Bloated, soft stars

Stars in the center and the corners are equally large and round, with soft edges. Check the linear stack before blaming the optics, because stretching can do this too.

1. 01

   Focus off

   The eye finds focus to within the range where stars look small. A measurement finds the bottom of the curve.

   FixFocus by measurement, using star size (HFR or FWHM) or a Bahtinov mask, and lock the focuser once focused.

   Session Checklist
2. 02

   Poor seeing

   Turbulent air blurs every star the same way. It changes through the night and from night to night.

   FixRefocus by measurement. If the stars do not shrink, the seeing is the limit; image on a steadier night.

3. 03

   Dew on the objective

   A film of dew softens every star, and it grows halos around the bright ones as it builds.

   FixCheck the glass with a red light. Turn dew heaters on before the optics cool to the dew point.

   Session Checklist
4. 04

   Temperature drift since the last focus

   Focus drifts as the tube cools, so focus set at dusk does not hold.

   FixRefocus as the temperature falls, or set autofocus to trigger on a temperature change.

   Session Checklist

• Star bloat from stretching

  If the stars are small in the linear stack, the stretch made them big. Check the stack before you touch focus.

• Dew on the optics

  Dew grows halos as the night goes on. Focus drift softens stars without growing halos.

• Lost to cloud

  Cloud drops the star count and leaves star size alone. Focus drift makes the stars larger as the count falls.

One frame cannot separate focus from seeing. A focus measurement taken now, and the HFR of each sub over the night, can.

HFR or FWHM that stays near its value just after focusing, through the whole session.

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.