Requirements and Design Thoughts

The ETX70 requires up to 500mA when running both motors at full speed and the autostar illumination being set to brightest possible. The telescope of mine is used in Alt/Az-mode with the autostar's illumination set to minimum (still rather bright) which results in a input current of about 200mA during tracking. My experience was, that the scope was running fine using accumulators, resulting in a voltage of  7.2V. I guessed that everything between 7V and 9V would be fine to run the scopes electronics.
During night, in particluar when the night is dark and therefore good for astronomy, the human eye's colour sight capacity is rather limited which would make it hard to distinguish between the positive and the negative connector of the external power source for the ETX. To overcome this, I added a rectifier bridge to the "design". The rectifier dissipates already Pdiss=(2×0.6V)×I when DC is running through two of it's four diodes, thereby heating up a little...
The voltage regulator family 78xx gives positive polarity regulation with a current up to 1A. The regulation is linear, meaning that the power Pdiss=(Uin-Uout)×I is being converted to head, making it necessary to apply a head sink to the regulator.
Example: The scope being run from a 12V lead gel accumulator. The "loss" at the rectifier being 1.2V. Therefore the 7808 (Uout=8V) "sees" just Uin=10.8V. With the scope just tracking (which it will do most of the time) the current will be about I=200mA, resulting in a dissipated power of  about Pdiss=0.56W, which will be turned to heat.
When running for hours, the heat sink reached a little above human body temperature.

---

Schematics

...click to see in full scale...