158,640 votes and 3,636 comments so far on Reddit
Article word count: 634
HN Discussion: https://news.ycombinator.com/item?id=19190153
Posted by sohkamyung
(karma: 23030)Post stats: Points: 185 - Comments: 37 - 2019-02-18T12:18:20Z
\#HackerNews #50000 #image
OP used an ASI224MC for the Moon exposures for this image. This is a camera which uses a similar sensor compared to your average DSLR, but it is smaller and has no moving parts, capable of taking dozens to hundreds of frames per second depending on what resolution you use.
The software for capture, stacking, and processing is all free, thankfully. Firecapture is a popular capture software - as the ASI224MC has no viewfinder it relies on laptop control for operation. Autostakkert!2 is a popular stacking software (it aligns the Moon for each image and averages the pixel values), and either Registax or Photoshop can be used for sharpening afterwards.
As for stacking, our atmosphere distorts incoming light, such as Moonlight. You can see this effect on a hot day when you look across a parking lot, the air will shimmer. This is compounded by the fact that even when the Moon is directly overhead, you are still looking through roughly 100 miles of air. The method then used to pick out those millisecond-long moments where the Moon is actually sharp and in focus is called Lucky Imaging, and it is why OP takes so many frames. For planetary imaging 50,000 is not too uncommon, though you can get decent enough results with as little as a few hundred with a calm atmosphere free of dust. As an example, I took this mosaic with the same camera OP used, using a total of about 100,000 frames, but spread through about 55 panels since my telescope is comparable to a ~2000mm lens (and since the camera sensor is so small, it only picks up a portion of that field of view). Click "full resolution" at your own peril, for those on mobile devices.
The other advantage of stacking multiple images is noise reduction. Cameras have some amount of random noise in each frame, which is the grainy effect you often see in darker images. Astrophotography is all about signal-to-noise ratios, with the primary goal being to increase this ratio through varying methods of noise reduction, such as image stacking. The great thing about random noise is that you can get rid of most of it by stacking about 40 images, with some decreasing benefit using higher numbers of images. Obviously 50,000 would reduce it to tiny levels, but in the case of planetary imaging, which uses very short exposures, the main advantage of stacking is to get around the atmospheric blurring. Here is an example of the benefits of stacking I found online.
Capturing the night sky via long exposure is another animal. Long exposure introduces not only random noise, but some heat-related noise caused by the camera taking so long for each exposure. This can be partially reduced through stacking (for long exposure, we use a free software called Deep Sky Stacker, which will identify and align images to detected stars), but also benefits from calibration images called Darks. These are exposures identical to the ones you took of the night sky except with the lens cap on. Using this method, you can capture only the camera noise, which is subtracted from each image of the night sky before then stacking them together. In this manner, you can remove the predictable noise as well as the random noise for a clean image.
Also in the case of night sky long exposure, you will require a tracking mount. For bright objects like the Moon or planets, you can get away with using a manually-driven telescope, but since the Earth rotates, the stars will move during the exposure and thus require a tracking mount to counteract this. Widefield lenses (14mm or so) can get away with static tripod imaging since the stars donʼt noticeably move as fast, but anything 50mm or beyond will likely require a tracking mount for decent imaging.
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