Back focus
List the parts between your flattener or reducer and the sensor. The calculator adds a third of each filter's thickness, then finds spacers that close what is left.
Read the guide: Back focus explained: getting the 55 mm right
Gap to close
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Spacer combinations
| Spacers | Shims | Total | Error |
|---|
From common sizes: 1, 2, 3, 5, 7.5, 10, 11, 15, 16.5, 20, 21, 25, 30 and 40 mm spacers, plus 0.1 to 0.5 mm shims.
How to use the back focus calculator
- Pick your flattener or reducer, or type its back focus. 55 mm is the most common.
- List every part between the flattener and the sensor, flattener side first. Add your camera last; its flange distance fills in from the preset.
- For filters, enter the glass thickness. Filters in a wheel or drawer have 0 length of their own.
- Read the gap and pick a spacer combination from the list.
Questions
What is back focus?
The distance from a flattener or reducer's rear thread to the camera sensor that gives a flat, sharp field. Most refractor flatteners want 55 mm. SCT reducers usually want 105 mm or more.
Why does a filter change the back focus?
Glass in a converging light beam pushes the focus back by about a third of its thickness. A 2 mm filter adds about 0.67 mm, a 3 mm filter 1 mm.
How precise does the spacing need to be?
Manufacturers rarely publish a tolerance, and it tightens as the focal ratio gets faster. As a working figure, about ±0.5 mm is often fine on slower refractors (f/6 and slower), while f/4 and faster, and full-frame sensors, can need ±0.1 to 0.2 mm. The calculator defaults to ±0.2 mm.
How do I know if my spacing is wrong?
Look at stars in all four corners at 100%. If they are stretched along lines through the centre of the frame, the spacing is off. Change it by about 1 mm and compare: keep going the way that improves the corners. If only one corner or side is bad, the cause is tilt, not spacing.
Where does the camera's own distance come from?
Each camera has a fixed distance from its front thread to the sensor, often 17.5 mm for cooled astronomy cameras with the standard adapter. Check your camera's manual, since tilt plates and adapters change it.
Method
A flattener or reducer is designed to put a flat focal plane a fixed distance behind its rear thread, usually 55 mm. Every part between that thread and the sensor adds its optical length. What is left over is the gap to fill with spacers.
Filter glass
Glass in a converging beam pushes the focus back by (n - 1) / n of its thickness. For filter glass with n near 1.5 that is one third:
target distance = required back focus + sum(filter thickness / 3) gap = target distance - sum(part lengths)
A 2 mm filter adds 0.67 mm, a 3 mm filter adds 1 mm. Only count filters that sit between the flattener and the sensor.
Spacer search
The calculator tries every combination of up to three spacers (repeats allowed) and up to two shims, keeps the ones within your tolerance, and lists the fewest parts first, then the smallest error.
Tolerance
Manufacturers rarely publish a tolerance. It tightens with the square of the focal ratio, so an f/4 system forgives about half the error of an f/5.6 one. As a working figure, about ±0.5 mm is often fine at f/6 and slower, while f/4 and faster, and full-frame sensors, can need ±0.1 to 0.2 mm. Star shapes in the corners are the final check. Sources disagree on which direction of stretch means too much or too little distance, so change the spacing by about 1 mm and keep going the way that improves the corners.
Camera flange distances
Presets use the published back focus of each camera with its standard adapter. Tilt plates and some adapters change it, so confirm with your camera's manual.