Tools Field of View & Framing
Field of View & Framing
A target drawn at its catalogue size against your sensor, at any camera angle. You get the field, the image scale and how much of the frame the target fills, and when it does not fit in one frame, the mosaic that covers it. Put a second rig beside the first to compare them on the same target.
Target sizes are OpenNGC’s catalogue extents, as published. The stars and the glow are synthetic, drawn in your browser; no survey imagery is used.
Telescope
250 mm. Source
Camera
11.31 × 11.31 mm, 3.76 µm pixels. Source
OpenNGC NGC7000: 120′ × 30′, no position angle in the table.
Camera angle
0° lays the sensor's width along the target's long axis. OpenNGC gives no position angle for NGC 7000 North America Nebula, so the angle is measured from the long axis as drawn, and the figure has no north.
Compare
Mosaic overlap and seeing
OpenNGC’s extents run small for faint nebulae: it lists NGC 7000 at 120′ × 30′, much narrower than the nebula in a long exposure. Leave room around the dashed outline.
William Optics RedCat 51 WIFD at 250 mm, f/4.9, with the ZWO ASI533MC Pro.
NGC 7000 North America Nebula fits one frame: its catalogue extent spans 77 % of the frame along the sensor's width.
- Turned 45° from its long axis, it spans 56 %: the least of any angle.
At 2.5″ seeing, 3.10″ per pixel puts 0.8 pixels across the seeing disc. Undersampled; the standard, deliberate wide-field trade.
The arithmetic for this rig
W = 2 × atan(11.31 / (2 × 250)) = 2.5916° = 155.50′
H = 2 × atan(11.31 / (2 × 250)) = 2.5916° = 155.50′
X = √(120² cos²0° + 30² sin²0°) = 120.00′
Y = √(120² sin²0° + 30² cos²0°) = 30.00′
fill = max(X / W, Y / H) = max(0.772, 0.193) = 0.772
panels 1 × 1 at o = 0.15
scale = 206.265 × 3.76 / 250 = 3.102″/pxHow this is computed
scale["/px] = 206.265 × pixel[µm] / focal length[mm]
W, H [°] = 2 × atan(sensor width, height[mm] / (2 × focal length[mm])) × 180/π
X = √(a² cos²θ + b² sin²θ) extent along the sensor's width
Y = √(a² sin²θ + b² cos²θ) extent along its height
frame filled = max(X / W, Y / H)
panels = 1 + ⌈(T − F) / (F × (1 − o))⌉ along each axis, 1 when T ≤ F
least fill tan²θ* = (a²/W² − b²/H²) / (a²/H² − b²/W²)
sampling = seeing FWHM["] / scale["/px]
a, b = OpenNGC's major and minor axes, θ = camera angle from the major axis,
T = X or Y, F = W or H, o = overlap between panelsField and scale use the exact forms; the worked example shows the arithmetic. The focal length is the telescope’s, times a corrector’s factor only where its maker states one. Where the maker publishes no factor, the figures stay at the native focal length and the tool says so.
Frame filled is linear: the target’s extent along whichever sensor axis it fills more, as a fraction of the field along that axis. 100 % means the catalogue outline touches both edges of that axis. It is not an area ratio, and at 0° it is the figure the Rig Reality Check and the rig pages print. Those two sort it into four bands for scoring; this tool reports the number and the panel count instead.
Turning the camera does not always make a long target fit worse. Between 0° and 90° the share along the width only falls and the share along the height only rises, so the larger of the two is smallest where they meet: for M31 on a 23.5 × 15.7 mm sensor at 400 mm, at 29°, where it spans 79 % against 88 % square to the galaxy. The panel count is a step function of the angle and is found by trying every whole degree.
A mosaic here covers the outline’s bounding box on a grid that turns with the camera, centered on the target. Panel steps are angles on the sky, which is what the mount slews; for fields of a few degrees the difference from a projected grid is far below the overlap. The default overlap is 15 %; set it to what your stitching software wants.
Position angles are OpenNGC’s, from HyperLEDA, for the 5 galaxies that have one (M31, M101, M51, M81, NGC 4565). For those the figure is drawn north up and east left, and the tool gives the position angle of the sensor’s width. The other 8 targets have none in the table, so their figure is drawn with the long axis level and no north.
The sampling line is a tradeoff, never a pass or a fail. Every band, as the physics reference sets them out:
| Seeing disc, px | What it means |
|---|---|
| < 0.8 | Badly undersampled: the pixel is wider than the seeing disc, so stars land as blocky squares |
| 0.8 – 1.4 | Undersampled; the standard, deliberate wide-field trade |
| 1.4 – 3.2 | Practical band for typical seeing |
| 3.2 – 4.5 | Oversampled: lower per-pixel signal and tighter guiding; binning recovers most of it |
| > 4.5 | Heavily oversampled: the atmosphere limits resolution long before the pixel does |
Target sizes: OpenNGC by Mattia Verga (tag v20260501), CC BY-SA 4.0. This table is shared under the same license. Source and license
Source: docs/03-physics-reference.md § 1, Optics and framing, and its Framing subsection with worked examples for M31 on an APS-C sensor at 250, 400 and 1000 mm. Implemented in packages/physics (optics.ts, framing.ts), with those examples as unit tests.
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