10  Sky Explorer and Session Planning

The SKY tab is where a session starts before any shutter opens: an offline sky map, target search against a bundled deep-sky catalog, altitude planning for the night, field-of-view framing, a mosaic planner, and the slew-and-center orchestration that puts the chosen target on the sensor. Everything essential works with no internet at the telescope; online services are used only as optional enrichments and fallbacks.

10.1 The sky map

The map itself is stellarium-web-engine, the engine behind the web version of the popular Stellarium planetarium, running as its own sandboxed sub-app (served under /sky/) embedded in the page.¹ Drag to pan; use the mouse wheel or a pinch gesture to zoom. The view aims wherever the host UI tells it: the mount’s current RA/Dec (right ascension and declination, the sky’s equivalent of longitude and latitude), a search hit, or the Centre on selected target buttons.

The bundled skydata/ directory (about 4.6 MB, shipped in the repo and in the publish output) covers, fully offline: stars from the Hipparcos and Tycho catalogs (the standard star catalogs from ESA’s Hipparcos mission) in the brighter naked-eye to binocular range, the NGC/IC/Messier catalog of DSOs (deep-sky objects: galaxies, nebulae, and star clusters) as labelled markers, the official IAU (International Astronomical Union) constellation lines and names (on by default), the 88 western constellation figure illustrations as toggleable artwork, a low-resolution Milky Way panorama, and the Sun, Moon, planets, asteroids, and comets.

WebGL2 is mandatory for the map; almost every desktop, laptop, and tablet browser from the last few years has it. On a host without it (for example, running Polaris’s local browser directly on a Raspberry Pi 2’s bare display), the SKY tab shows a graceful fallback banner; open Polaris from a desktop, laptop, or tablet browser instead.

Figure 10.1: The SKY map with a target card open: magnitude, coordinates, meridian and horizon times, tonight’s altitude curve, and the slew actions. On the map, the red rectangle is the target framing and the blue one is where the mount currently points.

10.2 Satellites and the major moons

The map is not limited to things that hold still.

Satellites. Polaris draws the ISS, Tiangong and the roughly 150 objects in CelesTrak’s “visual” group, the ones bright enough to follow by eye: Hubble, Envisat, Terra, the large spent rocket bodies. The map engine propagates the orbits itself with SGP4, the standard model these element sets are written for, from the TLE (two-line element) set the host serves. Search “ISS”, “Tiangong” or “Hubble” to find one, and clicking a satellite locks the view onto it so you can watch a pass unfold. One reading to know: a satellite shows magnitude 99 while it is below the horizon or inside the Earth’s shadow, which is the map’s way of saying “up there, but nothing to see”.

The 🚀 Satellites pill in the toolbar hides them, and the choice sticks on that browser.

Orbital elements go stale in days rather than months: a week past its epoch, the ISS is about a degree off. A snapshot ships inside each release, the host refreshes it from CelesTrak once a day when it has internet, and the Tonight’s Best tab reports the age of what the map is actually using (“Satellites: 156 from CelesTrak, orbits from today”) with an Update button beside it. In the mobile app the update also works when only the phone has internet: the app fetches the elements and hands them to the host. The map reloads after an update, because the engine reads the orbits once when it starts.

Moons. Zoom in on Jupiter and the four Galilean moons appear in their true positions, shadow transits included, labelled once the zoom is high enough not to crowd the planet; Saturn’s major moons behave the same. They are searchable by name (“Io”, “Europa”, “Ganymede”, “Callisto”, “Titan”). This is the map only: Center on body and the night planner know the planets, not their moons.

10.3 Offline deep-sky imagery (DSS)

Real deep-sky imagery, the “I can actually see the nebula” background familiar from the ASIAIR (ZWO’s all-in-one imaging controller), comes from a HiPS survey (sky images cut into small squares that load progressively as you zoom, like an online road map). DSS Color (the Digitized Sky Survey) is the default. A survey picker next to the DSS Img toggle switches the imagery layer to another full-sky survey - DSS2 red+blue, Mellinger wide-field colour, 2MASS near-infrared, or unWISE infrared - all streamed from CDS/alasky, keeping your current pan and zoom. Only DSS Colour ships an offline bundle; the others are online-only. Two modes exist for DSS Colour:

• Online (the default when no local bundle is present): tiles are streamed on demand from CDS Strasbourg (the astronomical data centre in France that hosts the survey). Needs a connection.
• Offline: provision the DSS bundle once; the bridge auto-detects it and prefers it, so the rich sky works at the telescope with no network.

The repo ships HEALPix order 3 and below as a baseline (the HEALPix order is the tile set’s zoom level; each extra order roughly quadruples the number of tiles), so the offline background works out of the box. For more detail, download a deeper tile set. Size scales roughly four times per order, so pick the ceiling that fits the memory card of your SBC (single-board computer, such as a Raspberry Pi):

max order	tiles	approx size	look
3	~1 020	~30 MB	big objects recognisable, soft on zoom
4	~4 100	~110 MB	most DSOs recognisable (good value)
5	~16 400	~400 MB	detailed, ASIAIR-like
6	~65 500	~1.5 GB	overkill for framing

The easiest route is Settings → Sky imagery (offline DSS), which downloads order 4 or 5 from inside the app. It fetches the tiles from CDS Strasbourg, or from the NASA/IPAC IRSA mirror (NASA’s infrared science archive, which keeps an identical copy of the survey) when CDS does not answer. The card counts an order as installed only when every one of its tiles is on disk; after an interrupted download it shows the order as incomplete with a tile count, and pressing the button again fetches only the missing tiles. From a shell, the provisioning scripts do the same (set DSS_REMOTE, or -Remote on Windows, to https://irsa.ipac.caltech.edu/data/hips/CDS/DSS2/color to use the mirror):

# Linux / macOS / Git-Bash  (args: MAX_ORDER [PARALLEL])
scripts/fetch-stellarium-dss.sh 4

# Windows
pwsh scripts/fetch-stellarium-dss.ps1 -MaxOrder 4

The script is resumable: it skips tiles already present, so you can re-run it later to top up to a higher order. It writes to src/NINA.Polaris/wwwroot/sky/data/skydata/surveys/dss/. The tiles are committed as ordinary files, so every clone has them and they ship in dotnet publish and the installer. The bundled survey stops at order 3 (about 45 MB) because each order up multiplies the tile count by four and every byte stays in the repository history. Contributors who fetch tiles commit them afterwards:

git add src/NINA.Polaris/wwwroot/sky/data/skydata/surveys/dss
git commit -m "skydata: bundle DSS Color HiPS (order 4)"

Attribution: DSS Color, STScI/NASA, HEALPixed by CDS Strasbourg.

10.4 DSO preview thumbnails

Search results, atlas cards, and the Tonight’s Best list each show a small DSS2 cutout of the object, an ASIAIR-style photo per target, so you can tell a galaxy from a nebula at a glance, fully offline. The repo bundles full coverage - every catalogued object with valid coordinates (about 17,700 images, roughly 215 MB, across M, C, NGC, IC, Arp, HH, LBN, LDN, Sh2, AGC, and HCG) under skydata/dso-thumbs/<SLUG>.jpg (for example M42.jpg, NGC7000.jpg, SH2279.jpg), tracked with Git LFS. The app derives each file name (the “slug”) from the result’s catalog and id, or by parsing the object name, and hides the image when none is bundled. Tonight’s Best uses these bundled cutouts first, so it works instantly and offline; the online NASA/Wikipedia lookup is only a fallback for objects with no bundled thumb, such as planets and comets.

To regenerate the set or widen coverage (--workers parallelises the fetch, ≤4 to stay polite to the shared CDS service):

python scripts/build-dso-thumbs.py --all-catalogs NGC,IC,AGC,HCG --workers 4

The script is resumable and sources DSS2 Color imagery via the CDS hips2fits service (STScI/NASA imagery). It is distinct from the online /api/sky/image NASA/Wikipedia lookup, which still works as a fallback when connected.

10.5 The bundled catalogs

Search, the atlas filter, and Tonight’s Best all draw from a bundled single-file database (SQLite, with an R*tree spatial index for fast sky searches) at wwwroot/catalogs/dso/dso.db (about 2.6 MB, roughly 14 500 objects):

Catalog	Entries	Source	License
NGC	~7570	OpenNGC	CC BY-SA 4.0
IC	~5000	OpenNGC (same file)	CC BY-SA 4.0
M (Messier)	107	OpenNGC cross-reference (M-tagged duplicates)	CC BY-SA 4.0
C (Caldwell)	104	Embedded Caldwell to NGC/IC mapping in the build script	Public domain
Arp	592	CDS Vizier VII/192A/arplist (Arp 1966)	Public domain
Sh2	313	CDS Vizier VII/20/catalog (Sharpless 1959)	Public domain
HCG	100	CDS Vizier VII/213/groups (Hickson 1982/89)	Public domain
AGC	767	CDS Vizier VII/110A/table3 (Abell-Corwin-Olowin 1989)	Public domain
HR / Star / WR	~9400	Bright Star Catalogue, IAU star names, Galactic Wolf-Rayet list	Public domain
Notable	~150	Hand-curated famous objects	Public domain

Two of those rows are not deep-sky catalogues in the usual sense.

The star rows make the naked-eye sky searchable: type “Vega”, “Albireo”, “HR 7001” or a Wolf-Rayet designation and the map takes you there, which matters for focusing, collimation and for pointing checks rather than for imaging targets.

The Notable rows are the objects people ask for by name that no NGC, IC or Messier number covers: Phoenix A, TON 618, 3C 273, the Vela pulsar, Cygnus X-1, Sagittarius A*, Markarian’s Chain, the Leo Triplet, the Coma and Perseus clusters, Wolf 359, TRAPPIST-1, the Hubble Deep Fields. They are drawn whatever the magnitude cap, since most are far fainter than any sensible cap, they carry their own types (“Quasar”, “Pulsar”, “Black Hole”, “Gravitational Lens”, “Dwarf Galaxy”), and Tonight’s Best lists them under a Notable chip with no brightness gate: every one that peaks above 30 degrees tonight, ordered by score. Half of them are targets for a photograph of a position rather than of a shape, which is its own kind of observing.

With the Notables and the comets the list can run to several hundred entries, so it shows 30 cards at a time. Scrolling near the end adds the next 30, as does the Show more button below the last card. Each card’s altitude chart and thumbnail load only when the card is shown, one chart at a time, so a tablet or a small host is not asked to draw hundreds of charts at once. Changing the filter starts again from the first 30.

The AGC entry is magnitude-trimmed at m10 < 17, keeping the brightest roughly 30% of the 2712-cluster catalog; fainter clusters require deep imaging beyond typical amateur reach. Full attribution and per-source license notes ship at wwwroot/catalogs/dso/LICENSE.txt.

To rebuild the bundle from the original sources, run:

python scripts/build-dso-catalog.py

The output overwrites src/NINA.Polaris/wwwroot/catalogs/dso/dso.db. The script needs only the Python 3.8+ standard library (urllib and sqlite3), no external dependencies; cached downloads live in scripts/.dso-cache/ for fast re-runs.

When dso.db is missing (a development clone without the bundle), the SKY tab silently falls back to a small list of about 150 hardcoded objects (the complete Messier catalog plus a handful of popular NGC entries), so the app still works, but search hits like “NGC 7331”, “Arp 273”, or “Sh2-279” come up empty.

10.6 Searching for a target

At the top of the tab sit a text input and a Search button. Names resolve against the bundled catalog (NGC, IC, M, C, Arp, Sh2, HCG, AGC, the Notable list, bright stars by name or HR number, plus common names like “Andromeda”), satellites and the major moons of Jupiter and Saturn by name, and you can also type RA/Dec coordinates directly. Matches appear as result cards showing the name with alternate designations, RA/Dec (in J2000, the standard reference epoch for sky coordinates), magnitude and apparent size in arcminutes, an object-type badge (Galaxy, Nebula, Cluster, and so on), and the constellation. Click a result and it overlays on the map, centred and highlighted.

When the box gets focus it lists your last 8 searches that found something, newest first; typing narrows the list, and tapping an entry runs that search again. The Sky screen of Basic mode shows the same list under its search box. Clear empties it. The list is kept in the browser (or app) you search from, not on the host.

The Filters button toggles a panel to narrow the results:

• Catalog dropdown, restricting to a single source. Hidden when the expanded database is not loaded.
• Object type dropdown (Galaxy, Globular Cluster, HII Region, Peculiar Galaxy, Planetary Nebula, Supernova Remnant, and more); the list of types comes live from whatever is in the catalog.
• Constellation, a free-text three-letter IAU abbreviation (“Cyg”, “Ori”, “And”). Hidden when the expanded database is not loaded.
• Magnitude range, minimum and maximum inputs.
• Dec range, minimum and maximum in degrees. Useful for filtering by hemisphere: set the minimum Dec to 0 to keep only northern targets, or the maximum to 0 for southern ones.

10.7 Planning the night: altitude chart, Tonight, and weather

Once a target is selected, the bottom of the SKY tab shows its altitude against UTC time, with twilight bands (astronomical, nautical, civil), a Moon-altitude overlay, and a highlight on the best window where the target rides highest.

For picking the target in the first place, the Tonight view ranks the best objects to image for your location and date, drawing altitude curves with the twilight and night windows. The ranking also surfaces large emission and dark nebulae that carry angular size but no stellar magnitude, so photogenic targets do not vanish just because they lack a catalog brightness. Galaxies get the same treatment: the ranking reaches down to a fainter magnitude for them and gates on object type rather than a single brightness cut, so faint but worthwhile galaxies show up instead of being trimmed away with the truly out-of-reach ones. A Weather view shows current conditions and the forecast for the observing site, so you can judge whether the plan survives the clouds.

Ranking for your sky: Bortle, SQM and the Moon

A faint, sprawling galaxy that is an easy target from a dark site can be nearly invisible from a city, while a bright planetary nebula or a star cluster comes through almost any sky. Tonight’s Best can rank for that if it knows how dark your sky is.

Open SETTINGS → Observatory → Sky quality (🌃) and either pick the Bortle class (the 1 to 9 scale of sky darkness, where 1 is a remote desert site and 9 an inner city) or type an SQM value. SQM is the reading of a Sky Quality Meter, the sky’s brightness in magnitudes per square arcsecond (mag/arcsec²): higher is darker, about 21.9 at a truly dark site and 18 under a city. If an SQM meter is connected, the card shows what it reads now and offers Use this reading; use it only on a clear, moonless night, since a reading under the Moon or cloud is not your site’s dark sky. If you do not know your Bortle class, a light pollution map such as lightpollutionmap.app shows it for your coordinates.

With the sky set, each deep-sky object in the list is compared with the sky at the moment it rides highest:

• The object’s surface brightness: its magnitude spread over its catalogued size, so a large object of a given magnitude is fainter per square arcsecond than a small one. Nebulae catalogued with a size but no magnitude (most Sharpless and LBN entries, and reflection nebulae such as the Witch Head) are taken as faint, about 24 mag/arcsec².
• The sky towards it: your site’s sky, brighter towards the horizon where the light crosses more air, plus the Moon’s scattered light from its phase and its distance to the target (the Krisciunas and Schaefer model used by most planners).

Objects much fainter than that sky lose points and drop down the list. Each card gets a tag; hover it for the two numbers:

• Great for your sky: comes out easily, faint outskirts included. Only a sky darker than about Bortle 5 gives this.
• Good for your sky: comes out well; under a bright sky this is the best there is.
• Possible under your sky: needs a long integration.
• Hard under your sky: out of reach in practice.

The thresholds were set against real sessions from two sites. From a Bortle 4 site, the Horsehead, the Flame Nebula, Barnard’s Loop and the Ghost of Cassiopeia show in minutes and are finished in three to four hours, so they are rated great there (good when low in the sky). From a Bortle 8 to 9 city those same nebulae are hard in broadband, while M31, M33, M42, the Pleiades, the Sculptor Galaxy (NGC 253), the Southern Pinwheel (M83), the Dumbbell (M27) and the Carina Nebula (even low in the sky) are good. Galaxies and bright nebulae such as M42 and Carina are judged by their bright inner parts, which carry most of their light, more than by their faint mean over the whole object. Star clusters are hurt far less than galaxies, since they are resolved stars rather than diffuse light. Planets, the Moon, comets and the Notable list are not affected.

A narrowband or dual-band filter (Hα, OIII, SII, L-eXtreme, SV220 and similar) cuts most of the light pollution and keeps the light of emission nebulae. When the active rig has one, on the filter wheel or as its attached filter, the list ranks emission and planetary nebulae accordingly; the Narrowband chip shows this and turns it on or off for the view. Galaxies and reflection nebulae are not helped by such a filter and keep their place. From the same Bortle 8 to 9 city, the Veil, the Crescent Nebula and the Eagle’s Pillars of Creation are hard in broadband and good through a narrowband filter.

Without a sky set, the list ranks as before and says so above the cards, with a button to set it.

10.8 Custom horizon

Trees, roofs, and hills all cut into the sky you can actually image, and a target that looks well-placed on the altitude chart is useless if it sits behind your neighbour’s roof. Polaris lets you draw your real local horizon and then respects it everywhere.

Open SETTINGS and find Custom horizon (🌄). It is an azimuth/altitude mask: a canvas where the horizontal axis is compass azimuth and the vertical axis is altitude. Click to add a point, drag to move one, right-click to delete. Two ways to build it:

• Capture from the mount. Point the scope at the top of a real obstacle (a tree, the roofline) and click Horizon point (also on the mount controls) to record the scope’s current azimuth and altitude as a point. Walk the horizon obstacle by obstacle and you trace your true skyline.
• Import a file. Load a horizon file (.hrz, .txt, or .csv, the common az/alt formats other planners export) with Import file, and Export to save yours back out.

Once set, the horizon is honoured across the app. It overlays on the sky map and the altitude chart so you can see at a glance where a target clears it. The Tonight ranking accounts for it, so objects that never rise above your obstructions are not recommended. And it feeds the mount’s slew-safety altitude floor (Chapter 7), so a GoTo below your horizon is refused rather than driving the tube into the trees.

10.9 Field-of-view overlays and framing

Polaris draws the camera footprints directly on the map so you can frame a composition before slewing:

• The blue rectangle is the mount FOV (field of view), the main camera’s footprint anchored where the scope is pointing. It is sized from the active rig’s focal length and the connected camera’s sensor, and rotated to the solved camera angle once a plate solve is available (plate solving matches the stars in a captured frame against a catalog to find exactly where, and at what angle, the camera is pointing).
• The red rectangle is the target framing box. While idle it is screen-anchored, so you drag the map to compose; while imaging with a recent solve it snaps to the solved sky position, so red converges on blue when you are framed correctly.
• The pink rectangle is the auxiliary camera’s FOV, shown when an Auxiliary Camera System is configured on the rig (aux focal length set, and the aux sensor reported once it connects; see Chapter 5). The aux rides the same mount, so it is anchored at the mount position.
• The yellow rectangles are the mosaic panels, described below.

When you run a plate solve from SKY (Solve & Sync), Polaris also fires a parallel solve on the aux camera if one is connected: it captures a single aux frame with the aux exposure, gain, and binning, and solves it on its own hardware, concurrently with the main solve. The pink rectangle then snaps onto the aux camera’s real solved rotation and scale, so you know for certain the field and angle the aux photo will come out with instead of assuming it matches the mount. A toast reports the solved aux rotation.

Framing rotation

The angle on the red box is the camera’s sky position angle, 0 with north up, the same number a plate solve reports. Normally it follows the solve. To choose it instead, click Rotation in the toolbar: a slider appears beside the box and turns it live, with single-degree nudges and a field for typing an angle. Follow solve hands the angle back to the plate solver.

What happens next depends on what can turn the camera.

With a rotator on the rig (Chapter 5), the chosen angle rides along with every Slew & Center from this tab. After each solve the loop turns the rotator by the difference and solves again to confirm, until the angle is inside half a degree. Rotate now does the same without moving the mount. The rotator’s mechanical angle differs from the sky angle by an offset nobody knows in advance, so the first move can go the wrong way once; the loop notices that the error grew, reverses, and remembers the direction for the rest of the session. A difference of 180 degrees counts as arrived, because a rectangular sensor frames the same field either way up.

With a rotator you turn by hand, a camera rotator ring or a motorised one with the motor off, that loop cannot run, so you are the loop. With no rotator connected and an angle chosen, the panel offers Measure framing: take a PREVIEW exposure, press it, and Polaris solves the frame and answers with an instruction, “turn the camera 20.0 degrees clockwise”, as seen looking at the back of the camera. Turn it, press it again, and repeat until it reports the framing is within a degree.

The direction comes from the parity of the solved field: a star diagonal, or any odd number of reflections, reverses which way the sky angle runs, and the solve’s CD matrix is what reveals that. What it cannot know is which side of the camera you are standing on, so if the instruction sends you the wrong way, press Other way; the choice is stored on the rig and applies to that optical train from then on. If a turn makes the error grow, Polaris flips the direction itself and says so.

A Slew & Center with no rotator connected still reports the framing error. It centres the mount as usual and never fails over an angle nothing on the host can change.

The chosen angle is kept either way: it is the rotation stored on a PLAN target and on targets added to a sequence.

10.10 Favourites and imported target lists

The target card carries a Favourite button. A starred object goes into a list kept on the host, alongside the plans rather than in the browser, so it is there on every tablet and for every rig. Favourites then surface in three places: the Favourites chip in Tonight’s Best, which lists every starred target that peaks above 30 degrees tonight with its peak time; the Favourites button in the PLAN target picker (Chapter 16); and that panel’s Map button, which frames the object back on the sky map.

The PLAN favourites panel also takes Import CSV, for a target list exported from Telescopius or assembled in a spreadsheet. The file needs a name column and right-ascension and declination columns, in sexagesimal or decimal form; the importer recognises both tidy headers (“Catalogue Entry”, “Right Ascension”, “Declination”) and plain ones (“name”, “ra”, “dec”). Targets you have already starred, matched by name or by position, are skipped, so re-importing a list you have curated does not duplicate it.

10.11 Slew & Center

The big workflow button. With a target selected, click Slew & Center and Polaris:

1. commands the mount to the target’s RA/Dec;
2. captures a plate-solve frame (5 s exposure by default);
3. solves it, using ASTAP (Kleijn, n.d.), the plate-solving program bundled with Polaris, as the primary solver, and falling back to PlateSolve3 or Astrometry.net (Lang et al. 2010), online or local;
4. computes the offset from the intended position;
5. re-slews to correct, repeating up to 5 iterations until the error is within tolerance (30 arcsec by default).

A status banner shows the phase live: “Slewing → Capturing → Solving → Centering → Centered (12 arcsec error)”.

Going to coordinates you type

Not every target has a catalogue entry: a comet from an ephemeris, a position from a paper, a spot a friend read out over the radio. GoTo coordinates, in the SKY toolbar and on the RIGS mount card, takes a pair of coordinates in whatever form you have them, sexagesimal (05:35:17 -05:23:28, 5h35m17s, 05 35 17, with degree marks) or decimal, and shows you how it parsed them before anything moves. A bare decimal right ascension is read as hours, or as degrees when it passes 24, when it carries a d, or when the unit switch says so.

From the parsed preview you can Slew & Center, Slew Only, or Show on sky map to check the field first. The SKY search box accepts the same coordinate pair as a target, so pasting one and pressing Search is the quick path.

Figure 10.2: Solve & Sync in action: the Plate solver panel streams the solver’s log live while the map shows the current field-of-view rectangle; Sync here applies the solved position to the mount.

10.12 Centering on the Sun, Moon, or a planet

Plate solving cannot lock onto solar-system objects: the Sun and Moon wash the frame out, and a planetary shot (long focal length, millisecond exposures) has no background stars to match, so Slew & Center fails on them. The Center on body picker on the map handles them with a solve-near-and-offset strategy instead:

1. Pick Moon, Sun, or a planet from the dropdown and click Center on body (the mount must be connected).
2. Polaris computes the object’s apparent topocentric position (where it appears from your exact spot on Earth) from its built-in ephemeris (tables of where solar-system bodies are at any given time), using your profile location and clock.
3. It slews a few degrees off to a nearby star field and runs the normal plate-solve and sync there, correcting the mount’s pointing model right next to the target without ever solving the object itself.
4. It re-reads the ephemeris (the Moon moves about 0.5 degrees per hour) and performs a precise GoTo (an automated slew to exact coordinates) onto the object. For the Moon and Sun it then switches the mount to lunar or solar tracking so the object stays centred.

A phase chip shows progress (Computing position → Solving nearby field → Slewing to target → Centered), and the offset-field solve streams to the SKY solver console like any other solve. The procedure needs a connected mount, a camera, and a working plate solver for the offset field; if the nearby field will not solve, raise the offset or pick a clearer patch of sky.

WarningThe Sun

Selecting the Sun pops a confirmation dialog. Only proceed with a certified full-aperture solar filter fitted. An unfiltered telescope pointed at the Sun destroys the camera instantly and can cause permanent eye damage. No software can protect against this.

10.13 Mosaic planner

For targets larger than one field of view, click Plan mosaic with a target selected. Polaris overlays an N by M grid of panels on the map, drawn live as you adjust the settings: panels per axis, overlap percentage, and the total grid size. A cos(δ) correction (δ being the declination) keeps panels at high declination from stretching, and the planner shows an estimated session time computed as panels times exposure times frames.

Camera angle turns the grid for a camera that is not mounted square to north. It starts at the angle of the last plate solve (or the angle of the target box on the map), and the panels and the steps between them both follow it, so a rotated camera keeps an even overlap at the corners instead of leaving wedge-shaped gaps. The exported sequence carries the angle on every panel.

Add to Sequence is how the plan leaves the map: it generates the AUTORUN rows for all panels, ordered in a serpentine slew pattern (back and forth, row by row, like mowing a lawn) so the mount walks the grid efficiently. From there the sequencer takes over; see Chapter 14. For multi-target scheduling across a whole night, the planner described in Chapter 16 builds on the same target selection.

10.14 MOSAIC mode

The planner above hands a mosaic to the sequencer to shoot later. The MOSAIC tab (in the left navigation, after LIVE) shoots one now and builds the picture on screen as the panels come in. It is marked experimental while it gathers field experience.

Pick a target with Find (a catalogue name or coordinates), SKY target (whatever is selected on the sky map), or Mount position (where the telescope points now). Then set the size of the final picture the way a smart telescope does, with the Final width and Final height sliders: each runs from one camera field up to eight, and shows the size in degrees and the panels it takes. The overlap is set automatically: Polaris takes the fewest panels that overlap by at least 20% and spreads them evenly across the field, so the actual overlap (shown under the sliders) is 20% or a little more. The grid, the field in degrees and an estimated time appear before anything moves. Exposure, gain and binning apply to every panel. Press the shutter to start; press it again to stop.

Until the first panel comes in, the picture area shows a small sky map around the target with the planned panels drawn on it in yellow, numbered in the order they will be shot. It redraws as you change the target or the field size. The panels take the camera’s angle from the last plate solve. With no solve yet, a Camera angle field turns them to the angle you type (north up when empty); either way the run lays the real grid out along the camera’s angle once it has centred. The panels only appear once the camera is connected, because their size comes from the camera and the rig.

What happens next:

1. Centring: Slew & Center (Section 10.11) puts the target in the middle of the frame. Its plate solve (working out exactly where the camera points from the star pattern) also gives the camera’s angle on the sky, and the grid is laid out along the sensor’s edges, so the overlap stays even whatever the rotation.
2. Each panel: the mount slews, waits two seconds to settle, takes one exposure, and plate-solves it. With Guide each panel on, guiding restarts on the panel and settles before the exposure (see below).
3. Placement: each panel lands on the picture where its own plate solve says it belongs, not where the mount was told to go, so pointing errors, field rotation and a meridian flip are all taken into account. Where panels overlap, each fades out toward its edge so the seam does not show, and each panel’s sky background is levelled first so panels shot through different sky glow match.

The picture fills in live: dashed outlines mark the panels still to come, the current one is highlighted, and a finished panel shows its real footprint. A panel whose plate solve fails is placed from the mount’s reported position instead and outlined in amber; its edges may not line up exactly. With Save the panel frames on, every panel is saved as a normal light frame in a “Target mosaic” folder, and the finished picture is saved next to them as a JPEG.

• Resolution: the on-screen picture is at most 2400 pixels on its longest side. It is a preview of the field, not the final image: stack and stitch the saved panels in your processing software for full resolution.
• Guiding: each panel is a slew, and a slew carries the guide star out of the guide camera’s view, so guiding always stops before the mount moves. What happens next depends on Guide each panel, which appears when a guider is connected:
  • On: after each slew Polaris picks a new guide star, starts guiding and waits until it settles (the star held within 1.5 pixels for 3 seconds) before the exposure. That usually takes a few seconds. A panel whose guiding does not start or settle is still shot, and the run lists it. When the mosaic completes, guiding keeps running on the last panel.
  • Off: guiding stays paused for the whole run and each panel is exposed on the mount’s tracking alone, so keep the exposures short enough for your mount to track unguided. When the mosaic completes, Polaris picks a new guide star and resumes guiding.
  The option starts on when guiding is running and the exposure is 30 seconds or more, and off for shorter exposures, where restarting guiding would take longer than the exposure itself; tick or clear it to choose. After a failure or a stop, guiding stays off either way.
• Plate solver: without one installed, every panel is placed from the mount position and the grid is laid out north-up, because the camera angle is unknown. The tab says so.
• Mount problems: if the mount refuses a slew (tracking off, a limit, a safety stop), the run stops there instead of trying the remaining panels.

10.15 Stellarium sync

If you have Stellarium (the popular free desktop planetarium program) open with its Remote Control plugin enabled, click Get from Stellarium. Polaris fetches Stellarium’s current selection over HTTP (a plain web request on your local network) and auto-populates the search box with the RA, Dec, and name.

10.16 Slew preview

While the mount is slewing and nothing is capturing, an inset card appears in the lower right showing a live camera feed, so you can watch the field sweep past during a GoTo. This is driven by SlewPreviewService, is on by default, and is polite to other camera consumers: it silently yields whenever a sequence, auto-focus, preview, or video recording grabs the camera.

10.17 Common pitfalls

Search returns nothing. The catalog is not loaded; refresh the page.

Slew & Center fails repeatedly. See the plate-solve section of Appendix C.

Mosaic panels overlap wrongly. The rig’s focal length or sensor size is wrong. Re-pick the OTA from the catalog in the RIGS tab (Chapter 5).

Kleijn, Han. n.d. ASTAP: Astrometric Stacking Program. Https://www.hnsky.org/astap.htm.

Lang, Dustin, David W. Hogg, Keir Mierle, Michael Blanton, and Sam Roweis. 2010. “Astrometry.net: Blind Astrometric Calibration of Arbitrary Astronomical Images.” The Astronomical Journal 139 (5): 1782–800. https://doi.org/10.1088/0004-6256/139/5/1782.

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1. For the technically curious: it runs in an iframe, draws with WebGL2 (the browser’s built-in 3D graphics capability), and the host page steers it with postMessage, the standard way two web pages exchange data.