10" Reverse Binocular Dobsonian Telescope

Designed and built by Jonathan Dubay

This page updated January 7, 2017

In case folks are interested in how this binocular telescope came about, I've outlined some of the details and steps on this journey...

Below you'll find: Attributes | Challenges | Design Constraints | Rough Design | Changes | Design | Adjustments | Continuing Issues | Tracking | About Eyepieces | A Filter Experiment

Attributes (or "Why build a bino?")

  • Lower cost and lighter, relative to a single-tube scope of comparable aperture. Two 10" mirrors gather the same light as a single 14" mirror. (Whether or not the visual perception of that light is the same, is the subject of debate. Here's an article by Arie Otte on the subject of Binocular Summation Factors.) Total weight of this scope with eyepieces and finders is 75 lbs (not including the plastic carrying box) versus a typical weight for a 14" homemade Dob of about 100lbs.
  • More comfortable viewing experience -- no squinting required.
  • More dramatic images due to parallax effect (though not true parallax) when using both eyes. Bright stars seem to come to the fore, dimmer stars recede.
  • Greater image clarity compared to a binocular eyepiece -- one's brain seems to pick out the best parts of each image.
Challenges
  • Alignment of two tubes -- structure needs to be rigid, yet adjustable.
  • Viewing position is limited to a straight-on approach, facing in the opposite direction of the telescope.
  • Requires purchasing matched eyepieces and filters.
  • Position for viewfinder can be awkward, due to the fact that the scope faces the opposite direction from the user.
Design constraints
  • Size -- must disassemble & fit into a Prius or similar-sized hatchback
  • Weight -- manageable by one person
  • Materials -- readily available at hardware stores
  • Construction -- require only basic woodworking skills and hand-held power tools & table/miter saw
  • Cost -- budget of $1500, includes $300 for wood and hardware, $1200 for optics.

Rough design

  • Modeled after Jerry Oltion's 12" reverse binocular telescope: Two OTAs are connected in a single truss for rigidity.
  • Secondary cells rotate to allow for adjustment of interpupillary distance without the need to refocus.
  • Collimation adjustment is made from the front of each primary mirror cell.

Changes

  • Minimize size of primary box.
  • Place box underneath the primary platform.
  • Primary mirror cells rotate independently for alignment of OTAs. The way I think of this, as each primary mirror rotates, any error in the system (such as mis-collimation, aberration in mirror terrain, wedge shape of the glass blank or a rotational axis that differs from the mirror's optical center) causes it to trace an ellipse in the sky. The position(s) where the two ellipses intersect is where the image in each eyepiece will match the other. (See diagram below for my visual representation of how this works. Read about Eric Royer's 360mm bino for a detailed explanation.)



  • Open-air secondary cage design with wire spiders.



  • Accordion-style configuration of truss tubes helps with single-person set up.


Design

  • Created 3D draft using SketchUp. I also used Newt for Web to calculate design specifications that minimize width and depth measurements yet optimize for my interocular distance of 2½ inches.



  • Purchased optics. Two 10" f/5 mirror sets from JMI (for a $20 up charge, they let me substitute the standard 62mm secondary mirrors with the 70mm ones I needed for the extra-long light path); Dual-speed focusers and star diagonals used as tertiary mirrors from Scopestuff.com.
  • Selected dimensional materials. Home Depot plastic tub 16" x 18" x 38" (internal) for carrying box, sets width and depth measurements for platforms, height measurements for primary box and secondary cages; black plastic pails and covers from Orchard for primary mirror cell housing; ½" apple-ply ash veneer plywood for platforms, rings and mirror cells; a single ¾" birch veneer plywood 30" diameter round from Readytocover.com for bearings; 1" diameter aluminum shower curtain rods for truss.

Construction

  • Checked primary mirrors with Ronchi test and measured focal length of each mirror. They differed by 1/8" over the 50" focal length, or 0.25%, well within the 3% difference in focal length to which a normal set of eyes can adapt.
  • Cut secondary cage rings, secondary platform and primary platform with router.
  • I used the cut-out scraps and sawed-off plastic buckets for the two primary cells.
  • Tied wire spiders.
  • Assembled the two platforms and measured the strut lengths between them.

    (Video below shows some of these steps, plus a time-lapse of full set up.)



     

  • Dowel tenons minimize attachment fasteners.
  • Constructed rocker/swivel base (eye hook attaches to spring balance assist) using a pre-cut 3/4" hardwood veneer round that looks a little like Jupiter
  • Adjustments and corrections

  • Tube length was too short. Needed approx ¾" extra height in struts. I solved this inexpensively by adding 3/4" plywood pads to the top of each strut pair.
  • Needed larger bearings and counterweights to balance heavy focusers and viewfinder. I had used springs to correct for the imbalance, but the amount of tension required caused the bearings to pull away from the base at low altitude. The original bearing diameter was 24" -- increased it to 30".
  • Needed collapsible fine adjustment rods for primary mirror cell rotation from eyepiece. Purchased timing belt and pulleys from automationdirect.com and added these to the primary platform, extending the height of the pulleys with left-over strut pieces. Turnbuckle adjusts tension on belts.





  • Needed string tensioner to minimize contortion of structure at low elevation.
  • Needed sight and viewfinder solutions. I adapted Jerry Oltion's split-pupil finder by adding a mirror. In my version, when one looks with one eye through the lens at the mirror, if the two glow-in-the-dark triangles become one, then what the triangle is pointing at is what the telescope is pointing at.


    Below is a picture of the mirrored split-pupil finder attached to the secondary platform (it detaches to fit into the box). I added a red-dot finder just in case my split-pupil finder mirror fogs up or doesn't provide enough detail. But using that re-dot finder requires walking around to the back side of the scope and entering a strenuous yoga-like position.



  • Added a detachable right-angle correct image viewfinder to the secondary platform.







  • I added fine-adjustment levers for interocular distance (see photo below). There are two levers -- one on each secondary cage-- which rotate the cage rings and thereby move the focusers, tertiary mirrors and eyepieces back and forth by about an inch.



  • Needed 7 pounds of counter weight, which I stashed under the primary mirror platform box.



  • My original three-screw secondary adjustment mechanism was not working well enough, so I have retrofit the secondary holder with a ball-and-socket attachemnt fashioned from an $8 cell phone holder (Okra MagnetMount2 OKUNI-VTM2-01). So far this unit has proven much easier to use for colimation and alignment -- I just point mirror where I want it. Eventually I'll restring the wire spider to repoosition the ball-and-socket to the center of the optical path.

Continuing issues

  • Still some loss of collimation at lower altitudes.
  • It is difficult to locate objects in my mirror site -- only the brightest stars are visible in the mirror. Consequently, I no longer use the split-view mirror sight but instead have installed a small red-dot finder in its place
  • Total weight of scope -- 75 lbs. -- is too much for me to lift all at once.
  • When adjusting interpupillary distance, some play in accordion strut system can change alignment of OTAs.

    Even with these issues, I can't wait to take it out and use it. Everything's in the box, ready to go!


Tracking

I built an equatorial platform based on a design by Don Peckham. Please see his webpage for details and a Sketchup model. After considering many different designs, this one seemed to fit the dimensions of my scope the best. To get the correct measurements, I used Sketchup to superimpose a cone over the telescope base, which allowed me to measure the bearing size accurately.

I used the Celestron Precision Motor Drive for PowerSeeker/AstroMaster EQ, which runs on a 9-volt battery. Amazingly enough, it has the torque needed to move this 75-pound telescope. I added a black stripe to help with rough alignment to Polaris, and plywood guides to keep the telescope base in place on the platform.

In addtion to providing motion to track objects over a 60-minute span, this platform gives the telescope about 6 inches of extra height, making viewing at medium and low altitudes much more comfortable -- no sitting on the ground necessary.

A word about eyepieces

One of the great advantages of a binocular telescope is the extra-wide apparent field of view (AF) it affords. With the appropriate wide-field eyepieces, the view gives one the impression of looking out a window, as opposed to looking through a tube (or worse, a keyhole, as with some eyepieces). However, because the interocular distance of most adults is around 2½ inches, eyepieces that have an outside diameter of more than 2 inches don't fit closely enough together. With a 1.25" barrel, as the focal length of the eyepiece increases, the available range of AF decreases. While there are a number of 100- to 82-degree AF eyepieces available in the 11mm range or below that will work for bino viewing, there are fewer choices at 19mm and 24mm in 68- or 72-degrees AF, and very few with AF above 72 degrees in that focal length or longer. Therefore, eventually I may retrofit this telescope with mirrors that are an f-ratio of 4 -- instead of f5 -- to open more possibilities for wide-view eyepieces.

To put it another way, with my current f5 mirrors, the eyepiece focal lengths I use are 11mm, 19mm and 24mm which give me magnifications of 114x, 68x and 52x respectively. The 19mm and 24mm eyepieces I use are the TeleVue Panoptics (68 degrees AFOV), which is about the widest AFOV there are (a few other manufacturers have bino-friendly eyepieces that go up to 70 degrees AFOV). But if I were to use f4 mirrors, I can get the same magnifications with 9mm, 15mm and 19mm. Being able able to use a 15mm eyepiece for my mid-range eyepiece would open up a number of eyepiece options over 68 degrees AVOF.

A Filter Experiment

Visual filters (broadband/deep-sky/LPR, narrowband/UHC/Ultrablock and line/OIII) can improve viewing contrast for nebulae. The questions I asked myself was, "Do I need matching filters for each eyepiece, or can I use different filters for each, and would doing so actaully improve the view?" The human brain does a good job of picking the best information from each eye to create a visual perception that is better than either of the single eye's view. Below is a transmission graph of the four visual filters I have chosen. I have paired them up in every permutation on multiple objects and my subjective impression is that if the filters provide a similar view in color and contrast, I can achieve a richer visual experience that will have a pleasing 3-D effect. But in situations where there is a bright star in the field which appears red in one filter but blue in the other, or where one filter dims more stars that another, the difference in color and contrast make for a confusing visual experience. So in most cases I have found that two matching filters are better. One un-paired situation where I liked the effect is viewing galaxies with an LPR filter in one eyepiece and no filter in the other. The small amount of extra contrast from the LRP filter darkens the filed in one eye view, while the other eye view brightens the target.

 

Acknowledgments & Contact

Special thanks to son Liam and father-in-law Dave VanDyke; Jerry Oltion and Rose City Astronomers Don Peckham, John DeLacy, Greg Jones and Ken Hose. Thanks to Ryan McDaniel for supplying eyepieces for exploration.

Let me know what you think! E-mail: