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Backfocus Solver

Nearly every corrector wants the sensor a fixed distance behind it, and being 2 mm out puts elongated stars in the corners of every frame you take. This works out the stack that gets you there from parts you already own, and tells you when it can’t.

Every figure below is re-keyed from a manufacturer page or drawing and carries a link to it. Where a number could not be confirmed, the row says so in the list before you pick it. There are no plausible-looking substitutes in this catalogue.

42 rows. 6 could not be confirmed against a manufacturer page or drawing and are marked in the list.

Sky-Watcher Esprit 100EDX Flattener (S20217)

Verified against the manufacturersource
Backfocus55.00mmmeasured from edge marking the start of the male thread on the flattener's exit (Esprit 100EDX manual)
Output thread
M66x1
Tolerance
±0.50 mm (working default; the manufacturer publishes none)
Image circle
40 mm
Designed for
Esprit 100EDX only

Manual (202505V1) p.5: "The 55mm distance is measured from the edge marking the start of the male thread on the field flattener, to the CCD sensor" (caliper photo reads 55.00). The same page's 69.8 mm is a different datum: "the distance between the last lenses to the sensor should be exactly 69.8mm". Exit thread M66x1 male, implied by the manual's custom coupling ring ("A female M66x1 thread, to screw and secure the ring to the field flattener exit"). No catalogued ring converts M66, so the solver reports the thread path as open. Image circle 40 mm (manual) vs 44 mm (skywatcher.com). Do not use Sky-Watcher USA's spec-table block for this scope: its 63 mm is the discontinued Esprit 100ED's figure.

70 rows. 11 could not be confirmed against a manufacturer page or drawing and are marked in the list.

ZWO ASI183MM Pro

Verified against the manufacturersource
Flange to sensor6.50mmcamera body front face to sensor, with the 11 mm T2 extender removed
Input thread
M42x0.75
Sensor
13.2 × 8.8 mm · 2.4 µm pixels

Sourced on its own from the shared MM/MC manual (V1.4, Aug 2021), whose model table lists "ASI183MM Pro Mono": "Back Focus Distance 6.5mm"; "When the attached 11mm T2 Extender is removed the optical distance from the sensor to the camera body is reduced to 6.5mm." "Image area 13.2mm*8.8mm", "Pixel Size 2.4μm". Same body and drawing as the MC.

Which configuration do you own?Answer this first

This product ships in more than one configuration and they are not interchangeable.

Between corrector and camera

Is there an off-axis guider?Answer this first

An OAG sits in front of any filter, where ZWO's charts put it. 6 configurations are catalogued: which adapter faces your camera decides what can follow it.

Is there a filter drawer or filter wheel in the train?Answer this first

It adds its own length and its filter's glass shift, and it often ships with a ring the train needs. 6 are catalogued so far, every one from the manufacturer's drawing or manual.

8 rows, every figure confirmed against a manufacturer source.

Only manufacturer-supplied kits are catalogued so far. Generic aftermarket spacer sets are not here yet, and an invented set of lengths would produce a confident stack out of rings you may not own.

Nothing solved yet

Still needed: the spacers you own, which configuration of the camera you have, whether an off-axis guider sits in the train, whether a filter drawer or wheel sits in the train.

Nothing is filled in on your behalf. Where a product ships in more than one configuration you are asked which one you own, because the two are not interchangeable and the difference is often larger than the tolerance.

Or open an example train

How this is computed

required[mm]  = flattenerBackfocus[mm] + Σ( glassThickness[mm] × (n − 1) / n )
fixed[mm]     = cameraFlangeToSensor[mm] + Σ components[mm]
residual[mm]  = required[mm] − fixed[mm] − Σ rings[mm]

rings: spacers, extenders and adapters you own, each used at most as
many times as you have it, where every ring's thread fits the next

The required distance is the corrector’s own published figure, measured from the face the manufacturer measured it from, which is why every row names that face: quote the same corrector from a different thread and the figure changes.

Filter glass shifts focus outward, so it makes the required distance longer, by t × (n − 1) / n. The familiar “one third of the thickness” is that expression at n = 1.5 and is wrong by about 2.5% for ordinary filter glass. Where a filter’s glass type is unpublished the solver assumes n = 1.52 and says so in the result.

Tolerance is the corrector’s published figure where it has one, and ±0.5 mm otherwise, the usual working figure for a 55 mm system. Faster systems have tighter tolerances.

Lengths and threads are solved together, because a ring of the right length can still have the wrong thread. ZWO’s ASI2600 Pro kit includes an extender that is M48 on one side and T2 on the other: an M48 corrector screws straight onto it, a T2 corrector needs the kit’s 0 mm ring first, and which of those is true decides the stack.

The stack is chosen fewest-pieces-first, because each extra joint adds a chance of tilt. When an exact stack exists that needs more rings, the result shows that one too. How the numbers are computed.

Source: docs/03-physics-reference.md § 5 · Agena, “A primer on back focus in astronomy” · ZWO, “The best solution of 55 mm back focal length”. Implemented in packages/physics/src/backfocus.ts, with the worked examples as unit tests.

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