22" binocular telescope

Even though the 8.4m Large Binocular Telescope (LBT) at Mt. Graham, Arizona, and the Meyer Binocular Telescope at Mt. Evans, Colorado (operated by the University of Denver) are both larger, neither converges images from each telescope for visual access. The LBT will combine light using interferometry. The Meyer telescope's dual-mounted 0.7m Ritchey-Chrétien's are used individually.

Introduction

The 22" Newtonian binocular described here shares a common CAD¹ design and many parts with a monocular telescope I also built. I completed both by August, 2003, after 2½ years of design and construction. The binocular won a Merit Award at the 2004 Riverside Telescope Makers Conference.

I'm indebted to others for some of the photos included here. I've credited them on their photos.

The telescopes are described in these sections:

Requirements — design criteria
Performance — how the binocular functions
Parts — material and parts
Mount —  major altitude and azimuth components
C-ring assembly holds optical support structure and moves in altitude
Optical support structure — struts, top ends, cells and connectors
Azimuth frame — moving azimuth part with motors, gears, clutches and controller
Azimuth ring — base and azimuth track
Focusers — focusers and their mount
Interpupillary adjustment
Convergence adjustment
Optics — who made them
Weight — component and total weight
Cost — comparisons to large monoculars

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Requirements

These were the design constraints:

1. Portability — the telescopes will be used often at remote dark sky sites for visual observations

2. Small footprint — the monocular must fit into a small Mazda hatchback sedan with a 21" high by 31" wide opening; the binocular must fit in a Toyota Sienna minivan

3. Low height — the lower the eyepiece, the shorter the climb up a ladder

4. Manageable weight — unaided lifting and setup imposes a weight limit of 60 pounds or less per component

5. Thin mirror — the cell must be designed to support a 48 pound, 1-5/8" thick mirror

6. Short monocular struts — in order to fit the struts into the hatchback, struts cannot exceed 48" in length

7. Drive — an automated "go to" system to find and track objects for better visual observation

8. Economy of material — aluminum costs go up proportionately with the weight of extruded stock or plate required

9. Stiffness — minimize flexure with rigid structures and a lightweight top end

10. Economy of machining — design as many parts as possible that can shared in both telescopes or cut with a miter saw instead of a milling machine

In addition, for the binocular:

1. Parallel alignment — provide a means of adjusting binocular convergence while at the eyepiece

2. Eyepieces — support for both 1¼" and 2" sizes

3. Interpupillary separation — support as wide a range as possible

4. Parts commonality — keep the same center of gravity as the monocular version

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Performance

The binocular telescope provides exceptionally satisfying views, providing levels of acuity, faint light detection, contrast and color equal to larger aperture monocular telescopes equipped with bino viewers.

Early performance feedback was obtained from the 2003 Oregon Star Party (OSP)².

In a test conducted October 24-25, 2003 at the Shingletown³ air strip in Northern California near Mt. Lassen — the Shingletown shootout — I was able to compare the 22" binocular to Dan Gray's innovative new 28" string telescope⁵.  We were joined by Shneor Sherman and Mel Bartels, both experienced observers. Our benchmark objects included the NGC 281 edge-on galaxy, the Andromeda galaxy, the Perseus double cluster and the Great Orion nebula.

Although the seeing was poor, and Dan's temporarily silvered mirror was slightly undercorrected and astigmatic, the consensus among the group was:

1. In general, views were essentially the same.
2. The 28" showed slightly brighter star images.
3. Contrast and brightness of galaxies and their dust lanes were similar.
4. Nebulosity in M42 was equally impressive, and the pink color in M42 was easy to detect in both telescopes.
5. The brain much prefers the comfort of binocular viewing — the sense of presence, the silkiness of faint, distended objects, the ability to concentrate, the removal of "background noise" by the brain were all noted as improvements due to the binocular view.

6. Low power 95X, 80° field of views were unique to the 22" binocular.

The best thing that happened at the shootout was the design of a new servo controller by Dan Gray and Mel Bartels ― they had to have something to do during daylight hours! I have since upgraded the drive system from Mel's stepper motor system⁴ to Dan's servo controller¹⁶.

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Parts

Most telescope parts were machined from standard 6061-T6 aluminum extrusions and plate⁶. Simpler parts were designed to be cut with a miter saw in order to reduce machine shop costs.  Stainless steel sheet was used for some laser-cut and folded parts⁷. Welding was used for many joints; finishes included powder and liquid coat painting and anodizing. Most hardware — machine screws, bearings, etc. — was purchased from McMaster-Carr Supply Company⁸. Prefabricated parts from JMI⁹ and Edmund Industrial Optics¹⁰ were also used.

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Mount

The mount consists of the optical support structure, two moving components—the C-ring assembly and the azimuth frame—and the azimuth ring. These as well as the focusers and adjustments for interpupillary distance and convergence are described in detail below.

C-ring assembly

The canonical³⁷ wooden Dobsonian mirror box is replaced by a welded aluminum C-ring assembly for stiffness. The heretical^{38, 39} cell — glued to its mirror with silicone to eliminate the sling and clips — is removable, and is lowered onto bolt holes located on the C-ring assembly. 

The assembly is open to promote air circulation and mirror cooling.

In the binocular, the two triangles are cantilevered outboard. Bracing is used to maintain a stiff structure. The telescopes are balanced around the altitude axis with lead brick counterweights¹¹ placed on shelves between the C-rings (two in the monocular and four in the binocular).
 

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Optical support structure

After seeing Greg Babcock's 24" reflector¹² at the 2000 Oregon Star Party, I was convinced six struts were a viable alternative to eight. In a given telescope, the struts are interchangeable.

The entire structure is assembled after transportation from separate struts, a top end, and a cell. Each strut is held in top and bottom end mounting blocks and locked in place with thumbscrews. Silicone O-rings under the thumbscrews compensate for the angle of the struts. O-rings are also placed between the struts and the strut mounting blocks to dampen vibrations transmitted through the struts.

Each top end holds the secondary, tertiary, interpupillary adjustment and the focuser. The underlying structure of each top end is a  lightweight steel hexagon; the focuser is permanently attached to it. The highly-evacuated hexagon design was inspired by the 60" telescope on Mt. Wilson²⁰.

The secondary is held in place by a single strand of 0.02" diameter (24 gauge) spring temper, stainless steel wire threaded through nine holes: three spider bolts attached to the hexagon and two three-point suspension fixtures mounted on the central secondary assembly. A single strand permits a uniform tension web; I have tightened it to about G above middle C.

The diffraction pattern of the spider web is more diffuse than the traditional three-van spider.

The 4½" secondary mirror is glued to a five-point cell with RTV. By adjusting the spider, the secondary can be positioned laterally to move the optical axis with respect to the mechanical axis of the tube. This adjustment is used to locate the optical axes of the telescopes so that the eyepieces are parallel to the ground; thus, the viewer doesn't have to tilt his or her head.

The secondary is collimated using three machine screws that pull against a ¼" rubber compression pad. No locking screw is required.

Two-inch minor axis tertiaries are mounted on an Edmund Optics mirror mount²⁷. The mount thumbscrews are used during collimation of the tertiary.

The bottom end of the optical support structure is the mirror cell. Cell blocks hold the struts to the cell.

The cell follows conventional whiffletree design, using three large triangles that each support three small triangles.

The location of the cell's 27 points was computed using David Lewis' Graphical Plop program²⁸. Each cell triangle pivots on stainless steel, Teflon-lined ball joint bearings. These bearings give the whiffletree flexibility, but prevent lateral displacement during use. They are glued with epoxy cement into recessed holes counterbored into the three large triangles of the whiffletree. The bearings fit snugly, insuring the dimensional stability of the cell regardless of its altitude position.

The mirror is glued with Dow Corning 832 Multi-Surface Adhesive RTV Sealant²⁹ to inverted weld nuts screwed to each small triangle. The cell is shown on the left just before gluing. Wooden spacers located next to the weld nut cell pads provide a 2mm gap for the glue, and stainless steel gauges ensure proper cell pad location. (The mirror can easily be removed from the cell for recoating by unscrewing the weld nuts in the small triangles. The weld nuts are then twisted off the mirror with a pair of pliers.)

When not in use, the mirror is protected by a cap made from ¼" white translucent acrylic plastic. It rests directly on the mirror. The cap's edge is wrapped with a soft rubber edge trim to avoid mirror damage. The cap can be removed or replaced any time after setup completion or before disassembly.

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Azimuth frame

The telescope's C-ring assembly rests on bearing pedestals mounted at the corners of a square frame, a minimal form of the traditional rocker box. The altitude bearings are mounted directly over azimuth bearings contained in the frame. In this innovative design — conceived by Ed Harvey and Dan Gray⁵ and sometimes called a "flex rocker" — the azimuth frame is thin enough to warp, if necessary, so the each azimuth bearing will make contact with the azimuth ring. This helps maintain a uniform distribution of weight on the azimuth ring through the four bearing pedestals.

Located at one corner, direct drive friction rollers move the telescopes in both axes. The rollers are clutched, and connected to two servo motor drives through a planetary gear reducer. The azimuth and altitude drive trains are identical.

Brushed DC servo motors with gearheads from Dynetic Systems¹⁴ are used. The gearmotors are NEMA 17 size — 1.5" diameter — with these characteristics: 12V DC, 3.25A, 30W, 70:1 gear ratio, 3.6 in-oz output, 10 arc-min backlash, 1000 line two-channel indexed optical encoder.

Both motors are connected to the palm-sized Sidereal Technology Dual Servo Controller¹⁶. This remarkably versatile controller, jointly developed by Dan Gray and Mel Bartels, allows standalone visual tracking and slewing, automated tracking and slewing using an Argo Navis⁴¹ or an ASCOM-compliant PC planetarium program, or it can be used with Mel Bartels' Scope II PC software¹⁷ for high precision tracking and automated "go to" control. The servo controller is powered with a small 12V gel cell.

U. S. Digital S-1000 optical encoders¹⁸  independently sense the telescope's position and provide it to the servo controller (no separate external encoder controller is required).

Azimuth ring

The azimuth ring is a welded structure consisting of a hexagonal frame sandwiched between two rings. The azimuth ring rests on triangular legs with self-leveling feet. It is reinforced by six braces. 

The top ring, or azimuth track, is 25" in diameter. It was machined to a flatness of .001" with a large Blanchard rotary grinder in a local machine shop. The azimuth track rollers, mounted at a 45º angle at the corner of the azimuth frame, rotate on the track. Idler bearings mounted on the azimuth frame roll laterally against the inside edge of the top ring and define the azimuth axis. 

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Focusers

The monocular's Feather Touch focuser²² is attached to a focuser shelf which is bolted to the top ring.

In the binocular, a pair of 2" JMI²³ RCF-1 reverse Crayford focusers slides back and forth for interpupillary adjustment and up and down for focus. The viewer looks down through the eyepieces and uses knobs to adjust both focus and interpupillary distance (or IPD). This approach to focuser mounting in a Newtonian binocular minimizes top end weight, but does require the user to re-focus whenever the IPD setting is changed.

For 1¼" eyepieces, custom machined eyepiece adapters with offset barrel holes are inserted into the RCF-1 focusers. To use these adapters, the telescopes' optical axes also need to be shifted toward the insides of the focusers so they are centered in the adapters' barrel holes. This is done by lowering the tertiaries a small amount, which are mounted on Versa-Mount linear guide blocks so they can slide up and down on the posts that hold them.

The binocular can support a wide range of eyepieces using the 2" focuser and its offset-center 1¼" adapter. For example, the Tele Vue²⁴ and BW-Optik eyepieces listed below could all be used:

For 1¼" eyepieces, the minimum separation depends on the eyepiece's maximum width. For any eyepiece whose maximum width is more than 1¾", the two eyepieces will touch at the minimum IPD setting. For any eyepiece whose width is less than this, the minimum IPD setting is fixed at 1¾", or 45 mm.

For 2" eyepieces, the minimum IPD setting is determined by the RCF-1 focuser, or 59 mm (assuming the eyepiece's maximum barrel diameter is less than 59 mm). Thus, the total IPD range supported is from around 41 to 80 mm. Most binocular viewers support a range of 55 to 75 mm; except for one manufacturer, all require 1¼" eyepieces.

Statistical studies²⁵ show that the "mean adult IPD is around 63 mm, the vast majority of adults have IPDs in the range 50–75 mm, the wider range of 45–80 mm is likely to include (almost) all adults, and the minimum IPD for children (down to five years old) is around 40 mm."

Many of my viewers at star parties are children and teens. Many of the them and a few adults are unable to use my BW-Optik eyepieces because their IPDs are less than 59 mm. For these users, other eyepieces are required.

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Interpupillary adjustment

The viewer adjusts interpupillary distance (IPD) to match the separation of his or her eyes by turning the end knob at either focuser (the left knob in the picture) and adjusts focus with the focusing knob built into each focuser (the right knob in the picture).

When the IPD is changed, the focal plane also moves. The user must then refocus. Because 3/8" Acme lead screws are used, the user can increase or decrease focuser separation rapidly in just a few turns.

The inner ends of the lead screw shafts are connected to the ends of a double-jointed, sliding-spine telescoping universal joint²⁶ (upper right). The focusers are suspended on linear motion ball bearing slides (upper left)²⁷. Turning the IPD knob on either end racks the focusers back and forth and moves them in equidistant and opposing directions.

The universal joint provides six-way tolerance for changes in relative position of the two telescope tubes during convergence adjustments. The center part slips together during setup (setscrews aren't required). 

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Convergence adjustments

The convergence, or binocular alignment of the twin telescopes is provided by the bottom pair of nuts on the cell bolt and an adjustable  bridge at the top. The thumbnuts are for coarse adjustment, performed during initial setup.

Subsequent refinements in convergence are performed using an adjusting bridge. The bridge actually torques the tubes slightly, moving the upper end optics with respect to the primary to move the optical axes. This adjustment offsets changes caused by natural gravitational deflection in the tubes as they change altitude position.

This approach to convergence provides a rigid connection between the two sides of the binocular, which is highly desirable because it minimizes the chance of differential flexure as the telescope moves to different positions. The bridge is within easy reach while at the eyepieces.

The bridge is used infrequently during the observing sessions, usually with high powers or after major changes in altitude that may induce flexure. Although the degree of change that can be made is less with the bridge than with the bottom end nuts, it has proven sufficient to take care of any flexure that has happened.

The essential parts of the bridge are two precision linear ball bearing slides²⁶ mounted at right angles to each other. By turning the knobs of the two slides, the top ends of the binocular can be moved slightly in whatever direction is needed to restore convergence. A quarter turn of travel is about .03", or 1/32". Once converged and collimated, the arms connecting each half of the bridge are tightened with thumbscrews.

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Optics

The 22", f5 primaries were made by Swayze Optical³⁰ and Deep Sky Optics³¹ in Australia. This is the second pair of primaries Steve has made for my binocular telescopes. A 4½" minor-axis secondary and 2" minor-axis tertiaries from Newport Glass Works and ProtoStar³² complete the binocular's optical train.
 

John Hall of Pegasus Optics³³ made a fine f4 mirror for the monocular. John is a knowledgeable and honest man: dealing with him was a pleasure. The monocular uses a 4" secondary, also supplied by Pegasus.

A QuikFinder³⁴ reflex sight is attached to a top ring. An erect-image, straight-through, 6X finder supplied with the Meade ETX-90 is glued to the side of the QuikFinder. This provides a little more precision when not using the drive system for slewing to a new object.

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Weight

The binocular is a much taller structure than the monocular for two reasons: the primaries are f5 instead of f4, and the eyepieces are located in the front instead of at the side. There are also elements unique to the binocular — tertiaries, interpupillary adjustment and adjustable bridge mechanisms — that add weight to the top end. In order to keep the same center of gravity as the monocular and use as many of the same parts as possible, the binocular telescope requires more counterweights than the monocular. 

The binocular telescope's total weight, including all optics and accessories, will be about 378 pounds; of this, the moving weight is about 240 pounds. Six lead brick counterweights, weighing a total of 120 pounds are included in the moving weight. 

The telescope breaks down into components that can all be carried without mechanical aids. The mirror and cell are the heaviest at 62 pounds.

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Cost

The best measure of power in a telescope is probably collecting area. It's also a good way to compare telescope costs. I configured a Starsplitter 30" ³⁵ at about $20,530 and an Equatorial Platforms 28" ³⁶ at about $20,595. These prices include a Denk II binocular viewer, a platform from Equatorial Platforms and digital setting circles, but no sales tax or shipping. 

While an ATM could certainly make a 30" monocular for less money than the cost of a commercial telescope, he or she could also certainly build a binocular for less than what I did.

Any ATM who is considering making a large aperture monocular should give a smaller-aperture binocular Newtonian serious consideration. While construction may be more complicated, the extra effort pays off. It's my belief that a binocular's views will always equal and often surpass those available from a larger monocular built for the same cost.

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Internet links:

1. DesignCAD 3D Max:
   IMSI 
   (See also DCUnleashed for more information)
2. Oregon Star Party
3. Also the location of the Shingletown Star Party
4. Mel Bartels' stepper drive system:
   BBAstro-Designs
5. 14" reflector:
   Dan Gray 
6. Machining:
   Zinola Manu-facturing

7. Welding, laser cutting, finishing:
   Quality Metal Fabrication

8. Hardware:
   McMaster-Carr

9. Focusers:
   Jim's Mobile, Inc.

10. Linear motion ball bearing slides; small mirror mount:
    Edmund Industrial Optics

11. Lead bricks:
    Biodex Medical Systems

12. 24" reflector:
    Greg Babcock 

13. Tube connectors:
    Superior Components

14. Servo motors and gear reducers:
    Dynetic Systems,
    PDF catalog  

15. Slip couplings:
    Polyclutch

16. Servo controller:
    Sidereal Technology
    Telescope controller:
    Argo Navis

17. Scope II software:
    BBAstro-Designs

18. Optical encoders:
    US Digital

19. Alucobond composite:
    Alcan Composites USA

20. Mt. Wilson's 60-inch telescope

21. Mel Bartels' wire spider on his
    Trilateral dob

22. Feather Touch focuser:
    Starlight Instruments

23. Focusers:
    Jim's Mobile, Inc.

24. Eyepieces:
    Tele Vue Optics

25. SPIE Proceedings Vol. 5241, pages 36-46 on interpupillary distance

26. Telescoping universal joint:
    Stock Drive Products

27. Linear motion ball bearing slides; small mirror mount:
    Edmund Industrial Optics

28. Graphical Plop:
    David Lewis

29. For a Dow Corning product data sheet:
    832 Multi-Surface Adhesive Sealant

30. Steve Swayze's
    Swayze Optical

31. Mark Suchting's
    Deep Sky Optics (Australia)

32. Binocular secondaries, tertiaries:
    ProtoStar
    Newport Glass Works, Ltd.

33. Monocular primary:
    Pegasus Optics  

34. Reflex sight:
    Rigel Systems

35. Large Dobsonians by
    Starsplitter

36. Tom Osypowski's SpicaEyes Newtonians, integrated with his platforms

37. Canonical (kə-nŏnʹĭ-kəl) adjective, conforming to a general rule or procedure; orthodox; law or decree
38. Heretical (hə-rĕtʹĭ-kəl) adjective, characterized by, revealing, or approaching departure from established beliefs or standards
39. Go to The case for gluing mirrors, on this Web site
40. 3RF's Comanche Springs Observatory
41. Argo Navis telescope controller from Wildcard Innovations, headquartered in the great and powerful Oz