[HN Gopher] LSST Camera: largest camera for astronomy
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LSST Camera: largest camera for astronomy
Author : defrost
Score : 59 points
Date : 2024-04-04 07:57 UTC (2 days ago)
(HTM) web link (www6.slac.stanford.edu)
(TXT) w3m dump (www6.slac.stanford.edu)
| porphyra wrote:
| 3200 megapixels custom CCD array cooled to about -100 degC sounds
| amazing.
|
| The 3.5 degree FoV is around 700 mm focal length equivalent on a
| full frame camera which makes this relatively wide field by
| telescope standards, allowing it to capture more of the sky per
| shot. But the array is 64 cm wide so the actual focal length is
| around 10 meters.
| perihelions wrote:
| It has the greatest light-gathering ability (product of field
| of view * aperture) of any telescope yet. That's the key figure
| of merit for a surveying telescope! (As I understand--I'm not a
| domain expert).
|
| - _" Combined with its large aperture (and thus light-
| collecting ability), this will give it a spectacularly large
| etendue of 319 m2[?]degree2.[6] This is more than three times
| the etendue of the largest-view existing telescopes, the Subaru
| Telescope with its Hyper Suprime Camera[36] and Pan-STARRS, and
| more than an order of magnitude better than most large
| telescopes.[37]"_
|
| https://en.wikipedia.org/wiki/Vera_C._Rubin_Observatory
| porphyra wrote:
| Wow yes, 10.31 m (f/1.23) overall (optical diameter of 8.360
| m). That's insane.
| mnw21cam wrote:
| Thanks. It boggles my mind that an article like this about
| a scientific camera can fail to list the focal length and
| aperture, which are basically the two most important
| numbers for any camera.
| ouptppr wrote:
| Maybe before you get too boggled, consider that the
| original article is about a scientific instrument
| designed to be attached to a telescope. The aperture and
| focal length given in the other source are properties of
| the telescope, not the instrument.
|
| It's not uncommon to discuss CCD cameras for large
| telescopes without making any mention of the focal length
| or aperture of the telescope. See, for example:
| https://www.eso.org/public/teles-instr/lasilla/ntt/susi2/
|
| Here's a page for another imaging device which lists the
| aperture and f-number of the telescope--and then gives a
| separate f-number for the instrument, without stating the
| instrument's aperture: https://www.eso.org/sci/facilities
| /lasilla/instruments/wfi/o...
|
| So I don't think the LSST media department dropped the
| ball on this one. The numbers (focal length and
| aperture), which you understandably think of as being
| essential info for any camera, just aren't as relevant
| here, partly because comparison with other options isn't
| in the forefront of the scientists minds (the camera is
| totally bespoke and there are many ways in which it is
| tailored to the telescope, which constrain its design,
| including in those two respects).
|
| Focal length and aperture of the LSST camera aren't
| mentioned here, either https://www.lsst.org/about/camera
| porphyra wrote:
| Yeah haha I mean even when you buy a consumer camera
| like, say, a Sony a7 or whatever, it doesn't come with
| the lens.
| Randalthorro wrote:
| Focal length and aperture literally mean nothing for this
| instrument (camera). Telescopes are multi instrument and
| it's the telescope plus instrument specific mounting
| configs that determine these. (Focal length is actually
| pointless except to make sure you design the light path
| correctly).
|
| Aperture is talked about in the light collecting area of
| the mirror, and often summarized to its diameter. 8m
| class telescope, is a telescope with an approximately 8m
| diameter mirror.
|
| Hubble is a little over 1m.
|
| Other things matter way more for a telescope and are much
| more interesting.
| ck2 wrote:
| It is fortunate that storing 15TB per night is now only $300,
| well before supporting hardware and mirroring.
|
| Imagine having to store that much every day a decade ago.
| imbusy111 wrote:
| So $600 a decade ago?
|
| [1] https://ourworldindata.org/grapher/historical-cost-of-
| comput...
| OldGuyInTheClub wrote:
| They've got fast fiber to get the data off the mountain and
| into multiple data processing centers in near real-time
| including alerts when something interesting happens.
|
| "The nightly pipelines are based on image subtraction, a
| process that highlights differences between two exposures of
| the same field, and are designed to rapidly detect interesting
| transient events in the image stream and send out alerts to the
| community within 60 seconds of completing the image readout. "
|
| https://www.lsst.org/about/dm
| Randalthorro wrote:
| well we don't have to imagine, there are meeting notes!
| dekhn wrote:
| A project I worked on donated compute cycles to LSST- it was a
| full scientific-grade ray tracer that takes star catalogs and
| uses them to render the image on the camera. It worked well
| enough, and early enough, that it caught a design error in the
| physical telescope. That work was done 10 years ago, I think they
| are wrapping up LSST's assembly now and they will have imaging
| going next year.
|
| It's very exciting because it's a survey which produces
| statistical quantities of information about a wide range of
| objects, and also it's great the US and other governments were
| willing to fund something at this scope for so long.
| venice_benice wrote:
| Do you recall the name of the software? I am aware only of
| PhoSim (https://www.phosim.org/)
| KennyBlanken wrote:
| ...significantly hampered by low-earth-orbit microsat
| constellations like Starlink. It boggles the mind that there are
| half a million microsats planned for launch in the next ten
| years. Starlink is already causing issues for launches, cutting
| down launch windows and even blocking ones that had been open
| because Starlink moves a satellite without clearing it with
| anyone first.
|
| https://www.lsst.org/content/lsst-statement-regarding-increa...
|
| TLDR: the telescope's field of view means that it will be nearly
| impossible for them to find a "clear" patch of sky, the
| brightness off satellites saturates the CCD, which in turn causes
| crosstalk during readout of the CCDs, and even just masking the
| primary streak is difficult. I'm sure there are optics issues as
| well from such bright objects, in the field of view and not.
|
| > During the nominal 30-second visit to a sky patch, satellites
| in 400-600km LEO orbits typically move about 15 degrees across
| the sky (about four times the diameter of Rubin Observatory's
| field of view), and are visible a few hours after sunset and
| before sunrise. With 400,000 satellites orbiting Earth, tens of
| thousands of satellites would be visible above the horizon and it
| would be difficult to find a circle of 9.6 square degrees
| anywhere on the sky that does not contain satellite streaks.
| Simulations of the LSST observing cadence and the full SpaceX
| satellite constellation show that as many as 30% of all LSST
| images would contain at least one Starlink satellite trail. With
| the planned constellations of 400,000 satellites at 400-600 km,
| all images in twilight will contain streaks. The OneWeb
| constellation at 1200 km will be visible all night long in
| Chilean summer. Measurements of the brightness of the current LEO
| satellites in their final orbits indicate that these trails would
| cause residual artifacts in the reduced data. If these LEO
| satellites can be darkened to 7th magnitude, then a new
| instrument signature removal algorithm can remove some of the
| residual artifacts. This is challenging due to apparent non-
| linear crosstalk between the 16 channels on each of the 189 CCDs,
| the cause of which is still under study. The bright main
| satellite trail would still be present, potentially creating
| bogus alerts and systematics at low surface brightness. Masking
| of these trails is not 100% perfect. This is a challenge for
| science data analysis, adding potentially significant effort.
| lawlessone wrote:
| >It boggles the mind that there are half a million microsats
| planned for launch in the next ten years.
|
| Great for Space X, they're creating a need for more space
| telescopes..
| mrbluecoat wrote:
| Looks like there was concern about this all the way back in
| 2016: https://twitter.com/lsstepo/status/804412225926463488
| porphyra wrote:
| > If these LEO satellites can be darkened to 7th magnitude,
| then a new instrument signature removal algorithm can remove
| some of the residual artifacts.
|
| The current generation of Starlink satellites is already above
| the 7th magnitude, so it no longer saturates the CCD. [1] Of
| course, darkening them further would always be good.
|
| [1] http://arxiv.org/abs/2306.06657
| perihelions wrote:
| There's however more considerations than just saturated
| pixels,
|
| https://www.lsst.org/content/lsst-statement-regarding-
| increa... ( _" Vera C. Rubin Observatory - Impact of
| Satellite Constellations"_)
| porphyra wrote:
| That's true, but this article was also from before the
| Starlink Generation 2 Mini satellites with magnitude > 7
| came out.
|
| > Darkening satellites to 7th magnitude would simplify
| removal of some artifacts in LSST images, but there is no
| guarantee most of the satellites will be limited in
| brightness to fainter than 7th magnitude.
|
| I'm curious if they were indeed able to implement the
| artifact removal or if it remains challenging even then.
| gammarator wrote:
| "Removal" here means "masking"--the pixels under the
| bright streaks will always be scientifically unusable
| because the streaks inflate the noise irrevocably. (You
| can't see stars in the daytime by "removing" the sun.)
|
| The 7th magnitude limit just minimizes the streak signal
| cross-talk through the rest of the camera. It also means
| the satellites are invisible to the naked eye.
| mjsweet wrote:
| I remember the original Google announcement where it was
| announced that Google would help with the huge storage
| requirements for the project, I just dug up the YouTube
| announcement. Pretty amazed that this video is 17 years old now,
| a testament to how long these projects are in development for...
| this 2007 video wasn't even when it began.
|
| https://www.youtube.com/watch?v=thPlpDcaewo
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