mdurkin I was expecting to see something like Shoestring adapter/GPUSB in the list of available mounts
Lets first clear up some fundamentals.
The reason for autoguiding is that you want to accurately track the position of an object for a long (say 100 to 200 seconds) duration. An object in the night sky moves about 15 degrees (angle) per hour of time. If not tracked, the object would have moved a whopping half a degree (the diameter of the Moon), in 120 seconds of time.
Fraunhofer invented a simple method of mounting a telescope (today called the "German mount"), where the stars can be tracked easily with a single clockwork driven shaft that is pointed perfectly at the north celestial pole. It requires two things: the shaft is driven at a precise rate and the shaft is pointed perfectly at the pole -- neither of which is particularly easy. The gears that drive the shaft (polar axis) will have imperfection that more or less repeat itself every revolution of the gear (each repeat is a period, and thus "Periodic Error.") On top of that, the object will also move in the declination of the mount unless the mount is perfectly aligned (this is the "drift," and causes an object to behave as if it is rotating in the field of view).
Thus, for long exposure astronomy, a long focal length guide telescope is strapped to the main telescope. It has illuminated cross hairs and you make small adjustments to the mount axes when a "guide star" moves away from the cross hairs. The easiest way to do this is to make the small adjustments to the RA and Declination motors.
The early attempts to make these adjustments is by intercepting the drive to the motors with small relays. When cameras became cheap so that anyone can afford to spare one for guiding, the process is automated by using a camera to measure the movements, instead of a human looking through the guide telescope.
Sometime in the late 1980s or early 1990s, SBIG (now owned by Diffraction Limited) introduced a turn-key guiding system to go with their model ST-4 camera. (SBIG cameras have an ST (and later, STF) prefix -- I still have the early ST-8300). There are two sets of relays, one (RA+) to move the RA axis in one direction, and an RA- wire to move in the other RA direction. Similarly DEC+ and DEC-. All sharing a common ground.
Since SBIG is known for expensive (but excellent) products, amateurs tried to cobble up simple relay operated boxes, either through parallel ports or serial ports of computers.
These devices hijacked the SBIG trademark, and were called "ST-4" interfaces.
The interface is so simple that the ST-4 circuitry came with many guide cameras, like the ones from Starlight Xpress; I still have an SVX-H9 too.). By then, the relays were also replaced by transistor switches. The ST-4 control is piggy-backed on the USB connection of the camera to the computer.
More and more mounts included the ST-4 interface (today, virtually every commercial mounts have them).
By then, Craig Stark had also made autoguiding easy with his PHD (push here, dummy) software, that ran on both DOS (later, Windows) and Mac OS. When Craig got tired of maintaining the code, he made it Open Sourced, and the program evolved into PHD2. Coincidentally, Craig himself has a Ph.D (but not in astronomy).
In the meantime, people have also discovered that with computer controlled mounts, you can use the slewing commands of the mounts to replace the ST-4 signals. You would change the slew rate of the motors to about the speed of sidereal rate. To adjust the mount oy a small amount, you would turn slewing on, and then off again after an amount of time. Each of these look like a pulse, and this method became known as "pulse guiding." Sidereal rate runs about 15 arc seconds per second of time. So, if you want to nudge the mount by 1 arc second at sidereal rate, you simply make the pulse 1/15 of a second long (67 milliseconds). If you slew by half a sidereal rate, the pulse would instead be 134 milliseconds long.
There are two methods to autoguide. One is by ST-4 and the other is by Pulse Guiding, if your mount is computer controllable.
The ASIAIR has a quirk where if you choose to control your mount (i.e., select a Mount protocol), it would assume guiding is done with Pulses. So, you cannot choose ST4 guiding while also controlling the mount.
If the mount is not computer controllable, then you would select "On-Camera-ST4" as your mount. When you do that, ASIAIR would assume that the guide camera supports ST-4 and all correction signals will be sent through ST-4 that is in the guide camera's USB interface. You simply add an RJ12 cable (looks like RJ-11 telephone cord, but has an extra pair of wires, so you can't simply grab a telephone cord) between the guide camera's ST-4 port and the mount's ST-4 port. You do not need any special rip-off "ST-4 cable." Any one-to-one RJ-12 cable will work, or make your own to make cables of any length (I still have a bag of RJ12 terminators, and a crimping tool, and a spool of 6-pair wires).
If you do not select "On-Camera-ST4", my guess is nothing is sent through the ST4 connector of the guide camera, but it is best to just leave the ST-4 cable out if you use pulse guiding. The extra cable just adds to the imbalance of the mount.
On other systems, you can choose mount controls and ST-4 for guiding instead of pulse guiding. On ASIAIR, if you choose to control the mount (for GOTO, meridian flip, etc), then you cannot use ST-4; the ASIAIR will force you to use pulse guiding. If you want to use ST-4 guiding, you will have to choose "On-Camera-ST4" as the mount, and forego computer controlling your mount (idiotic).
Chen
