The shutter cables are additional PC operated devices. They are needed for Canon EOS cameras with Digic 3 processor in order use mirror lock feature. For Digic 2 camera such cable is need to make both - to use mirror lock feature and to control the shutter. Also some Nikon models need such cable to control bulb exposure.
There are different types of shutter cables that are supported by APT:
"Do-It-Yourself" instructions for cable based on Relay Boards. No soldering needed!
Shoestring Astronomy DSUSB/DSUSB2 - Canon and Nikon
Shoestring Astronomy DSUSB/DSUSB2 long exposure shutter controller. Once a DSUSB or DSUSB2 option is selected from the “Long exposure control” list, APT will try to make a connection to the controller. More information about DSUSB/DSUSB2 can be found on http://store.shoestringastronomy.com/products_ds.htm
Serial Cable - Canon
Serial control cable (RS232) connected to COM port or to USB (with appropriate Serial to USB converter). If you don't want to make such cable yourself, here is a vendor.
Serial control cable (RS232) connected to COM port or to USB (with appropriate Serial to USB converter). Currently there is no way to buy a ready to use cable for Nikon camera. Here is a discussion how to make it yourself
