The Coulter Odessey 13.1 inch Dobsonian was my second telescope. It sits on a ply-wood base, and the OTA is housed in a 15" sonotube (heavy cardboard). I carry it around on a hand truck. Both parts together weigh in at about 100 lbs. The scope came from Coulter originally with a 1.25 inch focuser, but I upgraded it to a 2" helecial focuser. I have a crayford for it, but haven't gotten around to putting it on yet.
I have gotten wonderful views of Jupiter through the Nagler 13mm eyepiece. (about 114X) The image doesn't do well much above that. That evening, I spotted maybe 8 bands on the surface of Jupiter.
The scope is an f/4.5, and with the 13 inch mirror it has a focal length of 1486 mm.
With my suite of eyepieces, I get the following magnifications:
35mm Panoptic - 42x
20mm Nagler - 74x
13mm Nagler - 114x
10mm Nagler - 149x
In the summer of 1999, I added digital setting circles to the Coulter. The scope became very easy to use with this addition. Prior to that point, I relied on Star Hopping to find astronomical objects. Finding suitable stars to hop from is a problem in the Washington DC suburbs, and the scope didn't get a lot of use. That summer, I found a couple of 4000 tick encoders and began the process of mounting them to the coulter. For any scope to make use of a digital setting circle computer, there must be some way to mount encoders on the altitude axis, and a way to mount the encoder on the azamuth axis. The coulter provides neither. My solution to this problem for the altitude axis is shown in the next image. Basically, I cut two pieces of plywood 2"x2"x3/8", glued them together to make a 3/4" thick block of wood. I then reamed a 3/4" hole in the center of one side of the 2x2 so that it would fit over the bolt head of the altitude axis. On the other side of the 2x2 in the exact center of the block, I drilled a 1/4" hole all the way through. The 1/4" hole fits the encoder shaft. I then screwed the block (encoder hole side out) onto the center of the wooden bearing on the scope. I completed the assembly by making a simple holder for the encoder that velcros onto the base of the scope.
In the image at right, you will notice the transport dolly. The scope is held to the dolly with a purple strap. Below the strap at the center of the tube is the telrad mount. Above the strap on the right side of the tube is the JMI Micro-Max computer attached to a cable leading downward that connects to the encoders. On the right side of the scope in the vicinity of the altitude bearing, you will notice a wooden contraption. The altitude bearing is attached to this.
Beneath the red tube you will notice what looks like a small black disk atop a blond piece of wood. The black disk has a white mark at it's center. This is the azamuth encoder. At the very base of the telescope is the ground board.
The set of images which follow provide detailed pictures of each of these components.
Starting at the bottom, the azamuth encoder (black disk) with red wire connector is attached to a small piece of blond wood.
At each end, a small piece of wood is glued to provide a surface to attach the blond piece to the rotating base (rocker box) of the dobson mount. Below the encoder is a piece of 3/4 inch plywood which is epoxied to the head of a bolt which is firmly attached to the ground board. This peice of plywood has a 1/4 inch hole drilled in it's center. This hole mates with the shaft of the encoder. The ground board thus is affixed to the encoder shaft and does not rotate. As the dobson base rotates, the encoder housing rotates against the shaft causing electrical impulses to be produced. These signals are carred by the connecting wire to the computer.
The image at right shows the altitude bearing from the side. The image above showed the bearing from the front. Here you see the encoder fixed to a piece of blond plywood. At the left side of the piece of plywood, it is fixed to the azamuth base.
The fixing to the base is done with velcro and it keeps the encoder housing from rotating. The wires comming out of the housing are fed up to the computer.
Behind the housing board you can see a second piece of blond wood against the dark altitude bearing.
This piece of wood is shown at right. The wooden block is made up from two pieces of 3/8 inch plywood. The bottom piece is drilled in the center with a 1 inch hole to fit over the central bearing bold of the Coulter. This bottom piece is screwed onto the bearing over the bolt. The second piece of plywood is drilled with a 1/4 inch hole to mate with the encoder shaft. The second piece of plywood is bolted onto the first piece as shown.
The wires from the two encoders are spliced onto an 8 wire two line phone jack cable as shown at right. Click here for the wiring diagram.
And finally, the 8 pin jack is connected to the JMI Micro Max computer which has been velcroed to the side of the scope.
A note about setup. The JMI computer requires setup so that it knows the number of electronic pulses to expect from the encoder on each revolution. Setup also wants to know which direction the encoder is turning.
On the installation I described here, the azamuth shaft is fixed while the azamuth encoder housing is allowed to rotate, and the altitude encoder housing is fixed while the shaft is allowed to rotate. Both encoders produce 4000 pulses per 360 degree rotation. I carefully read the JMI documentation regarding the setting for encoder to determine the best way to set the computer. Placing the altitude encoder on the opposite bearing would have reversed one of the readings, so each installation will produce a different setup.
It took a few tries to get everything right. It took a few tries to get the azamuth readings right. The Coulter azamuth bearing is nothing more than a bolt that passes through from the bottom of the ground board through the rocker box. The holes through which the bolt is a bit larger than the bolt so the azamuth readings were error prone.
I cut a piece of transparency plastic and wrapped it around the bolt, and then used a mallet to tap the bolt up through the hole in the ground board. This very firmly held the bolt and kept it from rotating.
I cut a second piece of transparency plastic and wrapped it around the bolt and eased it up through hole in the bottom of the rocker box. I wanted the rocker box to be adjustable in azamuth, so the fit could not be as snug as it was for the ground board. With this in place, I then epoxied the nut onto the bearing bolt, and then epoxied the wooden encoder mate onto the nut. The azamuth encoder was then attached to the rocker box to complete the assembly.
35mm Panoptic - 42x
