The Space Shuttle Discovery (STS-133) and International Space Station (ISS) pass through the Big Dipper heading towards Leo. This image was shot with a Canon 18-55mm zoom lens at 18mm at f/3.5 with a 30 second exposure at ISO 400. Because of the rapid movement of the shuttle and ISS, the trails left long streaks during the time exposure. The ISS shone at magnitude -3 at this time. The shuttle had undocked from the ISS earlier in the day and was heading back to a landing on Earth to end its final mission in space.

In addition to the Moon, the Earth's natural satellite, there are also thousands of artificial satellites in orbit around the Earth.

The brighter ones can easily be seen reflecting the Sun's light as they pass overhead. They are usually visible in the morning and evening twilight and hours immediately after astronomical twilight has ended in the evening and before it begins in the morning, depending on the season and your viewing location. Some of the brightest ones can even be seen during bright twilight.

Satellites are not self illuminated with lights like an airplane. They are visible because they reflect the Sun's light. Although the Sun may have set for your observing location, it may still be visible from a high altitude above you. So while you are in the shadow of the Earth on the ground, a satellite overhead may still be illuminated by direct sunlight.

Some satellites have highly reflective parts, like the solar panels on the International Space Station, and the antennae on Iridium communications satellites.

For most satellites the reflected sunlight will maintain a fairly constant brightness until it goes into the shadow of the Earth when it will no longer be visible from the ground. The brightness of these satellites can be from very dim (but bright enough to be captured in a long-exposure astrophotograph) to fairly bright at first or second magnitude.

Some satellites and other objects in orbit such as rocket boosters, may vary in brightness as they tumble through space or as different parts reflect sunlight differently.

Iridium satellites can get as bright as magnitude -8 when they "flare" and direct sunlight is reflected off their mirror-like antenna panels.

The International Space Station can also get as bright as magnitude -3.3. Tiangong 1, the Chinese space module, can reach into the negative magnitudes also.

Heavens Above is an excellent resource for finding satellites that are visible from your observing location.

Tips for Satellite Trail Photography

• To shoot bright satellites as they cross the sky over you, all you need is a wide-angle lens and camera on a fixed tripod.

• Use Live View to focus the lens on a bright star. Then turn off autofocus.

• Set your exposure to about 30 seconds and take a test exposure, varying the ISO. If you are shooting from a light-polluted location, you will want to use a low ISO. If you are shooting from a dark-sky location, you can use a higher ISO if you want to record fainter stars in the background. Use daylight white balance.

• Focus and do the exposure tests before the satellite pass begins.

• Aim the camera and lens at the area of sky where the satellite will pass. You can find this on the Heavens Above web site.

• When you see the satellite enter the field, start the exposure. It helps to use a remote release so you don't touch the camera when opening it. You can also use the self-timer, but that can take 10 seconds before the shutter opens, so be careful with the timing.

Solar and Lunar Transits

Sometimes the orbit of the ISS and other satellites will take them directly in front of the Sun or Moon. These transits usually last for a second or less and can only be seen from a very narrow path on the Earth. When they do occur, they can make some very interesting pictures.

The International Space Station transits across the face of the Sun. Sunspot group 963 is also visible on the eastern limb of the Sun, just rotating on. Shot with a 5 inch refractor at f/8 at 1040 mm of focal length and a single 1/4000th second exposure at ISO 200 through a Baader white-light solar filter on July 8, 2007.

The best place to find predictions for ISS solar and lunar transits is CalSky. It will also give you a plot of the exact path that the transit is visible from. You can also set it up so that CalSky will notify you via email if a transit takes place near you and you can specify the distance radius from your observing location. For example, you can tell CalSky to notify you for any solar transits within 25 miles of your location.

The ISS orbits above the Earth at a height of about 330 to 410 kilometers (between 200 and 250 miles). This may seem very high compared to the altitude of commercial jets which normally fly at a maximum of about 40,000 feet (7.5 miles or 12 kilometers), but it is actually considered a low-Earth orbit. This means that when the ISS passes directly overhead for your particular observing location, it is seen lower in the sky as you move away from your location. Eventually it can not be seen because it is below the horizon. This circular area where it can be seen is actually more limited than you might think. For example, if the ISS is passing overhead in New York, it will not be visible at all from San Francisco.

When the ISS is directly overhead, it is also closest to you at your observing location. When it is closest, it also has its largest apparent size. As it moves away, or as seen from farther away, it is smaller. So the ISS will have the most detail and appear largest when it transits the Sun or Moon when it is closest and overhead.

The length of a transit is also the shortest when the ISS is closest and largest, usually on the order of about 1/2 of a second. When the Sun or Moon is lower in the sky and the ISS passes in front of them, the ISS will be farther away and the transit will last longer (around 1 second), but the ISS will be smaller.

You will have two main problems in shooting an ISS transit: being in the right location in the narrow path where it is visible, and timing your shot so that you get the ISS when it is actually in transit in front of the Sun or Moon. The latter requires a very accurate time signal.

CalSky will provide you with the correct location for the path to see the transit. There are several different approaches you can take to the timing.

Tips for Transit Photography

• Shoot regular high-definition video - Using your DSLRs normal high-definition video mode at 1920 x 1080 pixel resolution, you can shoot at 30 frames per second. Simply start the video several seconds before the predicted start of the transit and let it run for a while after that time. The problem is that you won't be able to get a really high-resolution shot of the ISS in front of the Sun with this method. It is cool for shooting the ISS in front of the full disk of the Sun or Moon however. This can also be a good method to use when the ISS is down range and not that large anyway.

• Shoot Live View or 640 x 480 Movie Crop Mode video - These methods will give you 1:1 pixel resolution and your best chance of capturing a high-resolution image of the ISS. The problem with Live View is that the effective shutter speed is about 1/30th of a second, which is not fast enough to freeze the motion of the ISS to reveal high-resolution details. With 640 x 480 Movie Crop Mode you do have access to real shutter speeds, so you want to get the shutter speed up to around 1/1,000th of a second.

  Another problem with Live View capture 640 x 480 Movie Crop Mode is that the field of view is usually tiny. Unless you are very sure of exactly where the ISS will transit the Sun or Moon, and unless you have your scope very carefully aimed, it may not even pass through your frame.

  You can use a telescope with a short focal length so that the full disk of the Sun or Moon fits in the frame, but then you won't get high resolution on the ISS itself. If you want the full disk, you are probably better off shooting 1080p high-definition video.

• Shoot one single frame - If you want to record the highest resolution image of the ISS, you need to shoot at the camera's native pixel resolution, which means shooting single frames instead of video. Watch the Sun or Moon through your scope, or an auxiliary scope, and when you see the ISS, quickly open the shutter with a remote release. If you put the camera in continuous shooting mode, you may even be lucky enough to get more than one frame. This method, however, is highly dependant on your reaction time. If the transit lasts only 1/2 second and your reaction time is slow, you risk missing it completely.

• Shoot a sequence of single frames based on time - Most DSLRs will shoot full-frame high-resolution single frame images continuously for a certain number of frames until the memory buffer in the camera fills up. Some may shoot at 3 frames per second. More expensive models will shoot at higher framing rates. The buffer fills up more quickly on less-expensive cameras. Shooting raw file format will also possibly give you a slower framing rate and fill the buffer up more quickly. With this method, you start shooting a second or two before the predicted time of the transit and let the camera continue until after the transit is over or until the buffer fills up. You should do a test ahead of time to see how many frames your camera will shoot before the buffer fills up. If you camera will only shoot at 3 fps and the buffer fills up at 6 frames, and you start 3 seconds before the transit, you are going to be out of luck and won't get a picture. Timing is critical here, and the most important consideration is having an accurate time signal. Phones and GPS devices are normally accurate, but can sometimes be off by a second or so. A short wave radio tuned to station WWV, which broadcasts an accurate time signal, should be used if possible.

Other Considerations

• Focus - Focusing in the daytime can be difficult. Use a dark cloth or even a winter coat over your head. See the section on focusing for more tips.

• Seeing - The quality of your high-resolution image will depend on the seeing. Seeing is normally worse in the daytime so going to a large aperture usually won't give you more resolution. Daytime seeing is usually around 1 arcsecond at best, so all you need is about a 5-inch scope. For an APS-C sensor, such as in most consumer-level DSLRs, you will need about 1,500 mm of focal length to make the Sun fill the short side of the frame.

• Polar Aligning / Tracking - A solar or lunar ISS transit is only going to last around a second at most, so it's not that important to be exactly polar aligned. However, if you are shooting with a lot of magnification, and you are not polar aligned, the Sun or Moon will tend to drift out of the frame over the time span of a couple of minutes while you are waiting for the transit to occur. It can be a pain to have to keep re-centering. You can rough polar align with a compass and the magnetic deviation for your observing location, and dial in the altitude with a protractor. You can even drift align on a sunspot with a high-power eyepiece with a cross-hair reticle. Don't forget to set your mount to lunar or solar tracking for the transit.

Iridium Satellite Flares

Iridium satellites have a mirror-like surfaces on their three door-sized antennas. If you happen to be in the correct location, you can see an Iridium satellite flare spectacularly as the mirror aims the Sun's reflection directly at you. These flares happen fairly often for a given location because there are a lot of Iridium satellites in orbit. Sometimes you can even get two Iridium flares close together in time and space.

Heavens Above will give predictions for Iridium flares for your observing location.

Two Iridium satellites flare into brilliant visibility in Ursa Minor and Draco in this 5-minute time exposure on a fixed tripod taken with A Canon XS (1000D) an 18-55mm zoom lens at 24 mm of focal length at ISO 400.

In the image immediately above, the Iridium flares, at magnitude -7 and -8, from Iridium numbers 59 and 96, occurred about 2 minutes apart in time, but very close to the same location in the sky. These satellites are only normally visible only at about magnitude 6, and the flare represents a brightening of some 13 to 14 magnitudes, a factor of about 158,000x to 398,000x.

The flares last only a couple of seconds and are visible from a path only about 10 km wide.

Tips for Iridium Flare Photography

• Use a wide-angle zoom lens on a fixed tripod.
• Focus the lens on infinity.
• Use a wide aperture.
• Point the camera at the area of sky where the satellite will flare.
• Use an exposure of 30 seconds to several minutes long and adjust the ISO with a test exposure.
• Open the shutter with a remote release if you have one, or use the camera's self timer.