Thursday, October 2, 2008

Night Sky Shooting

The current DSLRs are getting better characteristics making night sky photography viable: lower noise at higher ISOs and better long-exposure tolerance. Upon getting a Nikon D700, one of the first things I wanted to do was try out night sky photography like I used to do in the good old film days. You have two choices with long exposures of the night sky: either keep the camera stationary and get star trails or move the camera to compensate for the earth's rotation to get long exposures of faint sky features without smearing into trails.

It's the latter that I find most interesting usually. So, with a vacation based on a cottage overlooking a pond coming up, I hastily put together an equatorial mount to take night sky pictures if the weather cooperated.

An equatorial mount is simply a camera (or telescope) mount that allows you to rotate your gear precisely around an axis parallel to the earth's axis of rotation but in the opposite direction to compensate for the apparent rotation of the night sky as the earth turns.

You can build a primitive mount using some scrap lumber and some common hardware as I did. At its core, the mount is two boards joined at one end by a door hinge. The hinge is our rotation axis and will be aligned with earth's axis as described later. The bottom board is fixed in position so that it correctly aligns the hinge. At the end opposite the hinge on the bottom board we drill a hole through which a T-nut can be driven for a 1/4 X 20 bolt to be screwed in. (This means a bolt 1/4 inch in diameter with 20 threads per inch. The bolt should be at least 3 inches long.) In use we slowly (one revolution per minute) screw in the bolt so that it drives the upper board away around the hinge axis. This drive bolt is located 11.43 inches from the center of the hinge axis.

Here's why. (Math-phobes skip this paragraph, it will make your head hurt.) The earth completes one revolution every 23 hours, 56 minutes, and 4.1 seconds approximately. This is the sidereal or star day, not a solar day which is the familiar 24 hours, the extra four minutes of which are spent rotating a bit more to compensate for the earth's orbit around the sun. So, this means the earth rotates 360° every 23 hours, 56 minutes and 4.1 seconds or about 1436 minutes. That means the earth rotates about 0.25° every minute so we need to rotate the same amount in the opposite direction to compensate. Because we're using a bolt with 20 threads per inch, we can move our upper board 1/20 inch per minute. We need to find the distance between the hinge and the bolt that will cause the upper board to rotate 0.25° when we raise the far end 1/20 inch. Simple trigonometry shows that distance to be 0.05/tan(360/1436) or 11.43 inches. There's some heavy round off in these calculations but the error is well within our ability to drill holes with precision into lumber.

Next we need to mount our board assembly to keep our hinge axis in alignment with earth's. In my case, I knew I was going to be shooting pictures at 44° north latitude so the hinge needed to be angled up at 44°. I cut a couple of supports at this angle and screwed the bottom axis board to it. An adjustable setup would be better if you're going to use this at different locations. The angled supports were in turn screwed to a piece of plywood with a hole drilled in the center to thread a bolt through to secure the whole apparatus to a tripod. (You could simply set it on a level table top instead.)

The final tweak was to screw a piece of wood cut at 45° to the top rotating board to provide some elevation for mounting a ballhead and the camera, giving them clearance to point in any direction needed.

To align everything on site, it helps to have a compass and a good map that provides a couple of important pieces of information. You need your latitude so that the hinge can be angled up the proper amount, and you need the direction of true north that the hinge axis must be aligned with. Good maps, such as the USGS maps in the US, also include an indication of how far magnetic north deviates from true north. In my case, I was shooting at about 44° north latitude at a location where magnetic north was 14.5° west of true north. After adjusting the compass accordingly, I lined everything up beforehand, composed the approximate scene I wanted to capture, locked down the controls, and waited for dark. An alternative to all this is to eyeball along the hinge at night so it points in the direction of Polaris, the North Star, if you live in the northern hemisphere. In my case this wasn't an option because north was obscured by trees.

When shooting, keep a watch handy with a little light on it and use it to synchronize your rotation of the bolt with the seconds indicator of your watch—remember, we're screwing in the bolt at 1 complete revolution per minute. Trip your shutter and start slowly rotating the bolt and, if you've measured and aligned everything properly, you should get nice crisp star images.

Or not. As I found out, many wider-angle lenses don't do stars very well. There is a lot of coma and astigmatism that you don't notice in normal shooting. Experiment with lenses and f/stops to find the right combination to give you the best results.

In my case, I was getting good results at ISO 3200, 30-second exposures with a zoom set at 35 mm and f/4. I would shoot multiple exposures and then use stacking software to combine them to get nice clean (relatively) noise-free images.

Note that this solution is only accurate for a few minutes at a time, when the boards are closest together. As they rotate farther apart, the movement of the bolt pressing against the upper board has a progressively smaller effect on the rotation rate of the upper board. If you want a more robust solution, things get a lot more involved and you may want to simply buy a commercial equatorial mount (and maybe even a clock drive to turn it).

Update March, 2009: The image above, of the Milky Way over Osgood Pond in the Adirondacks, was the grand prize winner in the Adirondack Life magazine's annual photography contest. It was shot from the canoe house deck of the Gazebo, a gem of a rental cottage on the pond.