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12½" binocular telescope

Side viewThe portable binocular is shown here mounted on an aluminum platform built by Tom Osypowski, Equatorial Platforms, Grass Valley, California (photo courtesy Tom Osypowski)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Design factors

I wanted these features:

  1. No part should too large or heavy to lift and carry.

  2. All parts should fit into the back of my sports utility vehicle.
  3. Viewing close to the horizon should not require lying on the ground.
  4. Viewing at the zenith should not put the eyepieces over six feet off the ground.

  5. Interocular distances should be adjustable at least over the range 2.3" to 3.1" — my wife's and my interpupillary widths, respectively.

The tubes are conventional, permanently-assembled truss tubes, with single top and bottom rings. At setup, the bottom rings are bolted to the mirror cells. In turn, each cell is attached to a sliding linear stage built into a single mount platform; the sliding stages adjust for interpupillary separation. Each tube and mirror cell is mounted independently and is not connected at any point to the other tube.Binocular shown from the front.Budget and weight constraints limited the primary size to 12½". In order to keep the telescope as small as possible and convenient to use, I chose an ƒ4 focal ratio and a side-by-side mount configuration, with tertiary mirrors and eyepieces placed in front of the telescope.

Swayze Optical1 made the primary mirrors. I already had one Tele Vue2 22mm Panoptic eyepiece, thought it would perform well, and purchased another. The eyepiece has good eye relief, a wide apparent field of view and sharp images.

I wanted to use two Crayford focusers to provide individual focus for each eye. After discussions with JMI3, I found that their flat-based, NGF-mini2 1¼" focuser could be modified to meet my minimum interpupillary width if part of one side were removed. JMI machined off .3" from opposing sides of two focusers, relocated an eyepiece lock screw, and removed the focuser knobs on facing sides. These changes added only $50 to the cost of the focuser.

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Performance

At OSP telescope walkabout. (Panorama image)On telescope alley at RTMC 2000 (courtesy of Jeff Gortakowski)The telescope has proven successful. At the Oregon Star Party4 (OSP) early in September, 1999, many experienced telescope users had an opportunity to look through the telescope under dark skies. Close to 100 people stopped by throughout three nights to observe deep space objects through my 12½" binocular telescope.

 

Most observers were surprised at the richness and comfort of binocular viewing. Many perceived a three-dimensional effect; others described an unreal sense of presence or closeness to the object. Most spent more time than usual at the eyepieces.

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Optical design

Optical components of the binocular.To estimate secondary size and better understand vignetting, I used Dale Keller's Newt 2.5 shareware9. This program estimates the fall off in illumination as the distance from the center of the focal plane increases. Keller writes "The 100% zone is the area at the focal plane which is fully illuminated by the primary mirror. This area will have 100% of the brightness available from the primary mirror." The larger this area, the less the vignetting.

In order to keep the secondary under 4" and increase contrast, I sacrificed low-power, wide-angle viewing and used a Barlow lens mounted permanently just beyond the radius of the primary. This extends the focal plane through the tertiary to the eyepiece. I chose Meade's10 2x apochromatic triplet Barlow (Model #140); it has a 26mm clear aperture.

Here is the Newt solution for the Barlow case:

  1. Diagonal minor axis: 2.14"

  2. 100% illumination diameter: 0.58"
  3. 75% illumination diameter: 1.35"
  4. Secondary obstruction: 3%
  5. Front aperture diameter: 14.25"
  6. Required tertiary diameter: 1.25"

This solution minimized vignetting, and the dimensions of the diagonals are acceptable. I estimate magnification using the 22mm Nagler Panoptic eyepieces2 at around 150x. The Barlow lens does make finding objects more difficult.

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Construction details

Here are some photos of the details of the telescope. Hover the mouse over each picture for a detailed explanation.

To adjust interpupillary width, the observer turns a rod connected to a bevel gear box. The rod's motion is translated through left and right-handed screws to the sliding stage rings. The sliding stages move in opposite directions on rods inside the frame. This makes it easier for others to adjust the telescope to their eyes without losing the object in view.The secondary mirror, spider, tertiary mirror and Barlow lens are all attached to the focuser. This unique arrangement minimizes the weight of the top end and reduces focus travel by keeping the distance between the Barlow lens and the eyepiece constant.The user controls interpupillary separation by turning a knob that, through a bevel gear, moves the two sliding stages in opposite directions. The sliding stage's main component is a ring, which attaches to two rods in the frame through four Delrin-Teflon bearings. This bearing shaft supports the weight of the telescope tube and mirror cell.This is the ring that slides back and forth for interpupillary adjustment.

The cell's design is considerably simplified because the three adjustments usually present for cell collimation were transferred to the tube attachment. Three bolts are screwed into and locked to the main cell ring, beyond the diameter of the mirror. These bolts project above and below the mirror cell. The tube's bottom ring is lowered onto these three bolts. The tube's position on these bolts provides the collimation adjustment and is adjusted with thumbscrews above and below the tube's bottom ring.The mount consists of a tripod and a mount platform. The mount platform's altitude axes rest on four ball bearings attached to the tripod's azimuth plate. Two of the ball bearings are clutched with a Teflon washer. The azimuth bearing is a thin Teflon sheet about ten inches in diameter sandwiched between the azimuth plate and the tripod's top. I lubricated both sides of the Teflon with molybdenum disulfide grease. Gerry Logan inspired this bearing design; he has used it on several of his award-winning telescopes at RTMC. Lumicon 1¼" helical focusers, attached to the 90° mirror diagonals, hold the Meade #140 3-element Barlow lenses. I removed the upper part of the Meade Barlow that holds the eyepiece, leaving a short metal 1¼" housing that holds the optical elements. Each Barlow's housing couples the 90º mirror star diagonal with the helical focuser. A circular spider to hold the secondary is screwed to the helical focusers.

The focuser tube projects through a hole in the telescope tube's top ring. To minimize the distance between the Barlow lens and the eyepiece, I decided to mount the Orion 90º mirror star diagonal in the bottom of the JMI focuser tube. By mounting the Barlow lens housing in the other side of the diagonal mount, I could attach the circular spider to it using a small Lumicon helical focuser. Thus, all the optical components move under control of the focuser knob. Each mirror is glued to a nine-point, whiffletree cell with Dow Corning 93-076-2 aerospace silicone sealan. The nine points are located on three triangles, each of which has three mirror pads made from inverted weld T-nuts. These nuts bolt to the triangles and provide a ¾" pad for RTV gluing. The cell's triangles are bolted to rod end ball joints which let the triangle uniformly support the mirror but without any slack. Each rod end ball joint is screwed into the large inner hole of the cell's main ring.

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Internet links

  1. Swayze Optical primary mirrors
  2. Tele Vue eyepieces
  3. Jim's Mobile, Inc. (JMI) focusers
  1. Oregon Star Party
  2. Riverside Telescope Makers Conference
  3. Swayze Optical primary mirrors
  4. Tele Vue eyepieces
  5. Jim's Mobile, Inc. (JMI) focusers
  1. Dale Keller's Newt25 software
  2. Meade Instruments Corp. Barlow lenses
  3. Lumicon secondaries
    4.