The Worm Moon

My objective for this session was to shoot two two-panel mosaics of The Worm Moon in two runs. The first run would be at a 50% histogram, and the second would be at a 75% histogram. The two panels for each run will be stitched together in processing.

I had hoped to shoot this target last evening, but gave it up due to expected weather conditions by the time the moon had ascended above the trees at 2100, and I was very tired.

This morning I woke up just before 0400 and could see a bright moon hanging in a cloudless sky above the cottage. The scope was covered and had no camera attached, but it seemed that if I proceeded without delay I could complete my objective before loosing the moon behind the cottage.

The equipment for this session was G11/NP101/ASI178. The temperature was 40 degrees and the relative humidity was 68%. There was an occasional moderate breeze, and the sky was crystal clear. The Astrospheric forecast was for average transparency and poor seeing. I could detect the unsteady sky on the live FireCapture display. I was wearing a medium coat, but as expected, I was quite chilled by the end of the session.

I walked out the door to begin my work at 0413. Uncovering the scope, adding the camera, bringing up software and acquiring the target were complete by 0420. The shutter for all four runs was 0.082ms. The gain for the 50% histogram was 284, and for the 75% histogram is was 315.

I completed the four runs (2 panels x 2 histogram settings) by 042, but I noticed that I had not unchecked Debayer after I finished focusing. Not knowing if this setting degraded the data or not, I elected to re-shoot the four runs just to be on the safe side. I was finished with the second attempt by 0436, and I shut down the session.

The camera was removed from the scope and put away, and the scope was covered, and I was back in the house by 0442.

This session illustrates the value of practice. I could not have completed this session – from walking outside, setting up and starting up, shutting down and putting away – in 30 minutes without being well-practiced at my craft.

I noted the shape of the 50% and 75% histograms, and compared them to the previous session’s 30% histogram. Now I believe that I understand why there were muted colors and contrast in the previous session. In all three histograms, I could see a sky fog peak at the left edge of the histogram, followed by a wide hump to the right that included the entirety of the moon’s pixels. The lunar hump in the 50% histogram was wider than the lunar hump in the 30% histogram the night before. The lunar hump in the 75% histogram was even wider. I think issue is that the narrower 30% hump has less dynamic range, and that is what caused the muted colors.

Trying not to clip the whites in a lunar closeup of Aristarchus two sessions ago is how I ended up capturing with a 30% histogram in the first place, and that seemed to work well for a lunar closeup. Then I tried applying result that to the full disk of the moon on the next session and ended up with the muted colors.

I hope that by having data collected on the full lunar disk with 30%, 50% and 75% histograms will help me settle on an exposure that balances between clipping of whites, and enough dynamic range to bring out the colors.

The image selected for publishing with this report is the one shot with the 75% histogram.

The Worm Moon - 2023-03-07 09:37 UTC

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