Broad Strokes

In my getting started post, I described the design goals for this camera. I can tell you right off that one particular requirement somewhat contradicts all the others. How does one shove all that functionality into a lightweight sensor module? That also begs the preliminary question "How light?"

The answer: as light as possible. Remember, the sensor is going to attach to the telescope far away from the balance point for the optical tube. Every gram is multiplied by the moment arm away from the center of mass, becomes a burden on tracking motors, and can therefore be a real irritation in the field. When I attach a Nikon D80 camera body to my telescope (Celestron XLT120 refractor) I practically have to remount the tube to the equatorial mount to re-balance it. Heck, even fiddling with the focus might cause the balance to be thrown out, never mind adding a barlow lens. So I'm motivated to keep it light, light, light. (Can I get it as light as an eyepiece? Probably not, but that's an interesting impossible goal.)

Fortunately, the first major design decision follows rather naturally. The whole device can be broken into two major parts: the sensor module and the processing module. The sensor module need only contain the sensor chip and the bare minimum electronics to send the signal out through a cable to the processing module. The processing module can contain all the featurisms of the camera, and can be strapped to the telescope near its center of mass. The size and weight of the sensor module can be minimized by removing functionality to the processing module.

The sensor module needs to hold very little stuff. I can probably keep it down to only the CCD sensor, a few ASIC chips (including a tiny FPGA) and the discrete parts needed to handle some non-digital bits. It should be possible to keep the sensor circuit board down to little more then twice the area of the sensor itself. The sensor chip is going to be pretty large as semiconductor devices go, so this shouldn't be much of a challenge. Keeping chip count low (and hopefully minimize the power draw) should have the added advantage of limiting the heat sources in the sensor assembly. Sensors like cold.

The processing module need not have the size/weight limits, so I can go crazy there. And crazy is where I shall go. The basis can be a Gumstix running Linux. This gets me a whole host of possibilities for free, including networking software and hardware. To that I will attach an interface board that includes the electronics needed to communicate with the sensor board. I can also include on the interface board the components needed to manage power supplies (the sensor board will likely have tricky power needs) and even interface to stepper motors. This will allow me to interface not only to the sensor board, but with mount motors.

Actually, there will probably be a third box. The battery pack will likely be something off the shelf that generates some standard voltages (DC) that the processing module interface board can convert to the voltages needed to run the whole system. I like the idea of having Li-ion power for the whole system, but adding the weight of battery cells to the processor module really is a bit over the top. So batteries get a module of their own.

So that's the very broad stroke. A sensor module will contain the CCD sensor and minimal support electronics; a processor module will contain the interesting image processing hardware/software, the interfaces to the mount, and some power supply management; and a battery pack will supply the power. Next post, I will probably discuss my choice of sensor chip.