EQ-6 Polar Scope Illuminator

The EQ-6 has a built in polar finderscope, but it is hard to see its graticule against a dark sky. Later models have a built in polarscope illuminator, but all mine had was a suggestion in the instruction manual that you shine a red LED torch into the sky end of the polarscope. This is difficult to do while looking through it, unless you have very long arms. However, it’s quite easy to install a red LED inside the mount that shines on the polarscope whenever the mount is powered.

The photo shows my polar scope illuminator. After removing the drive electronics from the mount I added a small piece of perforated board on “stand-off” screws. This board is used to hold a small LED and two 10kΩ resistors, which restrict the current to about 0.5mA. Switched 12V is taken from suitable points on the main PCB. I used the smallest LED in my scrap parts bin to minimise the “central obstruction” it creates. I also carefully bent the leads so they are in line as seen through the polar scope.

When the drive electronics are reinstalled the LED should be positioned on the polar scope axis and shining directly at it. This can be checked by looking down the axis from the “sky” end. It is easy to bend the LED leads as required.

More recently I’ve installed the “SynScan” GOTO upgrade in my mount. This involves replacing the drive motors and electronics. The new circuit board has a 2 pin connector to provide power to a polar scope illuminator LED, so I removed the two resistors and connected my LED to this plug, after finding a suitable socket in a scrap computer.

Unfortunately the LED did not work with the new circuit board. The voltage across the LED was barely 1.2 volts, so I was back to shining a red torch into the scope until I investigated further.

Tests with a multimeter showed that the LED socket is connected to +5V via a 75kΩ resistor, called R2 on the circuit board. This limits the current to 66μA, not nearly enough for my old LED.

The solution was to solder a 6.8kΩ resistor in parallel with R2. Fortunately there are suitable holes in the circuit board: a through hole connecting R2 to +5V and an alternative mounting hole for the LED socket. You can see the extra resistor next to the word LED in the photograph. (Note that there is no need to remove R2, so no surface mount soldering skills are needed!)