Advanced Stargazing: Astrophotography Camera Types
When you’re moving beyond stargazing and into astrophotography, choosing the right camera for is often difficult. Three basic camera types allow you to do something a little different:
Single-lens reflex (SLR) cameras
Webcams and CCDs (charge-coupled devices)
Astrophotography camera types: SLRs
SLR cameras give you full control of the various manual settings, which is really important when you’re doing astrophotography. They also let you see through the viewfinder exactly what the camera is seeing, so that you get a much better idea of what kind of image you’ll get (unless you’re taking a long exposure shot, in which case the camera captures much more than your eye can see).
You need to have control over four basic settings if you want to get a good astro-image:
Shutter speed: Most SLRs give you the option for controlling the shutter speed, using the S setting on your camera. You can vary the shutter speed from very slow (maybe as slow as 30 seconds) to very fast (maybe as fast as 1⁄6000 of a second).
For astro-imaging, you usually want a slow shutter speed, which gives you a long exposure image and much more opportunity to collect more light and therefore get a more detailed, brighter image.
Sometimes you can use the slow S-settings on your camera to give you a shutter speed you’re happy with, but if you want much longer exposures – say, a five-minute exposure – you need to use the B setting, which stands for bulb. When you push the exposure button when using the bulb setting, you can keep the shutter open for as long as you like; the shutter stays open until you let go of the button again, allowing you to make exposures for minutes or even hours.
You should take lots of pictures of every scene at a variety of shutter speeds until you find an image you’re happy with. Keep track of the settings you use for each picture to build up a database of settings for future images. Keeping a database may seem time-consuming at first, but it will pay off later when setting up your camera becomes second nature, not a process of trial and error.
Aperture size: The aperture of the camera is the hole where the light comes in. The larger the aperture, the more light your camera collects, and the brighter and clearer objects are. SLR cameras let you manually set the aperture using the A setting. Camera apertures are described with an f-number, known as the f-stop. The usual f-stops on a camera are f/1.4, f/2, f/2.8, f/4, f/5.6 and f/8.
As you move up the list, each f-stop closes the aperture slightly and halves the amount of light entering the camera. For imaging faint stars in the night sky, you want wide apertures – so a low f-number.
Sensitivity (ISO numbers): One useful feature of most DSLR camera is that you can set the sensitivity of your camera yourself. The camera’s sensitivity is given by the ISO number. The higher the ISO number, the more sensitive you camera is to faint light, and so the brighter your image is (but the more grainy an image appears).
Usually you want a high ISO for capturing dim stars, but once again, it’s trial and error; take lots of images with different sensitivities to see what works best for you.
Manual focus: Most modern SLRs come with an autofocus feature, which you need to turn off while imaging the sky. Autofocus tends to get confused in very low light levels, and you probably won’t get an in-focus image. You should manually set the focus to infinity, represented by an infinity sign – ∞. To set the focus, turn the focus ring on your camera lens until the ∞ sign lies above the focus mark.
Flash: If you take astro-images, you do it at night, and your camera may automatically use the flash in such dim conditions. Turn off the flash! The flash will light up the foreground of your image and won’t help you see any more stars.
Astrophotography camera types: Point-and-click
While not everyone has an SLR camera, point-and-click cameras are much more readily available – and cheaper! The compromise you make if you use a point-and-click camera is the inability (usually) to manually set the shutter speed, aperture size, ISO sensitivity, and focus. This lack of control means that it is hard to get just the image you want. However, you can cheat and still get a rather good image.
Astrophotography camera types: Webcams and CCDs
Dedicated amateur astronomers use modified webcams and hi-tech devices called CCDs (charge-coupled devices) that use a digital chip to gather light and to image the night sky. You can fix these items in place on the eyepiece holder of your telescope and connect them to a laptop or computer indoors. This means that you can image from the warmth and comfort of your own home!
Even cheap webcams have chips in them that give good astronomical images. You need to buy a webcam with a relatively high number of pixels, and you mustn’t be afraid to modify it to fit into your telescope eyepiece.
Old 35mm camera film canisters are just the right diameter to slot into most telescope eyepiece sockets. Simply cut off the end of one of these canisters, glue the other end onto your webcam, and voilà – your very own astronomy webcam. If you image through the eyepiece of your telescope, you need to have a motorised mount set up correctly for tracking the stars. Otherwise, you’ll end up with very blurry images (or no image at all).