Comment by gammarator | Hacker News Reader

Comment by gammarator | Hacker News Reader

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> They're wide field of view, long-duration exposures. That used to be the way astronomers imaged space,

Sorry, but one of the two example images (https://noirlab.edu/public/images/iotw1946a/) is a single 333-second exposure with a modern survey camera, the Dark Energy Camera. This is not particularly long nor does it represent some outmoded observational strategy. Large, wide-field imaging sky surveys (such as the upcoming Rubin Observatory) are among the highest-profile ground-based astronomy projects today.

Masking and stacking can mitigate the problem but it does not of course compensate for the lost area and sensitivity. And the brightest satellites (like BlueWalker) saturate the readout electronics and spoil the whole exposure.

Narrow field instruments (such as spectrographs) have less geometric chance of seeing a satellite but tend to take longer exposures (tens of minutes), so there is a greater loss of telescope time when a streak does happen.

Even space telescopes are affected by streaks (https://www.space.com/hubble-images-spoiled-starlink-satelli...).

> It takes a bit more work and software

Equivalently, it takes more money and time. That just means less science, given flat to declining funding from Congress.

Where do you see that the full well of those CCDs are fully saturated by a single satellite streak?

Even if that was the case, that seems a very poor design for a sensor and not really an issue of satellites. If a satellite streak can saturate your well, then so could a decently dense star field.

Do they blow out the individual pixels? Sure. But you just stack.

You don't lose sensitivity just because your stack rejected pixels, that's not how stacking works. You wind up with a tiny bit more noise, how much more depends on your capture. But not lower sensitivity, stacking isn't just average an area

It's not really that it saturates the entire image, it's that bright objects creates non-linear and ghosting artifacts which degrade the sensitivity of the entire exposure, even after post-processing in software.

https://www.space.com/bluewalker-3-prototype-satellite-brigh...

https://www.nature.com/articles/s41586-023-06672-7

https://iopscience.iop.org/article/10.3847/1538-3881/abba3e/...

Thanks for providing all those links. Indeed, the key there is peak magnitude. BlueWalker 3 has a giant array that reflects light like a mirror. So when you happen to get the angle of incidence exactly equal to the angle of reflection, it shines quite brightly. Of course since it's reflecting all sunlight in one direction, that means from all other spots on earth it will be extremely dim.

Also note that BlueWalker 3, like most low-earth orbit satellites, is only visible during "terminator conditions", when the satellite itself is illuminated by the sun but the telescope is still in darkness. Those times are typically an hour or so after sundown or an hour or so before sunrise. So one solution is to schedule astronomy for the middle of the night, when none of these low earth satellites will lit by the sun at all. Which certainly increases the costs of astronomy, since you can't use your telescope for as many hours per day.

Or, just use your space object catalog to look elsewhere in the sky when bright satellites happen to be in the sky during terminator illumination conditions. Which I see is one of the things your last link, "Mitigation of LEO Satellite Brightness and Trail Effects on the Rubin Observatory LSST", suggests!

The BlueWalker 3 antenna is white, not mirrored. It will reflect sunlight like a lambertian reflector, not like a specular reflector as you suggest.

https://ast-science.com/wp-content/uploads/2022/07/2022-03-1...

SpaceX has offered their space-rated mirror technology at cost to other satellite manufacturers, but it seems AST SpaceMobile hasn't taken them up on the offer. I can't see any mirrored surfaces on BlueWalker 3.

  >that means from all other spots on earth it will be extremely dim

The second paper I linked (see Figure 1) shows a minimum magnitude after array deployment of 5.5 magnitude, from multiple observatories at multiple locations on the Earth. That's still 10 times brighter than the IAU limit.

  >just look elsewhere in the sky

The hottest thing right now is whole-sky surveys like Vera Rubin (which are important because they can alert other telescopes to transient events), which can't really do that because they're already looking at... the whole sky.

> So one solution is to schedule astronomy for the middle of the night, when none of these low earth satellites will lit by the sun at all.

Let me translate that a bit: "So one solution is to just eliminate 20% of all available observing time."

Not really. Astronomical dusk and dawn are different than civil dusk or dawn, you'd normally skip an hour or so after sunset and before sunrise anyway.

> Masking and stacking can mitigate the problem but it does not of course compensate for the lost area and sensitivity. And the brightest satellites (like BlueWalker) saturate the readout electronics and spoil the whole exposure.

Is this only a problem in systems that aren't aware of where the satellites will be? I naively assume that, if the system knew, it could start exposing in the next clear window. I naively assume that the window is almost always clear, especially for an individual sensor.

Even if they don’t know where the satellites will be, a simple motion detection algorithm in the stacker would delete the streak caused by the satellite. A given pixel will only be impacted for a few exposures, so just grab 5% more images.

I’d guess terrestrial light pollution is a much bigger problem.

  >so just grab 5% more images.

You say it like major observatory telescopes are sitting idle, but they're not. In practice what actually happens is you lose 5% of your scientific data return.

  >terrestrial light pollution

Problem is that it's global vs local, so our normal solution for terrestrial light pollution (build telescopes in a remotely populated area) doesn't work.

So just write all grant proposals with an extra 5% slop.

Why would they not already be trying to maximize grant size?

Budgets are fixed, so again this also boils down to "just do 5% less science."

Believe it or not, actually using 100% capacity on the big expensive telescope you paid for isn't some brilliant unheard-of suggestion like people seem to think. This Dunning-Kruger idea always seems to crop up whenever this particular topic is in the news.

There simply isn't any "slop in the system" that lets you get that 5% (and climbing!) back "for free." If there was, then that inefficiency should be fixed regardless of the situation with megaconstellations.

Doing 5% less science is an acceptable cost for full commercial space exploitation. I'd rather have internet service in 100% of the US than an extra few graduate theses.

And who knows, necessity is the mother of invention so one of those grad students could invent a way around it.

  >Doing 5% less science is an acceptable cost

That's certainly one choice we can make as a society.

However my point is we shouldn't delude ourselves that there's some "easy" fix, therefore the cost should actually by counted as zero. This defense mechanism is misguided and uninformed, yet it's shockingly common to have it (or some variation) crop up when discussing this particular topic.

Personally, I blame the current tribalize-all-the-things trend. You're either on Team Starlink or Team Astronomy. Only two options. Pick a side, we're at war!!!

  > And who knows,... one of those grad students could invent a way around it.

See? Shockingly common. :D

Even when we don't even have an idea, nevertheless we feel oddly compelled to suggest that maybe there's an easy ("grad student") solution that makes the scientific cost simply go away.

If we're willing to pay the cost in lost scientific data, then let's do it and say so. We shouldn't live in (oh so tempting!) denial about the downsides. That's all I'm saying.

Yes, but that's a system problem, not a user problem. The comment I replied to posited that a specific user would collect 5% less data. If you add ~5% to your grant proposal, you get the same amount of data.

As for the system problem, that is solved by building 5% more observing capacity. Or, more realistically, starting new telescope projects slightly earlier. This really isn't as complicated as you're trying make it out to be. Pot, meet kettle.

  >As for the system problem, that is solved by building 5% more observing capacity.

If this is to somehow come from existing funding sources, I'll need a moment to pick myself up off the floor laughing. ;D Obviously those sources are already at their budget appetite.

Or maybe this an Efficient Market / Pigouvian suggestion, where megaconstellaton operators pay (and pass on to their customers) into an Astronomical Reparation Fund worth ~5% of current global astronomy funding, to be distributed to astronomical grants and construction of new observatories? Because sure, that seems fair.

If you truly believe (as do I) that we value internet access more than the loss of scientific data, then you'll agree the internet users can easily reimburse scientists for the damage and still come out ahead. "You broke it, you buy it."

  > If you add ~5% to your grant proposal

..then I'll add 10% to mine!

It's a race to the bottom. Eventually folks get sick of it, so they enact rules and enforcement mechanisms to prevent padding. In the end all we accomplish is transforming an efficient & high-trust system into an overhead-laden, low-trust system.

if there were extra 5% budget they already would be using it

If it's just a matter of 5% inefficiency, I have to think that there must exist some possible mechanism for capturing the economic benefits of satellite internet (with a tax, perhaps) and redirecting a portion to astronomy to increase budgets there by 5% (or more) so that everybody wins.

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