Once you’ve captured your images of the night sky, you’re probably going to need to process them. As I keep saying on these pages, with very few exceptions, astrophotography stretches the limits of what cameras are designed to do. We collect tiny amounts of light from very faint objects, and to make it work we use very long exposure times and very high ISO settings, which maximises the amount of electrical noise generated by the sensor. This noise can be limited by cooling the sensor, but can never be fully eliminated. The result is a picture that is grainy, and which doesn’t look like much of anything at all. The next step is to cut out the unwanted signal and enhance those parts of the image that we want to see.
Once you’ve finished for the night and have a camera full of images, you then need to begin processing the data you’ve captured. There are a large number of techniques to reduce noise, bring out the fine details, and otherwise enhance the image, but these techniques must all be done in software on a computer. Most people are familiar with Photoshop, which remains popular for post-processing of astronomical images, but there are many other tools that can be used. Some are expensive, some are cheap, and many are free to download. Some are open source (many of the free packages, for example), while the rest are not. And some are general photo processing packages, while some are specifically designed with astrophotography in mind.
All of these packages need to be run on a suitable computer. Unfortunately, recommending the right computer is very hard because technology marches along at such a rapid pace. Yesterday’s supercomputer is today’s desktop workstation is tomorrow’s landfill. As computers improve, so software gets written to take advantage of the latest capabilities, which is why software written this year probably won’t work on a computer from fifteen years ago, and why today’s most powerful systems won’t be very useful a decade from now. This is why computer owners have become used to spending a lot of money on quality hardware, only to throw it away after a few years to buy a whole new system that never feels much better than the old system did when it was new.
But there is good news: old computers and old software don’t actually deteriorate over time (although old operating systems certainly do, as they fill up their filesystems with old temp files, stale registry settings, and the cruft generally builds up), so if you have a system which you can dedicate to image processing, and it hasn’t actually broken down, there is no need to replace it for something more modern. If you upgrade your camera to something with a higher bit depth, or more megapixels, you’ll find that the larger image sizes slow down processing, but the job will get done as effectively as before. But if you still need to get set up, then it’s worth looking at what’s most important for image processing so that you can spend your money where it’s needed.
Your computer is going to need enough disk space to store your raw images, your final printable images, and every temprorary image created as you go along. A single night’s observing and processing easily consumes 30GB with my current system. This doesn’t seem like much by today’s standards, but it adds up rapidly. You’ll want to keep all this stuff too, so that you can go back and try again with older images as you learn new techniques and become more experienced. If you have the money, it’s well worth it to spend the extra money on a solid state disk (SSD) instead of the traditional spinning magnetic disk type.
The more memory, or RAM, is installed in your computer, the faster it will be able to process images. This is because image processing involves working with very large amounts of data as image files are unpacked, decompressed, copied into temporary locations, and reassembled. This means lots and lots of data being written to and from the hard disk, and traditional hard disks are slow. To deal with this, computer operating systems allocate a large chunk of RAM to act as a temporary storage area, or cache, where frequently or recently accessed data can be left until the computer has a free moment to actually do the donkey work of writing it back to the disk. Meanwhile, if the file is needed again, it’s right there in memory and can be used without having to waste seconds trying to find it on the disk. SSD’s are significantly faster since they have no mechanical parts that have to be shunted over to physical locations on a disk, but they are still slower than working RAM. If your eyes glazed over reading that last paragraph, just remember this: You want lots of RAM – whatever your operating system specifies as “Recommended”, plus another gigabyte or so, would be a comfortable starting point. Less than this is going to slow you down.
What may be surprising is that the graphics card, usually touted as the most imortant (and powerful) component of a graphics-heavy systems, is completely unimportant for image processing. Modern graphics cards are really graphical accelerator cards – powerful co-processors that are heavily tuned for the cumbersome calculations needed to render photorealistic graphics for games and certain elements of modern computer interfaces. They are very good at calculating the position of millions of objects in a vierual three dimensional space, and creating an animated visual representation of them on your computer screen. However, image processing needs none of those capabilities – a single static image on a screen doesn’t need anything more powerful than a 1990’s vintage SVGA card (although technically those old cards couldn’t map enough pixels at a high enough bit depth to drive a modern monitor… but you get my point!). However, you really should get a good monitor. If you have a limited budget, spend as much money as possible on the monitor. And don’t just look at physical size – you want a bright picture, high contrast range, and good colour reproduction. These features are not what your typical electronics consumer looks for when buying a monitor, so you’ll either need to find a specialist shop, or do some homework on your own to find the most suitable model.
Finally, what is generally described as the most important part of a computer. The CPU, where all the actual work takes place. Sure, it’s critical, but when people talk about Moore’s Law and how the power of computers doubles every eighteen months, they’re specifically talking about this one component. You can spend years studying up on CPU architecture, reading specialist consumer reports and poring over benchmark results, but at the end of the day you can rest assured that even the cheapest low-power CPU on the market today is still more powerful and capable than the absolute top-of-the-line model used in supercomputers from a few years back. Don’t get too excited about this part, in other words. In my experience, the CPU is going to spend most of it’s time idle, waiting for those hard disk operations I was describing a few paragraphs back, so even a cheap budget part is not going to slow you down much.