The New Critical Focus Zone
When you rack a focuser in and out, stars stay tight over a small range of travel, then bloat. That range is the critical focus zone. For years it was estimated with CFZ = 4.88·λ·f², but Don Goldman showed that formula is flawed — it treats some focus error as "non-measurable," when in reality all focus error is measurable. His New Critical Focus Zone (NCFZ) instead asks: how much focus error has a negligible effect on the image, given your seeing?
The formula
NCFZ = 0.00225 × θ × √τ × A × f² (microns), where θ is total seeing FWHM in arcseconds, τ is the focus tolerance as a percentage of the seeing, A is aperture in millimetres, and f is the effective f-ratio. You can be off by half of it in either direction (±NCFZ⁄2). Example: a 106 mm f/5 scope in 3″ seeing at a 15% tolerance gives 0.00225 × 3 × √15 × 106 × 25 ≈ 69 µm (±35 µm).
Why seeing and tolerance matter
In poor seeing, stars are already fat, so a little defocus matters less — the NCFZ grows. In excellent seeing it shrinks, demanding tighter focus. The tolerance τ is your call: a stricter 5–10% keeps focus error well below the seeing for critical work; 15–20% is a practical everyday target. Because the zone scales with the square of f-ratio, fast systems are far more demanding.
Frequently asked questions
How is this different from the old CFZ?
The traditional 4.88·λ·f² depends only on wavelength and f-ratio and assumes a fixed wavefront criterion. Goldman's NCFZ replaces wavelength with your actual seeing and a chosen tolerance, so the answer reflects the conditions you're really imaging in.
What tolerance should I use?
15% is a sensible default for deep-sky imaging. Drop to 5–10% for premium refractors, planetary or lucky imaging where you want focus error well inside the seeing; relax toward 20% in poor conditions.
How does this relate to autofocus step size?
Pick an autofocus step that moves the focuser by roughly a third of the NCFZ, so several samples land inside the sharp zone and your V-curve is well defined. You'll need your focuser's microns-per-step to convert.
Does temperature change focus?
Yes — tubes shrink as they cool, shifting focus by tens of microns over a few degrees. If your NCFZ is small, plan to refocus through the night or use a temperature-compensated focuser.