2 x 10"/f5 Bino- Dobson
A dobsonian telescope for two eyes
During the telescope meeting at Vogelsberg (ITV) in May 2000 I had the chance to look through a medium size dobsonian telescope equiped with a binocular attachment, and the magnificent views impressed me deeply. There was no more stressed closing of one eye, but concentration on the essential and a relaxed observing with two eyes.
Thatīs the kind of telescope I want !
After returning home from ITV a binocular attachment immediately was ordered (the one from BW-Optik) and mounted to my 4"/f10 refractor.
The result did not meet the expectations, no chance for deep sky observing for lack of aperture and the posture while observing was not very comfortable.
But: bright objects showed more details.
So I changed to a 10"/f5 GSO- dobson and got brighter images and a comfortable observing position in reward. In order to compensate the additional optical length of the binocular attachment, the focusser of the GSO was changed to a sled focuser made from 3mm aluminium- sheet metal, that runs in Dry-Lin guidings manufactured by IGUS
Meanwhile I caught an infection called "glass- fever" and grinded a 6 inch/f8 mirror. After having finished this mirror and itīs telescope something was missing, and I decided to grind another mirror. My favourit choice would have been something like 16 inch/f5, but all the experts advices led me to a smaller project instead. So I set out to grind a 10 inch/f5 mirror. Taking this one and the mirror from my GSO- dobson, it should be possible to build a "real binocular".
Conception
Anyone who ever took a look through (poor) binocluars knows the problem of bringing the two images to congruence.
Doing this requires:
- the adjustability of the eyepiece- distance to the individual interpupilary distance
- the adjustability of the optical axises to each other.
My binocular telescope should consist of two newtonian telescopes, and one
feature of a newtonian telescope is, that it has to be well adjusted to reach its best performance. For that reason my bino consists of two complete newtonion truss tubes. Each telescope can be collimated like a conventional newtonian.
Moreover it should be possible to:
- adjust the interpupilary distance without affecting the adjustment of the optical axises or the collimation of the single telescopes
- adjust the optical axises until the two pictures come to congruence without affecting the interpupilary distance
- do these adjustments while looking through the telescope.
Structure
Each tube is a conventional truss design.
Each tube rests on two arc- shaped guide rails.
The radius and the position of these arcs is chosen in that way, that the position where one looks into the eyepiece is situated on a virtual line which meets the centers of the arcs. For this reason it is guaranteed that the position where one looks into the eyepiece stays in place when the tube is swiveled for adjusting.
The guidings for the two tubes are located in a manner, that the axises around which the tubes are swiveld enclose a right angle.
For this reason the inclination of the optical axises takes place in two plains enclosing a right angle and can be adjusted until the optical axises are parallel to each other.
One unit "tube + arc- shaped guiding" is mounted to a kind of guide slide, that can travel slightly sideways. This allows to adjust the interpupilary distance.
Design
Design for all mechanical parts was made with CAD. Compared to the 6"/f8 Dobson, which was designed with a handmade sketch, the construction of the bino required exact drawings and calculations. The additional work for designing is quickly made up for the time saved during construction and the chance to get an idea of what one is going to create before only one board is cut.
The finished Telescope
After nearly one year mirror grinding, designing and construction the bino was assembled for the first time completly a few days before christmas 2002.
The console visible below the eyepieces carries three turning knobs for adjusting axis- parallelity and interpupilary distance. The whole telescope can be moved very smoothly by pulling or pushing the tube frame
Each side is separatly focussed by turning the knob.
Drving the sled focuser by a threaded pin turned out to work not very well and was changed to a system with rack an pinion situated right beside the guiding rods. The pinions are connected to one shaft that is driven by the gear box (ratio 1:10). For preventing sliding down caused by gravity there is a little brake with a freewheel- clutch (inside the spring).
Adjustment
The mechanical components for swiveling and the rack for shifting (adjusting interpupilary distance) of the left tube. The shaft with the pinion drives another rack on the rear side to ensure constant movement. Inside the upper bore of the frame carrying the left tube one end of a bushing is visibale (Iglidur von IGUS). The bushings are inside all 8 bores of the frame and slide on the aluminium- tubes of 30mm diameter.
The mechanical components for swiveling the right tube. The bevel gears have been bought from Conrad Elektronik. The brass insert in the big bevel gear is got a M6 thread and moves a threaded pin that cannot turn. When the bevel wheel is turned, the right tube is pushed by the threaded pin.
In the lower part of this picture the arc- shaped guiding rail is visible in the side part of the frame. The groove, which carries the rail has been milled with a router used as a compass before assembling the frame.
The guiding rail is made from a tube with rectangular shape, glued to the groove and covered with Teflon. The upper part of the picture shows one of the two guiding grooves of the mirror box. The lower part of the groove can be taken off. Then the mirrorbox is inserted into the basic frame from above. The lower part of the groove then is inserted from below and gets fastened from inside the mirror box after adjusting the guiding to minimum tolerance by means of the pressing- screws.
The entire lower frame viewed from the rear side. The tailgates carrying the mirror cells can be folded backward for easy access to the mirrors.
Itīs quite difficult to find what you are looking for with your back standing towards it. Using the telrad finder makes it easy.
Handles for portability like discribed by Kriege/Berry. Working very well and are absolutely nessecary as the whole telescope turned out to be much more bulky than proposed (why is this always like that ?)
First Light
December 2002 the bino was ready for first light.
Adjustments were made with the help of a bright star. For adjusting the optical axises to each other one telescope is driven out of focus until large out of focus- discs get visible. The star visible in the other, still focussed telescope, then is "pushed" by means of the axis- adjustment right into the center of the out of focus- disc.
Axis- adjusting with both telescopes focussed is hard to do as the brain plays a trick to you: it merges the images long before real congruence is reached.
The first "real objekt" was M42. The sight was incredible ! Lots of details within the gaseous clouds, and, the whole complex appeared to be three- dimensional. Later on I turned over to Saturn, and the planet looked like he was soaring in the center of his rings accompanied by 5 visible moons.
ITV 2003
During ITV 2003 we had the chance to use the bino under dark sky and check out which eyepieces work well. Absolutely amazing it is to look at M13 through 9mm Nagler- Eyepieces, the views are really breathtaking.
Minimal interpupillary distance 2 inches: even the young ones can enjoy binocular vision.
Rüdiger Heins and another Dobson- follower seem to like what they see...
Home
The guidings for the two tubes are located in a manner, that the axises around which the tubes are swiveld enclose a right angle.
For this reason the inclination of the optical axises takes place in two plains enclosing a right angle and can be adjusted until the optical axises are parallel to each other.
Design for all mechanical parts was made with CAD. Compared to the 6"/f8 Dobson, which was designed with a handmade sketch, the construction of the bino required exact drawings and calculations. The additional work for designing is quickly made up for the time saved during construction and the chance to get an idea of what one is going to create before only one board is cut.
The finished Telescope
After nearly one year mirror grinding, designing and construction the bino was assembled for the first time completly a few days before christmas 2002.
The console visible below the eyepieces carries three turning knobs for adjusting axis- parallelity and interpupilary distance. The whole telescope can be moved very smoothly by pulling or pushing the tube frame
Each side is separatly focussed by turning the knob.
Drving the sled focuser by a threaded pin turned out to work not very well and was changed to a system with rack an pinion situated right beside the guiding rods. The pinions are connected to one shaft that is driven by the gear box (ratio 1:10). For preventing sliding down caused by gravity there is a little brake with a freewheel- clutch (inside the spring).
Adjustment
The mechanical components for swiveling and the rack for shifting (adjusting interpupilary distance) of the left tube. The shaft with the pinion drives another rack on the rear side to ensure constant movement. Inside the upper bore of the frame carrying the left tube one end of a bushing is visibale (Iglidur von IGUS). The bushings are inside all 8 bores of the frame and slide on the aluminium- tubes of 30mm diameter.
The mechanical components for swiveling the right tube. The bevel gears have been bought from Conrad Elektronik. The brass insert in the big bevel gear is got a M6 thread and moves a threaded pin that cannot turn. When the bevel wheel is turned, the right tube is pushed by the threaded pin.
In the lower part of this picture the arc- shaped guiding rail is visible in the side part of the frame. The groove, which carries the rail has been milled with a router used as a compass before assembling the frame.
The guiding rail is made from a tube with rectangular shape, glued to the groove and covered with Teflon. The upper part of the picture shows one of the two guiding grooves of the mirror box. The lower part of the groove can be taken off. Then the mirrorbox is inserted into the basic frame from above. The lower part of the groove then is inserted from below and gets fastened from inside the mirror box after adjusting the guiding to minimum tolerance by means of the pressing- screws.
The entire lower frame viewed from the rear side. The tailgates carrying the mirror cells can be folded backward for easy access to the mirrors.
Itīs quite difficult to find what you are looking for with your back standing towards it. Using the telrad finder makes it easy.
Handles for portability like discribed by Kriege/Berry. Working very well and are absolutely nessecary as the whole telescope turned out to be much more bulky than proposed (why is this always like that ?)
First Light
December 2002 the bino was ready for first light.
Minimal interpupillary distance 2 inches: even the young ones can enjoy binocular vision.
Rüdiger Heins and another Dobson- follower seem to like what they see...
Home