Two browser-based calculators that turn tape-measure numbers into the settings and clearances an observatory actually needs — a dome geometry tool that produces NINA dome-sync values, and a roll-off-roof planner that shows exactly which parts of the sky your building blocks. No install, no account, no data leaves the browser.
Download this overview as a PDF →
Observatory geometry is unforgiving. A pier a few inches off centre, a mount whose declination axis sits above the dome springline, or a wall two courses too high, and you lose sky you paid for — usually discovering it after the concrete has cured. Both tools let you settle that on screen first, with the numbers and the picture driven from the same maths.
Produces the four values NINA needs under Options → Dome, from measurements you can
take with a tape and a plumb bob. The dome-sync maths is ported directly from NINA's own
DomeSynchronization.cs, so the numbers match what the software will do.
Measure from each wall to the point below your mount's pivot — north, south, east, west. The signed offsets are derived for you and shown live. If you already have your NINA values, switch to direct entry instead.
Projects your actual aperture onto the dome surface as a ghost circle, so you can see whether the light path clears the shutter opening — not merely whether the dome is pointed roughly the right way.
Sweeps the whole sky and plots what the shutter can and cannot reach, on both sides of the meridian for an equatorial. Alt/Az collapses to a single plot, since there is no flip.
Side-by-side OTAs with a lateral axis offset, and a guide-scope correction that shifts the tracked optical axis. Clearance is checked for each tube independently.
Answers the question a roll-off roof actually poses: with these walls, this roof and the mount here, how much sky do I lose? Every wall, the roof in its parked position and your local terrain horizon are combined into one obstruction model.
Flat, peaked and wedge. The peaked roof takes a pitch ratio rather than a ridge height — enter 1:6 and the ridge is computed from the building span, then shown back to you.
Up to six walls at arbitrary angles and distances, so an L-shaped or clipped-corner building can be modelled properly instead of approximated by a box.
An equatorial sweeps a counterweight; an alt/az does not. Each is measured on its own terms, so a fork mount is not told it needs an equatorial's swept volume.
Walls, roof and terrain combine into a horizon profile you can export as a .hrz
file for your planning software.
Switching mount type changes the model and the maths together, not just a label.
| Equatorial (GEM) | Alt/Az or Fork | |
|---|---|---|
| 3D model | Latitude-tilted RA axis, RA and Dec rotors, counterweight shaft | PlaneWave L-500 direct drive — base drum, offset arm, altitude drive drum |
| Meridian flip | Pier side inverts the mount, aim unchanged | Not applicable — control hidden |
| Swing radius, 600 mm OTA | 979 mm | 651 mm |
| Swing radius, 2000 mm OTA | 2379 mm | 1220 mm |
| Sky coverage | Both sides of the meridian | Single plot |
The 3D view is not a decoration drawn alongside the numbers — it is driven from the same calculation. Across 36 pointing combinations per tool, spanning latitudes +60° to −33°, the rendered telescope matches each tool's own computed sky position to 0.0000° in both altitude and azimuth. If the picture and the numbers ever disagreed, that would be a bug, and it is tested as one.
Both tools are open source under the Mozilla Public License 2.0. The source is the HTML file itself — view source and read it.
CygnusWave builds observatory planning tools for amateur astronomers. The story behind the project, the people who helped shape it and how to get in touch are on the about page: