1. Triage the shape first
- One-sided tails / comet-like flare that gets worse toward the edges, center sharper → optical
alignment or sensor tilt. Keep reading.
- Uniform elongation in one direction across the whole frame → tracking/mechanical, not optics.
See tracking lag in the mount.
- Soft, bloated stars all over that worsen through the night → focus, not optics. See
- Round center, stretched corners only → field curvature / spacing (backfocus), often fixed with
the right flattener/reducer distance rather than alignment.
2. Diagnose
Do a defocused-star test on a medium-bright star near center: rack focus slightly out and look at
the disc. A well-aligned reflector shows concentric rings with the central obstruction centered; an
offset shadow means the optics need alignment. For refractors, a centered star that still shows a
consistent directional flare across the frame usually points to sensor tilt in the imaging train,
not the lens.
Repeat near a corner. If only corners are bad, suspect spacing/tilt; if the whole frame is bad and
directional, suspect alignment.
3. Fix
- Reflectors: align the secondary under the focuser, then the primary, using a sight tube /
Cheshire or a laser, finishing on a real defocused star. Re-check after transport — it drifts.
- Refractors / imaging train: measure and correct sensor tilt (tilt adapter or shimming) and
set the backfocus to spec for your flattener/reducer; small spacing errors produce edge
elongation that mimics alignment.
- Re-verify with a fresh defocused-star test and a full-frame sub before declaring it solved.
When it isn't optics
If round stars suddenly become directionally smeared only during longer subs, that's tracking, not
alignment — go to tracking lag in the mount. If sharpness fades
gradually as the night cools, that's focus.
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Background on the optics involved: how a telescope forms an image.
Considering different optics for cleaner stars? Choosing a first deep-sky telescope.
Common questions
Why are my stars not round (one-sided tails or flares)?
One-sided tails/comet flares that worsen toward the corners point to optical alignment (collimation) or sensor tilt; uniform elongation is tracking, soft all-over is focus, and stretched-corners-only is backfocus/field curvature.
How do I fix elongated or distorted stars?
Run a defocused-star test at center and a corner to separate alignment from tilt from spacing, then collimate a reflector (secondary then primary) or correct sensor tilt and set backfocus on a refractor train.