Hi Paul
This is straight out of the Manual:
4.2.3 Connecting an Autoguider to an Older Gemini
4.2.3.1 Connecting an Autoguider with Relay Output
Autoguiders with relay outputs (such as the SBIG ST-4) can be connected to the Gemini
system directly. Simply connect one side of each of the four relay contacts to the
Autoguider input common pin, and connect the other side of each relay to the appropriate
direction input pins of the autoguider port. Cables are available for most compatible
autoguiders to plug directly between the autoguider and Gemini.
4.2.3.2 Connecting an Autoguider with TTL Output
The typical "open collector" output stage (e.g. the ST-7/8/9/10, user-modified ST-4) has
high impedance while inactive and pulls down the direction signals to a common ground
when active. This is exactly the opposite of the way the Hand Controller and autoguider
ports of the older version Gemini system work.
A relay (or optocoupler) decouples the switching device (the Autoguider) from the
controlled device (the Gemini), thereby avoiding a direct connection between the two
systems and protecting both systems against high currents that could cause damage.
IMPORTANT NOTICE:
(1) Never connect TTL output stages directly to the Hand Controller or autoguider ports
of an older version Gemini! Severe damage may occur to the connected devices. Use a
Relay Box (as provided by SBIG) or the optional Losmandy Optocoupler unit instead.
By so doing, you get the additional advantage of providing electrical protection to these
sensitive units.
(2) The use of two separate power supplies without a coupler unit is strongly
discouraged. To do so would connect the ground of one system to the supply voltage of
the other system, causing the two grounds to be at different potentials. If the grounds of
the two systems are then connected, severe damage is likely to occur.
Detailed instructions for connecting an SBIG camera with TTL outputs to the older
version of Gemini can be found in Appendix 8.6 (Autoguider Port).
I should add to that, as far as I am aware, the Shoestring thingy is an Optocoupler anyway.
Cheers
John G