"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.'"

Comments

Re:Apparently...

[Anonymous Coward]

Apparently it was so awesome, they just skipped the secondary mirror and went straight to tertiary. :)

The optical design is somewhat unusual as it has
three mirrors, but this is required to get a very
large field of view (with diameter equal to 7 full Moons).
The secondary mirror will be made separately.
If you are interested in more details
about LSST, please take a look at our website,
http://www.lsst.org,
and a review paper
http://lanl.arxiv.org/abs/0805.2366

      Cheers,
      Zeljko

Re:No such thing as "perfect"...

[hey!]

In optics, you get to the point where further "perfection" doesn't give you any pratical benefit. That is being "diffraction limited". Diffraction limited optics are for practical purposes as "perfect" as you can get.

For a telescope operating through the Earth's atmosphere, you run out of marginal advantage before you reach diffraction limitation. Therefore for such a system, unless special techniques such as adaptive optics are used, practical "perfection" is considerably lower.

I don't know much about the LSST, except that it is a fast (short focal length relative to aperture) optical system. Such systems are much more difficult to get right. Long focal lengths are much more forgiving. Therefore to reach practical perfection in such an aggressive design is quite an achievement. Of course, we aren't there yet. There's three absolutely huge surfaces to grind to very price specifications. But simply casting a blank this size is a huge technical challenge. The amount of heat energy in twenty six tons of molten glass is mind-boggling. Getting it cast into a shape that can be ground and polished into an optical mirror is an engineering tour de force in itself.

Mirror and Camera

[stewardwildcat]

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.

Re:why not an array?

[edremy]

LSST isn't interesting because of the mirror diameter, it's interesting because of its incredibly wide field of view and amazingly fast optics. This thing has a field of view of almost 10 square degrees and can image down to ~24th magnitude every 15 seconds. Nothing else built or planned even comes close. PAN-STARRS4 will be the nearest thing to LSST and it has an etendue* that's something like 1/6th of LSST, although the PAN-STARRS people like to point out it's also something like a 5th of the cost of the LSST. (* a measure of mirror diameter*field of view. Bigger is better for survey telescopes)

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.