Backfocus in Astrophotography: The Complete Guide

In astrophotography, “backfocus” is used to mean two different things. This article uses the second meaning unless we say otherwise.

Getting the second meaning right is what turns soft, bloated corners into pinpoint stars across the whole frame.

Correctors and reducers are designed to flatten and sharpen a specific region of space behind them. Place the sensor at that distance and the optic performs as intended. Place it too close or too far and the stars away from the optical axis lose sharpness, even when the stars in the centre of the frame look fine.

Backfocus error is one of the most common reasons new astrophotographers see round stars in the centre and egg-shaped or bloated stars at the edge of their images.

You will often see “astro cameras need 55 mm backfocus.” That figure became popular because many full-frame flatteners and reducers were designed around a DSLR body, whose Canon EF or Nikon F lens mount is about 44 mm from flange to sensor, plus roughly 11 mm of adapters, filter drawers and tolerances.

The distance is measured from the corrector's reference surface to the sensor plane. Not the front of the camera. Not the filter wheel face. Not the start of the threads. The sensor.

In its simplest form, working out backfocus is primary-school arithmetic.

All values below are illustrative examples. Use your own equipment specifications.

A filter does not just add its physical thickness to the path. Because glass slows light, the effective optical path is shorter than the physical thickness. A common approximation is:

If your filter sits inside the specified backfocus, this correction matters. If the manufacturer already states their backfocus “with a 2 mm filter in place,” do not add the correction again.

DSLR and mirrorless bodies

DSLR and mirrorless cameras have a fixed flange distance from lens mount to sensor. A Canon EF mount, for example, is 44.0 mm. That distance is non-negotiable unless you physically modify the camera. You build the rest of the train around it.

Dedicated astronomy cameras

These give you a “backfocus budget” to play with. The camera body itself consumes some of the required spacing, and you fill the rest with adapters, filter wheels, off-axis guiders and spacer rings.

Each corrector design expects a specific distance behind it.

• Field flatteners for refractors often want 55 mm, 67.5 mm or another fixed value to the sensor.
• Focal reducers change the focal length and usually demand tight tolerances, sometimes ±0.5 mm.
• Coma correctors for Newtonians are measured from the shoulder or the top of the corrector body to the sensor.

Always use the value and tolerance quoted by the manufacturer for your exact optic and sensor size.

Petzval, quadruplet and some quintuplet refractors have their field correction built into the telescope tube. They often have generous backfocus and only require you to reach focus within a wide range. You may still want to fine-tune for the flattest field, but the spacing is far less critical than with an external flattener.

• Use digital callipers for component thicknesses and thread lengths.
• Measure from the corrector's stated reference surface. If the manual says “from the M48 rear thread shoulder,” start there.
• Add the camera's published sensor-to-front distance. Do not guess it.
• Account for any filter, glass window or adapter that changes the optical path.

Astrophotography trains are usually built from M42, M48 or M54 threaded adapters and T-thread or C-thread fittings. Always check:

• Thread pitch. M42 can be 0.75 mm or 1.0 mm pitch and they are not interchangeable.
• Whether the adapter adds or subtracts spacing. A male-to-male adapter can eat into your backfocus budget.
• Filter thread or filter drawer depth, including any locking ring.

Aluminium spacer rings are inexpensive and let you trim the total distance in fine steps.

Calculation gets you close. The sky tells you the truth. Use this iterative process:

1. Calculate and assemble the train to the nearest millimetre.
2. Point at a field with bright stars near the edge, such as a dense Milky Way region.
3. Focus carefully on a star in the centre of the frame.
4. Take short exposures and inspect corners. Adjust spacing in small increments and re-test.

A change of 0.5 mm can be visible with fast refractors. Keep notes of each adjustment and the resulting corner star shape.

The patterns below are rough guides. Actual star shapes depend on the optical design, focal ratio and whether the error is positive or negative.

Use the calculators below to estimate your imaging train and filter correction. Remember: these are planning tools. Real hardware tolerances and optical effects will differ slightly.

Print this worksheet and fill it in at the bench. It is faster than hunting through browser tabs when your hands are full of adapters.

• Confirm the required backfocus from the optic manual
• Identify the exact reference surface
• List every component thickness to the sensor
• Apply filter optical-path correction if needed
• Add or remove spacers to reach the target
• Check thread compatibility and engagement
• Focus on a central star and inspect corners
• Adjust in small steps and re-test
• Rule out tilt, collimation and poor focus
• Record the final spacing for next time

• Manufacturer backfocus specifications for your specific flattener, reducer or coma corrector.
• Camera sensor-to-front and sensor-to-thread dimensions from the camera manual.
• Filter optical-path discussion in amateur astrophotography literature; the Δ ≈ t(1 − 1/n) approximation is widely used for filter glass near normal incidence.
• General optical design references on field curvature, Petzval sum and corrective optics.