1. Confirm it's thermal, not something else
- Gradual, whole-frame softening that tracks falling temperature → thermal focus drift. Keep
reading.
- Sudden focus loss → drawtube slip, mirror flop, or a bump — mechanical, not thermal.
- One-sided star distortion (not uniform softening) → alignment/tilt, see
diagnosing distorted star shapes.
2. Diagnose
Plot HFR/FWHM (star size) against time and temperature across the session. A steady rise in star
size that correlates with dropping temperature is the signature: most tubes shrink as they cool,
shifting the focal point. Note how many degrees of cooling it takes to visibly soften (it varies by
tube material and focal ratio — faster scopes are less forgiving).
3. Keep focus locked
- Refocus on a cadence (e.g. every N minutes or every few °C) — the simplest fix.
- Temperature compensation: if you have a motorized focuser, set a compensation coefficient so it
nudges focus as temperature falls.
- Autofocus routine between targets/filters (each filter may need its own offset).
- Reduce mechanical slip: lock the drawtube, address mirror flop, balance the train so the
focuser isn't fighting gravity.
4. Verify
After enabling a refocus cadence or compensation, the HFR-vs-time plot should stay flat instead of
climbing. Confirm across a temperature swing, not a short clip.
When it isn't focus
If stars are distorted directionally rather than uniformly soft, that's optical, not focus — see
---
How focusing works: focusing in the imaging train.
Considering a motorized solution to automate this? Choosing a focusing approach.