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By: Ken Diller, BAS memberSeptember 21, 2011

OutlinePurpose 3Newtonian Telescope Origination & Today 4Newtonian Telescope Pro’s / Con’s 7Buy or Make 8Telescope Size Considerations 9Newtonian Telescope Key Components & Layout 10Newtonian Telescope Design Example: 8 Inch f/5 12

Criteria 13Process & Calculations 14Telescope Design Software 17Mirror Making & Testing 20

Telescope Construction 37Telescope Construction 37Telescope Mounts 39Accessories 40Collimation 41Collimation 41Summary 42References 43 2

Purpose

The purpose of this presentation is to:

1. Introduce you to Newtonian Telescope characteristics.

2. Summarize optical design of a Newtonian Telescope.p g p

3. Summarize Mirror Making activities.

4 S i t ti f N t i T l4. Summarize construction of a Newtonian Telescope.

NOTE: Presentation concentrates on Newtonian Telescope “Optical Tube Assembly” (OTA) and Mirror Making.

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Newtonian Telescope Origination

In 1668 while attempting to improve telescope designs, Sir Isaac Newton made 1st Newtonian Telescope 1 3 Inch f/51st Newtonian Telescope, 1.3 Inch f/5.

In 1672, 2nd telescope presented to Royal Society of London.

His telescope mirror was made with a spherical metal mirror (tin & copper).

R li f 1st N t i T lReplica of 1st Newtonian Telescope

Kings College in Grantham, England. Attended by Sir Isaac Newton

Sir Isaac Newton

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What is a Newtonian Telescope?A “Cl i l” N t i T lA “Classical” Newtonian Telescope:

Is the simplest type of telescope.

M t l h b ilt t t l Most commonly home-built amateur telescope.

Uses a primary concave mirror to gather light, and a smaller flat secondary mirror which directs converging cone of light to side of tube for visual or photographic use.

5Basic Newtonian Telescope

Newtonian Telescopes of Today

• Hundreds of different Newtonian Telescope variations existsvariations exists.

• Most Newtonian Telescope designs use solid tube or truss tube.uss ube

Dobsonian in solid tube.16” Dobsonian truss tube.

Dobsonian truss tube.

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8” Equatorial mounted solid tube.4.5” Equatorial mounted.

Newtonian Telescope Pro’s / Con’sAd t d di d t f N t i T lAdvantages and disadvantages of Newtonian Telescopes:• Pro’s: Less expensive to make/buy than refractors of similar size

* M f h* Most aperture for the money. No Chromatic Aberrations: Color wavelengths focus at same

image plane. “Correct-Reading” image rather than mirror image

Coma

Correct-Reading image rather than mirror image.

• Con’s: f/6 or faster mirrors exhibit increased off-axis coma (dominant

aberration); fanning of stars away from optical axis Comaaberration); fanning of stars away from optical axis. Coma makes extended object edges slightly less distinct.

Requires regular collimation (alignment) for best performance. Open tube ends allow direct entry for dust and dirt on optics.

Effect of Coma on stars Off-Axisp y p

Mirror will likely require recoating every 10 - 20 years. Central Obstruction (secondary mirror) reduces contrast but

can be minimal.

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Convection effects: Air currents inside tube affect image. Diffraction Spikes from spider vanes, though can be pleasing. Diffraction Spikes from 4 Vanes

Buy or Make?Things to consider when deciding to buy or make a Newtonian:

• Do you have the time?

Depending on extent of your involvement, buying components may reduce your time to complete.

Grinding, polishing and figuring a primary mirror tremendously increases g, p g g g p y yeffort needed. Patience and persistence will be needed.

• Do you like to construct things?

Will need workspace tools basic construction ability and math skills Will need workspace, tools, basic construction ability and math skills.

• Costs:

May keep low cost with used parts, or making parts from raw material.

Or, you can make/buy a more elaborate system using more expensive custom parts.

May be cheaper to buy comparable telescope/parts when effort (time) is considered.

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Newtonian Telescope Size ConsiderationWh h i ki N t i T l b t t d id h tWhen choosing or making a Newtonian Telescope, best to decide what you want to observe the most.• Mirror size diameter and focal ratio (f/#) affects performance:

T Ob C f/# d Mi di tTo Observe Common f/# and Mirror diameter Planetary / Lunar Observation f/10 and higher, 3” dia. and up. Multi - Purpose f/6 to f/10, 4.5” dia. and up. Rich Field / Wide Views f/6 and lower, 3” to 8” dia.

Notes: Smaller f/# gives wider field of view.Mirror size and f/#’s vary from above list.

• Mirror Diameter: Larger size quality optics provide more light

gathering and higher resolution under goodi Al lifi b d i

Table-Top 4.5” scope

seeing. Also amplifies bad seeing. Larger telescopes with large mirrors can be

cumbersome. How much portability do youwant?

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For each incremental increase in f/#,telescope length increases by 1 mirror dia.

1-scope garage!

Newtonian Telescope Key ComponentsTube: Rigidly holds components and keeps mirrors in alignment. Made of metal, wood, PVC, carbon-fiber, or combinations. Typically Enclosed or Truss Tube type.

Primary Mirror: Has paraboloid curve, gathers incoming light and directs

to eyepiece and/or camera. Made of Pyrex Glass (has lower Coefficient of Thermal

Expansion), or Plate Glass (lower cost). Special reflective coating on surface (Aluminized).

Primary Mirror Cell Holder:Primary Mirror Cell Holder: Holds mirror and has collimation screws.

Secondary Mirror: Redirects light 45° from primary mirror out to eyepiece or Redirects light 45 from primary mirror out to eyepiece or

camera.Spider Vane & Secondary Mirror Holder: Holds secondary mirror and has collimation crews.

Focuser: Holds eyepiece and/or camera and provides focus

adjustment (common sizes: 1.25” and 2”). 10

Newtonian Telescope Component LayoutP i Mirror

Focuser & Eyepiece Primary Mirror

Primary Mirror Cell

Mirror Collimation Screws (3)

Secondary Mirror & Holder

Spider Vanes

Mirror Collimation Screws (3)

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Tube

Screws (3)

Newtonian Telescope Design Example:Newtonian Telescope Design Example:

8 Inch f/58 Inch f/5

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Telescope Design Example: Criteria

Desire Telescope designed to meet following goals:

Desire “Rich Field Telescope” to observe large star fields, galaxies & clusters, comets, etc., therefore “fast” f/5 ratio for wide field observing.

For Visual and Imaging use.g g Reasonable portability, lightweight. Image size: 0.8” at 100% Illumination.

Primary Mirror surface accuracy: Less than 1/4 λ (lambda: wavelength) from ideal paraboloid curve. Known as Rayleigh Criterion. (Surface must not depart 5.5 millionths of-an-inch from required shape)(Surface must not depart 5.5 millionths of an inch from required shape) Better to have 1/8 λ or better since reflected defects are doubled at

image plane.

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Newtonian Telescope Design: Process

General Design Process:

Choose mirror diameter and f/#.

Ch t b di f i / h i ht / t l Choose tube dia., focuser size / height / travel.

If making mirror, involves additional calculations & testing.

Calculate Secondary Mirror minor axis size Calculate Secondary Mirror minor axis size.

• Choose desired Image Size (typically < 0.5” for visual, > 0.5” photographic).

• Optimize for visual and/or photographic use (Image Size & Illumination).

• Keep size (obstruction) lower for planetary type scopes.

• Consider eyepieces to be used (i.e., Field of View, magnification, etc.).

Design / choose mechanical components & rigid construction around optical design.

• Draw layout (paper or computer) prior to cutting, drilling, etc.

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• Verify design.

Newtonian Telescope Calculations: Diagonal size

Calculations shown for 8 Inch f/5 design:• Perform by hand, and/or use software programs.

Diagonal Size: Depends on F.L., tube size, focuser height, and image size.• Minor Axis: a = (D – d) ℓ ÷ F.L. + d

= (7.9” – 0.8”) 7.85” ÷ 40.125” + 0.8”( 9 0 8 ) 85 0 5 0 8= 2.19” (use standard size 2.14” diagonal)

(Observe in formula that longer F.L. will reduce diagonal size)Parameters:Parameters:D = Mirror diameter 8.0” minus bevel = (7.9”)d = Image size fully illuminated (desire 0.8” at 100% illumination)ℓ = Focal plane to diagonal dist. (1/2 tube dia. + focuser ht + in-travel = 7.85”)ℓ Focal plane to diagonal dist. (1/2 tube dia. focuser ht in travel 7.85 )F.L. = Focal Length (40.125”)

• Offset: For large and/or fast f/# telescope calculate diagonal offset.Offset moves mirror away from focuser and towards primary mirror.= (D x a) ÷ 4 x F.L. = (7.9” x 2.14”) ÷ 4 x 40.125” = 0.1”

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Newtonian Telescope Design: OpticalGoal: Optimize diagonal size to meet desired image size & illumination.

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Newtonian Telescope Design SoftwareVarious optical design software applications available (free or purchased):

“Newtonian Telescope Design Planner” by Jim Fly: Free, online, interactive software allows operator numeric field entries, then it calculates dimensions. Link at: http://www.atmsite.org/contrib/Fly/scopecalc/designie5.html

See next 2 slides for example.

“Newt for the Web” by Dale Keller: Free, downloadable file for designing a e o e eb by a e e e ee, do oadab e e o des g g aNewtonian Telescope. Link at: http://stellafane.org/tm/newt-web/newt-web.html

Old Version (Newt25)

Others are available.17

(Newt25)

New Version

Newtonian Telescope Layout ParametersU i “N t i T l D i Pl ” f 8 I h f/5 E lUsing “Newtonian Telescope Design Planner” for 8 Inch f/5 Example:

• Useful program to determine and verify OTA dimensionsand verify OTA dimensions.

• Additional info included at link.

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Newtonian Telescope Layout ParametersU i “N t i T l D i Pl ” f 8 I h f/5 E l ( ti d)Using “Newtonian Telescope Design Planner” for 8 Inch f/5 Example (continued):

• Calculates illuminations based on design.• Brightness tapers off toward edge of view.

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Mirror Making

Mirror Kit from Willmann-Bell, Inc. Completed 8 Inch f/5 Mirror

Mirror Kit typically consists of: Purpose:Mirror Kit typically consists of: Purpose:• Mirror blanks (Pyrex glass) 1 for mirror, 1 for tool (you chose)• Silicon Carbide (sizes 80 to 500 grit) Rough & Fine Grinding• Aluminum Oxide (12 to 3 microns) Powder for Smoothing

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Aluminum Oxide (12 to 3 microns) Powder for Smoothing• Cerium Oxide Polishing & Figuring Compound• Pitch Lap for Polishing & Figuring

Mirror Making Outline

Primary Mirror Making Steps Summarized:

P f C l l ti K l f ti iti d i t ti Perform Calculations. Keep log of activities and mirror testing.

Chose parts to be “mirror” and “tool”. (some people use tile pieces for tool).

4 Stages to Grind & Polish a Mirror: 4 Stages to Grind & Polish a Mirror:

1. Grind Mirror to desired depth and spherical profile. Measure and verify.

2. Smooth Mirror. Measure and verify.

3. Polish Mirror. Measure and verify.

4. Figure mirror to paraboloid. For long F.L. telescopes (slower than f/10), can keep spherical since it will not depart ¼ λ from paraboloidcan keep spherical since it will not depart ¼ λ from paraboloid.

NOTE: Perform Foucault Test (Knife Edge) alternatively “figuring” and “testing” until at correct paraboloid. Verify within ¼ λ limits (or better).

Package mirror and send to Mirror Coating service to have surface Aluminized.

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Initial Mirror Making CalculationsS itt G l i t l l t h i l d th f i t

Description Formula Example (using 8” f/5)

F l L h (F L ) D f/# 8” 5 40” (1016 )

Sagitta: Goal is to calculate physical depth of mirror center.

Focal Length (F.L.) D x f/# 8” x 5 = 40” (1016 mm)f/# (F ratio) F.L./D 40“ / 8” = f/5Radius of Curvature (R) 2 x F.L. 2 x 40” = 80” (2032 mm)Depth of curve (s: Sagitta) r2/2R 42/(2 x 80”) = 0.10” (2.54 mm)

(“r” used for Sagitta will be radius of spherometer if used)

D = Mirror Diameterr = radius of mirrorr radius of mirror

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With an 8” mirror and desired f/5 curve, need to grind away glass to a depth of 0.1 inch at center. This gets you near desired F.L.

Mirror Grinding: Coarse & FineMirror Grinding Process: Goal is to grind glass to sphere at desired depth.

1. Choose glass to use as mirror and tool. (use best of 4 surfaces for mirror).

2. Chamfer mirror edge.

g g g p p

2. Chamfer mirror edge.3. Grind using various strokes, switch mirror on top/bottom to

reach “sagitta” depth. Typically mirror on top more often: Mix coarse Silicon Carbide grit (80 grit) with water and

Note:• “Mirror” on top g ( g )

grind until near sagitta depth. Continue grinding with finer grits (incrementally to 500 grit). Grind until no “pits” from previous size grits exist.

Between grit sizes always thoroughly clean mirror/tool

becomes concave.• “Tool” on bottom

becomes convex. Between grit sizes, always thoroughly clean mirror/tool

and workspace to prevent cross-contamination of previous size grit.

Adjust F.L.: Mirror on top to shorten, on bottom to l th Ch k itt ith t l t tlengthen. Check sagitta with template, gauge, etc.

23Note: Grinding with grit causes pieces of glass to be removed approx. same size as grit.

Grit size = Grains per Inch)

Mirror Smoothing

Mirror Smoothing Process: Goal is to smooth glass sufficiently prior to polishing.

1. Continue “smoothing”, alternatively using mirror on top/bottom:g , y g p

Mix Aluminum Oxide Lapping Powder (12 microns) with water and polish until no pits are seen from fine grinding. Continue polishing with finer Al. Ox. (incrementally from 12 to 3 microns).( y )

Between powder sizes, again, always thoroughly clean mirror/tool and workspace.

M F l L th (i fl ti t t ith b lb) d t Measure Focal Length (i.e., reflection test with sun or bulb) and correct as necessary (mirror on top to shorten F.L.).

Correct for surface errors (i.e., holes, hills, etc. by various corrective strokes).

After polishing with 3 micron Al. Ox., use magnifying glass and verify smooth surface. Thereafter during Polishing and Figuring, new method will be used to check focal length and surface profile.

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Mirror Polishing

Mirror Polishing Process: Goal is to polish glass to a highly reflective spherical surface.

1. From existing “tool”, make new polishing pads from “pitch”. Heat, pour into mold, cool and cut, add to tool, then press/form.

2. Continue “polishing”, alternatively using mirror on top/bottom to polish mirror:

Mix Cerium Oxide Polishing Compound with water and polish Mix Cerium Oxide Polishing Compound with water and polish.

• Removes glass at fractions of wavelength.

Prevent other compound contamination.

Mirror surface becomes increasingly reflective until surface "peaks" and "valleys" are less than wavelength of visible light.

Periodically check surface profile and F.L. with Knife Edge Test or RonchiGrating.

• Surface errors, i.e. holes, hills, hyperbola, zonal errors, etc.: Correct by various polishing techniques. (see multiple sources for techniques).

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When mirror is highly polished, continue with “Figuring”.

Mirror FiguringMi Fi i P G l i t h i f h i l t b l id fMirror Figuring Process: Goal is to change mirror from spherical to paraboloid surface:

Mirror “figuring” involves making corrective polishing strokes to reach a defined paraboloid:

Keep log. Various “Stroke” patterns used to correct mirror:

p

Various Stroke patterns used to correct mirror:• “W” stroke, 1/3 stroke• Vary stroke lengths

Radial tangent circular strokesTool with Pitch Lap

• Radial, tangent, circular strokes• Accented pressure

May need to “recharge” lap due to compressing.

NOTE:• Spherical surface: Reflects

light coming from a single After each figuring cycle, do mirror knife edge test. Repeat figuring and testing until at paraboloid.

(see next slides).

point back onto itself. • Paraboloid surface: Reflects

light coming from distant astronomical objects to

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astronomical objects to single focal point.

Mirror Testing Calculations• Determine ideal knife edge displacements and tolerance for each zoneDetermine ideal knife edge displacements and tolerance for each zone.• Tabulate and Plot tolerance for each zone.• Make mask for test measurements.

Goals:

Description Parameter with example valueMirror Diameter D = 7.9” (200.82 mm)Mirror radius r = 3.95” (100.4 mm)F l L th F L 40 125” (1019 18 )Focal Length F.L. = 40.125” (1019.18mm)Radius of Curvature R = 80.25” (2038.36 mm)Wavelength of light (Green) λ = 0.00055 mmNumber of zones n = 5Zone # i = 1, 2, 3, 4, 5

Formulas required

Ai di k di t f (1 22) λ F L / DAiry disk radius at focus ρ = (1.22) λ F.L. / DFoucault tolerance for each zone (λ /4 criteria) τ = 2 ρ R / reff

Knife Edge displacement for each zone ΔX = (reff )2 / R√

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Effective Radius reff = r √(2i - 1) / (2 n)Outer boundary for zones z = r √ i / n

Mirror Testing Calculations (cont.)8” f/5 Mi E l8” f/5 Mirror Example:

= 2038.35= 200.82= 100.41= 1019 18

Input Mirror Parameters (mm):

Radius of Curvature Mirror Diameter Mirror radius Focal Length = 1019.18

= 5.08= 0.00341= 5Number of Zones

Focal Length Focal Ratio Airy Disk Radius at Focus

Mask Properties (mm):

Zone Outer Radius (z)

Effective Radius

1 44.90 31.752 63 50 55 00

Mask Properties (mm):

2 63.50 55.003 77.78 71.004 89.81 84.015 100.41 95.26

Zone Expected Measured Foucault Tolerance deltaXmin deltaXmax

1 0.495 0.437 0.057 0.9322 1 484 0 252 1 231 1 736

Knife Edge Displacements (mm):

Make mask with this data

No data measured yet

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2 1.484 0.252 1.231 1.7363 2.473 0.196 2.278 2.6694 3.462 0.165 3.297 3.6285 4.452 0.146 4.306 4.597

Mirror Testing: Couder Screen

Draw & Cut “Couder Screen”: Mask used to measure focus at multiple mirror radii (typ. 3 to 5).

3 Zone Mask

For a paraboloid, treat as several concentric spherical mirrors with different radius of

Zone 1 & progressing outward to zone 5

mirrors with different radius of curvature at each zone.

29Example: 8” f/5 Couder Screen

Zone 1, & progressing outward to zone 5.

Mirror Testing: Foucault Test

Foucault Test, also known as “Knife Edge Test”:

Mask over Mirror

, g

• Uses simple test device which you can make or buy.

• Detects 1/10 λ surface errors.

• Requires math to calculate mirror figure.

• Taking zonal measurements is a delicate operation.

M di f h d• Measure radius of curvature at each zone and compare to calculated paraboloid.

• Described in 1858 by French physicist Léon Foucault.

Knife Edge

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Tester

Mirror Testing: Knife Edge Test

Examples of knife edge cutting into light

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Mirror Testing: Knife Edge Test

Center Zone 1 Image Typical “doughnut” view at 70% zone.

Example: Zone 3 Measurement:• Notice start of graying in zone 3• Notice start of graying in zone 3.• Knife Edge “cutting” into returned light at

zone 3 radius of curvature.• For this mask zone 3 is also 70% zone

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• For this mask, zone 3 is also 70% zone. Remove mask and you see image above.Zone 3

Mirror Testing: Knife Edge TestImportance of stable air during Knife Edge Test

Stable, no air flow in path. With air flow in path, li htl diff t t t

p

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slightly different temperature.

Mirror Test Results8” f/5 Mirror Example: Input Mirror Parameters (mm):8 f/5 Mirror Example:

= 2038.35= 200.82= 100.41= 1019.18= 5.08

p ( )

Radius of Curvature Mirror Diameter Mirror radius Focal Length Focal Ratio

= 0.00341= 5Number of Zones

Airy Disk Radius at Focus

Mask Properties (mm):

Zone Outer Radius (z)

Effective Radius

1 44.90 31.752 63.50 55.003 77.78 71.004 89.81 84.015 100.41 95.26

Knife Edge Displacements (mm):

Measured data for 8” f/5 mirror. Average of

multiple test samples

Zone Expected Measured Foucault Tolerance deltaXmin deltaXmax

1 0.495 0.5 0.437 0.057 0.9322 1.484 1.579 0.252 1.231 1.7363 2 473 2 633 0 196 2 278 2 669

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multiple test samples. 3 2.473 2.633 0.196 2.278 2.6694 3.462 3.319 0.165 3.297 3.6285 4.452 4.475 0.146 4.306 4.597

Additional Mirror Test Methods

“Sun” Test: Easy check to test mirror focal length. Measure with tape measure/ruler.

Ronchi Grating Test Method: Quick check of mirror surface profile.

Others: See other literature, websites, etc.

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Mirror Coating & Mirror Making Experiences

After mirror polishing & figuring is complete, package mirror & send to be “Aluminized”: Safely protect mirror in shipping package. Safely protect mirror in shipping package. Thin film coating made in vacuum chamber. Various levels of reflection quality available: 89% to 98% typical.

Issues encountered: Measured Sagitta incorrectly first time. Off by factor of 2: was f/10. Accidently chipped edge of mirror while removing mirror from tool Accidently chipped edge of mirror while removing mirror from tool

(seized together). No detrimental affect to mirror (cosmetic). During mirror polishing made errors in surface figure: Hole, Hill,

Hyperboloid, Oblate Spheroid, etc. Part of learning curve.Hyperboloid, Oblate Spheroid, etc. Part of learning curve. Slight (< 1/16”) turned edge.

For additional Mirror Making info, see BAS Website “Mirror Making 101” at:

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http://www.brevardastro.org/mirrorcourse.php , or many other commercially available texts and websites.

Telescope ConstructionTypical steps to construct a reflector telescope OTA:Typical steps to construct a reflector telescope OTA:1. Design telescope and chose components.2. Obtain and/or make individual parts.3 A t l k l l t d t l ti t b ( t t l t3. Accurately mark calculated component locations on tube (create templates,

mark holes, etc.).4. Cut / Drill holes, etc.5 Install main components for “fit” check5. Install main components for “fit” check.6. Collimate and check optics for initial performance (eyepieces reach focus).7. If needed, take apart and complete final preparations of tube, etc.

Contrast improvement: • Baffles: Best choice but more laborious.• Flock paper: 2nd best choice.• Flat black paint: Of 3 choices is least effective.

8. Reassemble all parts.9. Place in desired telescope mount.10. Collimate.11. Ready for observing.

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Telescope Construction

M hi i T l E d Initial OTA fit check:Wyck and Ken

Machining Telescope End Ring at Wyck’s shop.

Cutting tube to length

If installing flock paper, recommend cutting into sectionscutting into sections and then install; not 1 piece as shown.

38Completed 8” F/5 Newtonian Telescope

Telescope MountsD id h t t f t t i t t ti OTA• Decide what type of mount you want prior to constructing OTA.

• Will affect OTA completed design.• Choose type and stability as needed for visual, and/or astrophotography.

Common mounts for Newtonian Telescopes:

Equatorial: On tripod, pier, etc. Equatorial: On tripod, pier, etc. German Equatorial. Fork Equatorial. Yoke mounted.

Dobsonian: Version of altazimuth Dobsonian: Version of altazimuth. Box mount & sets low to ground.

Altazimuth (altitude-azimuth). Typically for small telescopes & general

German Equatorial Mount Dobsonian Mount

yp ca y o s a e escopes & ge e aterrestrial viewing.

Split Ring: Variation of equatorial.

Other types also exist Other types also exist.

39Altazimuth on Pier Mount Split-Ring Mount

Basic Accessories Eyepieces: Precision lenses that magnify image from primary mirror Eyepieces: Precision lenses that magnify image from primary mirror.

• Various types and quality of eyepieces exist.• Magnification = F.L.(tel.) ÷ f.l.(eyepiece): Longer f.l. = lower magnification.• Use lowest power to view detailUse lowest power to view detail.

Star Chart: Guide to locating celestial objects.

Finder: Optical aid to assist locating celestial objects.• Wider field of view, low power, typically with cross-hairs.• Many types available: 1x Reflex, small refractor, right-angle, illuminated,

laser, etc.

Collimation Tools: Optical aid used during collimation process.• Sight Tube• Cheshire Eyepiece• Laser CollimatorLaser Collimator

Other useful accessories beyond basic items:• Barlow Lens• Coma CorrectorComa Corrector• Filters• Cases & Covers

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Collimation

Necessity and Improves performance.

Collimation: Alignment of each optical component of a telescope with regard to each other.

y p p

There are 3 steps to Collimate a Newtonian Telescope:1. Align and center secondary mirror under focuser tube

Adj t d h ld d/ f• Adjust secondary holder and/or focuser.2. Adjust tilt of secondary mirror to aim focuser axis at center of primary mirror.

• Adjust secondary screws so primary mirror reflection (center dot) is t d i fi ld f icentered in field of view.

3. Adjust tilt of primary mirror to center it’s optical axis back to the focuser.• Most critical part of alignment.

Star Test: Ultimate test of performance. • Defocus star and perfectly center secondary shadow, then view airy disk.

Refer to many other articles for detailed collimation and Star Test procedures

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Refer to many other articles for detailed collimation and Star Test procedures.

Summary

In summary, to make a Newtonian Telescope:

1. Pick size and f/# you want (per expected usage).

2. Do calculations (optical, image & diagonal size, mirror if making).

3 D i h t b t th t3. Design or choose tube, truss, other type.

4. Buy or Make & Test mirror.

5 Choose remaining components5. Choose remaining components.

6. Draw telescope layout & verify.

7 Build telescope7. Build telescope.

8. Collimate……. often.

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Also many thanks to Wyck Hoffler (BAS) for final OTA design advice and assistance, and Joseph Lawrence (IPFW / FWAS) for Mirror Making guidance.

ReferencesBooks:• How to Make a Telescope, Jean Texereau, 2nd Ed., 1984.• Making Your Own Telescope, Allyn J. Thompson, 1973.• Telescope Optics, Evaluation and Design, Harrie Rutten and Martin van Venrooij,1988.• All About Telescopes, Sam Brown, 1975.• Newtonian Notes, Designing and Building a High Performance Telescope from

Commercial Components Peter Francis 1979Commercial Components, Peter Francis,1979.• Stargazer, the Life and Times of the Telescope, Fred Watson, 2004.Articles:• Mirror Grinding 101 BAS WebsiteMirror Grinding 101, BAS WebsiteSoftware:• Newtonian Telescope Design Planner (v/IE1.3), Jim Fly, 2003• Newt for the Web Dale A Keller 2011Newt for the Web, Dale A. Keller, 2011Drawings & Images:• Websites: Willmann-Bell, Inc. (Mirror Making Kit), El Camino College (mirror grinding),

Stellafane.org (knife-edge principle), BAS Website (various photos), Wikipedia (Telescope Compnent Layout)

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