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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

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Building a newtonian astrograph l.jpg

Building aNewtonian Astrograph

Curt Walker

Presented October 2, 2009

for the MARS Club


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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


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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


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Examples of currently mass-produced telescopes that are (or could be) considered astrographs:

Meade’s Schmidt Newtonians


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Examples of currently mass-produced telescopes that are (or could be) considered astrographs:

Vixen R200SS


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Examples of currently mass-produced telescopes that are (or could be) considered astrographs:

Takahashi Epsilon 160 & 180

Epsilon 160


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Examples of currently mass-produced telescopes that are (or could be) considered astrographs:

Borg’s Refractor Astrographic Systems


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Examples of currently mass-produced telescopes that are (or could be) considered astrographs:

TeleVue’s Nagler/Peztval APO Refractors


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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


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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.


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Design Tools

NEWT 2.5


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Design Tools

TurboCAD 14 Deluxe


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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


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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


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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


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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


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Design Highlights

Planning for a fully illuminated imager:

100%

1.194”


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Design Highlights

Planning for a fully illuminated imager:

QHY8

100%

APS-C

(DSLR)

SBIG

ST-2000XM

Orion

StarShoot


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Materials & Components

  • Telescope Tube – Hastings Aluminum Pipe

  • 10” OD, 0.064” wall

  • Both ends ‘rolled’

  • Heavy but relatively cheap and available

  • Variety of finishes


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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


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Materials & Components

  • Focuser – Moonlight Crayford

  • 3-point compression ring

  • Hi-Res Stepper Motor

  • Cercis Controller

  • Supports 2 focusers

  • Supports 2 temp probes and profiles


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Materials & Components

  • Primary – Royce Conical

  • 8” F/5

  • 3.5 lbs

  • Very secure mounting (no epoxy necessary)

  • Cools quickly


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Fabrication of Custom Parts


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Fabrication of Custom Parts


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Fabrication of Custom Parts


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Assembly


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Assembly


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Assembly


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Assembly


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First Light & Focus Test


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Assembly


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First Photographic Results

M51 – Whirlpool Galaxy

3x300


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First Photographic Results

NGC5139 – Omega Centauri

13x120


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First Photographic Results

M8 – Lagoon Nebula

12x300


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Finishing Touches


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Finishing Touches


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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


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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


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Thank You

Questions?


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