Turn the exposure right down in daylight

Out in a gale this afternoon with the Moon sitting at 55 degrees in a blue sky, and the first thing the camera gave me was a completely white screen while I could see the Moon perfectly well through the finder with my own eye. I spent a while treating that as a camera fault. It was not.

Night exposures are the wrong starting point in daylight

The camera was sitting at 27 ms (exposure_time_absolute=270) when I looked. That is already shorter than anything I use on the Moon at night — my recent runs have landed between 45 and 115 ms. So whatever was filling that frame was brighter than the lunar surface, which the Moon itself cannot be.

That is the thing worth writing down. The Moon needs exactly the same exposure in daylight as at night — it is a sunlit rock lit by the same Sun at the same distance, and its surface brightness does not care what time it is here. What changes is everything around it. So if the screen is white at an exposure the Moon would find too short, the Moon is not what you are looking at.

Black means lost at night. White means lost in the day.

This is the part I keep having to relearn in a new costume.

At night a scope pointed at nothing reads black, and I know instantly I have lost it. I have the whole of 17 September written up as an evening of chasing a black frame that turned out to be an eyepiece still sitting in the focuser.

In daylight a scope pointed at nothing reads white — a sunlit roof, a wall, the ground, even bright sky. My own pre-flight gate says it in one line: sunlit ground in frame pins p99.5 at 255. And white does not feel like "lost". White feels like "too much exposure", so the instinct is to fiddle with the camera instead of checking where the tube is aimed. Two opposite symptoms, one cause, and the daylight one is the liar.

So the diagnostic is: wind the exposure right down and see what appears. The SV205 goes to exposure_time_absolute=1, which is 0.1 ms. Then read the result:

• Sharp terrestrial detail — a roof edge, branches — means the tube is not on the Moon and the finder needs aligning to the Mak.
• Lunar structure appearing means it really was just exposure, and I can work back up.
• Still uniform white at 1 means light is flooding the tube and I need the dew shield on as a sun shade.

I ended up at 22 ms and got a picture, but I also moved the arrows to correct the line up at about the same time, so I cannot honestly say which of the two fixed it. That is my own fault for changing two things at once and it is exactly why this note exists — next time, exposure first, all the way down, then touch the pointing.

Set these before anything else in daylight

• Auto exposure off. It was already on Manual, which is right. A meter looking at a big bright sky will underexpose the Moon every time.
• Auto white balance off. It was still on today (white_balance_automatic=1). A daylight sky is strongly blue and AWB will chase it, drifting between panels — which is poison for the per-panel gain solve in the compositor, since that assumes panels are photometrically comparable.
• Gamma neutral. Sitting at 300 of 100-500. The gain solve does least-squares on overlap medians and that arithmetic wants a linear response.
• Gain: there isn't one. The SV205 has no gain control. Exposure is the only lever, which is why the sky pedestal costs real dynamic range on 8 bits and there is no buying it back.

What I am still not sure about

Whether the white was bright sky or sunlit ground. It matters more than it sounds. If clear sky through this f/12.7 train really does saturate at 27 ms, then the daytime sky is comparable in brightness to the lunar surface — not a modest pedestal under the Moon but a genuine rival to it — and daytime lunar imaging on this rig is contrast-starved rather than merely range-limited. That would be a real limit, not a nuisance.

One clean test settles it, and I should do it deliberately rather than in a gale: point at blank sky well away from the Moon, wind the exposure down, and find where the sky stops clipping. If that number is anywhere near the Moon's own exposure, I have my answer.

The one genuine daylight advantage

Focus. I can rough-focus on a TV aerial or a tree line a kilometre off, which is enormously easier than hunting focus on a limb, and it gets me close enough that focus-grab.sh only has to refine. That is not available at night at any price.