---
title: "Diagnosing and correcting distorted star shapes"
canonical: https://stackingstarlight.com/troubleshooting/collimation
description: "If your stars aren't round — tails to one side, flaring or \"wings\" toward the corners, a faint one-sided spike — this page walks the diagnosis and the fix. Start by separating *optical* problems (which this page covers) from *tracking* and *focus* problems (which look different and live elsewhere)."
---

# Diagnosing and correcting distorted star shapes

> If your stars aren't round — tails to one side, flaring or "wings" toward the corners, a faint one-sided spike — this page walks the diagnosis and the fix. Start by separating *optical* problems (which this page covers) from *tracking* and *focus* problems (which look different and live elsewhere).

## 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](/troubleshooting/mount-backlash).
- **Soft, bloated stars all over that worsen through the night** → focus, not optics. See
  [holding focus](/troubleshooting/focus-drift).
- **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](/troubleshooting/mount-backlash). If sharpness fades
gradually as the night cools, that's [focus](/troubleshooting/focus-drift).

---

Background on the optics involved: [how a telescope forms an image](/equipment/telescope).
Considering different optics for cleaner stars? [Choosing a first deep-sky telescope](/buy/telescope).

---

Source: [https://stackingstarlight.com/troubleshooting/collimation](https://stackingstarlight.com/troubleshooting/collimation) · Part of [Stacking Starlight](https://stackingstarlight.com) by Michael Kalika.

LLM resources: [LLM index (llms.txt)](https://stackingstarlight.com/llms.txt) · [Complete LLM text (llms-full.txt)](https://stackingstarlight.com/llms-full.txt) · [Markdown homepage (index.md)](https://stackingstarlight.com/index.md)