Elongated Stars: Is It Your Tracking, or Your Optics?

Elongated stars are the most common complaint in astrophotography and the most often misdiagnosed. Two completely different faults produce a symptom that looks the same in a single frame. The usual forum answer is “improve your guiding”, which is right about half the time and expensive when it is wrong.

There is a clean test, and it takes one look at three frames.

The test

Look at the same corner of three frames taken an hour apart.

• If the stretch changes between them (different length, different angle, present in one and gone in another), the mount moved.
• If the stretch is identical in all three, same direction and same amount, it is the optics.

The logic is simple. Your optics did not move between frames, so whatever they do to a star they do in every frame of the night, the same way. The mount is the part that moves.

Why the distinction matters

The two fixes have almost nothing in common.

Tracking faults are dealt with by polar alignment, guiding, cable management, balance, and not asking a mount to carry more than it can. Some of that is technique and costs nothing. Some of it is a guide scope and a camera.

Optical faults are dealt with by a coma corrector, a field flattener, backfocus spacing measured to the millimeter, tilt adjustment, collimation, and sometimes a sturdier focuser. None of that is guiding equipment, and a guide camera will not fix any of it.

People buy an autoguider to fix a tilted sensor all the time. They spend real money and see no improvement at all.

Reading it off a single frame

You can often tell from one frame if you look in the right place.

Optical faults usually depend on where you are in the field. Coma, field curvature and tilt all get worse away from the optical axis, so compare the center with the corners:

• Center sharp, corners stretched radially (pointing outwards from the middle): coma, or a missing corrector.
• Center sharp, corners soft but round: field curvature. Everything is at a slightly different focus depending on where it sits.
• One corner or one side worse than the rest: tilt. The sensor is not square to the light path, or the focuser is sagging under the camera’s weight.
• Everything uniformly soft, corners no worse than the middle: not an optical aberration. That is focus or seeing.

Tracking faults usually affect the whole field equally. The whole frame drifts together, so stars in the middle are stretched about as much as stars at the edge, and all in the same direction, the direction the mount moved.

That last point is the most useful thing to look for. Radial elongation, fanning outwards, is optics. Parallel elongation, all the trails pointing the same way, is motion.

Field rotation

One case is neither, and it catches people with alt-azimuth mounts and setups without a wedge: field rotation.

Stars near the center are round, stars at the edge are stretched into short arcs, and the arcs curve around the center of the frame. This is the field turning during the exposure, because the mount follows the sky in altitude and azimuth rather than about a polar axis. It is neither a tracking error nor an aberration.

The fix is shorter subs and more of them, or a wedge.

Why software should not reject frames for this

This is where automatic frame rejection goes badly wrong, and it is worth understanding if you use any stacking software.

Most quality scoring includes star shape, and reasonably so, because a trailed frame is a worse frame. But if the elongation comes from the optics, it is in every frame equally. A rule that rejects frames for elongation then either rejects all of them and leaves you with nothing, or rejects an arbitrary fraction and leaves you with a thinner stack and no explanation.

The right behavior is to notice that the fault is shared across the session:

Akastroid measures elongation across the whole session, not frame by frame. When the same stretch is present in all of them, it reports the eccentricity, says that this is the optics rather than the tracking, and rejects nothing for it. It then stacks the session, because slightly oval stars in every frame still make a picture and an empty session does not.

The measurement is “these stars are 0.97 eccentric”. The judgment is “since that is true of all 200 frames, it is not a reason to throw any of them away.” Those are two separate things.

What to do tonight

1. Take three frames an hour apart and compare the same corner.
2. Changing → mount. Start with polar alignment, then cables and balance, then guiding.
3. Identical → optics. Check spacing first, because it costs nothing; then tilt; then whether this scope needs a corrector.
4. Curved arcs around the frame center → field rotation. Use shorter subs.
5. Round in the middle, radial at the edges → you need a coma corrector or a flattener. Guiding will not substitute for one.

Measure rather than squint. Image Doctor reports eccentricity per frame and across the session, so you can see whether the stretch is shared or varying without flicking between frames at two in the morning.

Related: why are my stars trailing, a symptom-by-symptom guide to bad astrophotos, reading a quality score.