The Vignetting Calculator tells you how wide each adapter, spacer or filter between your telescope and camera must be for the corners of the image to lose no light, and how much light the corners lose when one is too narrow. Enter each one’s clear aperture and its distance to the sensor, and the tool draws the light path, the falloff across the sensor and the Flat Frame those adapters would make.
How to use it
- Describe the telescope: pick it from the catalogue or type its aperture and focal length in mm, and its central obstruction as a percentage of the aperture, 0 for a refractor.
- Choose a reducer preset, or none, or type its factor, from 0.30 to 1.00. Enter its backfocus in mm: the distance from its shoulder to the sensor, as its maker specifies it. Leave the exit pupil distance on Estimated unless you know it.
- Describe the camera: pick it from the catalogue or type its pixel size in µm and its resolution in pixels.
- Add every adapter between the telescope and the sensor; a spacer, a filter or a thread each counts as one. For each, type its clear aperture, the narrowest inside diameter, and the distance from the sensor to that narrowest point, both in mm. Or start from a preset, and measure yours when you can.
- Read the four figures: the corner illumination and its grade, the fully lit circle, the clear aperture the tightest adapter needs, and the focal ratio at the sensor. Each adapter says whether it is wide enough.
- Then look at the light path, where hatching marks the light an adapter cuts off; the falloff chart; and the simulated Flat Frame. Stretch the Flat Frame’s contrast to make a small falloff visible, or view it as recorded.
How it works
Each point of the sensor is lit by its own cone of light. Its width depends on the focal ratio N: at a distance d in front of the sensor it is d ÷ N across. The cone for a corner is pushed sideways from the axis, because the light arrives at a slant, and the shift grows with the corner’s distance h from the centre.
For no light to be lost, an adapter must be at least as wide as the whole beam where it sits: the cone plus the corner’s shift on both sides. That is the minimal clear aperture. Without a reducer the light appears to come from the telescope’s objective and the shift is small. A reducer changes where it appears to come from, so the tool estimates the exit pupil distance from the reducer’s factor and backfocus, unless you type it.
When an adapter is narrower than required, the tool counts how much of the beam still gets through, using 1,600 sample points across the beam and leaving the central obstruction empty. That share is the corner illumination, measured against an unobstructed beam, and it is graded from no vignetting (99.5% or more) to heavy vignetting (under 75%). Only the adapters and filters are simulated; the telescope’s own illumination is not.
Formula
D = d ÷ N + 2h × |1 − d ÷ L|
D is the minimal clear aperture, in mm. d is the adapter’s distance to the sensor, in mm. N is the focal ratio at the sensor, after any reducer. h is the corner’s distance from the centre of the sensor, half its diagonal, in mm. L is the exit pupil distance, in mm: positive in front of the sensor, negative behind it.
Worked example
An 80 mm aperture and 480 mm focal length with a 0.8× reducer give N = 4.8. A 23.5 × 15.7 mm sensor has a 28.26 mm diagonal, so h = 14.13 mm. With a 55 mm backfocus the tool estimates L = −775 mm, behind the sensor. For an M42 thread at d = 45 mm, d ÷ N = 9.375 mm and 2h × |1 − 45 ÷ (−775)| = 29.90 mm, so D = 39.28 mm. A thread with a 38 mm opening is 1.28 mm too narrow, and the corners receive 96.6% of an unobstructed beam.
Common questions
How wide must an adapter or filter be to avoid vignetting?
At least as wide as the beam is where it sits: the cone, d ÷ N across, plus the sideways shift of the corner’s beam. The tool works this out for each adapter and says by how much each one falls short. A larger sensor or a faster focal ratio calls for a wider opening.
Why does a reducer make vignetting worse?
A reducer lowers the focal ratio, so the cone of light is wider at every adapter. It also changes where the light appears to come from, which pushes the corner’s beam farther from the axis. An adapter that is wide enough without the reducer can therefore cut the corners with it.
Is a filter’s thread the same as its clear aperture?
No. The thread is the outside size of the filter’s cell; the glass, or the opening in the cell, is smaller. A 1.25-inch filter typically clears about 26 mm and a 2-inch filter about 44 mm, but brands differ, so measure yours where you can.
What does the calculator leave out?
The telescope’s own illumination. The tool simulates only what the adapters and filters take from a beam that would otherwise arrive whole. It is also geometric, so it does not model the baffles inside the telescope, the glass of the reducer, the shape of a filter cell or dust. With a reducer, the exit pupil distance is an estimate. Confirm the result with a real Flat Frame.
How much vignetting is too much?
The tool grades the corners by the share of the light they receive: no vignetting at 99.5% or more, slight from 90%, visible from 75%, and heavy below that. Calibrating with a Flat Frame can correct a smooth falloff, but the corrected corners are noisier.
Related tools
In the store
Equipment this tool helps you choose, at David Astro: