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"Perfect" Mirrors Cast For LSST
eldavojohn writes "The Large Synoptic Survey Telescope (which was partially funded by Gates & Co.) announced a world record casting for its single-piece primary and tertiary mirror blanks, cast at the University of Arizona. From the announcement: 'The Mirror Lab team opened the furnace for a close-up look at the cooled 51,900-pound mirror blank, which consists of an outer 27.5-foot diameter (8.4-meter) primary mirror and an inner 16.5-foot (5-meter) third mirror cast in one mold. It is the first time a combined primary and tertiary mirror has been produced on such a large scale.'"
Apparently it was so awesome, they just skipped the secondary mirror and went straight to tertiary. :)
Let's hope the grinding is more accurate than the Hubble mirror.
http://www.newscientist.com/article/mg12717301.000-the-testing-error-that-led-to-hubble-mirror-fiasco-.html
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... perfection is only a limitation of the measurement process used to find flaws.
I don't understand, what is this post about? It looks like random mindless babbling, not really a structured conspiracy theory or criticism or anything.
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Microsoft products aren't worth buying until they get to the third release. So they just skipped straight to #3 this time.
Weaselmancer
rediculous.
The digital camera in this thing generates 15TB of data a day from its 3200megapixel camera. I'm assuming it has an array of sensors, but thats still a ridiculous amazing pixel count.
I am an astronomer at the UA and the mirror is a major feat of engineering. It will be the first telescope to have the tertiary and the primary mirror on the same piece of glass. They will have to grind both parts to be perfectly aligned (point to the same place) as well as make the transition area as small as possible. The secondary mirror is a doughnut shape that will be placed above the primary and will have the $100M camera behind it. The camera itself will be the size of a small car and will be as stated before a 3.2 Gigapixel ccd. It will have 200+ 4k by 4k CCD chips that will be read out in 2 seconds. This coupled with the fact it will image the night sky in 5 colors every week will lead to petabytes of data by the programs terminus. Its basically the coolest telescope that will ever be built. ESPECIALLY since the data is set to be public (for US residents) the moment it is processed each morning.
...all the ants on the planet screamed out in horror at the same time, then suddenly went silent.
"perfect" is a malleable term for "flawless within the constraints and granularity of one's ability to measure". It may be perfect on the molecular level, and on the atomic, but ... some of thoze particalzorz haz a deviant spin!!
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Casting 8-meter-class blanks simply isn't that uncommon any more. The Large Binocular Telescope has a pair of 8.4-meter primaries; Subaru has an 8.3-meter; VLT has four 8.2-meter, Gemini North and South each have an 8.1-meter. Oh, and the Giant Magellan Telescope is planned to have seven 8.4-meter mirrors.
The LSST is unusual in that its light path is more "folded", hitting 3 mirror surfaces on the way to its primary camera, which means that relatively run-of-the-mill 8-meter-class blank has to be ground pretty uniquely. (And I wish them the best of luck with the process.)
Also, its secondary mirror is absofreakinglutely huge, at 5 meters. To put this in context, just ten years ago there was only one operational telescope in the whole world with a primary mirror larger than 5 meters.
And f/1.25 is crazy fast, yes. The newest, fastest survey scopes out there right now are VISTA at f/3.25 and Pan-STARRS PS1 at f/4. SDSS is f/5, and VLT is f/5.5.
So there you have it - what's really cool about LSST, from a guy who drives a boring old f/10 2.2-meter. ;)
There's a difference between a segmented or even sparse primary mirror and an interferometer.
A telescope with a segmented mirror works almost exactly the same way as a normal telescope, except its easier to manufacture mirrors. Of course, this is with the cost of making it harder to keep aligned, and introduce unnecessary complexity for a small mirror, but as the sizes grow it becomes more and more cost effective to segment.
A sparse mirror with a well designed layout (say a Golay array) will be very effective also as a traditional telescope. The array is designed to gather all the spatial frequencies (think of a telescope as an analog Fourier transform) with as few elements as possible. Thus, though gathering less light, it will create an image of the same resolution. Of course less light leads to lower SNRs which can be tricky and is why you don't see too many sparse systems right now.
An interferometer, while conceptually similar to a sparse aperture system, only measures a single frequency component at a time, by taking the light from two distant telescopes and interfering them to determine the "fringes" (Young's experiment) which measure how similar the light beams are. It is thus very precise, but also very limited. Given enough time and patience you could move the relative positions of the telescopes to fill out the Fourier transform, but this is usually not very practical given that alignments need to be maintened within 10s of nanometers.
It's easy to understand once you realize there are four simultaneous days in each rotation of the earth!
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The UT system isn't even the same idea- the main mirror can't even be moved in elevation and doesn't cover the entire sky- it only sees 70% of it. Hobby-Eberly is a spectroscope, designed to look at specific targets for a long time to get the spectrum of the target. LSST is a survey telescope- it's going to scan the visible sky every 3 days in multiple wavelengths, so you have to have an entirely different grade of mount, support structure and drive system. As any amateur astronomer will tell you, cheaping out on the mount will save you quite a few bucks. :^) (Although looking over the Hobby-Eberly, they did some really neat stuff with the mount to get it to track.)
Entirely different missions, different optics, different mounts, etc etc.
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Yes, it's the same. A shorter focal ratio ("fast") allows for a larger field of view with the same size CCD-chip. The "side-effect" of this is that a larger bit of sky falls onto the individual pixels, which means you sacrifice resolving power for sensitivity.
A telescope mirror needs a number of special properties, from rigidity and weight, but also thermal stability and the ability to polish it efficiently.
For nearly 50 years the largest mirror was the 5 meter Hale telescope, but in the late '80s materials science and casting techniques had evolved to the point where we could reliably cast larger, lighter telescope mirrors, and computing power to the point where active suspension of thinner mirrors is possible.
However, this doesn't mean we can create weightless mirrors, and an 8.4m mirror with a short focal length and two different surfaces still requires quite a bit of internal strength. Glass still has a higher density than water.
An 8.4m mirror has a surface area of 220 square meters, even assuming the density of water (1000 kg/m3) 25 tons corresponds to a thickness of only about 12 cm, or less than 5 inches, which is very very thin, and as I said, glass has a density higher than water, so the actual thickness would be substantially less.
this is a fast wide-field telescope, it's designed to be wide-angled and low magnification, most other scopes are narrow-field and high magnification. This one will take pictures of the whole observable sky over and over so changes over time can be easily seen, hell they could even make time-lapse movies!
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hmm, I think you did a d^2 instead of an r^2 in your area calculations. The actual area is 55 m^2. With glass density around 2.5 g/cm^3 you get about 20 cm thickness. What's an astonishing aspect ratio, about the shape of a saucer.
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