Telescope Design The W.M. Keck (I & II) Telescopes

1. Telescope DesignThe W.M. Keck (I & II) Telescopes Jana Hunt & Kent Van ME250 Precision Machine Design April 8, 2003

2. Telescope Design • Introduction/Purpose of Keck Telescope • Precision Engineering Applications • Keck Telescope Design • Dome and Building Design • Conclusion

3. 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

4. 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 mirror Primary Mirror configuration Light path

5. 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

6. Hexagonal Mirror Segments Passive 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

7. Wiffle Tree Support 12-pt Wiffle Tree (x3) Diaphragm & Radial Support Post

8. Segment Fabrication Stressed 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

9. Mirror Sensors & Actuators Controls 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

10. Dome Design To 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

11. 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

12. ?’s

13. 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

14. Appendix • Mirror profile • Improvement from use of active optics • Interferometer plans

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

16. 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

17. D 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