographing: galaxies, nebulae, and star clusters. The SKY tab’s atlas catalogs them (Chapter 10).
EAA (electronically assisted astronomy). Watching a stacked image build up on screen live during the session, rather than processing offline later. Polaris’s LIVE tab is purpose-built for it (Chapter 17).
Ephemeris. A table of predicted positions of a celestial object over time. Polaris computes ephemerides for Tonight’s Best and the altitude charts (Chapter 16).
Field of view (FOV). How much sky the camera and telescope combination covers, calculated from sensor size and focal length. Polaris shows yours on the Main Telescope card.
Field rotation. The slow rotation of the star field around the frame center that appears when the mount’s polar axis is misaligned (or on an alt-azimuth mount). It breaks frame alignment in stacking.
Filter offsets. Per-filter focus position differences (in focuser steps) relative to a reference filter, usually luminance. Stored per rig, so a filter change can adjust focus without a full auto-focus run (Chapter 11).
Filter wheel. A motorized carousel that rotates filters in front of a monochrome camera’s sensor, letting one camera shoot through L, R, G, B, or narrowband filters in sequence.
FITS. The Flexible Image Transport System, astronomy’s standard image format: a text header plus a raw pixel array, usually 16-bit (Wells et al. 1981; Pence et al. 2010). Polaris reads and writes FITS by default.
Flat frame. An exposure of an evenly illuminated surface (a flat panel, twilight sky, or a T-shirt over the aperture), used to correct vignetting and dust shadows.
Flexure. Slight bending between the imaging telescope and the guide scope as the mount moves, so the two see slightly different motion. A classic cause of drift that guiding cannot see.
Focal ratio. Focal length divided by aperture diameter (the f-number). Lower is “faster”: it gathers light over a wider cone and needs shorter exposures for the same signal.
FWHM (full width at half maximum). A star-size measure: the width of the star’s brightness profile at half its peak. Like HFR, smaller means sharper; the two metrics track each other closely.
Gain. The camera’s amplification setting, the astro-camera equivalent of ISO. Higher gain brightens faint signal at the cost of dynamic range.
German equatorial mount (GEM). The classic counterweighted equatorial mount design. Its geometry forces a meridian flip when a target crosses from the east side of the sky to the west (Chapter 7).
Goto. Slewing the mount to a specific (RA, Dec) target on command.
GraXpert. The free AI-based astro image correction tool (The GraXpert Team, n.d.) whose trained models Polaris uses for background extraction, denoising, and deconvolution, in the browser or through the host-side CLI (Chapter 23).
Guide camera. The secondary camera, on a guide scope or off-axis guider, that the guiding software uses to track a star and derive correction pulses. PHD2 owns it; Polaris does not manage it directly.
Guide rate. How fast the mount moves in response to guide pulses, usually 0.5× sidereal (about 7.5 arcseconds per second).
Guide scope / off-axis guider. The two ways to feed a guide camera: a small separate telescope riding on the main one, or a prism (the off-axis guider, OAG) that picks a sliver of light from the main telescope’s own beam.
HFR (half-flux radius). A focus metric: the radius, in pixels, containing half of a star’s light. Smaller is sharper. Auto-focus minimizes HFR via a V-curve sweep, and live stacking can trigger a refocus when HFR degrades (Chapter 11).
HiPS. Hierarchical Progressive Surveys, the tiled all-sky image format the SKY tab uses: the bundled offline tiles first, the CDS Aladin servers for deeper zooms online (Centre de Données astronomiques de Strasbourg (CDS), n.d.).
Histogram. The distribution of pixel brightness in an image. In astrophotography almost everything piles up at the dark end until a stretch spreads it out.
Hot pixel. A pixel that reads far too bright regardless of the sky, from sensor defects and thermal noise. Removed by dark subtraction, cosmetic correction, and dithering.
INDI. The Linux-first protocol and driver ecosystem for astronomy devices (Downey, n.d.). Polaris is INDI-native; indiserver runs on the host on port 7624.
ISO. Sensor sensitivity on DSLR and mirrorless bodies. Polaris exposes ISO selectors for vendor-driver cameras (Canon, Nikon, Sony; Chapter 6).
Kappa-sigma clipping. A stacking rejection method: pixels more than kappa standard deviations (sigma) from the mean across frames are thrown out, removing satellite trails, plane lights, and cosmic ray hits.
Laplacian variance. A sharpness metric (Pertuz et al. 2013) Polaris uses in the planetary pipeline to rank video frames for lucky imaging. Higher means sharper.
Light frame. An actual exposure of the target, as opposed to the calibration frames that correct it. The frames you keep.
LRGB. The monochrome-camera workflow that shoots luminance (L) for detail plus red, green, and blue filters for color, then combines the four stacks into one color image (Chapter 21).
LST (local sidereal time). The right ascension currently crossing your meridian. Polaris computes it to drive meridian-flip warnings.
Lucky imaging. Capturing thousands of short video frames of a planet and keeping only the sharpest few percent, beating the atmosphere by luck (Law et al. 2006). The VIDEO tab implements it (Chapter 18).
Luminance. The brightness-only channel of an image, or the clear filter (L) that captures it. Carries the detail in an LRGB combination.
Master frame. The average of many calibration frames of one kind (master dark, master flat, master bias), smoother than any single frame. STUDIO builds and applies them (Chapter 20).
Meridian. The imaginary north-south line passing directly overhead. Targets are highest, and imaging is best, as they cross it.
Meridian flip. The side swap a German equatorial mount must perform as a target crosses the meridian, to keep the telescope from colliding with the pier. Polaris automates the whole pause, flip, re-solve, re-center, resume cycle (Chapter 7).
Monochrome camera. A camera with no color filter array; every pixel sees all light. More sensitive than a color camera, but needs filters and a filter wheel to produce color (Chapter 21).
Mosaic. Covering a target too large for one field of view with a grid of overlapping panels, later stitched together. The SKY tab’s framing assistant plans them (Chapter 10).
MTF (midtone transfer function). The non-linear stretch curve used to display astro images. Polaris’s WebGL renderer applies it on the GPU at preview time without touching the underlying data.
Narrowband. Filters that pass only a few nanometers around one emission line, most commonly hydrogen-alpha (Ha), doubly ionized oxygen (OIII), and singly ionized sulfur (SII). They cut through light pollution and feed false-color palettes such as SHO and HOO (Chapter 21).
Offset. A small constant the camera adds to every pixel before digitizing, so read noise never clips to zero. Set once per gain value and left alone.
OSC (one-shot color). A camera with a Bayer matrix that captures color in a single exposure, as opposed to a monochrome camera with filters. Simpler; somewhat less sensitive.
PHD2. The de-facto standard autoguiding program (Stark et al., n.d.). Polaris controls it over its network command interface on TCP port 4400 and can embed its full GUI in the browser (Chapter 12).
Photometric color calibration (PCC / SPCC). Setting an image’s color balance by comparing the measured colors of its stars against their true catalog colors, using APASS (Henden and Munari 2014) or Gaia (Gaia Collaboration et al. 2023) photometry. Polaris bundles the catalog data (Chapter 22).
Pixel scale. Arcseconds of sky per pixel: pixel size in microns times 206.265, divided by focal length in millimeters. It defines your resolution and feeds plate solving and guiding tuning.
Plate solving. Working out the exact (RA, Dec) a frame is pointed at by matching its stars against a catalog, then using that to re-center the mount. Polaris uses ASTAP by default, with PlateSolve3 and Astrometry.net as fallbacks (Appendix A).
Polar alignment. Aiming the mount’s rotation axis at the celestial pole so tracking follows the sky exactly. Polaris offers TPPA and a rudimentary method for sites without a view of the pole (Chapter 9).
Polaris (this app). Not to be confused with the star. Named after the user, kind of.
Rig. Polaris’s term for a saved bundle of equipment selections and per-setup defaults (cooler temperature, focus step, focal length, guiding preset, and so on). Switch rigs in one click from the RIGS tab; each keeps its own configuration (Chapter 5).
Right ascension (RA). The celestial equivalent of longitude, measured in hours (0 to 24) rather than degrees. Half of an (RA, Dec) coordinate pair.
RMS (guiding). The root-mean-square of guiding corrections, the single number that summarizes guiding quality. Usually quoted in arcseconds; smaller is better.
ROI (region of interest). A subframe of the camera sensor read out instead of the whole chip, for faster frame rates; essential in planetary imaging (Chapter 18).
Rotator. A motorized device that rotates the camera to a commanded angle, so a planned framing can be reproduced exactly.
Seeing. The steadiness of the atmosphere. Poor seeing smears stars and limits resolution no matter how good the optics; it is the “seeing” column in the weather forecast.
SER. A raw video format designed for planetary capture: a header, uncompressed frames, and an optional timestamp trailer (The SER Astronomical Video File Format, n.d.). Polaris writes SER from the VIDEO tab, and dedicated planetary stacking programs read it.
Settling. The pause after a dither or slew while the mount stops oscillating and guiding locks back on, before the next exposure starts.
Sidereal rate. The speed of the sky’s apparent rotation, one full turn in 23 hours 56 minutes. Tracking at sidereal rate holds stars still on the sensor.
Siril. A free astronomical image-processing application (The Siril Team, n.d.). Polaris can hand pre-processing and stacking to Siril scripts as an alternative to its built-in STUDIO pipeline (Chapter 20).
Sky atlas. The offline catalog of deep-sky objects browsable in the SKY tab, with search, filters, and framing tools (Chapter 10).
Slew. Moving the mount fast to a new position, as opposed to tracking. Polaris chains slew, plate solve, and re-center into one operation.
SNR (signal-to-noise ratio). How far the real signal stands above the noise. It grows with total integration time; live stacking shows a running SNR readout (Chapter 17).
Star removal. Separating an image into a starless layer and a stars-only layer, in the manner of StarNet (Misiura, n.d.), so nebulosity can be stretched hard without bloating stars, then recombined (Chapter 24).
Stretch. Converting linear pixel values to display-friendly brightness through a non-linear curve (autostretch, manual sliders, or MTF). Necessary because the raw data is linear and nearly black.
Target. In sequencing, a (name, RA, Dec) tuple the engine slews to and shoots a set of frames of.
TPPA (three-point polar alignment). The polar-alignment method that plate-solves three mount positions to measure the axis error, then guides you through correcting it live (Chapter 9).
Tracking. The mount’s continuous compensation for Earth’s rotation, usually at sidereal rate, keeping the target stationary on the sensor.
V-curve. The plot of HFR against focuser position that auto-focus builds: a sweep of positions around the current one, a parabola fit through the samples, and a move to the vertex (Chapter 11).
Vignetting. The gradual darkening toward an image’s corners imposed by the optical path. Corrected by flat frames.
WebSocket. A long-lived, two-way browser-to-server connection. Polaris uses one for the once-per-second status broadcast and another for the live image stream (Chapter 31).
White balance. Per-channel gain multipliers (red and blue relative to green) for color cameras. The VIDEO tab exposes sliders when the camera supports INDI’s WB_R and WB_B properties.
XISF. PixInsight’s image format. Polaris can write it as an alternative to FITS.
xpra. A Linux tool that shows a native program’s window inside a web page (The Xpra project contributors, n.d.). Polaris uses it to embed PHD2’s full GUI in the GUIDE tab for the dialogs PHD2’s network interface does not expose (Chapter 12).