Wide field

Wide field imaging is fairly simple, I use a DSLR camera with 50mm lens, tripod and remote shutter release with intervalometer.

Attach the camera to the tripod and plug in the shutter release, now set the camera to manual mode and put the exposure time to Bulb, this will allow the remote shutter release to control the exposure time and number of exposures to take. I generally use 25 second exposures, any longer and you will get noticeable star trails. Take around 20 or more images for stacking.

Set the camera’s ISO ( sensitivity ) to 1600. I leave the white balance on auto and put the lens in manual focus; and one small point. TURN THE FLASH OFF.

Point the camera at a bright star, if the camera has live view use that to look at the star and zoom in. Use the focus ring on the lens to get the best image you can, the focus will be somewhere near the infinity symbol ( ∞ ) on the lens. Set the exposure time on the shutter release, number of shots to take and start the imaging session.

In this way you can take pictures of constellations and the Milky Way. This is also the method used to capture meteors as they burn up during meteor showers.

You will note that taking longer exposures will lead to star trailing and many people take star trail images, especially if there is a nice backdrop – a tree or an old building for instance. The exposure time will depend on the level of your local light pollution: if you have a lot of light pollution your exposure times will be reduced or your images will appear orange. So if possible take your kit away from the light pollution and get out into the countryside.

You will want to dress warmly and take a few things with you to be comfortable like a folding chair, flask of hot drink and plenty of batteries for the camera; keep these in your pocket so they stay warm as well.

If you mount the camera on a polar aligned German Equatorial tracking mount you can follow the stars and have much longer exposure times and use longer focal length lenses. This is the favoured way of imaging objects like the Andromeda galaxy and the Orion constellation showing Barnard’s loop.

A word on focal ratios- usually the focal ratio ( f ratio ) is set low, this means that the lens iris is wide open to allow as much light as possible through to the camera’s sensor in much the same way your eyes pupil opens to allow more light through in the dark. However many lenses especially cheaper ones do not perform well with very low f ratios and the stars tend to look bloated and stretched around the edges of the image. If this is the case, set the f ratio a bit higher. This closes the iris and gives better images – you will have to find out what f ratio works best for your lens.  

F ratio   

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