Astrophotography II

In the last astrophotography installment, we discussed opportunities to do astrophotography in the daytime. Although interesting and challenging, shooting celestial objects in the daytime is probably not what most folks relate to astrophotography. So in this blog, we’re going to be turning down the light and shooting at night.

Shooting a camera at night poses some challenges. Most DSLR’s are not sensitive enough to take quick snapshots of the night sky. The intensity levels of most celestial objects are very low. There are some new mirrorless cameras that have incredible ISO ranges and may provide additional opportunities. But since most of us have DSLR’s, we’ll be looking to get the most out of them, in night shooting situations.

As many folks are aware, shooting in low light can create some challenges to getting a good image. The aperture needs to be opened up, the exposure lengthened, and the ISO increased.  Sometimes a combination of all three is required. This will allow you to get an image, but usually at a cost.

Longer exposures will sometimes have hot pixels. These are pixels that will appear brighter than the surrounding ones. They are usually colored and stand out. In addition, shooting at high ISO’s will increase the image noise. And opening the aperture makes it more critical to get the object in precise focus. So there are some trade-off’s. There are ways to deal with some of this in post processing. Newer cameras are becoming increasingly cleaner, and also, employ in-camera features to reduce these artifacts. Most cameras can take advantage of internal noise reduction, where a dark frame is recorded and subtracted from the original. This will cover some of the issues with nighttime shooting, at the expense of extra time on the camera (a 5-minute exposure required a 5-minute dark frame). Getting quality dark sky time can be rare, so noise removal and clean-up is usually handled off-line, so to speak. There are several post-processing techniques that will improve the results dramatically. Some post-processing and noise reduction techniques will be discussed in a future installment. The following is a crop of an image that exhibits noise and hot pixels. Noise is usually displayed as a grain or a blotchy color pattern.

One of the most iconic and easiest of night sky targets is the Moon. Because of the Moon’s brightness, shorter exposures can be used and still record very good images. However, this brightness can also make it challenging to get dimmer objects in the same scene. For example, stars will usually not be visible in Moon images. The exposure requirements for the Moon will be vastly different than that for stars and other dimmer objects.

Seeing a full Moon is pretty impressive. Some interesting photos can be shot, especially when the Moon is at low elevations and just rising or setting. Adding scenery in the foreground is a great addition to full Moon shots. This image is of a Supermoon, when the Moon is full and at its closest point to the Earth.

However, when shooting the Moon higher in the sky, it can sometimes be more interesting to capture it in a partial phase. This will tend to show more detail on the terminator, the line along the light and dark sides. This was shot with the same set-up as above. Notice the deep mountain and crater shadows along the terminator.

Both the Moon shots were photographed with a Canon 7D through a 530mm f/5 telescope, on a fixed tripod. At long focal lengths, the use of a shutter release is increasingly more important.

One aspect of night photography that I find particularly rewarding and challenging is shooting satellites. Satellites can include the ISS (International Space Station), but I’ve also captured the space shuttles and other orbiting objects. These long exposure shots can be done on a simple fixed tripod, pointing at an area that will capture the transit. The paths of the ISS and many other satellites can be obtained online. Since the ISS is very bright, it’s a popular target for photographers. Signing up to alerts on Calsky.com can provide ample warning of an ISS pass, along with where the ISS will appear in the sky. There are other satellite predicting sites, if you’re looking to shoot more obscure satellites.  The smaller satellites will be much dimmer and more difficult to capture.

You can go out after sunset and see many satellites pass overhead. Spend a little time, look up and watch the entire sky. Eventually, your eye will catch what appears to be a dim star in motion. If it doesn’t have any blinking lights, it’s probably a satellite or another orbiting object. The ISS is a great first target, since its passes are predictable and its high brightness makes it easy to spot. Set up your camera and tripod in an area free of obstructions. Once you know when and where to shoot, aim in the general area and set the exposure, aperture and ISO. The longer the exposure, the longer the satellite and star trails will be. I usually shoot with exposure lengths of between 10-30 seconds. When you see the satellite approaching, adjust the framing, wait for the appropriate time and click the shutter button. A remote release is very helpful here. I generally shoot some test shots in the minutes before the arrival of the target. This will help ensure that the exposure is correct.  Below is a shot of the ISS.  Though not visible here, the shuttle Endeavor was attached, at the time.

This next photo is the ISS and the space shuttle Atlantis shortly after it separated from the ISS.  Both of these shots were under 30 seconds in duration.

Another very interesting series of targets to shoot are the Iridium satellites. Getting good shots of these are more challenging because the timing needs to be so precise. These satellites handle the Iridium satellite phone service and have very predictable, intense flashes of reflected sunlight called flares. To shoot these satellites, you’ll need a way to get the exact time. The peak flare time is listed on the Calsky notices. I use a GPS, as it’s precise and displays the time, including seconds. But it may also be acceptable to use a cell phone. Some experimentation may be needed with the timing sources.

I generally set up and aim the camera at the area of the predicted flare. This usually requires a compass to determine azimuth, and the means to measure the elevation. Some tripods may have heads with degree divisions. If not, a digital protractor from your local home improvement store is a great tool for this.  As with other satellite passes, take some test shots in the minutes before the approach, so the proper exposure can be set. The most visible Iridium flares usually occur around sunset and sunrise. So the exposure requirements are constantly changing. You will need to be prepared to make adjustments right up to the time of the actual flare.  I like to take the exposure symmetrically around the predicted time of the flare. So I may start the exposure 10 seconds before the flare and continue it for 10 seconds after. But longer exposures are also interesting and will capture more of the satellite’s flight before and after the flare. Here are a couple examples.

Here’s another shot of an Iridium flare.  Many of these flares can reach very high brightness, for a short period of time.

Satellites, although interesting to shoot, can also be a significant hindrance to astrophotography. Many times, they will show up in frames where they are not wanted. This becomes a bigger problem when shooting long exposure deep sky images. There are processing techniques that can be used to clean up images with these artifacts, which will be discussed in a future installment. Here’s an example of a tumbling, out of control, Iridium satellite that passed in front of a long exposure image of Markarian’s chain.  This particular one was designated IR 914tum.

A great first target of astrophotographers is the Orion Nebula. It’s bright enough to make it an easy target, with good success. Unfortunately, it sits right on the celestial equator. There are many geosynchronous satellites that are present in this area. Different than other satellites, these remain stationary. So as you’re tracking the target, the satellites pass through the frame and can ruin the image.

For those interested, geosynchronous satellites can also be another interesting target to shoot. Since they remain referenced to the Earth, they can be picked out of a long exposure image. Here’s an example.  This was a long exposure shot through a telescope with the tracking turned off.

The bright dots are the geosynchronous satellites. Interestingly, this image also captured some curved trails, which turned out to be a spent rocket body and other orbital debris. There are lots of interesting things to see in the night sky.

In this astrophotography segment, we looked at several objects that can be photographed on a fixed tripod. However, doing dedicated astrophotography in this way can be fairly limiting. So in the next segment, we’ll be examining what is needed for long exposure tracking astrophotography.  See you then…