Introduction

Iridium flares are the name given to satellites from Iridium Co. frequently flaring up due to reflected sunlight off of one of their three antennae. The Iridium network consists of 66 satellites for global cell-phone communication, and the satellites are positioned very accurately about their vertical axis. Due to that accurate attitude, the reflections of sunlight from their antennae are very predictable, up to a few seconds accuracy days ahead of time.

Flares range in magnitude down to -9, which is about a factor 100 brighter (!) than Venus can get. A -9 flare briefly outshines all other stars and planets in the sky and is worthwhile to watch.

Two Iridium flares of magnitude -4 and -1 low over the horizon.

Observing Iridium flares

To observe or photograph a flare, the first thing you should do is look up when a flare will be due at your location. A good site to learn more about these flares and obtain predictions for any location on Earth, is the heavens-above website.

You may notice that usually all the flares happen in the early morning and evening, and not in the middle of the night. This is due to the fact that the satellites will only flare if they are not within the shadow of Earth, so the sun cannot be too low below the horizon.

When a good flare is due, check the weather. The sky should be clear, not too hazy or cloudy. Go out about 5 to 10 minutes before a flare is predicted, and if you will be photographing setup a camera with standard lens (50mm) on a tripod, and cable release.

The prediction of a flare is given by its time, azimuth and elevation. You will have to estimate where in the sky this will be, beforehand. A star map valid for the flare time is really helpful in identifying stars which will be near to the flare.

Photographing Iridium flares

You should know the location in the sky where the flare will have maximum brightness (the center of the flare) to about 5 degrees accuracy, if you will be photographing. You also need to know the track of the satellite (in what direction it will be going): this is always due north or due south, because the orbital planes of the satellites have a high inclination. Orient the camera view so that the flare will run from one side or corner to the opposite. This reduces the chance that the flare path will be in view only partly.

Sometimes a flare is followed or preceded a few minutes by another flare that wasn't predicted. There are some Iridium satellites in orbit that are not controllable but still flare. Be wary of such flares so you don't open the shutter too late or close it too early, as happened here.

Some low angle flares (near the horizon) will be visible longer along the satellite's track than flares higher up in the sky. A flare path for a mag. -9 flare may be as long as 15 or 20 degrees, so it is really necessary to aim the camera well, or use a wide angle lens if you are unsure. The field of view of a 50mm standard lens is about 30 degrees.

Use 100 or 200 speed film and an aperture of f/2.8 or f/4. Set the camera on 'B' setting (Bulb mode) and open the shutter about 30 seconds to one minute before the flare is due. There is no time to readjust the camera when you actually start seeing the flare, so if it appears in a different place than expected, you will just have to take the picture as is and try again another time.

Keep the camera shutter open (using the cable release) until the flare reaches maximal brightness and then leaves the camera field completely (this may take about a minute or more depending on the lens used and the altitude of the flare).

After the flare is gone, you may want to keep the shutter open a little longer. Although the stars will produce tracks, sometimes there may be another flare very near the first flare (from another Iridium satellite) that wasn't predicted.

With the method described above, you will get small star trails in your photo. A better way to photograph an Iridium flare is to guide your exposure (use a star-guiding setup) to eliminate the star trails. Using a telescope with azimuth/elevation mount makes it even easier, since you can piggy-back the camera on the telescope and use the setting circles to aim the camera precisely to the predicted center of the flare, and then let the telescope mount track the stars.