The Ubiquitous ST-4 Port

Many telescope mounts have these ST-4 autoguider ports, including my Celestron CPC-800. My ASI120MC-S camera includes a “ST-4 compatible” autoguider port and came with a cable for connecting it to the telescope mount. I wanted to know more about how the telescope responded to inputs on the ST-4 port.

There’s lots of information about the storied history of the ST-4 port from its appearance on the SBIG cameras and before that. I like this simple description from shoestring astronomy. Other descriptions abound on the web and they mostly talk about the hardware connections and cabling, and mention that the auto-guider motions are controlled by simple switch (or solid-state) contact closures.

But what does a contact closure do and how does it relate to setting up an autoguider? And, if the telescope is already tracking at the usual sidereal rate, how much motion is added or subtracted? What happens if the tracking is turned off? I set out to answer these questions.

I decided to make a breakout cable to do the simplest of tests on my CPC-800. I brought out the four connections and ground wire (pin 2 of the RJ-12 connection), and shorted the connections one-by-one to the ground lead as I watched a distance fixed object through the telescope. One could also wire these to four switches to make a manual fine guiding control box. Yep, it worked – the telescope executed fine motions.

Connecting each of the four lines DEC+, DEC- and RA+, RA- added-to or subtracted-from the sidereal motion of the telescope. What wasn’t immediately obvious to me, is that the autoguide rate is settable from a menu in the CPC-800 hand controller. It’s expressed as a rate from 0 to 99% and mine defaulted to 50%. I tried it at 99% to make the motions more apparent and I set the scope up for equatorial tracking.

So, if the the scope tracking along at the normal sidereal rate, closing one of the RA connections caused the scope to speed up to about twice its normal rate for as long as I held the connection. Closing the opposite one, well – stopped the motion? Pretty much. It reduced the motion by 99% of its normal speed, and this essentially stopped moving. But I noticed that every once and a while there would be a slight lurch. Perhaps if there were a 100% autoguide rate, the motion would stop completely? For declination, the telescope moved up or down by about what I perceived to be the sidereal rate (in equatorial tracking mode, the declination axis normally doesn’t move, as it’s assumed the telescope is polar aligned and need only move in right ascension).

This all pretty much made sense, but I found it instructional to observe in real life while I was directly in control of the four connections. I also set the autoguide rate to 10%. That did about what I expected – i.e. it slowed down or sped up by about what I perceived to be 10% in right ascension, and moved up or down by about 10% the sidereal rate in declination.

This helps set into context for me some of the settings available the lin_guider software that’s part of iAstroHub. I’ll be experimenting with this for sure if the weather ever improves! It’s April 6 as I write this. It’s 33 degrees Fahrenheit and light snow is falling! Happy spring!