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 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. From a shell, the provisioning scripts do the same:

# 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/, a path tracked with Git LFS² (see .gitattributes) so the binary tiles do not bloat a working clone but still ship in dotnet publish and the installer. 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.

10.3 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.

10.4 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

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.

10.5 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, plus common names like “Andromeda”), and you can also enter manual RA/Dec coordinates. 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.

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.

10.6 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. A Weather view shows current conditions and the forecast for the observing site, so you can judge whether the plan survives the clouds.

10.7 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.

10.8 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)”.

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.

10.9 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.

10.10 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.

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.

10.11 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.

10.12 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.

10.13 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.

---

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.

2. Git LFS (Large File Storage) is an extension to the Git version-control system that keeps big binary files out of the normal repository history and downloads them only when needed.