If you’re about to choose a charged-couple device, or CCD, camera for astrophotography, then there’s a seemingly bewildering set of factors that you have to take into account before you make your final decision.

Obviously, cost is the major one, but even if you don’t have a huge budget, there’ll be a few models available to you, so what else do you look at?

The different types of CCD cameras

There are several different types of CCD camera and the main differences are to be found in the sensors, although there’ll also be other features that will make a particular camera more or less useful and attractive to you.

The sensor size

The bigger the CCD sensor, the bigger the area of the sky it’ll image. However, if you’re using a smaller sensor with a short focal length telescope than it might have a wider field of vision than a long focal length telescope combined with a larger CCD. It depends on what you’re imaging, really – a smaller sensor may be enough. You may want a bigger sensor, but if the numbers don’t add up… You also need to factor in the size of the chip.

The pixel size

This is the subject of many a lively debate. Some think that there’s a kind of sweet spot where CCD pixel size and telescope focal length meet to produce optimum results. This idea has more standing in scientific applications, where the highest resolution is needed and aimed for. If all you’re after is nice-looking images of galaxies, then pixel size isn’t a big deal.

Smaller pixels make for higher resolution, but larger pixels can take in more light, so there’s always going to be a balancing act. In general, the number of pixels and the chip size are more important than pixel size when it comes to astrophotography.

The number of pixels

Having more pixels is better, most of the time. Having more pixels can lead to slower download and processing times later on when the image is being manipulated and enhanced, but this is becoming a thing of the past. Many CCD cameras now have USB connectors and our computers are only becoming faster, so this problem will fade into the distance. The advantage of more pixels is larger images on your PC monitor, larger prints and better-looking images even when they’re compressed for web usage.

Do you want one-shot colour or tri-colour imaging?

A recent innovation in CCD imaging has been one-shot colour. Generally, CCD chips have been black and white and they have needed to use coloured filters (green, red and blue) and at least three exposures (one through each filter) to create a colour image. These new CCDs have chips that are divided into red-, green- and blue-sensitive pixels so just one exposure results in a colour image. These new chips can be a bit less sensitive than their B&W counterparts, but then again, only one exposure is needed. Additionally, these colour chips are becoming ever more sensitive.

There’s still a place for B&W CCDs, though. They’re better for narrowband imaging and applications like asteroid searching and imaging. Again, it depends on what you’re planning to do.