Modern Telescopes AS4100 Astrofisika Pengamatan Prodi Astronomi 2007/2008 B. Dermawan

To Dreamers, Then, Now, and Always Majewski

Trends in Modern Telescope Development Light gathering power Quality & sensitivity of the instruments  efficiency Resolution (image quality & sensitivity)  efficiency Majewski Bely

Trends in Modern Telescope Development Instrument Sensitivity - Improvement in optical design, optics, detectors - New inventions (fiver optics, holographic gratings) - Finding appropriate sites Resolution - Quick & automatic guiding - Improved understanding of the effects of seeing - Locating observing sites for better seeing - Improvement in mirror & dome design - Active mirror figure & atmospheric compensation - Space-based platforms - Interferometry Majewski Bely

Large Mirrors: Shapes, Materials & Types Telescope f-ratio Majewski Fast primaries: cost driver, smaller tube  stiffer, smaller wind cross-section, secondary mirror smaller Bely Difficult decisions: type & construction of primary mirror, rest of optical train, tel. Structure/size/control, instrument optical design, size of dome, potentially the site of the tel., overall cost

Large Mirrors: Shapes, Materials & Types Mirror substrate Majewski Considerations: Available in large quantities, castable in large size Long term stability of shape Polishable & be able to be coated Rigidity: sustain cooling, withstand constant motion, not deformable, segmented mirrors are low-rigidity such as space telescopes Good thermal behavior: homogeneous mirror  focal length, low Coefficient of Thermal Expansion (CTE), high thermal conductivity & low density & spec. heat, for active shape mirror  high thermal diffusivity

Large Mirrors: Shapes, Materials & Types Mirror substrate Majewski Bely

Large Mirrors: Shapes, Materials & Types Lightweighting mirror Majewski Lighter mirrors without sacrificing rigidity - Compression on the top surface and tension on the bottom - Middle section can be removed  honeycomb Bely

Large Mirrors: Shapes, Materials & Types Lightweighting mirror Majewski HST LBT, abell.as.arizona.edu/~hill/mirror Bely

Large Mirrors: Shapes, Materials & Types Segmented mirrors Majewski Size limited of monolithic mirrors: fabrication facilities, difficult to achieve substrate homogeneity, handling & transport difficulties, max. 4-m sized space telescope Advantages of segmented mirrors: lower mass, shorter thermal time constants compared to monolithic mirror of same size, ability to "change out" segments, unlimited aperture sizes

Large Mirrors: Shapes, Materials & Types Segmented mirrors Majewski However: all segments must be figured to be parts of one parent shape (“off- axis" paraboloidal segments tricky and expensive), all segments must be kept precisely and actively aligned despite changing gravity, thermal effects, wind, etc. Segmentation geometry: “petals" / "keystone" -- radial/azimuthal segments, hexagons (put down in rings) Bely

Large Mirrors: Shapes, Materials & Types Segmented mirrors Majewski 1.8-m Guido Horn-d'Arturo 6-mirror (~4.5-m) to a single mirror 6.5-m MMT

Large Mirrors: Shapes, Materials & Types Segmented mirrors Majewski Hobby-Eberly Telescope (HET) South Africa Large Telescope (SALT)

Large Mirrors: Shapes, Materials & Types Segmented mirrors Majewski Keck I & II

Large Mirrors: Shapes, Materials & Types Segmented mirrors Majewski Large Binocular Telescope (LBT)

Large Mirrors: Shapes, Materials & Types Tubes, Trusses, & Baffling Majewski Heavy, under gravity  decollimate the optics, presents a large wind cross-section, prevents air from flowing across & cooling mirror Serruir Truss: open structure based on isosceles triangles on a square base When vertical triangles deflect, the parallelogram of horizontal triangles constrains the tube ends to move in a parallel plane  mass inefficiency & use of active optics Multi-bay structure HST Keck Gemini Bely

Large Mirrors: Shapes, Materials & Types Tubes, Trusses, & Baffling Majewski Preventing scattered light  baffling Generally conical or cylindrical tubes enclosing parts of the beam Often include perpendicular vanes to force radiation to make multiple scatters Bely Demands for wide field imaging are more severe

Large Mirrors: Shapes, Materials & Types Tubes, Trusses, & Baffling Majewski Reflections off of the primary or secondary: scattering of off-axis rays off of dust Critically important for space telescope HST: numerous vanes and both secondary and primary conical baffles (all black) Bely

Large Mirrors: Shapes, Materials & Types Mounts Majewski Before 1980 nearly all telescopes were mounted with an equatorial mount: counteract Earth rotation by motion only on one, polar axis, simple correction with single speed, no field rotation in focal plane Now most telescopes are built with altitude-azimuth (alt- az) mounts: neither axis changes direction with respect to gravity, structurally sturdier than equatorial, less massive, less expensive BUT: three axes of rotation needed: altitude (h), azimuth (A), and field rotation, all three axes move with variable speed, could only do this with fast computers Bely

Large Mirrors: Shapes, Materials & Types The biggest existing telescopes Majewski IFA, Univ of Hawaii

Large Mirrors: Shapes, Materials & Types The biggest existing telescopes Majewski Bely

Large Mirrors: Shapes, Materials & Types Some Proposed/Planned Large, Ground-based Telescopes Majewski Large Synoptic Survey Telescope (LSST) Proposed 8.4-m telescope with enormous 10 square degree field 3 billion pixel camera Will cover the entire sky with 10 second integrations every three nights Find fast moving or variable objects Build up a deep survey image of the sky in multiple wavelengths

Large Mirrors: Shapes, Materials & Types Some Proposed/Planned Large, Ground-based Telescopes Majewski Giant Magellan Telescope Seven 8.4-m Arizona Mirror Lab borosilicate honeycomb mirrors Light gathering power equivalent to a 21.4-m filled aperture Diffraction limited resolution equivalent to a 24.5-m filled aperture f/8.4 Gregorian with adaptive optics secondary Chile Partners: Carnegie Observatories, Harvard, MIT, SAO, Texas A&M, Arizona, Michigan, Texas

Large Mirrors: Shapes, Materials & Types Some Proposed/Planned Large, Ground-based Telescopes Majewski Thirty Meter Telescope (TMT) project A joining of several separate efforts: California Extremely Large Telescope (CELT) -- Caltech/UC Giant Segmented Mirror Telescope (GSMT) -- AURA Very Large Optical Telescope (VLOT) -- Canada

Large Mirrors: Shapes, Materials & Types Scientific productivity of telescopes Majewski Bely Suggests that scientific productivity scales by collecting area But cost is roughly proportional to diameter 2 or diameter 3, so the cost- effectiveness of a ground-based telescope is roughly independent of size, or maybe even somewhat favoring smaller apertures Of course, there is some science that simply demands the largest telescopes

Spherical & Aspherical Surfaces Spherical surfaces are so much simpler to build Optical Society of America Majewski

Spherical & Aspherical Surfaces Aspherical surfaces are typically very hard to make and to measure Computer-Controlled Optical Surfacing A computer controls a non-rotating but orbiting grinding tool The computer controls 6 motions: three positional, two tilt and one orientation (which is a fixed angle to the optical axis) The lap actually wears itself to the correct shape as it moves over different radial zones Generally limited size of laps to work well -- takes long time and can create high spatial frequency irregularities if not controlled very well Majewski

Spherical & Aspherical Surfaces Membrane and strip polishing Majewski Wilson

Spherical & Aspherical Surfaces Rotating furnace Majewski TECH.php?navi=cast /furnace.html

Spherical & Aspherical Surfaces Stressed-lap polishing Majewski

Spherical & Aspherical Surfaces Stressed-workpiece polishing Majewski Deform the workpiece, e.g., against another surface like a steel plate (e.g., with a vaccum) Grind the opposite surface as a flat or a sphere Release the stresses Used, e.g., in creating the Schmidt corrector plates for the Meade telescopes Wilson

Spherical & Aspherical Surfaces Ion beam figuring Majewski Sputters material from the workpiece at the atomic level, by momentum of a directed ion beam bombarding the surface Wilson

Spherical & Aspherical Surfaces Liquid mirror telescopes (LMTs) Majewski Newton originally proposed using a rotating liquid (e.g., mercury) itself as a perfect paraboloid, but first done for 35 cm telescope in 1872 Revived in last few decades (primarily by Canadian collaborations) as technical problems overcome Primary limitation is that they can only look at zenith: limits science to survey type projects, with drift-scan CCD imaging articles/sciam/1299musserbox6.html

Spherical & Aspherical Surfaces Primary technical challenge is suppression of ripples on surface from: wind, vibrations, misalignment of rotational axis Air bearings are one modern solution to smooth, accurate rotation Primary practical problem is that mercury vapors and oxides are very toxic But a big advantage is cost: The Large Zenith Telescope, a 6-m LMT, is being built at a cost of about $500,000 (which is 1% the cost of a conventional telescope of similar aperture) Majewski articles/sciam/1299musserbox6.html