Telescope DesignThe W.M. Keck (I & II) Telescopes

Jana Hunt & Kent Van

ME250 Precision Machine Design

April 8, 2003

Telescope Design

• Introduction/Purpose of Keck Telescope

• Precision Engineering Applications

• Keck Telescope Design

• Dome and Building Design

• Conclusion

W.M. Keck Telescope• Type: Optical reflecting, Schmidt-Cassegrain design

– Uses the primary hyperboloid mirror to focus incoming light onto the convex circular secondary mirror which sends the light back through a hole in the primary mirror to the eyepiece, located at the rear of the telescope…tertiary mirror...

• Sits on summit of Mauna Kea, Hawaii• Purpose: To gather light to help astrophysicists observe the universe.• Various instruments are attached to analyze gathered light• Material Zerodur• Mount: Altazimuth• Overall height: 24.6 meters

World’s Largest Reflecting Telescope• Single piece large mirror would be extremely difficult to make & maintain

• Deflection is function of diameter (d) cubed

• Array of 36 hexagonal mirrors

– 1.8m across, 0.075 m thick

– Individually mounted and adjusted

– reduce deformation & mass of mirrorEI

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Primary Mirror configuration

Light path

Why is Precision Important for Keck Telescopes?

Produce high quality image & reduce image blur– Collimation - Alignment of optics

– Serrurier truss design- sets of vees gives parallel motion of upper and lower tube - no miscollimation due to secondary tilt

– Adjust relative stiffness of the upper and lower tubes to maintain adequate focus in direction parallel and perpendicular to optical axis

Hexagonal Mirror SegmentsPassive Support System

– Axial Support (piston - x y z)• Three 12-point wiffle trees w/flex rods epoxied to back of segment

– Radial Support (‘tilt’-x, spin-y, azimuthal-z)• Radial Support Post• .25m-diameter Flexible Diaphragm - permit small amount of tilt and piston

motion required by control system

Diaphragm & Radial Support Post

12-pt Wiffle Tree (x3)

Wiffle Tree Support

Segment FabricationStressed Mirror Polishing

• Forces and moments applied to Zerodur blank to form desired non-axisymmetric shape

• Sphere is ground and polished into blank, forces are removed

• Polished surface deforms elastically into desired shape (provided no hysteresis)

• Segment cut into hexagonal shape

• Warping harnesses used to correct surface shape (reduced surface error to 90nm rms)

• Ion-Figuring- ionized argon beam removed glass molecules from surface (15nm rms!)

Surface Profile of Segment after Warping Harnesses are used

Mirror Sensors & ActuatorsControls compensates for small errors

• Requires active control system to maintain segments in proper alignment• Adjustments made 2x per second• Redundancy – 168 sensors, 108 actuators• Precision linear actuators

Dome DesignTo protect telescope and help

maintain precision

– Important in maintaining temperature stability

– Height, Width: 30.8 x 37 meters – Moving weight: 635 tons– Total air-replacement: 5 minutes– Geographic stability and vibration

isolation pg9-2

W.M. Keck Telescope

• Conclusion (Kent)– Review precision applications adapted on a larger

scale– Summarize how/why precision engineering was

used for Keck telescope design– 1080 segment (CELT) telescope being planned

?’s

References• W. M. Keck Observatory. [Online] Available: http://www.astro.caltech.edu/mirror/keck/

• Keck Telescope's adaptive optics let astronomers study volcanic activity on Io from armchair on Earth. [Online] Available: http://www.berkeley.edu/news/media/releases/2002/06/03_keck.html

• The Keck Telescope Space Craft SCIENCE Kit. [Online] Available: http://scikits.com/Keck.html• The Keck Telescope. [Online] Available: //www.ngst.nasa.gov/science/meetings/Keck.html• An Introduction to Interferometry. [Online] Available:

http://www.mtwilson.edu/Education/Presentations/Interferometry/• Kodak Supplies Optical Quality Mirrors to the W.M. Keck Observatory. [Online] Available:

http://www.kodak.com/US/en/government/ias/optics/ion.shtml• Nelson, Mast, and Faber. The Design of the Keck Observatory and Telescope (Ten Meter

Telescope), Keck Observator Report No. 90, January 1985• “Advanced Technology Optical Telescopes IV,” SPIE Volume 1236, 1990• Terry Mast and Jerry Nelson, Warping Harnesses for CELT. CELT Technical Note No. 6. [Online]

Available: celt.ucolick.org/reports/technote01_6.doc, February 2002• Terry Mast and Jerry Nelson, and Gary E. Sommargren, Primary Mirror Segment Fabrication for

CELT. [Online] Available: http://celt.ucolick.org/reports/report00_5.pdf

Appendix

• Mirror profile

• Improvement from use of active optics

• Interferometer plans

Figure 1: Decenter and tilt of secondary mirror and its effects on image quality and image displacement (blur).

Mirror Alignment• Active controls of mounts

required due to Earth’s motion– Adjustments made 2x per

second

– Adjustments to 4nm possible

• Adaptive optics installed on Keck II– 670 adjustments per second

– 10x focus improvement

Interferometer• Future plans:

– Multiple telescopes become a massive interferometer

– Light combined at single point in tunnel

– Resolution angle improved as function of 1/D:

• Θ=λ/D

D