optics for photo-visual alt-az telescopes dallas workshop dave rowe october 27, 2007
TRANSCRIPT
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
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
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
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.
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