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Digital Setting Circles


Shortly after completing a truss type Dob telescope, a friend loaned me a set of low resolution encoders and an old Advanced AstroMaster Digital Setting Circle (DSC) computer. He said, “You can use these ‘til the cows come home.” Even though I was getting very good at finding objects using my atlas, PDA with Planetarium and hopping my RACI finder, I decided to give them a try.

As fate would have it, that old Advanced AstroMaster would power up, but just did not work any more. Too many years in the garage I guess. (Those cows never left the barn.) At first I wasn’t sure which was broken, the AstroMaster or the encoders? Testing with a multi-meter determined the encoders were working fine. Old Advanced Astromater Digital Setting Circles 
Typical PIC and Encoder Output Signal 

I looked on the internet for a way to connect the encoders to my Laptop and/or PDA. I found several very informative websites that discuss how digital setting circles worked. Dave Ek had a very useful site, . The heart of Dave’s digital setting circles design is a PIC. The PIC monitors two encoders, one for altitude and the other for azimuth. It also is used to report their relative positions to another external computer, a laptop or PDA, using a serial communication port. The encoders used by Ek were optical type. Optical encoders have two channels, A and B. The PIC monitors both channels, determines if an encoder is turning one way or the other and it counts the square wave pulses produced by the etched wheel inside the encoder breaking a beam of light. The PIC’s program can determine which direction the wheel turned because the two channels are out of phase by ½ of a square wave pulse. Optical encoders are available through US Digital,, and can be purchased with as many as 10,000 countable events per revolution


As mentioned above, the encoder counting circuit communicates with an external computer, a laptop or Palm Pilot via a serial communication port. All of the heavy duty astronomical calculations are done by the software on the laptop and/or in the Palm. All the “box” has to do is count up or down, remember those counts and send them back to the external computer when ask to. This is the important part. The “box” just counts, up, or down, over and over. It only transmits the encoder position when instructed to so by the external computer. That’s it! Easy, right? The trick is, the DSC unit has to count very fast to keep up with the encoder pulses. It also has to count while communicating with the main computer without missing a beat.

US Digital Optical Encoder 
 First Bread Board Phototype

Ek’s DSC system used an older PIC that was very popular at the time. Ek’s program is available online in HEX format, but I found it did not work with the PIC I had. Since I had done a few other hobby projects using PIC’s, I felt I might be able to “clone” Ek’s box with the parts I already had in the drawer.

Turns out, this was easier-said-that-done. I use a 16F127 PIC instead of one Ek used. The 16F127 has a built-in UART (serial port) making serial communication very easy. I had a bench top “clone" working within a few days. It didn’t work well, but it worked. The basic circuit has very few components. It has the PIC, and a Max232 IC which protects the PIC from serial port voltage levels. The Max232 has a few associated capacitors. There is a voltage regulator too, that keeps the supply voltage at a steady +5V. This regulator is also flanked by two capacitors. 

Circuit Schmatic  

I came across another very useful website, Doug Braun wrote an excellent Palm OS application, PalmDSC. It did exactly what I needed. PalmDSC is freeware and can communicate via the Palm’s serial (or Bluetooth) port to a DSC box like Ek’s or my clone.

I decided to try connecting my Palm IIIx to the bench top clone. After tracking down, two bad soldered joints in the cable I made, I had my old Palm connected to the “clone” (Doug is a good guy and was willing to give me lots of help troubleshooting my system from start to finish.)

My bench top version had two problems. Firstly, the code I wrote could not count fast enough. Sometimes the bench top unit would get lost, especially during rapid encoder movements. Other times it would lock up. Mike, a true programming expert, provided a suggested fix for the slow counting issues. It worked like a charm. My son, another smart guy, was able to eliminate the conflict between the PIC serial port and encoder monitoring routines. He did this using an interrupt protocol giving priority to the encoder counting routines while buffering the serial port. The final code was bullet proof thanks to Mike and my son.  


Bread Board Circuit and PDA
Printed Circuit Board Layout

It was time to move up from the bread board to a prototype unit. This required designing and making a printed circuit board. I used a program called Eagle which can be found at the following site, (If you don’t like Eagle there are other programs available for free on the internet. Most printed circuit board suppliers have design software available for free.)

I purchased a 2 x 4 “box” at my local Radio Shack and designed the prototype PCB around it. I had read that you can use a laser printer to print your PCB traces and then transfer that image onto the copper of a PCB using an iron. This only works with a laser printer that uses toner and a fuser system. Ironing re-softens the toner so it sticks tightly to the copper. Make sure the copper is bright and shiny first. Brillo works for that. It took some experimenting to figure out how to get the paper off the toner without lifting the toner off the copper. The best method I found was to wet the paper. With careful rubbing and scratching between traces, most of the paper can be removed without damaging the toner. Some of the paper fibers remain stuck to the toner, but not enough to stop the copper etch from getting to the copper between the toner covered traces. I like to drill all the holes before etching. This seems to work best for me. I drilled each hole by hand using a drill set purchased at Radio Shack. This took a little time and patience. Then into the etching solution it went. After etching, the toner was gently removed using a Brillo pad. The PCB board was trimmed to fit the enclosure. Next I populated the board. During testing I found I had several traces that looked fine, but actually had breaks in them. Before I was done I checked every trace end to end to find all the breaks. I was able to fix all of them with solder. Again, Ok for a prototype.  Etched Board Preparation 
 Finished Black Box Assembly Encoder Mounting Locations
Final Installation

The last task was to mount the encoders on the telescope. I used a standard arrangement for the altitude encoder. I found a way to make mounting hardware with little additional expense. I purchased two plastic radio knobs each having a set screw. Encoders have ¼” shafts. The plastic radio knobs fit nicely on them and were could be anchored to the shafts using the set screws. Each knob cost $2. I Gorilla glued one into a plywood fixture mounted on the center axis of the altitude bearing. The altitude encoder was mounted on a long thin plywood arm. The loose end of the arm was gently restrained, but not anchored. The final version the azimuth encoder mount was actually in a very similar too the altitude mounting, except the plastic radio knob was Gorilla glued to a long plastic arm and the encoder was hard mounted to the center of the azimuth box using thin plywood. [Finding a successful azimuth mounting detail took several attempts. When I designed and built this scope I really never planned on installing DSC’s.] 

I velcro’d the “box” inside the mirror box and the Palm IIIx just above the focuser.

While I was at NEAF this year, 2013, I stopped at the SOUTHERN STAR table to speak with them about their product, SkySafari.  I had been following this "App" online for some time but was unwilling to take the plunge without talking  "technical" to a real person.  After a few minutes with the SOUTHERN STAR folks, I decided to give it a try especially after they confirmed that their software was compatible with Dave Ek's DSC box.  [My PIC was able to emulate Dave Ek's box perfectly.  I was fairly sure it should be able to communicate with SkySafari like an Ek Box would. ]  


You need to purchase SkySafari Plus or Pro to get the telescope control capability.  I went with the Plus version since is has most of the objects I normally go for with my Dob.

Adriod Image

US Converter Bluetooth to Serial

Which serial to bluetooth converter to buy?

I found the USCONVERTERS had the best pricing.

It can be powered using an external power source, through a mini USB or through pin 9 of the serial connector.  I selected to power mine using pin 9.

My DSC system is powered by a rechargeable 9 volt battery.  Since the PIC is a 5 volt device my DSC board already has a 5 volt regulator onboard. I used that 5 volt source to power my converter through a spare wire I had in my DSC cable.

The converter is shipped with factory default serial protocol settings.  I reset several of these using my desktop computer so that the converter serial port setting matched those already used by my DSC box.  

After some trial and error, I was able to connect my Droid smartphone to SkySafari through this new bluetooth connection.  SkySafari works very well needing only a two star alignment to synchronize the Dob mount with the sky.

Once aligned, 9 times out of 10, I can find the object I am looking for well within the eyepiece after locating it with SkySafari Plus.  

If it is not within the eyepiece, I simply find a nearby bright star, point the Dob at it then I trade that star for one of the two alignment stars I started with and BAAAM...object I am looking for is dead center in the eyepiece when I go back to it.