A walk around
one horizon.
An invented garden, one fixed tripod position.
Take bearings from where the telescope will sit.
Swipe the panorama, or choose a direction.
Start from true north.
Azimuth is your horizontal direction. Use 0° for north and 90° for east. Altitude is the vertical angle from a level horizon: 0° at the horizon, 90° straight overhead.
Our north reading is 0° / 18°. A compass in magnetic-north mode is not the same reference; set it to true north or apply a verified local correction. SkyMargin does not read a compass. NOAA explains magnetic declination.
Take the first reading
Place a compass and inclinometer at the planned optical height. Follow their calibration instructions. Find a direction, then sight the highest obstruction along it and record its elevation above level. A tilt reading from a device aimed along the ground is not the angle to a roofline. Record the instrument and estimated uncertainty in your own notes.
- Azimuth → altitude
- 0° → 18°
- 40° → 12°
- 70° → 12°
- 72° → 38°
Keep both sides of an edge.
One reading at each cardinal direction would miss this roof. Record the low side at 70° / 12° and the high side at 72° / 38°. The slope between them is steep but explicit.
At 71°, linear interpolation gives 25°. At 90°, the roof stays at 38°. Do not draw a vertical step with two different heights at the same bearing.
Sample the shape, not a timetable
A regular 30° grid can miss a narrow chimney. Add readings before and after each roof corner, ridge or gap; use more points where the boundary changes quickly. Interpolation fills unknown directions with straight slopes in azimuth/altitude space. It does not discover an obstacle between your measurements.
- Azimuth → altitude
- 90° → 38° (interpolated)
- 110° → 38°
- 112° → 14°
- 180° → 8°
A gap is not a prediction.
The illustrative reading at south is 180° / 8°. It describes an obstruction, not the quality of the view. Haze, light pollution, weather and your planner's own altitude floor can still rule out a target.
Measure at telescope height. Moving closer to a wall changes its angular height; a profile belongs to one observing position. The Seestar S50 Pro manual also discusses keeping sky clear of surrounding obstructions; it does not make this sample a visibility guarantee.
Leave room for uncertainty
If an illustrative reading is 18° with about 2° uncertainty, use 20° as a conservative obstruction estimate if that matches your purpose. This is your chosen margin, not a calibrated correction. A target centre clearing a line does not mean the entire image frame clears it. Seasonal foliage and branches moving in wind deserve a fresh check.
- Azimuth → altitude
- 180° → 8°
- 225° → 10°
- 265° → 30°
Join the same north twice.
The file ends at 360° / 18°, matching 0° / 18°. These are the same direction. The final slope wraps from 340° / 30° down to north; at 350° the example is 24° high.
Check the first and last lines together. A mismatch makes an artificial discontinuity exactly where the panorama joins. If you do not enter north, the editor calculates it between the last and first bearings and writes the same value at both file ends.
Know when a line is insufficient
A single horizon assumes everything below the line is blocked and everything above it is open. It cannot describe the underside of an overhanging roof, a window aperture or a gap with another obstruction above it. Move the observing position or use a richer mask in a compatible planner. Do not interpret this circle as a camera view.
- Azimuth → altitude
- 300° → 22°
- 340° → 30°
- 350° → 24° (interpolated)
- 360° → 18° (same north)
A SECOND, INVENTED SURVEY
Two valid files.
One missing chimney.
Format checks cannot tell whether you measured enough directions. These two original practice files use the same 12° background. Only the detailed one records a narrow obstruction between 145° and 149°.
Sparse readings
0 12 180 12 360 12
Interpolated height at 147°: 12°. The straight line contains no evidence of a chimney.
Keep the sparse practice file ↓Edges retained
0 12 144 12 145 52 149 52 150 12 180 12 360 12
Interpolated height at 147°: 52°. The high segment remains between the two measured edges.
Keep the edge practice file ↓Try the discrepancy in the editor.
Save your own draft before opening either practice file. Open the sparse file in Your sky, then the edge file. Look just southeast of the east bearing: the second profile reaches further towards the centre because a higher obstruction removes more low sky. The centre of this polar disc is overhead, not the observing position on a map.
A hypothetical point target at 147° azimuth and 30° altitude is above the sparse line but below the detailed one. The difference is 40° of recorded obstruction height; it is not a measured telescope error or a prediction of observing time. No physical chimney or instrument was surveyed for this exercise.
Repair the survey, not only the drawing.
If your real skyline has a narrow feature, return to the same observing position and measure both edges and the high part. Adding a decorative peak from memory changes the picture without improving the evidence. Keep the original readings alongside the revised file, with the date, optical height and reason for the change.
Check north again after editing. Both practice files deliberately begin and end at 12°; changing only the last line would create a false seam. Also distinguish an obstacle's shape from its measurement uncertainty. A blanket margin can raise a known roofline, but it cannot supply the location of an unmeasured chimney.
DSO Tonight provides a custom-horizon upload. A successful upload establishes that the planner accepted your file; the outdoor measurements remain your responsibility.
The same horizon,
in a useful file.
Download the complete example .txt ↓
Fictional coordinates, identical to the starting drawing. The panorama above is linear in bearing and elevation; the home disc is an overhead polar projection. Their silhouettes differ because their axes differ.
Use it in DSO Tonight
Save a TXT from Your sky, then open DSO Tonight ↗. Open Settings → Horizon → Upload, choose the file and confirm its name is selected. Its parser accepts two columns: azimuth then altitude, separated by spaces. Lines starting with # are comments.
Set your location and observing time in the planner, then inspect a known obstructed direction. SkyMargin does not set those values or control a telescope. This is a third-party planning file, not a claim that the Seestar app imports it.
File rejected, or skyline facing the wrong way?
Use plain text, two decimal-degree numbers per line and at least two distinct bearings. SkyMargin accepts 0–90° altitude and up to four decimal places. It rejects repeated bearings, mismatched 0°/360° heights, extra columns and malformed lines rather than silently dropping them. A valid but rotated horizon usually needs its north convention checked; do not exchange the altitude and azimuth columns to compensate.
Save a corrected copy and reload it in your planner. A successful file import proves format acceptance, not the accuracy of your outdoor measurements.
Share the observing site carefully
For your own astronomy club's public observing page, keep the profile's position and date beside it. Check that the download and directions work in WebTraceCheck. For a non-sensitive test invitation from your own club signup, use CleanTempMail to inspect delivery. Keep important club or telescope accounts on a permanent inbox.
Sources & scope
The 25 August 2026 S50 Pro launch provides the current product context. The manufacturer's user manual, p49, discusses sky obstructions and observing conditions. This site makes no telescope-performance claim.
Stellarium's horizon-format reference uses azimuth from true north towards east. SkyMargin exports TXT, not a complete Stellarium landscape package. The example is original and invented; no outdoor instrument or native telescope import has been tested.