Stars trail partway through a sub

Every star has the same tail, starting abruptly from a sharp core. The frames before and after it are usually fine.

1. 01

   Cable snag

   A cable pulled taut by the mount's motion tugs the rig during the exposure. A snag mid-frame streaks one sub.

   FixDress cables so nothing is taut at any orientation. After any change to the cabling, slew through the night's range in daylight and watch every cable.

   Session Checklist
2. 02

   Gust

   One gust shakes the rig for part of the exposure, and the stars streak while it lasts.

   FixCheck the wind forecast for the hours you will image, and drop the spoiled subs before stacking.

3. 03

   Dew heaters browning out the mount

   Heaters cycling on a marginal supply can brown out the mount mid-frame.

   FixCheck the supply voltage under load, with the dew heaters and the camera cooler running, against your mount maker's stated minimum input.

   Session Checklist
4. 04

   Mount slip

   Part of the mount or the load shifted under its own weight partway through the exposure.

   FixCheck that every clutch and clamp is tight before the sequence starts.

• Satellite trails

  A satellite leaves one thin line across the frame and the stars stay round. Here every star has a tail.

• Star trails

  A full trail has no sharp core. Stars that start sharp and then streak were tracked for part of the sub.

One frame shows that the rig moved. Which cause moved it needs the guide log or the power log for the same minute.

No streaked subs through a full night, including the slews and the meridian flip.

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.