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?