Autoguiding is a multi-step function. First, the autoguider camera takes an image. Next, the software measures the position of a guide star in the field, and calculates the movement required to recenter it. Next, a command is sent to the telescope to cause that motion to happen. Once the motion has fully completed, the autoguider camera is triggered to take another exposure.
This command must be sent to the telescope mount somehow. Often this takes the form of an electrical pulse, which essentially "pushes the button" on the telescope mount's hand paddle. The longer the pulse, the farther the telescope moves. The telescope must be set up for guiding speed, e.g. 0.1X to 1X sidereal, in order for this to work. Typically the 0.5X rate is recommended, if it is adjustable. Some telescope mount models have a dedicated "CCD" jack that always provides guide speed; for others you must set up the hand paddle appropriately before starting to guide.
Over the years, a number of different connection methods have been devised to send the autoguider pulses, or to emulate sending the pulses digitally. You must choose a method and set it up using the Control Via function on the Guide tab Settings. The most popular methods are as follows:
Guider Relays or Main Relays
Some cameras include a built-in port that can be connected directly to a telescope mount, using an RJ-11 (telephone cable) connector. Cameras supporting this feature include all SBIG models, current-model Starlight Xpress cameras, and some special-purpose autoguider cameras such as the Orion StarShoot Autoguider. Normally Control Via is set to Guider Relays, but if you happen to have a main camera with an autoguider port, you can use it instead by selecting Main Relays.
Telescope Control
Many ASCOM-compatible GOTO telescopes accommodate a special mode called PulseGuide. This allows the pulse commands to be sent via the RS-232 or USB cable used to control the mount. Most modern GOTO telescopes accommodate this; however, there are exceptions. Common exceptions include some older-model Celestron telescopes and any telescope controlled via TheSky planetarium software.
In order to use this method, the Observatory Control window must be linked to the mount. Set Control Via to Telescope.
Note that this method does not send GOTO commands to the mount. In the vast majority of cases, the mount has a minimum slew distance that is far too large to be used for autoguiding.
ASCOM Direct
Certain third-party pulse output devices use ASCOM Telescope drivers, which cannot GOTO but can PulseGuide. ASCOM Direct is used for accessing these devices.
Shoestring Adapter
The Shoestring Astronomy GPUSB is a small device commonly used for sending autoguider pulses to the mount. It is a small box that connects directly between the computer and the telescope. Select GPUSB to connect to this device.
MicroGuide
A very small number of telescope mounts can perform a GOTO operation small enough to be useful for autoguiding. In this case, you can use the MicroGuide mode. MicroGuide sends commands through the Observatory Control window and the ASCOM driver, but it actually sends GOTO slew commands rather than PulseGuide commands. (Note: as of this writing, we recommend using the Telescope option instead of MicroGuide.)
Other Methods
Other methods include the Mt. Stromlo serial protocol, SX serial protocol, Cookbook camera serial protocol, LX200 serial protocol, and the Aquest AstroHub. Some third-party autoguider interface devices emulate the Cookbook or LX200 protocols; please refer to the manufacturer's guide for more information. For additional detail please refer to Guide tab Settings.