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Astrophotography Basics

Astrophotography Basics


So you fancy taking some images of the night sky? Well, it's much easier than you would think but does require a few basic requirements.


Camera Functions


The first thing you need is a camera! This can be anything, even a camera phone. In reality though we shall discount everything apart from SLR cameras, as these will offer you the most flexibility and offer functionality that will allow you to take the biggest range of astro images. We shall also discount film cameras because the availability of special film types, and the requirements for subsequent image processing make it less relevant these days.

Having decided to focus on DSLR's, you need to make sure they have some basic functionality as follows;

1) They have a manual exposure mode. This allows you to take pictures using your own aperture and shutter speed settings.

2) A 'B' or 'Bulb' setting. When set to this, the shutter will open when you press the shutter button, and close when you release it. Therefore if you hold it for 2 minutes the shutter stays open for that long.

3) A connection for an external shutter release. This will allow you to use an electronic timer / shutter release with your camera. In combination with the cameras bulb setting, this allows you to automate taking a sequence of pictures which have a specific shutter period. i.e. 10 frames each lasting 2 minutes each

4) Your camera should have a Live View function. This will allow you to focus more accuratley. You can zoom into a star and get the focus absolutley spot on.




As far as camera brands are concerned, there are no preferences. If you look at astronomical publications and websites, you may get the impression that Canon cameras are the brand of choice. This is slightly misleading and more a historical legacy.


In the early days of digital SLR astro imaging, when both Nikon and Canon cameras were the main brands, it was discovered that for some odd reason. you could not make a Nikon cameras shutter open for more than 30 secs via a remote shutter control / timer. This immedialtey rendered Nikon cameras useless for Astrophotography as you need to open a shutter for minutes at a time. In addition to this, a few rumours began to float about that Nikon cameras processed their 'RAW' files to remove noise and in doing so removed stars! Of course, Canon cameras also had there share of issues but they are less well publicised. Today there is nothing to choose between any of the main camera brands in terms of controllability or how they deal with images. If anything, Nikon currently have an advantage in how much noise images taken at high ISO settings have, but its minimal. The only real advantage to using a Canon DSLR is that an industry has grown up modifying them. The modification consists of removing the cameras internal IR / UV filter and replacing it with a special type (which are only made for specific Canon cameras). This allows you to take pictures of light (H-Alpha) which is normally filtered out by the standard filter. This is only really relevant for specific Deep Sky objects.


Our advice would be to use whatever camera you have. If you decide you want to start taking H-Alpha images then buy a secondhand Canon camera and have it modified. If your a Nikon user (like me) then you can use a mount adaptor to let you fit Nikon lenses to a Canon camera. Of course you may have the Astrophotography bug so bad by then that you decide to invest in specific Astro imaging cameras. But thats another story....


Lens Choices


So you have a camera. Now you need to choose a lens. You may already be thinking, 'don't I need a huge telephoto lens to take pictures of the universe?'. Answer, 'No'.


The Universe is a very big place. Everything in it is big. Many things in it are so big you will find even your cameras standard lens does not have a wide enough angle of view to photograph your chosen subject. Therefore, start with whatever lens or lenses you have. Typical camera lenses useful for astro imaging are;


24mm or less (36mm or less for DX format lenses) - Useful for Aurora, Constellations and the Milky Way

50mm (75mm DX format) - Constellations (The Constellation Orion will just fit in a 50mm lens frame)

100-200mm (150-300mm DX format) - Sections of Constellations or large Deep Sky areas

300-400mm (450-500mm DX format) - Galaxies, Clusters and Nebula


Just a point to note with DSLR's with APS-C or DX format sensors. There is a common misconception that if you use a normal 35mm or full frame (FX) lens with them that you somehow get more magification from a given lens. i.e. a 50mm lens (x1 magnification) becomes a 75mm which is 1.5x magnification. This is nonsense. Because of the 'crop factor' (a DX format sensor if typically 1.5x smaller than an FX sensor) it only records a smaller portion of the image. However that image is not magnified, it is just cropped.


What are you trying to achieve?


Astrophotography is all about capturing light. When you look up at the night sky from a heavily lit city you see stars. If you look from a remote countryside location, you see a lot more stars and the sky will appear darker. This is because there is less artificial light competing with star light. The reason you don't see stars in the day is because the light from the Sun is brighter. You can however see the Moon in the day because its very bright. Go to a dark location and the stars get a chance to shine.


There are many stars and objects you can't see because they are very faint. Taking images with long shutter speeds captures this faint light. As you gain experience in astro imaging you will be amazed at just how many stars and other objects there are, you just can't see them without a camera!


So you have your camera and lens. Whats next? Your first picture of course!