Stacking Starlight

Stars Softening / Losing Focus Through the Night (Temperature Drift): Fix

Fixing focus that drifts as the night cools — early subs crisp, later ones softening as stars bloat: how to confirm it's thermal (plot HFR/FWHM star size vs temperature), then hold focus with a refocus cadence, temperature compensation, or an autofocus routine between filters.

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

distorted star shapes.

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How focusing works: focusing in the imaging train.

Considering a motorized solution to automate this? Choosing a focusing approach.

Common questions

Why do my stars get softer through the night?

Gradual whole-frame softening that tracks falling temperature is thermal focus drift — the tube shrinks as it cools and shifts the focal point; sudden loss is drawtube slip or mirror flop instead.

How do I keep focus locked as the temperature drops?

Refocus on a cadence (every N minutes or few °C), enable temperature compensation on a motorized focuser, run autofocus between targets/filters, and lock the drawtube to reduce slip.