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Science Forum

  1. Standard member sonhouse
    Fast and Curious
    26 May '17 21:32
    https://phys.org/news/2017-05-secondary-mirror-elt-successfully-castlargest.html#nRlv

    That is just the SECONDARY! The main reflector is almost 40 METERS across. Maybe 20 times the resolution of Hubble.

    Starts running in 2024.
  2. 27 May '17 05:50
    Originally posted by sonhouse
    https://phys.org/news/2017-05-secondary-mirror-elt-successfully-castlargest.html#nRlv

    That is just the SECONDARY! The main reflector is almost 40 METERS across. Maybe 20 times the resolution of Hubble.

    Starts running in 2024.
    A big problem with large mirrors is its weight. The gravitation will deform them so they lose its shape. The precision here is only 15 nanometres. How is this done...?
  3. Standard member sonhouse
    Fast and Curious
    28 May '17 15:47
    Originally posted by FabianFnas
    A big problem with large mirrors is its weight. The gravitation will deform them so they lose its shape. The precision here is only 15 nanometres. How is this done...?
    Only 15 nm? That is 150 Angsroms, so accuracy is within about 50 atoms.

    I assume it will have an adjustable mount so stress can be minimized.

    Since it is a mirror and not a lens it can also be supported (upside down but supported by a structure)
  4. 28 May '17 17:44 / 1 edit
    Originally posted by sonhouse
    Only 15 nm? That is 150 Angsroms, so accuracy is within about 50 atoms.

    I assume it will have an adjustable mount so stress can be minimized.

    Since it is a mirror and not a lens it can also be supported (upside down but supported by a structure)
    40 meter in diameter, 3.5 tonnes! Hanging upside down! 15 nm in precision everywhere! Anti-gravitational support from above!
    Oh, I want to know more!
  5. 28 May '17 17:54
    Originally posted by FabianFnas
    40 meter in diameter, 3.5 tonnes! Hanging upside down! 15 nm in precision everywhere! Anti-gravitational support from above!
    Oh, I want to know more!
    The main mirror (40m) will not hang upside down.
    as for the hanging one, if it is thick enough gravity will not distort it significantly.
    In some cases where it is necessary to have an extra thin mirror, they design them out of shape so that when they are mounted they move into shape.

    I believe many large telescope actually have actuators to subtly distort the mirror to correct for atmospheric effects, but I am not sure if that is done on the main mirror or later on.
  6. 28 May '17 17:56
    http://www2.lbl.gov/Science-Articles/Archive/keck-telescope.html
  7. Standard member sonhouse
    Fast and Curious
    28 May '17 19:24
    Originally posted by twhitehead
    The main mirror (40m) will not hang upside down.
    as for the hanging one, if it is thick enough gravity will not distort it significantly.
    In some cases where it is necessary to have an extra thin mirror, they design them out of shape so that when they are mounted they move into shape.

    I believe many large telescope actually have actuators to subtly d ...[text shortened]... rrect for atmospheric effects, but I am not sure if that is done on the main mirror or later on.
    Anyway, the main mirror will have adoptive optics so it will actively correct for bad curvature AND respond to the laser for maximum focus on stars. It should be 20 times the res power of Hubble, but time will tell.
  8. 28 May '17 19:31
    Originally posted by sonhouse
    Anyway, the main mirror will have adoptive optics so it will actively correct for bad curvature AND respond to the laser for maximum focus on stars. It should be 20 times the res power of Hubble, but time will tell.
    Not sure why you compare it to Hubble. Hubble is special for being outside the atmosphere not for having a large mirror (its not all that large).
  9. Standard member sonhouse
    Fast and Curious
    29 May '17 13:06 / 2 edits
    Originally posted by twhitehead
    Not sure why you compare it to Hubble. Hubble is special for being outside the atmosphere not for having a large mirror (its not all that large).
    I compare it so I can do an estimate of the res of the larger scopes. For instance, Keck II uses adoptive optics and has some very high res images of the center of the galaxy letting them chart the course of stars around the central black hole, they have been doing that for 20 years and are looking for changes in orbits that would indicate holes in Relativity.

    It uses a 10 meter dish, one fourth the size of this baby.

    All things being equal, Hubble has about a 2 meter dish and clocks in at 0,05 arc seconds or so of res.

    So I would expect Keck to have about 5 times that or 0.01 arc seconds and the new one at about 40 meters should clock in at 2.5 MILLIarcseconds of res. That will make for even sharper images of the central black hole region for just that one example.

    But no matter what size Earth bound scopes are, 100 meter, whatever we come up with in the next 50 or 100 years, Hubble still has one advantage no Earth bound scope has, the ability to see IR and UV much better than anything that has to point through the atmosphere.

    Of course the Webb scope will supersede Hubble in the IR band when it gets launched and if it works as planned. If it fails there is no quickie fix like Hubble getting all those repair runs via the Space Shuttle. And even now there is no quick fix to Hubble if it fails since we chucked the Shuttle.