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). 

4.2.4  Connecting an Autoguider to a Newer Gemini 

The Autoguider port on the newer versions of Gemini is plug compatible with a large 

variety of autoguiders.  You can use an autoguider with relay outputs such as the SBIG 

ST 4 and STV or an autoguider with TTL outputs such as the SBIG ST 7/8/9/10 directly 

using the appropriate cable.  Use SBIG' s   TIC   cable for the ST 4 and STV, and SBIG' s 

  TIC 78   cable for cameras with TTL output. 

The SBIG relay box can be used, but is not necessary.  The Losmandy Optocoupler is not 

compatible with the newer Gemini. 

4.2.5  Software Guiding  

An alternative to using the autoguider port is to write your own software to send guiding 

commands over the serial connection using the LX200 protocol. Although there may be 

small delay or jitter caused by the serial transmission, it should be adequate for most 

purposes.  

4.3  Periodic Error Control (PEC) 

4.3.1  What is Periodic Error? 

The Gemini servomotor on the RA axis turns a worm that meshes with a worm gear that 

moves the telescope.  Ideally, the interaction between the worm and worm gear is such 

that for a constant speed of the worm, the worm gear also turns at a constant speed.  

Gemini Users Manual 

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