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Building a Newtonian Astrograph

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Building a Newtonian Astrograph Building a Newtonian Astrograph Curt Walker Presented October 2, 2009 for the MARS Club Topics Covered • What is an astrograph? • Fabricating Custom Parts • Building vs. Buying • Assembly • Design Tools • First Light & Focus Test • Design Highlights • First Photographic Results • Materials & Components • Finishing Touches What is an ‘astrograph’ ? An astrograph (astrographic camera) is a telescope designed for the sole purpose of astrophotography. Familiar forms of astrographs: - Schmidt Camera (different than Schmidt Newt) - Short FL, low F-ratio APO refractors - Ritchey-Chretien Examples of currently mass-produced telescopes that are (or could be) considered astrographs: Meade’s Schmidt Newtonians Examples of currently mass-produced telescopes that are (or could be) considered astrographs: Vixen R200SS Examples of currently mass-produced telescopes that are (or could be) considered astrographs: Takahashi Epsilon 160 & 180 Epsilon 160 Examples of currently mass-produced telescopes that are (or could be) considered astrographs: Borg’s Refractor Astrographic Systems Examples of currently mass-produced telescopes that are (or could be) considered astrographs: TeleVue’s Nagler/Peztval APO Refractors Examples of currently mass-produced telescopes that are (or could be) considered astrographs: Various Ritchey-Chretien OTAs RC Optical Systems 16" f/8.4 CF OTA RC Optical Systems 20" f/8.1 Carbon Truss Buying vs. Rolling Your Own Reasons that influenced my decision to build: - Conversations with other ATMs - Luxury of choosing each component to suit needs/wants - Quality assurance (…ha ha) - No perfect matches with my needs/wants among commercially- made astrographs (all require upgrades) - My compulsive need to build stuff. Design Tools NEWT 2.5 Design Tools TurboCAD 14 Deluxe Design Highlights ….a telescope designed for the sole purpose of astrophotography. Needs & Wants: Manifested in specs: • 8” aperture • Thin or lightweight parabolic primary • 1000mm focal length • Motorized stepper focuser • Classic newtonian • Spacings for camera focus • Fine focusing control (extenders may be required for • Imaging first, visual second visual) • Optical accessories • Spacings with room for optical (barlow or coma correction) accessories • Minimal vignetting • Fully illuminated imaging plane Design Highlights Planning for a fully illuminated imager: • The focal plane is generally fully illuminated in the center, and gradually tapers off in brightness toward the edge. • The 100% zone will have all of the brightness available from the primary mirror. • The 75% zone is the area at the focal plane which is 3/4 illuminated by the primary mirror. Source – NEWT 2.5 Help Files Design Highlights Planning for a fully illuminated imager: Differing design principals for visual vs. imaging: Visual: - The larger the eyepiece field lens, the larger areas of 100% and 75% illumination required. - However, To see fine details in planetary images and faint nebulae alike, you need the maximum contrast possible. - In a newtonian telescope, one of the biggest contrast killers is an oversized diagonal mirror. - If possible, the diagonal minor axis should be kept under 20% of the diameter of the primary mirror. Source – NEWT 2.5 Help Files Design Highlights Planning for a fully illuminated imager: Differing design principals for visual vs. imaging: Imaging: - To attain the brightest image possible and help avoid vignetting, the imaging plane should be as fully illuminated as possible. - This requires substantially larger secondary mirrors than in similarly sized scopes intended for visual observing. Typically, a telescope optimized for imaging will not perform well for visual observing. Source – NEWT 2.5 Help Files Design Highlights Planning for a fully illuminated imager: 100% 1.194” Design Highlights Planning for a fully illuminated imager: QHY8 100% APS-C (DSLR) SBIG ST-2000XM Orion StarShoot Materials & Components Telescope Tube – Hastings Aluminum Pipe - 10” OD, 0.064” wall - Both ends ‘rolled’ - Heavy but relatively cheap and available - Variety of finishes Materials & Components Spider Vane – Protostar Secondary Mirror – Protostar - Straight 4-vane - 2.60” minor axis - Built-in secondary offset - ULS Quartz - Internal dew heater - Interferogram says 1/14th Materials & Components Focuser – Moonlight Crayford - 3-point compression ring - Hi-Res Stepper Motor - Cercis Controller - Supports 2 focusers - Supports 2 temp probes and profiles Materials & Components Primary – Royce Conical - 8” F/5 - 3.5 lbs - Very secure mounting (no epoxy necessary) - Cools quickly Fabrication of Custom Parts Fabrication of Custom Parts Fabrication of Custom Parts Assembly Assembly Assembly Assembly First Light & Focus Test Assembly First Photographic Results M51 – Whirlpool Galaxy 3x300 First Photographic Results NGC5139 – Omega Centauri 13x120 First Photographic Results M8 – Lagoon Nebula 12x300 Finishing Touches Finishing Touches What’s Next? Currently saving up for a complete imaging rig: - Mount…….GM11? Mach1GTO??? - Switch to a self-guided monochrome imager (i.e. SBIG ST-2000XM or similar) - Transition to MaximDL or CCDSoft for capture and processing References & Education • The Dobsonian Handbook: A Practical Manual for Building Large Aperture Telescopes By Kriege & Berry • TurboCAD 14 Deluxe Help Files & online user forum • NEWT 2.5 Help Files • Bryan Greer of Protostar • Bob Royce of R. F. Royce Precision Optical Components • Daniel Mounsey of Woodland Hills Telescopes • Fellow ATMs across the globe via Cloudy Nights ATM Forum Thank You Questions? .
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