Astrophotography

Astrophotography (literally photographing stars, but it usually includes all kinds of celestial bodies, such as nebulae and galaxies) is a great hobby, but it can be quite challenging for the beginner. However, with a bit (lot) of patience and the right equipment most difficulties can be overcome. Here's a brief introduction;

1. Camera

The first thing you'll need is a single lens reflex (SLR) camera, with either a long exposure setting or a manual shutter release. Compact cameras are unsuitable, as they usually lack the possibility to do long exposure or manual shutter release, and getting the right focus might be problematic as well.

2. Mount

The next step is to get a stable mount for the camera. Since we're talking about very low light levels, you'll need a camera that can cope with long exposure times. With long exposure times, however, the image is very susceptible to vibration and movement, and it's easy to end up with bright streaks instead of stars. And, to complicate things further, the celestial sphere rotates (15 degrees per hour), so when dealing with exposure times of more than, say, 30 seconds, you actually need to rotate the camera along with the sky. See below.

When taking pictures you can rest the camera against anything solid, such as the car, or place it on the ground in a stable position. It's recommended to get a tripod, though, as you'll have much better control of the aim and field of view.

3. Barn door mount

If you want to take shots of really dim objects, such as nebulae, clusters, comets and galaxies, or just generally spice up your pictures, you might need to use exposure times of anything from several minutes, perhaps up to a few hours. But as mentioned, the sky rotates, so we'll need something that rotates along with it. The "barn door mount" to the rescue!

Making your own barn door mount is pretty easy. You'll need two pieces of wood, a good hinge, some screws and a ball-and-socket camera mount. The two pieces of wood are joined by the hinge, the camera is mounted on the top side, and the tripod is mounted underneat. (If you search the net for "barn door mount" you can easily find more detailed building plans.) The principle is the same as with a telescope on an equatorial mount; if you point the axis of the hinge towards the pole star, the "door" will "swing open" in the same direction as the sky rotates. You will need to use a screw to control the angle, though, as you need to be pretty accurate.

4. Taking it up a notch

More experienced astrophotographers might try different approaches, such as "piggybacking", i.e. mounting your camera onto the tube of a telescope, photographing through the telescope (adapters can be found for most cameras), and digital photography using either a regular digital camera or a CCD camera specialized for low light levels. Other tricks of the trade include filters, to remove unwanted light pollution or to emphasise certain parts of the spectrum.

5. Conclusion

All in all, it doesn't have to be very difficult, nor expensive, but it will take a bit of trial and error. But the reward for patience is stunning pictures, and impressed friends.

Initial issues

Whenever shooting night exposures it is often suggested to start off with a normal day photograph. This serves two purposes: to give you a reference for color values and the exposures necessary and also gives the photo finisher a reference where to cut the film (this is critical for slide film).

Do not use the largest aperture. Yes, you want to use a large one but not wide open. In most cases there is some distortions and aberrations in a lens at the widest aperture. This may cause some flare which will cause the photograph to lose contrast. While these aberrations are most common in fast lenses there are some that correct for this - most notably the Nikkor 55mm f/1.2 Noct (which sells for $1,500 or there abouts)

Even at night there is a blue halo around stars. We see this by day as the color of the sky. At night, several orders of magnitude dimmer these halos are only seen around the stars themselves. To correct for this it is suggested to use a pale yellow filter (such as the Wratten 2B or 2E) to reduce this halo. Comets have a bluish gas tail and a yellow dust tail. To emphasize the dust tail use a Wratten 21, and a Wratten 47A to emphasize the gas tail.

In almost every case, we are talking about photographs that last more than just a flick of the lens. This makes a tripod critical and a cable release very nice.

Always tell the photo shop that these photographs are of the night sky. Many photo labs will try to render the black of the sky as a deep gray instead. If they don't know what you are talking about, go to another photo lab.

The specifics

There are two types of photographs of the night sky

• photographs of stars as points
• photographs of stars as arcs (often known as star trails)

Stars as points

With no ambient light (far away from the city - or at least on the other side of the mountains) a lens opened to f/2 with ISO 400 film will give nice photographs of the Milky Way in about 15 minutes. 1600 film will give those photographs in less than a minute (15 - 30 seconds will give very nice photographs of the Milky Way). Watch out for the moon too - a full moon can be quite bright. Realize that with the faster film there will be a noticeable grain that may distract from the photograph.

To prevent star trails lacking a telescope tracking mechanism the maxim exposure length should be:
600/focal length = maximum exposure in seconds
At 50mm, a the maximum exposure to completely avoid star trails would be 12 seconds.

Stars as arcs

The minimum time to get a reasonable star trail is at least four times the maximum length to avoid them. Chances are, the minimals are not what you are after - you want a good arc in the sky. We are talking an hour at the minimal side to all night. At the north pole, in winter, a 24h exposure would capture a full 360 circle. However, few people are willing to or able to do such a photograph. Assuming about 8 hours of "usable" night (after disk and before dawn) such an exposure would make arcs of the stars that cover 120 degrees (360/24/8) while a 6 hour exposure would make star trails that are 90 degrees (360/24/6). Even a three hour exposure can be impressive with 45 degree arcs (http://www.photo.net/photodb/photo?photo_id=80368).

With photographs that range in the hours, the moon is even more of a hassle and light pollution is deadly. Try to avoid being underneath an airplane fight corridor too.

Bring an extra set of batteries with an automatic camera. Many of these cameras will drain the batteries all the period of photographing the sky. By morning, the batteries will be dead.

Print film is recommended because it has a lower contrast. Dim stars will be captured along with the bright ones. The slower film is more useful here with the finer grain. Philip Greenspun from photo.net recommends 25 or 100 speed film. If you happen to have a medium format camera these work even better, capturing even more detail than a 35mm frame.

Artistically, adding a tree or the horizon within the frame makes an interesting picture. Also, consider having the north star in the photograph. Comets are very interesting in star trail photographs showing up as fuzzy stars that move notably across the sky - while all the other celestial bodies make clean circles the comet may move across these.

Consider getting a 'dew chaser' assembly for the camera lens. This will keep the lens warmer than the dew point to avoid moisture condensing on the lens and fogging up the photograph as the night goes on.

The longer the exposure the more "sky fog" exposes the film and thus this places a limit on the dimmest a star can be. Likewise, the larger the aperture, the dimmer the star but also the more "sky fog". A longer lens captures less sky and thus less sky fog. The focal length and aperture affect the maximum magnitude of the star that can be captured.

A bit of both?

Consider, combining both photographic techniques of stars in one photograph. There are two approaches to this: changing the f-stop for the last few minutes or using a double exposure. For this to work properly, the first portion (the star trails) would have to have a smaller aperture than the second portion opening up the final stars frozen. In earth case, make certain that the camera is very firmly secured to the tripod and does not budge a millimeter. Failing to do so will mean that the second portion of the photograph is mis-aligned to the first.

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The Nikon Field Guide - A photographer's Portable Reference

As`tro*pho*tog"ra*phy (#), n. [Astro- + photography.]

The application of photography to the delineation of the sun, moon, and stars.

 

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