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posted by hubie on Wednesday September 21 2022, @08:35AM   Printer-friendly
from the buh-bye-night-sky dept.

The imminent launch of a BlueWalker satellite, with a giant phased array antenna, portends a brightening night sky:

The prototype of a new constellation of extremely bright Earth-orbiting satellites is due to launch in early- to mid-September. The AST SpaceMobile company plans to orbit more than 100 of these spacecraft by the end of 2024. Astronomers at the Vera Rubin Observatory and the International Astronomical Union's Centre for the Protection of Dark and Quiet Skies from Satellite Constellation Interference (IAU CPS) are concerned because these new spacecraft will interfere with celestial observations, adding to the problems already caused by other constellations.

The first member of this new group, called BlueWalker 3, will feature a giant antenna array covering an area of 64 square meters (689 square feet). Observers on the ground will see bright sunlight reflected from this structure. After on-orbit tests of BlueWalker 3 are completed, the operational satellites, called BlueBirds, will be launched. BlueBirds may produce even more glaring light pollution since they are significantly larger. The commercial appeal of these satellites is that they will link directly to cell phones without the need of a cell tower. AST SpaceMobile has already secured a license from the Federal Communications Commission to test the prototype.

[...] Other bright satellites are waiting in the wings: 30,000 second-generation Starlink satellites are currently awaiting FCC approval. Like the BlueBirds, the new Starlinks may carry antennas for direct connection to cell phones; the antennas are slightly smaller at "only" 25 square meters, but the satellites would be far more numerous than the BlueBird constellation. That development would be very bad news for astronomy.

BlueWalker 3 is expected to be among the brightest objects in the night sky after the antenna unfolds. Amateur astronomers can help record this satellite's brightness, bringing awareness to bright satellites' effects on our night sky and on astronomy.

[...] Astrophotographers can also play an important role in the study of artificial satellites, by uploading celestial images impacted by satellite streaks to the TrailBlazer website. Meredith Rawls and Dino Bektešević (both at University of Washington) are developing this data archive as part of the IAU's response to the problems posed by spacecraft. Trailblazer stores the impacted images and records selected metadata, so users can search for satellite-streaked images by date, location, and other parameters such as sky position and telescope.

See also:
    AST SpaceMobile video describing the phased array satellite.
    NASA APOD showing satellite streaks over a two hour period.

    SpaceX Has Had 'Promising Conversations' With Apple About iPhone Satellite Service
    AST SpaceMobile Gets US Approval to Test Satellite-based Cellular Broadband

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  • (Score: 3, Informative) by JoeMerchant on Wednesday September 21 2022, @01:28PM (7 children)

    by JoeMerchant (3937) on Wednesday September 21 2022, @01:28PM (#1272750)

    >What is this algorithm that can do this?

    Bright thing, not in FOV, then enters and moves across at anything near "satellite speed" which is considerably slower than cosmic rays, erase it, and pixels close to it, and maybe following it on subsequent frames if it's bright enough to dazzle the sensor.

    You don't need to, but I bet the big boys actually have orbital schedules for known objects, know 99.999%+ of the objects that would interfere with the images they are capturing, and can remove them without even looking for bright streaks in the image - they just know where to expect them. That's a big database to maintain, but one copy of that database serves the entire community. Also, this isn't too far off from how new object discovery is done. The satellites move faster than planetisimals, asteroids and other known objects, but it's basically the same computations.

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  • (Score: 5, Informative) by Immerman on Wednesday September 21 2022, @02:26PM (5 children)

    by Immerman (3985) on Wednesday September 21 2022, @02:26PM (#1272769)

    Quick sanity check:
    From what I can find a typical amateur telescope has a field of view of about 15', or 1/14,400th of a great circle.

    A typical LEO satellite has an orbital period of about 90 minutes, so it will be in frame for at most 6 seconds. Professional telescopes tend to have a much smaller FoV (=much higher magnification), so the transit time for them will be much shorter.

    I don't do astronomy, but it seems to me 6 seconds is a pretty short exposure time if you want to be able to see *anything* more than the brightest stars. So you're unlikely to get any sense of motion from a satellite - you'll just see a bright streak across the frame - and the internal reflections of that light, which is orders of magnitude brighter than everything else in the sky, will likely drown out anything else of interest in that frame.

    • (Score: 1, Informative) by Anonymous Coward on Wednesday September 21 2022, @02:35PM

      by Anonymous Coward on Wednesday September 21 2022, @02:35PM (#1272773)

      If you are doing multiple exposures, it would be pretty easy to dump the one (or two) frames that streak is in.

    • (Score: 2) by JoeMerchant on Wednesday September 21 2022, @03:39PM

      by JoeMerchant (3937) on Wednesday September 21 2022, @03:39PM (#1272788)

      As AC said: think in terms of frame stacking: not a single 60 second (or 60+ minutes) exposure, but a series of exposures at whatever the optimal time for the sensor is, maybe one second?

      Some people are doing multi night captures with thousands of frames captured per night, stacked into a single image.

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    • (Score: 2) by JoeMerchant on Wednesday September 21 2022, @06:29PM (2 children)

      by JoeMerchant (3937) on Wednesday September 21 2022, @06:29PM (#1272834)

      Just like film: excessively long exposures also lead to saturation and clipping. This is a good read, if you're truly interested: []

      They seem to be in the region of 30-120 seconds per exposure, with clipping already setting in for the subjects in their examples (note the moon at 1/800th of a second). Of course, if you want to go deep-field not into the denser parts of the Milky Way, longer exposures would make sense.

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      • (Score: 2) by Immerman on Wednesday September 21 2022, @07:35PM (1 child)

        by Immerman (3985) on Wednesday September 21 2022, @07:35PM (#1272849)

        Keep in mind that, unlike for the "pretty night sky photos" you link to, you're almost guaranteed to *want* a lot of saturation and clipping in an astronomical photo. Or at least be unable to avoid it because you can't frame your shot to avoid all of the stars that are much brighter than your target. (aka practically everything visible in a pretty night sky photo). Hence the bright blooms on Webb images where it gets so ridiculously oversaturated that a single pinprick of light becomes a huge "lens flare".

        But sure, a few minutes sounds like a plausible exposure limit for for earthbound astronomy - your telescope is after all spinning at one revolution per day, and it's all but impossible to compensate for that without introducing so much vibration that you can't get a clear image anyway.

        • (Score: 2) by JoeMerchant on Wednesday September 21 2022, @08:08PM

          by JoeMerchant (3937) on Wednesday September 21 2022, @08:08PM (#1272860)

          That's what the image stackers are really good at, if you can keep each individual image reasonably sharp then they can rotate and align and even distort the subsequent images to align with the first (or whichever) one and then average them together to bring up the SNR, pretty dramatically if your individual exposures are 60 seconds and you've got 360 of them. As I said elsewhere, some people go even further and shoot the same part of the sky night after night... not so good for planetary images, but 10 nights in a row knocks down your noise by another factor of 10 (or, at least lets you get a decent number of shots in-between the clouds...)

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  • (Score: 0) by Anonymous Coward on Wednesday September 21 2022, @03:22PM

    by Anonymous Coward on Wednesday September 21 2022, @03:22PM (#1272786)

    There are all sorts of clever approaches one could take, including what you mention, but none of them are trivial. Recognizing these streaks falls into the bucket for things that are easy for humans are hard for machines. The way you'd have to approach it is very situationally dependent. If you have long exposure frames and have long tracks, that is different than if you have lots of very short integration times with very short tracks, but once they are racked and stacked, make a long track. The more recent work that I've heard (and I don't actively follow this area), like a lot of things these days, waive the magic machine learning wand to say they do or will find and correct them this way.

    For your second paragraph, I wouldn't take that bet. Remember that this is astronomy we're talking about here, so don't assume anyone has sufficient budget to do much more than is necessary. Predicting where and when something will be in a future observation is also not easy. The positions and locations of satellites is not very well known with precision. You can download the ephemerides of them and figure out where they'll be and when, but the error bars on that are not small, and you can't extrapolate too far into the future for predictions because of all the external forces on the satellites, especially the lower orbital ones due to things like atmospheric drag and gravitational nonuniformities.