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A Metric / Imperial toggle sits in the top-right header. Switching units converts all building, roof, mount position, height, axis offset, pier gap, and custom wall fields in place. OTA tube length and aperture always remain in mm regardless of the unit setting — telescope optics are always specified in millimetres.
In imperial mode each field shows a fractional-inch hint (e.g. 13'1 1/2") beside it, updating live as you type. The summary chips at the top of the page (Footprint, Wall Height, Roof Travel, Mount Offset) also display in the chosen unit.
The same page rearranges rather than hiding anything. On a tablet the rail moves above the 3D view instead of beside it; on a phone everything becomes a single column and the measurement fields, the reasoning and the sky notes each drop to one column of their own.
Nothing is removed at any width, and there is no sideways scrolling — the page simply gets taller.
A measured horizon profile is saved with the rest of your setup, not just the flat limit. Loading that file brings back the profile, the custom mode, and the name of the .hrz file it came from — so the status line tells you which horizon was restored rather than just how many points it found.
Drag the grip along the bottom edge of the 3D view — the chevrons above and below it show which way it moves, and it turns green while you are dragging to make it taller or shorter, trading height with the figures below it. Double-click the bar to go back to the default. With the bar focused, the arrow keys nudge it and Home resets it.
Whatever height you settle on is saved with your setup. On a phone the control is hidden, since the layout stacks and the page scrolls instead.
The five Astro-Physics park positions are on the pointing bar as AP PARK 1–5, computed from your latitude where the definition depends on it.
The pier side follows the hour angle: at any given hour angle only one side is reachable, and the other would put the counterweight above the tube. The change is animated rather than instant.
The outline is drawn in the wall colour and traces your building’s actual obstruction, so a long thin building pinches the plot across its narrow axis. Sky the building blocks is still shown but subdued, so you can see what the walls cost you without it competing with the sky you keep.
A peaked or wedge roof can be given either way, and the two stay in step — use whichever your plans quote.
Rise:run is the builder’s form: 4:12 means four of rise for every twelve of run. Type a ratio and the angle updates beside it, leaving your numbers exactly as entered.
Degrees is the other way round: type an angle and the ratio is rewritten as rise-over-twelve, which is the form drawings use. A 30° roof comes out as 6.93:12.
Only the ratio drives the geometry; the angle is a second way of writing the same thing. Both are saved with your setup.
The roof is only the panel that slides away. Whatever the walls are, they are still standing when it has gone — so it is the walls, not the roof, that limit your sky every night.
Choose flat and the extra fields stay out of the way.
Two limits pull opposite ways, and the setup check reports both as a range.
Anywhere between the two works. If your walls sit above the ceiling the check says how much sky they are costing and how far they could come down; below the floor, it says the roof will not close.
Sometimes the two cross — the mount needs more height than your terrain will forgive. That is worth knowing before the footings go in: raise the pier, use a shorter tube, or accept the loss deliberately rather than discovering it afterwards.
Everything else here assumes the roof is off, because that is when you observe. This is the other question: with the roof rolled back on, does the mount clear it where you leave it parked?
Usable height above wall top is how far into the roof structure the mount may reach. It depends entirely on what carries the roof:
What the roof itself can offer depends on its shape:
How much of that you may actually use depends on what carries the roof:
The tool shows the roof’s rise above the wall top beside the field, worked out from your span and pitch. That figure is a hard ceiling — the field will not accept more than the roof offers, and pulls anything larger back. It is a ceiling, not a promise — it is the most any structure on that roof could leave you, and anything entered above it is capped. How high a tie may safely go is a structural question involving span, timber grade, connections and your local snow and wind loading; it is not derivable from the roof shape, and this tool does not attempt it. Ask whoever engineers the trusses.
The usable space is drawn in the 3D as a blue volume sitting on the wall top. On a pitched roof it follows the pitch — full height through the middle, tapering to nothing at the eaves — because that is the shape of the room you actually have. Where the structure limits you before the roof does, the top runs flat at that height instead. The setup check reports the headroom for whatever position the mount is in, so try each park position — they are different poses and one may fit where another does not. Close the roof on the pointing bar slides the roof on; if the mount would foul it, the roof stops short and turns red rather than driving through, and the button offers to reopen, which slides it back out at the same pace. Clear the obstruction while it is sitting there and it goes back to its normal colour and finishes closing on its own.
The chip at the top right shows the version and a four-character build stamp — v8.14.000 33f9. The version says what changed; the stamp identifies the exact file, and changes whenever the file does. If you are reporting something, quote both.
Everything sits on one screen; there is nothing to scroll past.
Enter the building's width, length and wall height, then locate the mount by measuring from each wall to directly below the RA/Dec pivot on the floor — North, South, East and West. The signed offsets are derived automatically:
Both are shown in orange below the fields and update as you type. Because the building size is also entered, the four measurements are over-determined: N+S should equal the building's North/South span and E+W its East/West span. If they disagree, a warning appears naming the conflict. The offset itself comes from the difference, so it stays correct even when the sums are off.
Building width, length, wall height, and mount pivot position — height above floor, and N/S and E/W offset from the building's center. All in centimetres.
For anything other than a simple rectangle — angled corners, an extension, an irregular footprint — turn on "Use a custom wall shape" to describe the building as up to 6 walls, each with its own angle from North and distance from center.
Pick which wall the roof rolls off past and how far. Three roof shapes:
A scaled floor plan of the building sits above the Telescopes card. Click anywhere inside the building outline to place a telescope pier at that position. Drag to reposition. Each enabled telescope appears as a colour-coded dot with a faint dashed swing-radius circle. The NS/EW number fields in the Telescopes card stay in sync — move a pier and the numbers update, or type in a number and the dot moves. Use the floor plan for initial placement and the number fields for fine-tuning.
Each telescope is on its own independent mount. Telescope 1 uses the mount type and optics from the Optical Axis card, but its own position (NS/EW from building centre, in cm) and mount height defined in the Telescopes card. Enable Telescopes 2–4 for additional mounts — each gets its own position, mount height, mount type (GEM/Alt-Az), GEM axis length or Dec horizontal offset, tube length (mm), and aperture (mm). Note: OTA tube length and aperture are always in mm even in imperial mode.
Use the Pointing telescope dropdown in Step 4 to choose which mount drives the live pointing preview, obstruction check, and sky coverage sweep.
Each telescope's swing radius is checked independently at its own mount position against the nearest wall, floor, and ceiling — since each is on a separate mount, their clearance envelopes don't share a pivot. A separate check catches telescopes hitting each other: two mounts collide when the distance between their pivots is less than the sum of their swing radii plus the minimum walkway gap you specify.
The 3D view shows a wireframe sphere per telescope at its actual swing radius. The selected telescope's OTA tube tracks the live pointing direction; other scopes point straight up in a neutral pose.
Same concept as the dome tool. The plot is stretched to your building's actual width/length ratio. Includes an Export as Horizon File (.hrz) button.
Enable Dual saddle in the Optical System card to mount two tubes side by side. Enter how far each sits from the mount axis (half the centre-to-centre spacing) plus OTA 2's length and aperture. Both tubes are rigidly mounted, so they share the same pointing — only their position differs.
The offset feeds the clearance check: the swing radius uses the larger tube plus the offset, so the wall, floor and ceiling limits reflect the outer tube's reach rather than the mount axis alone.
Swing clearance is driven by the tube body, so the Optical System card asks for the OTA's outer diameter as well as its clear aperture. Left at 0 it is estimated at 1.3× the aperture, roughly right for a CDK once rings and dovetail are included — but if you are building to tight clearance, measure across your own tube and saddle and enter it.
The swept radius is the hypotenuse of how far the tube reaches along the optical axis and how far its far corner sits across it. Wall and roof limits include the aperture radius, so the whole light cone clears rather than just the centre ray.
The tube is usually balanced so its centre sits on the declination axis, but not always. OTA offset is the distance between the centre of the whole assembly — tube plus dew shield — and that axis, measured along the tube. Positive slides it toward the aperture, negative toward the focuser. Leave it at 0 if the assembly is balanced on the axis.
It drives how far the tube reaches when it swings, so an unbalanced tube is no longer assumed to be a typical one.
The Optical System card takes a dew shield length. It is added to the tube and drawn ahead of it in a lighter tone. It counts toward the swept reach, because a shield is the part most likely to catch a wall or a shutter edge. Leave it at 0 if you do not use one.
This is your own limit, not an obstruction: how low you are willing to observe. It starts at 0, which applies no limit, so a pointing is only reported as blocked when the building or dome actually blocks it.
Raise it if you would rather not observe close to the ground, or switch to a measured horizon profile if terrain is the real constraint. Either way the tool will tell you which of the two is stopping you.
Under the status banner, four lines explain why the setup does or does not hold together: the pointing model, swing clearance, what the walls cost you against what your terrain already blocks, and how far off centre the mount sits. Each states the reason rather than just a verdict, and the heading takes the worst of the four.
The swing clearance line is the one to read before building. It compares the radius your tube sweeps against the tightest obstruction — nearest wall, floor, or the parked roof — names which is tightest, and gives the margin. Under about six inches it says so is worth measuring on site. If the tube would strike, it says which obstruction and by how much.
Beneath the plot, the losses are attributed structure by structure, so you can see which wall costs what, in which direction, and to what altitude. Raising the mount lifts every limit at once; lowering a single wall only helps on that side.
With Dual saddle enabled, OTA 2 is described with the same figures as the first — tube length, clear aperture, outer diameter and offset along the tube. Each tube's reach is worked out separately and the larger governs the swing clearance, rather than mixing the longest length with the widest aperture from different tubes. A guide scope or wide-field refractor riding alongside can easily be the tube that reaches furthest.
If your mount is not in the list, or is listed without dimensions, measure it against the diagram, tick Use my own values, and press Save this mount. It is added to the list under Your mounts and stays there next time you open the tool.
Export my mounts writes them to a file you can keep or pass to someone else; Import mounts reads one back. Anyone who owns a mount the manufacturer does not publish figures for can measure it once and share it.
Show swept volume in the 3D view draws the space the telescope and counterweight sweep through as they turn. It is the shape the clearance figures describe, so you can see it against the walls or the dome shell rather than working from the numbers alone.
It draws blue when everything clears and red when something fouls the pier, matching the pier-strike line in the setup check.
Switching pier side rotates the mount through the pole: the tube and the counterweight change sides, and the tube rolls 180° about its own axis. It looks upside down because it is — that is what a German equatorial does, and it is why a meridian flip rotates your field of view and needs re-framing.
The pointing does not change. What does change is the required dome azimuth, because the optical axis sits off the dome centre: the same target needs the shutter in a different place depending on which side the telescope is on. That difference is the reason this tool exists.
The setup check reports whether the counterweight clears the pier as the mount turns, and by how much. A margin under about six inches is flagged as worth confirming on the real thing. On a harmonic mount, which carries no counterweight, the same line reports the back of the tube instead — that is what comes closest to the pier on those.
The list is grouped by how much is actually known about each mount, because more mounts are named than dimensioned:
A partial entry fills in what is known and deliberately leaves the rest alone rather than inventing a figure. If your mount is in one of the lower groups, measure it and enter the figures under Use my own values; the tool then treats it exactly like a full entry.
Setups and exported horizon files carry the date and time they were saved — dome-geometry-setup_2026-07-28_1432.json. Year first, so a folder of them sorts into order by name, and local time so the stamp matches the clock you were looking at.
The counterweight sweeps its own circle about the RA axis, and on a short optical tube it is the widest thing on the mount — the part nearest the pier and the floor. Enter the distance from the RA axis to the far end of the shaft. Choosing a library mount fills this from that mount's published shaft length.
Set it to 0 for a harmonic mount run without one. Whichever reaches further, telescope or counterweight, sets the swept radius, and the clearance line names which.
Strain-wave mounts — the ZWO AM series and similar — are drawn and modelled as their own machine rather than as an equatorial with the weights hidden. No counterweight shaft, no meridian flip, and the swept envelope is the tube alone, so the same telescope needs materially less room than it would on a German equatorial.
The counterweight reach is held at 0 and locked for these, since the mount does not carry a bar.
The Mount System card carries a small library of common mounts. Choosing one fills in its geometry — the mount head height and, for an equatorial, the declination-axis offset — and switches the mount type to match. Generic is the default and changes nothing, so measured values are never overwritten unless you ask for a preset.
An equatorial's declination axis sits at A + B × sin(latitude) above the pier flange, since dimension B runs along the polar axis. Presets re-apply when you change latitude so the figure stays right for your site.
Preset dimensions are community measurements, not manufacturer specifications, and every field they fill remains editable. Where sources disagree the card says so. Measure your own mount before building to tight clearances.
Select Measure my own and the card lets you type into the dimension boxes beside the labelled diagram. Generic equatorial is a plausible mid-size mount for getting started; picking any library mount fills the same boxes, read-only. Tick Use my own values to unlock them and adjust from there; untick to put the published figures back. Measure A vertically from the pier flange to the RA pivot, B from that pivot to the declination axis along the polar axis, and C from the declination axis across to the optical axis. I, the counterweight shaft, is optional. For an alt/az mount there is a single measurement, H, from the pier flange to the altitude axis.
Because B runs along the polar axis, the declination axis sits at A + B × sin(latitude) above the flange, so the figure follows your site. Any box left at 0 leaves the corresponding value alone.
Selecting Alt/Az sets the pier height to 12″, the shortest PlaneWave offers for the L-500 (piers come in 12″ steps up to 48″). Your equatorial pier height is restored if you switch back.
Equatorial (GEM) draws a German equatorial: latitude-tilted RA axis, RA and Dec rotors, and a counterweight shaft. Swing clearance is measured from the RA axis and includes the counterweight sweep.
Alt/Az or Fork draws a PlaneWave L-500 direct drive: wide base drum, single offset arm and altitude drive drum. There is no counterweight, and the tube pivots about the altitude axis near its balance point, so the swept radius is the longer half of the tube plus the aperture radius — floored at the L-500's published 650.748 mm swing-through. This is materially smaller than the equatorial figure, so an alt/az is not asked to clear a swept volume it never occupies.
The pier and telescope are drawn as a real German equatorial mount: the head tilts to the site latitude so the RA axis points at the celestial pole, the RA rotor turns with Right Ascension, and the Dec rotor swings the tube about the declination axis. At RA 0h / Dec 90° the mount sits in the park position — counterweight hanging down, tube along the polar axis.
The pier base always rests on the floor; changing the pier height, diameter or mount-head height rebuilds the model to match.
Same Hour Angle/Dec or Alt/Az controls, jog pad, and pier-side handling as the dome tool. The 3D view shows the actual building shape, the parked roof, cardinal directions, the celestial equator, and wireframe swing-radius spheres per telescope (red if any fails to clear).