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 commerciallymade 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
• Classic newtonian
• Motorized stepper focuser
• Imaging first, visual second
• Spacings for camera focus
(extenders may be required for
visual)
• Optical accessories
(barlow or coma correction)
• Spacings with room for optical
accessories
• Minimal vignetting
• Fully illuminated imaging plane
• Fine focusing control
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
- Straight 4-vane
- Built-in secondary offset
- Internal dew heater
Secondary Mirror – Protostar
- 2.60” minor axis
- ULS Quartz
- 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?