This is a very complex topic. The complexity can be ignored by just putting any camera on any telescope, click the shutter and see what you get. With some subjects, this may work and in fact, work well. Generally, anything you can already see with the naked eye or with the use of a low powered telescope may yield truly satisfying and sometimes glorious results. However, for the majority of faint subjects, this approach won't work; you can land up with images so disappointing that you give up CCD imaging forever and simply rely on what the good old HST produces for us after a simple Google search. I prefer the Google option, but for those who like pain, read on....

How a CCD Camera Works

A CCD (Charge Couple Device) sensor is a flat wafer on top of which is placed light sensitive “pixels” arranged in rows and columns like the cells of a spreadsheet. Each pixel has the ability to receive photons from a light source and change them into electrons. The electrons in turn build a charge which can be stored temporarily by capacitors. From here, the charge is moved down to a gate and then off to an Analogue/Digital (A/D) converter. The A/D converter then sends a digital picture in the form of a file full of numbers (just like a spreadsheet again) off to a computer for further processing. Most astronomical CCD devices have 16 bit A/D converters, whereas your basic consumer digital camera may have 8,10,12 or 14 bit A/D converters. The higher the bits, the more distinct shades of grey that can be rendered by the sensor; more is always better but results in much larger files.

Each pixel can only collect so many photons before it becomes “full”. At this “full well” capacity, the shade of grey rendered by that pixel is pure white. Any more photons collected by this pixel (due to over exposure)  will cause “blooming” or the spillage of charge into the neighbouring pixels. The full well capacity depends on the size of the pixel – the larger the pixel, the more photons it can collect before becoming full. Blooming makes itself known as a bright white line ejecting from the centre of a star, much like a side on image of Saturn's rings as seen below.

There are two main types of CCD cameras: Anti Blooming Gate (ABG) and Non Anti-Blooming Gate (NABG) cameras. The ABG cameras bleed off charge from each pixel so they can never bloom, but are about 30% less sensitive as their NAGB counterparts. They each have their place in astro-photography, but it is good to start with an ABG type sensor. Fortunately, our SBIG ST7XE is just such a sensor!

All CCDs and CMOS sensors can only “see” shades of grey. To get colour, various techniques have to be employed such as placing colour filters over groups of pixels to render differing shades of Red, Green and Blue (RBG). While your pocket digital camera has these filters, many advanced astronomical CCD cameras do not. You have to add these in manually which adds to the complexity and length of time for taking colour images in astronomy.

An image is rendered on your computer's screen by simply reading the raw image file containing numbers and converting those numbers into shades of grey on the screen's pixels. This is a little simplified, but good enough for now. So for an 8 bit image that has only 256 shades of grey per pixel, (or 256 shades each of R,G,B for a colour image), black is represented by the number 0 and white by the number 255. It turns out that 256 shades of grey are good enough for the human eye to visualise the shades/colours as life-like, though our eyes can resolve many more shades. Your basic JPEG internet image uses just 8 bits worth of shades for their RGB components. Our CCD can produce 65,536(!) shades of grey, so it can render those delicate deep sky objects way better than the most expensive DSLR camera around. Oh wait, I think the Hasseblad has a 16bit A/D converter too – but you have to give up a liver for one of those.