Getting Started in Astrophotography

by Michael Oates

The aim of this article to give some basic advice to those who would like to have a go at taking some photographs of the night sky, and who don't want, at this stage, to spend much money on equipment. So I will mention where possible any alternative methods or equipment that can be used.

It is also aimed at those who have little or no knowledge in astronomy and/or photography, but I do assume you know how to load the camera with film etc.

Camera

To get started in astrophotography you don't need much in the way of equipment. A camera is required of course, and can be of any type, as long as it has manual control of the aperture (or at least some manual override) and a "B" shutter setting. This shutter setting enables the shutter to be kept open for as long as you press the shutter release.

Although almost any camera can be used the best type is an SLR Single Lens Reflex that allows the changing of the lenses, so that a telephoto lens can be used to show a smaller area of the sky. It is best to avoid automatic cameras as the electronics, or at least the battery can fail in the low temperatures that the camera can be subjected to in the cold nights of winter. Having said this to get started, so long as there is a "B" setting any camera will do.

If the camera allows interchangeable lenses you should use either the standard lens or wide angle. To use telephoto lenses requires some form of driven mount.

Shutter Cable Release

As you can imagine, if you used your finger on the shutter button you will almost certainly move the camera. So to avoid this a shutter release cable is used. This is a flexible cable that screws into the release button, it has a plunger at the end which when pressed operates opens the shutter. Some of these cables have the feature of being able to lock the plunger down during the exposure so that you don't have to keep pressing the plunger for the whole of the exposure. The above described a manual cable release, there are now electronic versions which are basically a button on the end of a length wire.

Tripod

A tripod is required so that the camera can be pointed at the region of sky you wish to photograph. But before spending money on a tripod you may wish to 'have a go' without. You could fix the camera to something secure such as a garden fence with nothing more than some adhesive tape, but it does make it more difficult to aim the camera and also to operate it. (make sure the tape does not leave the adhesive on the camera when you remove it.

If a tripod is do be used the camera must have a tripod bush on its underside. There is really no substitute for a good strong tripod that does not wobble when touched. If the tripod is not firm the resulting photograph will be disappointing. Its a good idea not to extend the tripod too far, so be prepared to get down on your knees.

Film

I should mention here one of the main difficulties in taking astro-photos. The Earth is continually spinning, which causes the stars to appear to move across the sky. This limits the maximum exposure that we can give before the star images start to trail on the film forming arcs instead of point images. In order to give long exposures, typically longer than 30 seconds with a standard lens, a special type of mounting needs to be used to move the camera in the opposite direction of the Earth's rotation. The use of such a mounting is out of the scope of this getting started article, and will be addressed in a further article.

As the exposures need to be fairly short and the stars are faint this necessitates the use of a fast film, typically over 400 ISO. It you tried to use a slow film you may only record the brightest of the stars, or none at all.

A good film to start with would be around 1000 ISO slide film, I say slide film in preference to print film because most processing labs do not do justice to images of the night sky.

Exposure

As the maximum exposure time is limited by the Earth's rotation, we generally try to give the maximum exposure possible to record as many stars as possible. The lens aperture also needs to be open wide, so set the aperture to its maximum setting, i.e. if the lens has apertures f2.8 to f22 set the lens to f2.8.

A typical exposure that does not show trailing of the stars is about 20-30 seconds. You may be able to extend this time to 45 seconds. The maximum length of time you can expose for is somewhat dependant on where the camera is pointing. If you are photographing the sky near the celestial pole, (near the Pole star) then longer exposures can be taken as the stars move shorter distances for a period of time, where as near the celestial equator the stars are moving further in the same time span.

Star Trails

This photo is taken near the Pole star, the stars nearest the pole have very short arcs and the further away from the pole you go the longer the arcs become. To get an image like this, all you need to do is reduce the aperture of the lens down a few stops, then expose for a long period, this was for 46 mins with a 50mm standard lens. If you include the landscape in the picture or a building you can get quite pleasant and novel photographs.

When the shutter is opened, even with a cable release there may be some movement of the camera. To stop this from spoiling the photo, you need a sheet of black card. Hold the card over the front of the lens, (with out touching it) open the shutter and wait a few seconds for any vibrations to die down, quickly move the card away for the length of the exposure. Before closing the shutter at the end of the exposure move the card over the lens first. This method is commonly known as the "top hat method" and is to be recommended for any astrophoto, even those taken with more advanced equipment.

Hyades, Pleiades & Jupiter.

This photo was taken with an 85mm fl lens at full aperture f2 for 15 seconds on Scotch 3M 1000 slide film. 6th Dec 1988 23:30 UT.

Notes

When taking astrophoto's try and find a site away from lights, the darker the sky the better, but the main thing is don't let any lights actually shine on your camera, this causes internal reflections and glare which will ruin you photo.

Take a few photographs at different exposure times, and also change the lens aperture. After you have had the film processed you will know next time what the ideal exposure should be.

By taking more shots of the same subject you stand a better chance of getting an image that is sharp. No matter how careful you are some images will be blurred due to wind or even kicking the tripod with your foot in the dark !

If the camera has been out side for a while you may get due forming on the lens. This will degrade the image a great deal. The due can be prevented to some extent by using a lens hood on the lens. But if it gets too bad it can be removed by gently application of a hair drier, and I do mean gentle do not let it get too hot you may risk damaging the lens.

Summary

• Use a fast film in the camera.
• Set aperture to maximum opening.
• Set focus to infinity.
• Use a cable release.
• Mount the camera on the tripod and make sure it is secure.
• Use the "Top Hat" method of taking the exposure.

I still remember the first time I took my astrophotographs, it was of Comet Halley in 1985. The thrill of capturing light from Comet Halley on my film with an 85mm lens, even though it was recorded as a faint smudge, with stay with me forever.

Michael Oates