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Comment by crazygringo | Hacker News Reader

Incredibly cool. I love this so much, both artistically as well as how it demonstrate the equation of time [1] -- the fact that the changes in sunrise and sunset are not symmetrical.

On the other hand, it's driving me absolutely crazy that he centers the image at 4:00 rather than midnight. Or maybe that's to show the shimmer of sunlight a little after noon on the right hand side?

I can't figure out why it's "bluest" closest to dawn and dusk. I'm guessing the exposure makes a huge difference, and obviously the night part is way more exposed than the daylight part, or else it would be much darker. Wondering if the camera used automatic exposure, and how much of the brightness of colors in the image are artifacts because of that? Also if he locked the white point hopefully?

[1] https://en.wikipedia.org/wiki/Equation_of_time

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One of the commenters noticed this as well, the author says it is something automatic in the exposure.

https://fediscience.org/@Birk_lab/113770845539931892

Oh that's a real shame then. The resulting composite images are certainly artistically interesting to look at, and you can see the big-picture effects like sunrise/sunset and moon, but that explains why you can't see the gradual brightening at dawn, or degrees of darkness at night.

It seems like if you wanted to do this accurately, you'd need to lock exposure to handle a bright blue sky without blowing out -- both aperture and shutter speed. And lock white balance. The question is whether that would allow for sufficient sensitivity at night. But if you're just averaging color values across a section of sky and mainly looking for moon and moonlit clouds, I think it would, since pixel noise will get averaged out and the moon is bright.

Yeah, I agree this should be using a fixed exposure (possibly on a schedule) with locked white balance. They are using an astro camera so the sensor is very sensitive, they can get away with extremely low exposure times.

> get away with extremely low exposure times

For night sky photography with an astro cam you’re still looking at exposure times of 20-60s at night (possibly also increasing the gain at night) and milliseconds during the day. The dynamic range is immense.

As someone who has struggled with this for my own allskycam, it’s extremly difficult to have white balance settings that perform well at all times of the cycle, especially with a camera designed to be more sensitive in the IR part of the spectrum (which will always look unrealistic). Settings that give you lovely white clouds and blue skies during the day tend to give you purple skies and green clouds at night. The quality of light is different so the white balance is different.

You can use autobalance or different white balance profiles for day and night but they each have issues.

Additionally to locking the exposrue time and aperture, one could also take multiple exposures, figure out the camera's light response function and fuse multiple exposures together into a single higher dynamic range (HDR) image (see OpenCV tutorial on that or Debevec et al. 1997) Assuming you can find the camera response for the very long exposure times at night _and_ the very short during the day, one could relate them to each other and display both for accurate visual comparison.

#1 rule of timelapse: manual white balance, manual shutter speed, manual aperture, manual agc/iso, manual focus.

I think the blueness is because clouds at dawn and dusk reflect atmospheric colors more, whereas midday the clouds light up as more of a white from diffusing sunlight. 100% made up theory on my part, but I think it makes sense?

I asked o1 to estimate colors by hour and its reasoning and estimates seem fairly convincing[1], and also show more saturated blues dawn and dusk, though it did not model clouds.

1. https://chatgpt.com/share/67795fe3-9ac8-8009-9922-153f40c509...

Not sure about the accuracy of the results but the theory’s correct. The colour of sunlight changes as it passes through different thicknesses of the atmosphere and the proportion of direct light, refracted light and bounced light changes.

>he centers the image at 4:00[pm]

I think because the night sky offers interesting variation that looks nice stitched together. the sunny day, not so much. I will also point out, OCD notwithstanding, that 0hour/12am is an arbitrary time to start the day, and you should really look at the days/nights all strung together in a long continuous strip

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