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The 9to5Google article Exclusive: Official Pixel 4 camera samples show astrophotography, selfies, more [Gallery] shows some surprisingly nice (at least to me) photos of the Milky Way.

Every phone camera is different but I imagine the apertures are only a few millimeters, so their ability to capture light from dim, diffuse areas like the milky way (seen in a short focal-length camera) is impressive.

I have a few related questions.

  1. Is this a "first" or have people been photographing the Milky Way with cell phones regularly and I just never heard about it?
  2. Is there some "secret sauce" or special image processing that makes it possible to see these dim, diffuse areas above sensor noise?
  3. Does the Pixel 4 have an unusually large aperture compared to other cell phone cameras in order to make this possible?

Related questions:

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This article on googleblog goes into details how the Pixel's Night Sight mode works.

Basically, it is using the well-known stacking technique, which Michael C already mentioned, together with some sophisticated algorithms to determine exposure time, number of exposures, and which parts of the images to merge. The article talks about exposures up to 1s on tripods.

enter image description here

Left: Crop from a handheld Night Sight shot of the sky (full resolution image here). There was slight handshake, so Night Sight chose 333ms x 15 frames = 5.0 seconds of capture. Right: Tripod shot (full resolution image here). No handshake was detected, so Night Sight used 1.0 second x 6 frames = 6.0 seconds. The sky is cleaner (less noise), and you can see more stars. (Florian Kainz)

Stacking many images has the advantage of averaging out noise while increasing signal, but adds the problem of aligning images taken some time apart. The camera's software (which ought to be largely independent of the phone hardware) aims to handle this intelligently.

  • Okay this is extremely helpful! I've added in a key point that I found particularly helpful, I hope you don't mind. – uhoh Oct 3 at 9:04
  • Results look curiously similar to what "Multiframe NR" on any old a6000 does .... – rackandboneman Oct 7 at 0:38
  • some recent, related links 9to5google, Matt Gonzalez tweet, Matt Gonzalez video – uhoh Nov 7 at 1:35
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Is there some "secret sauce" or special image processing that makes it possible to see these dim, diffuse areas above sensor noise?

Regardless of what type of camera one is using, the "secret" to getting good Milky Way images is to shoot from an area with virtually no light pollution at a time when the moon is below the horizon and the sun is at least 18° below the horizon. In other words, you want to have very dark skies. High altitude, low amounts of moisture or dust in the air, and cooler temperatures that result in less atmospheric turbulence also help.

The next requirement for a camera with limited light gathering capability is to gather enough light.

Since light gathering in astronomy has nothing to do with f-number and everything to do with absolute aperture (the actual diameter of the entrance pupil), cell phones are extremely limited at this point.

There are mainly two ways to gather more light in astrophotography.

  • Mount the camera on a device, called a tracking mount, that counteracts the apparent motion of the sky so that long exposures may be used without getting 'star trails'.
  • Take multiple shorter exposures and "stack" them using a post processing application made for combining multiple images. Each frame will need to be aligned with the others and the edges may be cropped to exclude parts of the sky that are not included in all of the frames.

Ideally, one can combine both methods.

In either case, the sky and the foreground will need to be captured separately. The first of your two examples shows some fairly obvious masking on the edges of the trees in the foreground.

With all of that out of the way, let's talk about the camera(s) in the Pixel 4.

No one knows anything for sure yet. The phone has not been officially released. It's quite possible that the phone is doing multiple exposures and combining the images to reduce the influence of Poisson distribution (shot) noise. It's possible that other computational photography is being applied. But all of that is purely speculative at this point.

There's also another method many folks use to get astro photos with camera phones:

  • Use a bright, wide field telescope with a large aperture and set the camera at the proper distance from the eyepiece for the image to be in focus. There are accessories available that fit specific eyepieces to hold the camera in the correct position.
  • This is a nice general review of astrophotography, but I'm not convinced that cell phone photos require a tracking mount, and close examination of the stars in each of these photos show every star elongated in nearly the same direction, so probably no tracking mount was used here. Your discussion about telescopes seems totally unrelated to the question asked, and there's no discussion of cell phone camera apertures, light gathering power for diffuse, extended things like the milky way, or how to see it above sensor noise. Nonetheless, thanks for the answer. – uhoh Oct 3 at 7:05
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    @uhoh For the Milky Way, it is all about dark skies. Without that, nothing else matters. The presence of elongation does not necessarily mean a tracking mount was not used, it might only mean that the tracker was not perfectly aligned with the earth's axis, or the timer was not properly calibrated, or a DYI 'barn door' tracker with a straight bolt was used. F-number is not the key issue when doing astronomy. Absolute aperture (the actual diameter) is what determines how much light is collected from a source far enough that its light is collimated. Any cell phone is extremely limited there. – Michael C Oct 3 at 8:12
  • @uhoh It's fairly difficult to discuss specifics of the camera in a phone that has not yet been officially announced, much less released. – Michael C Oct 3 at 8:28
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Cameras in smart phones have a small front lens but since they also have a short focal length (5mm) they have a relatively wide aperture. Looking at the EXIF, the camera on my $200 Xiaomi Note 7 opens a f/1.8 at least which makes it a rather fast lens.

In addition when you take pictures with a wide angle lens the exposure can last several seconds before you start having visible star trails.

So yes, someone with some skill, the right camera application and at the right place at the right time (very dark skies) can likely take Milky Ways pictures with a smartphone.

  • This Matt Gonzales video mentions an exposure time of 4 minutes! Per this answer the phone uses frame stacking, so it's possible that the star trails might be suppressed. How they could do that without blurring static objects on the ground at the same time I don't know. – uhoh Nov 7 at 1:38
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There are few steps you can follow:

  1. Make sure it is few hours past the sunset and your mobile lens is clean.
  2. Go to the less populated place away from a city (Hill station preferable)
  3. Set your mobile on a tripod
  4. Use Google Sky Map if Milky Way is not visible with naked eye.
  5. set shutter speed to max (more than 15 seconds if mobile supports)
  6. set ISO to Max or 1 f-stop less for less noise.
  7. set focus manually keeping brightest star in view where it appears sharpest (not usually at infinity)
  8. set timer and wait for it to capture.

(Note: RAW format offers more data so it is a good practice to capture galaxy in RAW)

  • Are these steps possible on a phone? – uhoh Nov 7 at 1:36
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    @uhoh while not as extreme as this answer states, there are some Android camera apps that allow customizing the setting more freely as long as the hardware & OS supports it (particularly Camera2 API). Also on SoftwareRecs.SE – Andrew T. Nov 8 at 3:57
  • @AndrewT. Thanks, I had no idea all of this was possible! – uhoh Nov 8 at 6:02

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