LEARNED IT · 09/26
Daytime focus tests need far more focuser travel than a star does
A target at 50 m needs about 10 mm more outward focuser travel than infinity. Run out of travel on a streetlight and you'll wrongly conclude the camera can't reach focus at all.
Testing a new tube in daylight is sensible — you can see what you're doing. But a nearby target does not focus where a star does, and the gap is bigger than it feels.
Extra extension needed is f^2 / (d - f). At 700 mm:
| target distance | extra focuser travel vs a star |
|---|---|
| 30 m | 16.7 mm |
| 50 m | 9.9 mm |
| 100 m | 4.9 mm |
| 200 m | 2.5 mm |
| infinity | 0 |
Ten millimetres is a lot on a small focuser. If a nearby target won't come to focus, that says nothing about whether a star will — the star needs less travel, not more.
Two practical consequences:
- Pick the furthest target available for a daytime focus check. Past about 200 m the correction stops mattering.
- Never conclude "the camera can't reach focus on this scope" from a close-range test. That verdict requires something at infinity.
And there is a floor on what daylight can tell you anyway: at 80 m across open ground, heat shimmer put a 5 arcsec blur on everything, three times worse than the telescope's own 1.53" limit. Daylight can prove a setup is broken. It cannot prove one is good.