Elongated stars

Compare a star in the center with one in each corner. Each is an oval of about the same length, pointing the same way.

1. 01

   Guiding error

   The guider is not holding the star where it should. Excursions on the guide graph that line up with the worst subs point here. A guide scope that flexes against the main scope stretches stars while the graph looks fine.

   FixWatch the guide graph for a few minutes before the sequence starts and read the RMS against your image scale. Calibrate near the celestial equator and the meridian. With a guide scope, tighten its rings and the guide camera so nothing flexes.

   Session Checklist
2. 02

   Balance

   A rig out of balance loads the drive unevenly and leaves the guider chasing it. Balancing without the camera, cables or dew straps attached leaves the rig out of balance once they go on.

   FixRebalance with everything that rides on the mount attached, declination first and then right ascension. Adjusting one axis moves the center of gravity for the other.

   Session Checklist
3. 03

   Wind

   Gusts shake the rig during the exposure, so the damage varies from sub to sub with the wind.

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

4. 04

   Periodic error

   Every mount's drive has a small tracking error that repeats on a cycle. On a guided rig, guiding causes far more elongated stars than periodic error does.

   FixGuide, if you don't already. Unguided, keep subs short enough that the error does not show. A mount with less periodic error comes last, after balance and guiding are right.

5. 05

   Polar alignment drift

   A polar axis that is off, or that shifts during the night, lets stars drift across the sensor during each sub.

   FixPolar align with a software routine. On a pier, measure the error and compare it with where you left it before re-aligning.

   Session Checklist

• One side soft

  Tilt stretches stars on one side of the frame only. Guiding and balance stretch them the same way everywhere.

• Coma

  Coma stretches stars outward from the center and leaves the center sharp. Here the center is as bad as the corners.

• Star trails

  Star trails are long streaks with no round core. Elongated stars are short ovals.

One frame shows the stretch and its direction. Telling guiding from balance or wind needs the guide log for the same minutes.

Stars round in the center and in every corner of each sub, and a guide graph with no excursions that line up with bad subs.

Rules of thumb for guiding RMS against image scale disagree, and so do the imagers applying them. Wide-field rigs tolerate far more than long focal lengths. Judge by star shape in the subs before any single ratio.

• PHD2 manual, “Basic use”: calibration
• Starizona, “Balancing for imaging”