Optics for Photo-Visual Alt-Az Telescopes Dallas Workshop Dave Rowe October 27, 2007

Requirements for Photo-visual Alt-Az Telescopes Convenient access to instruments and eyepiece –Newtonian focus or tertiary Cassegrain focus Excellent images over large, flat field –should support 37 mm X 37 mm CCDs –400 nm to 850 nm Adequate back focus for imager, filters, OAG and deviator –> 90 mm BFL Affordable Compact and lightweight Ghost free

Selected Optical Systems Hyperbolic Newtonian –excellent wide-field optical performance –Convenient Newtonian focus –Requires overcorrected primary Wynne-corrector + parabolic mirror –An acceptable alternative to the HN in some situations –Lack of back focus can be a serious limitation –Corrector is large, expensive and difficult to make Tertiary-focus Corrected Dall-Kirkham (CDK) for larger telescopes –Optics are very easy to make relative to the RC Cassegrain –Easier to collimate –Excellent wide-field performance

Two-element and Three-element HN Systems Two-element corrected systems are acceptable for fields up to 40 mm in diameter Three-element corrected systems can have excellent performance over much wider fields

HN2 Corrector Prescription and Layout

HN2 Image Characteristics 2-element corrector, 625 mm f/3.5 primary, f/3.4 EFR

HN3 Corrector Layout 95 mm C.A. D-K9LH-ZK21H-QK3 BFL = 100 mm

HN3 Prescription 625 mm f/3.5 primary, 30% overcorrected Glass from GCDM Glass Co., Ltd. Schott or Ohara glass works equally well after re-optimization flat field generous back focus

HN3 Image Characteristics Scale = 0.5” / sq. = 5 microns / sq. 625 mm f/3.5 primary, SC= element corrector Inexpensive glass relatively insensitive to fabrication errors EFL = f/3.25 BFL = 100 mm <10 micron RMS diameter over 65 mm field extremely well corrected from 400 nm to 850 nm ghost free <3% above background for 3 rd mag star

HN3 Host Analysis Image Diameter = 8 mm Type 1 Ghosts Reflection from CCD and One Glass-Air Surface

HN3 Host Analysis Type 2 Ghosts Two Reflections from Glass-Air Surfaces

HN3 Optical Layout

HN3 Upper End Concept

HN3 Upper End Concept (cont.)

HN3 OTA Concept

One Meter Corrected Dall-Kirkham

Telescope Layout D. Rowe

Corrector Layout D. Rowe

Optical Design Primary –Diameter = 1 meter –f/3.3 –Conic deformation = -0.7 Secondary –Diameter = 350 mm –Spherical –ROC = 5200 mm Tertiary –Minor Axis = 160 mm Corrector –Two-element all spherical –BK7 glass –80 mm clear aperture D. Rowe

Characteristics Effective focal length = 5930 mm (f/5.9) Fully baffled over 55-mm image circle Flat field Geometric RMS spot diameter < 10 microns over full field Extremely well corrected from 375 nm to 1000 nm Eyepiece height less than 1.8 meters (70 inches) Back focus > 200 mm Spherical secondary is easy to collimate Much less expensive than other Cass alternatives D. Rowe

CDK Optical Performance D. Rowe

OTA Concept D. Rowe

Telescope Conceptual Sketch D. Rowe

An Alternate Approach

Lightweight Mirrors Tong Liu of Hubble Optics holding a prototype 500 mm lightweight fused sandwich mirror blank. Joe Haberman of PlaneWave Instruments holding a Wangsness Optics 1070 mm lightweight fused cellular mirror blank.

FEA Optimized Design Courtesy Hubble Optics

Highly Scalable Design Thermal and structural optimized open core Rapid (practically constant!) thermal response time regardless of the mirror size Completely scalable to any reasonable size Unprecedented price/performance Courtesy Hubble Optics

Wangsness Optics Peter Wangsness and 48” LIDAR Blank Cellular construction Fused Pyrex ~ 40% of solid blank

Wangsness 1070 mm blank at El Camino CC