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=-1.30• 3-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 3rd mag star

HN3 Host Analysis

Image Diameter = 8

mm

Type 1 GhostsReflection from CCD and One Glass-Air Surface

HN3 Host Analysis

Type 2 GhostsTwo 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 9-21-07

Corrector Layout

D. Rowe 9-21-07

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 9-21-07

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 9-21-07

CDK Optical Performance

D. Rowe 9-21-07

OTA Concept

D. Rowe 9-21-07

Telescope Conceptual Sketch

D. Rowe 9-21-07

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