Description
A focus miss is one of the most elementary defects and yet one of the most frequent in astrophotography.
When the focal plane of the telescope does not coincide exactly with the sensor surface, every point of light in the image (star, planetary detail) is rendered not as a point but as a small fuzzy disk: the circle of confusion.
The entire image loses sharpness: stars become uniformly large and soft, fine details of nebulae and galaxies disappear, and the separation of tight stars in clusters is compromised.
Unlike optical defects (coma, astigmatism, backfocus error), a focus miss affects the whole field uniformly with no directional signature, which is what distinguishes it.
It can be obvious (manifestly blurry image) or subtle ("almost good" focus that silently degrades every session).
On long exposures, gradual thermal drift of the tube and focuser slowly drives the image out of optimal focus, requiring automatic or manual refocusing during the night.
Visual signature
Stars appear round but large, with a high FWHM (3-8 pixels) well above what the local seeing would justify.
On a strongly defocused Newtonian, the star becomes a ring (donut) revealing the central obstruction of the secondary mirror.
On a refractor or SCT, the defect produces a uniformly diffuse disk.
Contrast loss is global: fine structures of galaxies (spiral arms, dust lanes) are crushed, nebulae appear bloated, tightly packed stars merge into diffuse blobs.
No directionality: stars are equally round at the center and at the edges (unlike an optical defect).
On a visible diffraction figure (bright star, Bahtinov mask), the spikes do not cross at the center but are offset to one side or the other depending on the direction of defocus.
Differential diagnosis
Distinct from poor seeing (stars also round and large, but with visible scintillation on the subs and variable FWHM from frame to frame: seeing changes every second, focus shifts over minutes).
Do not confuse with an incorrect backfocus (stars symmetrically elongated in the corners but sharp at center: focus is locally correct but the focal plane is not flat).
Different from coma (comet-like tails pointing toward the center, more pronounced at the periphery).
Do not mix with a collimation error (stars asymmetric even at center, with a preferred direction).
Dew on optics produces diffuse blur with additional bright halos around bright stars, not just uniform enlargement.
Also check for persistent strong wind or chronic vibrations (stars bloated by accumulated micro-displacements, but inspection of subs shows trailed rather than round stars).
Probable causes
- Initial focus set by eye or approximately, without a Bahtinov mask or autofocus
- Thermal drift of the tube (mechanical contraction with the nightly temperature drop)
- Tube not thermalized at session start, focus set too early
- Focuser with mechanical play or slip (weight of the optical train causes the drawtube to slide)
- No automatic refocus scheduled during a long session (over 2-3 hours)
- Focus set without a filter, then filter inserted (each filter has a slightly different parfocal distance)
- Significant temperature change (over 3-5 degrees C) without refocusing
- Autofocus step size too coarse to reach the true optimum
- Bahtinov mask misread (insufficient visual sensitivity to detect the spike offset)
Course of action
- Systematically use a Bahtinov or Tri-Bahtinov mask for the initial focus
- Enable automatic autofocus (NINA, SGP, APT, Voyager) with a V-curve routine every 0.5-1 degree C of thermal variation
- Set up a rigorous autofocus routine: step size approximately 30-50 microns, 7-11 data points, 5-10 s exposure on an unsaturated star
- Define a thermal trigger (refocus at every 1 degree C change) and a time trigger (refocus every 1-2 hours)
- Systematic refocus at every filter change (unless filters are verified parfocal)
- Pre-cool and thermalize the tube for 30-60 minutes before the first frame
- Choose a quality motorized focuser (ZWO EAF, Pegasus FocusCube, Esatto) with position memory
- For a non-motorized focuser, use a Bahtinov mask with HFR analysis in NINA in manual mode
- Check for mechanical play in the focuser drawtube before each session
- On an already-affected image, BlurXTerminator can partially recover, but no tool restores a severely missed focus
The Doc's advice
Focus miss is the least glamorous defect to diagnose but the one that silently ruins the most sessions. The good focus at the start of the evening is not the good focus at 2 a.m.: a 5 degree C drop on an aluminum tube means several tens of microns of focuser drift. Automatic autofocus with a thermal trigger is non-negotiable on long sessions. And if you have no motorization, set a reminder every 90 minutes to redo a Bahtinov check: less glamorous than watching a film, but it makes a real difference to the final images.
Think you can see this defect in your image?
Run a diagnosisFrequently asked questions
How can I tell whether my focus is truly optimal?
The most reliable method is measuring HFR (Half Flux Radius) or FWHM on an unsaturated star. In NINA, SGP, or Siril, a V-curve autofocus plots HFR evolution around the optimal position and identifies the minimum mathematically. Failing that, the Bahtinov mask shows three diffraction spikes whose central one must be perfectly centered between the two outer ones; a visible offset already corresponds to several tens of microns of error. For the demanding: validate after the fact by measuring the average FWHM on the master and comparing it to the theoretical limits of the setup (sampling and seeing combined).
How often should I refocus during a session?
This depends mainly on the thermal drift of the setup. On an aluminum tube with a metal focuser, expect roughly 10-30 microns of drift per degree C, requiring refocus every 0.5-1 degree C of variation. On a carbon tube (more thermally stable), the interval can extend to every 2 degrees C. In practice: schedule a refocus at every 1 degree C change in outdoor temperature and every 60-90 minutes as a time-based backup, in addition to filter changes. A well-dialed setup refocuses 3 to 8 times over a full night.
Should I refocus at every filter change?
Unless the filters are guaranteed parfocal to within 10 microns (Astrodon, Astronomik LRGB-XT, Chroma LRGB), yes. Narrowband filters (Ha, OIII, SII) typically diverge by 20-50 microns from the L filter on standard sets (Optolong, Baader, ZWO). On a heavily over-sampled sensor, this offset is enough to visibly degrade FWHM. Pragmatic solution: systematic autofocus at every filter change ("Autofocus on Filter Change" option in NINA), or measure a per-filter offset once for all and apply it automatically thereafter.
Is the Bahtinov mask still useful with a motorized autofocus?
Yes, as a validation and fallback tool. The Bahtinov mask remains more precise than a poorly calibrated HFR routine and allows confirming that an automatic autofocus has truly converged to the optimum (rather than to a false minimum caused by noise or a poorly chosen star). It is also the indispensable tool for calibrating the approximate starting position before launching the first autofocus sequence, and for optical testing (collimation, defect identification). Keeping a Bahtinov mask sized for your tube diameter is a 30-50 euro investment that easily earns its place in the field kit.