This free critical focus zone (CFZ) calculator works out the depth of focus for your telescope using the modern NCFZ formula, so you know exactly how precise your focus must be for pin-sharp stars.
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.
