Diagnose 06 of 29 Acquisition quality

Triangular corner stars

Corner stars are triangles or little comets while the center is sharp, and all four corners look about the same.

A labeled frame from our own archive, with its acquisition data, is coming to this page.

Unverified

We have not confirmed that wrong spacing alone makes stars triangular. If the stars in the center are triangular too, look past spacing.

Drawing, not a photo. Round stars in the center, and comet shapes that grow toward every corner, about the same in each.

At 100 %: Look at all four corners of a focused sub. If the distortion is about equal in each and grows with distance from the center, start with spacing.
Causes, most likely first: Backfocus wrong
A corrector or flattener works at one distance from the sensor. Too much or too little spacing distorts the corners, and it does so symmetrically.
FixWork out the spacing your corrector needs with every part of the train counted, then add or remove spacers to match.
Backfocus Solver
Filter glass not accounted for
A filter in the converging light path pushes focus back, so a train spaced without it is off by that much.
FixCount the filter's glass in the spacing. The Backfocus Solver includes it.
Backfocus Solver
Looks like, but isn't: One side soft
Tilt is asymmetric: one corner or one edge is worse than its opposite. Spacing errors look about the same in every corner.
Coma
Coma is what a fast Newtonian does with no corrector in the train. With a corrector or flattener fitted, symmetric corner distortion points at its spacing.
One frame can't tell you: One frame, checked corner by corner, separates spacing from tilt. To learn which way to move the spacing, change it and compare a second frame.
How to confirm the fix: Corner stars as round as the center in all four corners of a focused sub.
Sources: No outside source is cited here. The signature, the causes and their order are the atlas’s own, from imaging experience. The order is a judgment of likelihood; no measured frequency is behind it.

In the symptom picker

Corner stars are triangles or little comets while the center is sharpweight 3
Stars stretch outward from the center, more toward the edges, like seagullsweight 2

Open the picker with its signature ticked

Tools for this

Backfocus Solver

Last updated 2026-10-12All 29 failure modes, and the symptom picker