Assuming you mean an LX200R, and the mainboard is similar to the LX200GPS, then a mob in the states has reverse engineered the boards and supplied circuit diags.
http://www.telescopeservice.com/lx200gps.htm
For the 12V output, based on these diags ( Mainboard IO ), there is an inline 5ohm resistor, and the switching is provided via a P channel mosfet.
Thus ohms law will tell you what you can draw before the voltage drop across the resistor gets too high, or the bits fry.
Ie if you draw 500mA, you will get a 2.5V drop across the resistor
so if yr supply is 12V, you will only be getting 9.5V out
So, its not real useful for any large loads