Building a

Newtonian Astrograph

Curt Walker

Presented October 2, 2009

for the MARS Club

Topics Covered

• What is an astrograph?

• Building vs. Buying

• Design Tools

• Design Highlights

• Materials & Components

• Fabricating Custom Parts

• Assembly

• First Light & Focus Test

• First Photographic Results

• 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:

• 8” aperture

• 1000mm focal length

• Classic newtonian

• Fine focusing control

• Imaging first, visual second

• Optical accessories

(barlow or coma correction)

• Minimal vignetting

Manifested in specs:

• Thin or lightweight parabolic

primary

• Motorized stepper focuser

• Spacings for camera focus

(extenders may be required for

visual)

• Spacings with room for optical

accessories

• Fully illuminated imaging plane

Design Highlights

Planning for a fully illuminated imager:

Source – NEWT 2.5 Help Files

• 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.

Design Highlights

Planning for a fully illuminated imager:

Source – NEWT 2.5 Help Files

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.

Design Highlights

Planning for a fully illuminated imager:

Source – NEWT 2.5 Help Files

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.

Design Highlights

Planning for a fully illuminated imager:

100%

1.194”

Design Highlights

Planning for a fully illuminated imager:

QHY8

APS-C

(DSLR)

SBIG

ST-2000XM

Orion

StarShoot

100%

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?