The most essential accessory for astrophotography, after a camera, scope and mount, is a remote shutter release to reduce vibration. A remote control is even more useful if it can trigger a series of exposures at preset intervals and of preset duration.
With my Nikon Coolpix 4500 I used a Palm PDA and serial lead with the excellent CoolRemote software. I've recently bought a Canon 350d DSLR (this model is also sold under the ghastly name "Digital Rebel xt") and soon found some software called PalmDSLR. However, some electronics is needed to connect the Palm serial port to the camera's remote control port. There are several web sites showing how to build an interface. I found those from Cyrille Thieullet (the author of PalmDSLR) and Michael Covington particularly helpful.
I decided to make a dual purpose remote control. It has push buttons for manual operation as well as an RS232 socket to connect to a Palm PDA (or other computer) for automated control. Here is a circuit diagram:
This diagram is also available as a PDF file that can be printed out at higher quality.
None of the components is particularly hard to find, and Covington lists some suitable alternatives for the 4N25 opto-isolator.
I built the adapter in a small plastic box, about 80x50x20mm in size. For neatness I wanted to use a lead with a moulded on 2.5mm jack plug. I considered buying a suitable ready made audio or video lead and cutting the unwanted plugs off, but found I had a suitable lead already, attached to an unused mobile phone "hands free" set. The lead enters the box through a grommet to give some protection.
Inside the box is a lot of empty space! All the electronic components are mounted on a small piece of circuit board in the middle of the box.
Here is the remote control connected to camera and PDA. The camera can be focused by pressing the black button, then the shutter can be operated by pressing the red button or by the PalmDSLR program running on the PDA.
Finally, a brief word about choice of Palm model. I had intended to use PalmDSLR with my shiny new Tungsten E2, but soon discovered that this model, and others with the "multi-connector", only has the RS232 data wires (2, 3 & 5) connected and so cannot control the CTS wire. Luckily I still have my old Palm Vx, which does work with PalmDSLR — leaving my E2 free to control the telescope mount.