What is backfocus?
Backfocus (or back focus distance) is the exact optical distance between the last optical element of a field corrector (reducer, flattener) and the sensitive surface of the sensor.
Each corrector is designed for a precise distance, typically 55 mm for modern standards (APS-C and full-frame CCD/CMOS), sometimes 56.5 mm, 65 mm or more depending on the model. When this distance is not respected, typically within about 0.5 mm for demanding setups and 1-2 mm for more tolerant ones, the corrector no longer performs its function and field flatness degrades at the corners.
Stars that remain point-like at center elongate progressively toward the edges in a characteristic radial pattern: crescents pointing outward or inward depending on the direction of the error.
The defect is purely geometric and entirely reversible by adjusting extension rings, provided the direction and magnitude of the error are correctly identified.
Recognizing a backfocus problem
At the center of the field, stars are perfectly round (the paraxial zone is little affected by backfocus errors).
Toward the edges, stars progressively elongate, and at the four corners the defect reaches its maximum: stars shaped like commas, crescents, or small arrows oriented radially.
The pattern is symmetric at all four corners: every corner shows the same type and amplitude of deformation, pointing toward or away from center depending on the direction of the error.
Backfocus too short (sensor too close to the corrector): stars elongated radially outward.
Backfocus too long (sensor too far): stars elongated tangentially, sometimes appearing as double dots or crescents pointing toward center.
Inspection with Aberration Inspector (PixInsight) or crops of the four corners immediately reveals the pattern.
Differential diagnosis
To be distinguished from uncorrected coma (stars also deformed into comma shapes at the edges, but the pattern differs: tails all pointing toward the optical center, and the defect exists even toward the center periphery, coma increases linearly with distance from center, whereas backfocus error is more pronounced specifically at the extremities).
Not to be confused with sensor tilt (clear asymmetry: one corner much worse than the opposite corner, whereas backfocus produces a symmetric defect at all 4 corners).
Different from optical astigmatism (stars elongated in a direction that varies with position and focus).
Not to be mixed with collimation error (pattern similar to coma but shifted asymmetrically).
Also check whether it is a decentered corrector (stars elongated asymmetrically, more in one direction than another).
Primary diagnostic test: all corners must show the same type and amplitude of deformation to conclude it is a backfocus issue.
Probable causes
- Corrector-to-sensor distance not matching the manufacturer's specification
- Incorrect extension rings or miscalculated number of rings
- Camera flange focal distance not accounted for (varies by manufacturer: ZWO 6.5 mm, QHY 17.5 mm, Touptek variable)
- Inserted filter not included in the calculation (a 2 mm thick filter adds ~0.67 mm of optical backfocus)
- Filter wheel or filter drawer whose thickness has not been subtracted
- Rotator or off-axis guider modifying the optical path
- Change of sensor without recalculation (APS-C to full-frame switch, or vice versa)
- Dedicated corrector replaced by a generic corrector with a different backfocus requirement
- Manufacturer tolerance underestimated (some correctors require within 0.3 mm only)
How to correct this defect
- Check the official backfocus specification of the corrector (manufacturer manual or website)
- Calculate the actual backfocus: sum the ring lengths + camera flange focal distance + filter thickness / 3
- For a 2 mm filter, add ~0.67 mm to the mechanical spacing to compensate for the optical shift
- Use fine-adjustment rings (0.5 mm, 1 mm, 2 mm) to reach the exact distance
- Test with short exposures after each adjustment, inspect corners via Aberration Inspector
- Adjustment direction by iteration: vary the spacing by 0.5 to 1 mm and keep the setting that improves the corners (the exact radial/tangential correspondence is not universal and depends on the corrector)
- Document the validated configuration (written diagram of rings used) for reproducibility
- On a setup with a rotator or OAG, recalculate whenever the optical train is modified
- For correctors with tight backfocus tolerances (FSQ, Esprit, some RCs), consider custom-machined rings
The Doc's advice
Backfocus is basic math, not magic: add up every element between the corrector and the sensor, full stop. The trap is that everyone forgets filter thickness (one third of the physical thickness counts toward optical backfocus). If your corners are misbehaving symmetrically, check your spreadsheet before suspecting your equipment. And invest in a set of fine-adjustment rings: five dollars' worth of rings saves you 50 hours of reprocessing.
Think you can see this defect in your image?
Run a diagnosisFrequently asked questions
Why does a filter add optical backfocus?
Because the filter glass, with a refractive index higher than air (~1.52 vs 1.00), slows light and virtually shifts the focal plane. The standard rule of thumb: a filter of thickness e adds approximately e/3 to the required optical backfocus. For a 2 mm filter, you therefore need to increase the mechanical spacing by ~0.67 mm to keep focus at the sensor plane. This rule applies to standard filters (BK7 glass, BOROFLOAT); very thick filters or exotic substrates may differ slightly.
How do I determine whether my backfocus is too short or too long?
The visual test guides the diagnosis (the exact radial/tangential correspondence can be reversed depending on the corrector, confirm by varying the spacing). Crop the four corners of the image and examine the orientation of star elongations: if corner stars are elongated radially (axis pointing toward or away from the field center), backfocus is too short: move the sensor farther. If stars are elongated tangentially (perpendicular to the radial axis), backfocus is too long: move the sensor closer. In PixInsight, the Aberration Inspector script displays all four corners side by side for direct comparison.
What tolerance is acceptable for backfocus?
Tolerance depends on the corrector and sensor. For common amateur correctors (Skywatcher, TS Optics, William Optics) with an APS-C sensor, within 1 mm generally gives acceptable corners. For high-end correctors (Takahashi, Tele Vue, Riccardi) with a full-frame sensor, tolerance drops to 0.3-0.5 mm. On RC astrographs (CCT, ASA, GSO), typically 0.5 mm. Validation rule: if Aberration Inspector shows corners equivalent to center in FWHM and eccentricity (<10% difference), backfocus is correct.
Does the optimal backfocus depend on the sensor used?
No, the optimal backfocus is defined by the corrector alone; it sets the distance between its last optical surface and the corrected focal plane. However, the mechanical calculation (ring lengths) depends on the camera's flange focal distance: a ZWO ASI sensor with a 6.5 mm flange and a QHY sensor with a 17.5 mm flange require very different ring lengths to achieve the same optical backfocus. When you change cameras, you do not change the target backfocus, you recalculate the mechanical rings to reach it with the new flange.