LEARNED IT · 11/26
Focus cannot fix the air
Soft planet pictures are either focus or seeing, and the recordings can tell them apart. Seeing changes from one fiftieth of a second to the next and moves the planet around; focus is the same in every frame. If the sharpest frames are sharp and the middling ones smeared, it is the air, and no focuser, processing or frame selection will get past it.
On 8 November Saturn came out soft with the settings that had been sharp on 3 November. The question was whether to refocus or give up on the night.
How to tell from the recording
- The planet wobbles. Take out the slow drift and see how far it jumps frame to frame. Steady air holds it under 0.3 arcsec; that night it was 0.45 to 0.8.
- Sharpness changes fast. A run's sharpness, frame by frame, had no memory beyond half a second. The focus cannot do that.
- Put the best frame beside a middling one. Same recording, same focus: one showed the ring gap, the other was smeared and doubled.
The trap
Every frame was lower than on the good night, the best ones too, and a loss on every frame reads like focus. I refocused. The next four runs scored exactly the same. The air was lower all night, the lucky moments included.
What to do
Check the wobble before touching the focuser. If the air is poor, take the night's best and stop: keeping the sharpest tenth instead of a quarter, harder deconvolution and wavelets all came out the same or worse. See stacking is selection then sharpening, diffraction is the real ceiling, altitude beats everything.