The Ubiquitous ST-4 Port

Many telescope mounts have ST-4 auto-guider ports, including my Celestron CPC-800. My ASI120MC-S camera includes a “ST-4 compatible” auto-guider port and came with a cable for connection 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 earlier. 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 contact closures.

But what does a contact closure do and how does it relate to setting up an auto-guider? 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 from the RJ-12 connector to do the simplest of tests on my CPC-800. I brought out the four declination and right ascension connections (DEC+, DEC- and RA+, RA-) and the return wire, and shorted the connections one-by-one to the return lead as I watched a distant fixed object through the telescope. Yep, it worked Рthe telescope responded with each connection. Motion was added-to or subtracted-from the sidereal motion of the telescope.

What wasn’t immediately obvious is that the auto-guide rate is settable from a menu in the CPC-800 hand controller. It’s expressed as a value from 0 to 99% and mine defaulted to 50%. I tried it at 99% to make the motions more apparent. I had set up the scope in equatorial tracking mode so there should normally only be movement of the right ascension axis – which is the way one would use guiding in real life.

So, with the scope tracking along at its default sidereal rate, closing one of the RA connections caused the scope to speed up to about twice that rate for as long as I made the connection. Closing the opposite one, well – stopped the motion? Pretty much. It apparently reduced the motion by 99% of its normal speed, and this essentially stopped the tracking while I held the connection closed. But I noticed that every once and a while there would be a slight, very fine jog of the telescope. Perhaps if there were a 100% autoguide rate and with this opposite RA contact closed, the motion would stop completely? For declination, the telescope moved up or down by about what I perceived to be the sidereal rate.

This all pretty much made sense, but I found it instructional to observe the behavior with me in control. I also tried setting the auto-guide 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 to set into context some of the settings available the lin_guider software that’s part of iAstroHub running on my Raspberry Pi.

Another take-away from this – what does the camera have to do with controlling the auto-guider port? It’s apparently just a convenient interface so that the auto-guider software (running on the Raspberry Pi in response to images from the guide camera) doesn’t need a separate interface or connection to the mount. I think of it as a USB to ST-4 converter, conveniently included as part of the guide camera with its existing USB interface. The camera itself doesn’t contribute to auto-guiding beyond feeding images to the Raspberry Pi and providing this interface.

I’ll be experimenting with this further 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!

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