Two free, open-source geometry calculators for astrophotographers planning or building an observatory — designed to give you real answers about whether your setup will physically work before anything gets bolted down or cut.
Setting up dome-sync in NINA requires knowing four signed numbers with the right sign conventions. Getting them wrong is easy and the feedback loop is slow — configure, run NINA, watch the dome miss, tweak, repeat, on a cold night with dew forming. The dome calculator grew out of that frustration.
The roll-off-roof calculator came from the same question in a different form. A roll-off roof is a genuinely different geometric problem from a dome, and the astrophotography software ecosystem has good dome-sync support but comparatively little for the equally real problem of whether your ROR's walls, parked roof, and floor let your assembly swing freely.
The dome calculator's core azimuth math is ported from NINA's DomeSynchronization.cs, using the same matrix conventions. Both tools are licensed under the Mozilla Public License 2.0 — the same license NINA uses — in recognition of that shared foundation.
These tools are not affiliated with, endorsed by, or supported by the NINA project or its developers.
The Mount System card carries dimensions for a number of mounts. Where those figures came from matters, because they are not all equally solid — some are taken from a manufacturer's own published drawings, others are community measurements that have been circulating in amateur observatory spreadsheets for years. Each is listed below so you can check the original rather than take our word for it.
Dimension C throughout is measured to the mounting surface, not to the optical axis. Your rings raise the tube above the saddle, so C should be increased by that amount for your own setup.
astro-physics.com · Technical Support
Status: C and the counterweight shafts are manufacturer-published. A and B are not — they do not reproduce Astro-Physics' own declination-axis heights under any single latitude, so they remain community estimates.
The drawings describe the pre-Series-6 mounts, so those entries are named accordingly. Software Bisque's current Series 6 models are listed separately and are not dimensioned: Series 6 changed the declination axis on the MX and the bearings on the MYT, so the earlier figures do not carry across.
Status: A and B are manufacturer-derived. C is confirmed for the MX, flagged for confirmation on the ME II, and not published for the MYT. The drawings do not state which generation they describe, so their applicability to the MX+ and Series 6 models is unconfirmed.
The HEQ5 Pro, EQ6-R Pro, AZ-EQ6-GT, EQ8-R Pro and CQ350 Pro are listed, with no geometry filled in. Sky-Watcher publish payload capacities, saddle types and counterweight shaft diameters, but not the axis dimensions this tool works from.
Status: named, not dimensioned.
zwoastro.com · AM5 User Manual
The AM5, AM5N and AM7 are listed, but with no geometry filled in. ZWO's manual does carry a Structural Dimensions section, and it is a diagram rather than a table, so none of the three figures can be transcribed from it. Selecting one of these changes nothing; the card says so and tells you what to measure.
What is taken from the manual is the surrounding detail: 0–90° latitude range, dual equatorial and alt-azimuth operation, and ZWO's advice that a counterweight, if fitted, should sit 20–30 cm out. The counterweight reach defaults to 0 for these, since harmonic mounts are commonly run without one.
These use a purpose-built strain-wave model in the 3D preview, not a German equatorial with the weights hidden: no counterweight shaft, no meridian flip, and the swept envelope is the tube alone.
Status: named and described, not dimensioned. Three measurements would make any of them a full entry.
The published GM HPS dimensions sheet draws every mount at a single latitude of 45°. That gives one equation for two unknowns, so the declination-axis height cannot be separated into the constants this tool normally uses.
The GM2000, GM3000 and GM4000 are therefore included as fixed-latitude entries: they carry 10 Micron's published 45° figure and do not follow your site latitude. The card says so whenever one is selected, and estimates the error if your site is more than a few degrees from 45° — around 1.3″ at 33° or 60°. The GM1000 is absent because the sheet does not carry that dimension for it, and the declination-axis to saddle distance is not identifiable for any of them, so none of these presets touch the optical-axis offset.
Selecting one of these also holds the site latitude at 45° and locks the field, so the sky figures and the mount geometry describe the same site rather than quietly disagreeing. The page says so while the lock is active, and choosing any other mount restores your own latitude.
Status: published by the manufacturer, but for one latitude only. If you own one of these mounts, two measurements would upgrade it to a full entry.
The L-350, L-500 and L-600 altitude-axis heights come from the community spreadsheet below, not from PlaneWave. The L-500 figure in particular has two conflicting values in that source and carries an on-screen caution.
The Astro-Physics A200, the Paramount ME, the PlaneWave figures, and the A and B values for the Astro-Physics mounts come from a Pier-Height / Dome Calculator workbook (dated 8 October 2020) that circulates among amateur observatory builders. It is careful work and much of it has held up against manufacturer sources — but one of its own entries is annotated "guestimate", and its Paramount figures were found to be measuring the wrong quantity entirely, which is why those were replaced.
No manufacturer listed here is affiliated with, endorses, or has reviewed these tools. Product names are used only to identify which mount a set of dimensions describes. Every figure a preset fills remains editable, and anyone building to tight clearances should measure their own mount.
These calculators are free and always will be. If they saved you a wasted weekend or a wrongly poured pier, consider putting something toward one of these instead — neither has anything to do with this project, which is rather the point.
Both calculators are fully client-side HTML — no server, no database, no data collection. The source is the page itself. Released under the MPL 2.0; contributions and forks welcome under those terms.
Astronomy fanatic. Hooked since childhood.
I can still remember the first time I saw Saturn's rings through a telescope. That was it — done. Space had me completely. As an adult I've been lucky enough to build my own observatory, and the hobby has given me so much over the years that I wanted to find a way to give something back. If these tools help even one person spend less time fighting their setup and more time under the stars, that's exactly what they were built for.
My astrophotography work — still very much a work in progress — lives here: M51_MyDestination on AstroBin →
Travellingbears & AndrewBares @ Cloudy Nights
For their inspiration, feedback, and generosity with their time. These tools are sharper, more useful, and more honest about what they're actually solving because of their input. Thank you.