What is back-focus (back focal distance) on a field flattener or reducer?

It is the distance from the rear mounting face of the flattener or reducer to the camera sensor at which the optic delivers a flat, corrected field. Most modern refractor flatteners are designed for 55.0 mm; Schmidt–Cassegrain reducers often need 105 mm or more. Everything you bolt in between — filter wheel, off-axis guider, drawer, adapters, the camera body itself — has to add up to that number.

Why does a filter change the back-focus?

Light slows down inside the glass, so a filter of thickness t pushes the focal plane back by t × (n − 1) / n. With n = 1.5 that is one third of the glass thickness: a 2.0 mm filter needs +0.67 mm more spacing, a 3.0 mm filter +1.0 mm. This tool adds that shift to the target before working out what is missing.

How exact does the spacing need to be?

Manufacturers usually quote ±0.5 mm, and at f/6–f/7 that is comfortable. Fast systems (f/4 and below) show star shape errors in the corners with half-millimetre mistakes, which is why 0.3 / 0.5 / 1.0 mm shim rings exist. You can tighten the tolerance in the tool; the default is ±0.5 mm.

Does a full-frame sensor vignette through an M42 (T2) thread?

Yes. The clear bore of an M42×0.75 thread is only about 37–38 mm, smaller than the 43.3 mm diagonal of a full-frame sensor, so the extreme corners are physically cut off whatever the f-ratio. M48 passes the diagonal but still clips the light cone of faster systems; M54 and larger are the safe choice for full frame.

Where do the component thicknesses come from?

Every entry in the catalog carries the URL of the manufacturer specification (or, where the maker publishes none, a retailer listing) it was read from, plus the retrieval date. Items that could not be verified are kept out of the pickers rather than guessed. Measure your own parts with calipers before buying shims — production tolerances exist.

Imaging train · back-focus · spacers

Back-focus calculator

Pick the flattener, the camera and whatever sits between them. The tool checks that every thread actually mates, adds the focus shift from your filter glass, tells you to a tenth of a millimetre what spacing is still missing, and flags where a full-frame sensor will hit a narrow bore. Every thickness links to the page it was read from. Done? Copy the whole train as Markdown or BBCode for a forum post, or download the cross-section as a PNG.

Loading the configurator…

Common trains, pre-filled (62)

Each page opens the calculator with the parts already in place — telescope or flattener, camera, wheel or drawer, filter — and the numbers worked out live from the same catalog. Grouped by the optic.

Askar

William Optics

ZWO

Starizona

Sky-Watcher

Celestron

Takahashi

How the numbers are worked out

Target. The flattener or reducer states a back-focus (55.0 mm for most refractor flatteners, 105 mm for the classic Celestron f/6.3 reducer, more for EdgeHD). That is the distance its rear face wants to the sensor.
Filter compensation. Each filter in the path adds t × (n − 1) / n to the target, with n = 1.5: +0.62 mm for a 1.85 mm Optolong, +0.67 mm for 2.0 mm glass, +1.0 mm for 3.0 mm Chroma.
Sum of parts. Filter wheel, OAG, drawer, extension rings and the camera's own sensor-to-flange distance are added, in the order you place them. If two neighbours do not share a thread standard, the matching adapter is inserted and its thickness counted too.
Missing spacing. (target + Σ shift) − Σ parts, to 0.1 mm, then the fewest stock rings (0.3 / 0.5 / 1 / 2 / 3 / 5 / 10 / 11 / 15 / 16.5 / 20 / 21 mm) that close it.
Vignetting. Each bore is compared with the sensor diagonal, then with the full light cone diagonal + distance / f-ratio. Smaller than the diagonal is a hard cut-off; smaller than the cone is corner fall-off that flats can mostly fix.

Honesty notes: the calculator is geometry, not optics design — it cannot tell you a flattener's true optimum for your exact telescope, which some makers tune by ±1–2 mm per model. Treat the result as the starting point, then confirm with a star test at the corners. Prices are not shown because they change daily; the shop links open a live search.

Questions people ask

What is back-focus (back focal distance) on a field flattener or reducer?: It is the distance from the rear mounting face of the flattener or reducer to the camera sensor at which the optic delivers a flat, corrected field. Most modern refractor flatteners are designed for 55.0 mm; Schmidt–Cassegrain reducers often need 105 mm or more. Everything you bolt in between — filter wheel, off-axis guider, drawer, adapters, the camera body itself — has to add up to that number.
Why does a filter change the back-focus?: Light slows down inside the glass, so a filter of thickness t pushes the focal plane back by t × (n − 1) / n. With n = 1.5 that is one third of the glass thickness: a 2.0 mm filter needs +0.67 mm more spacing, a 3.0 mm filter +1.0 mm. This tool adds that shift to the target before working out what is missing.
How exact does the spacing need to be?: Manufacturers usually quote ±0.5 mm, and at f/6–f/7 that is comfortable. Fast systems (f/4 and below) show star shape errors in the corners with half-millimetre mistakes, which is why 0.3 / 0.5 / 1.0 mm shim rings exist. You can tighten the tolerance in the tool; the default is ±0.5 mm.
Does a full-frame sensor vignette through an M42 (T2) thread?: Yes. The clear bore of an M42×0.75 thread is only about 37–38 mm, smaller than the 43.3 mm diagonal of a full-frame sensor, so the extreme corners are physically cut off whatever the f-ratio. M48 passes the diagonal but still clips the light cone of faster systems; M54 and larger are the safe choice for full frame.
Where do the component thicknesses come from?: Every entry in the catalog carries the URL of the manufacturer specification (or, where the maker publishes none, a retailer listing) it was read from, plus the retrieval date. Items that could not be verified are kept out of the pickers rather than guessed. Measure your own parts with calipers before buying shims — production tolerances exist.