Simple Wide-field Astrophotography

by David Lee

 

Astrophotography is often characterized by narrow-field images of distant galaxies taken over long periods of time. This has been made easier in the last decade with the advent of digital techniques. These are often beautiful images given that we don't get to see these everyday with our naked eyes.

As a photographer I don't have many telephoto lenses in my photo toolkit. Why? I think I've always wanted to see everything in the image, perhaps trying to recreate the experience more fully. In my toolkit I have an extremely wide 16mm, wide 20mm and moderate 35mm. Yes I do have longer lenses but I don't find I use them as often.

Lately I find myself more intrigued by untracked (less than 45 seconds) wide-field astronomy images. I've heard people say they appear more natural, more accessible. Using very simple techniques this can be a great way to enhance those observing sessions with a record of the evening sky. For the beginner they can be a visual diary of the constellations that are visible during different seasons. Some images are definitely wide-field ... constellations, the recent alignment of planets, meteor showers and aurora.

What do you need?

The Camera

You may have one of these in your closet from the '70s. The 35mm single lens reflex was very common during this period. Why not a brand new auto-everything camera? Unless it has manual controls this degree of sophistication will not be an asset. Older cameras with manual settings are far more useful. Manual settings are required because astrophotography images are not "normal". There's a lot of dark sky that will reproduce like the daytime sky if you use automatic settings. So much for those pretty stars. Older cameras can often be purchased for as little as $150. While you're at it get a cable release so you can mimimize any vibrations when you start and end the exposure.

Digital cameras can also be used but will suffer from long exposure noise. Greater than 1 or 2 seconds and you will start to see random bright pixels mar the image. Some cameras are worst than others. By the time your exposure is 8 seconds most cameras will have noise. Electronics is improving but if you're interested in high quality wide-field astro images, film is still the best bet. This doesn't mean you can't scan and edit the film afterwards. You have the best of both worlds.

The Lens

Lenses come in different focal lengths. For a 35mm camera 50mm is considered an average field of view. My preference is for a wideangle around 35mm or wider, remember I'm the wide-angle guy, but if you already have a 50mm and nothing else use it. There is an advantage to normal lenses, they tend to be higher speed f/2 or faster and this is a good thing. The higher the speed the more information captured for that less than a minute time exposure. A 35mm f/2 would be ideal.

The Tripod

Get the best one you can afford. The tripod should be stable extended as far as you expect to use it. The better ones are rigid fully extended. If you're nervous about leaving your camera on the tripod for extended periods it's the wrong one. Price will be $80 and up.

High Speed Film

High speed film is unbelievably sharp and grainless today. Even ISO 400 film is very fine grain. There are good ones in virtually any of the major brands such as Fuji, Kodak and Konica. My all time favourite is still the Fuji films. Remember this is a personal preference. There are colour differences between the films. The Fuji films tend to have a brown black whereas the Kodak films have a blue black. Using digital post processing techniques this is also tunable.

Technique

Ok, you have the camera, cable release, tripod and film. Now what? Wait for that clear night and be prepared.

Time exposures are special. All movement is recorded during the duration of the exposure. This means the apparent movement of celestial objects and the inadvertent kick of your tripod. If the shutter is faster than the motion everything looks still. We're going for as much information as possible from the night sky so stay away from the tripod and keep the exposure fast enough not to show movement. We call this movement "trailing".

The maximum exposures given in the table may show some trailing under moderate enlargement.What looks good on a 4x6 print may not look as good on an enlargement. Experiment ... use shorter exposures for sharper images.

For aperture settings use the maximum opening eg. f/2 or f/2.8. For sharper and more evenly illuminated results try 1 stop down from maximum. Unless you paid a lot of money for your lenses it's unlikely they will perform as well optically when used at their maximum aperture. These are guidelines for night conditions.

If you are photographing just after sunset try using shorter exposures such as 4 seconds to 1/30 second at f/2.8 for ISO 400 film. These are suitable for planetary conjunctions or alignments. You can also try measuring the sky area around the planets for your exposure. This will reproduce the sky as a medium density blue. Experiment with the exposure and wait longer after dusk to get more contrast between the planet and background sky. For these types of planetary portraits do start your exposures before it is fully dark. Wide-field shots can benefit from some sky tone as opposed to a jet black sky. Foreground objects like trees and buildings will still appear as silhouettes. It's a careful balance ... so experiment.

Keep records of your exposures and the conditions eg. Cameras lens, f/stop, shutter speed, date and time, weather conditions. This will help fine-tune your settings for that next roll of film.