Photographing Comet Pan-STARRS

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3/17 Update – I managed to see comet PanSTARRS last night from Sunset Hill Park in Ballard along with about half a dozen other intrepid stargazers, and capture a few decent photos as well.

In case you haven’t heard, there’s a comet now visible in the northern hemisphere just after sunset. This is one of those cases where, with a bit of effort and luck with the weather, you can take a great photo of an astronomical object without much more than just a camera and a tripod.

For the next few days Pan-STARRS will be low on the western horizon, visible about 45 minutes after sunset. This is a great chance to frame a comet with the landscape. Go stake out your favorite sunset watching venue, and then stick around for the after-show. Pan-STARRS will be about a magnitude 3 or 2 object, but not easily visible to the naked eye due to the afterglow of the sunset, so pack along some binocular or a spotting scope. You shouldn’t, however, need a particularly long camera lens to get a memorable photo of this rare event. (Ignoring, for a moment, that we’ll have another, possibly much brighter comet passing at the end of this year.)

I’d be busy getting ready to take my own photos right now, but the sky in the Emerald City is rather grey and soggy. So in the mean time, here are a few tips to help you prepare for Photographing Fairies Comet Pan-STARRS:

1. Use Program or Manual Mode

Point and shoot cameras generally have a program mode that will let you control the exposure length and ISO. While you might be able to make out the comet in a photo taken in automatic mode, it’s worth taking a few minutes to understand how to use the program mode – with a little practice, you can get a much more satisfying result.

For a DSLR, manual mode is the way to go. For a primer on all the different manual exposure settings for astrophotography, see Jerry Lodriguss’ Astropix website, in particular the excerpt from his book, A Beginner’s Guide to DSLR Astrophotography that talks about DSLR Camera Settings for Astrophotography.

2. The Rule of 400

If you take a long exposure photo of the stars from a fixed camera, after a while the stars becomes streaks. An easy way to avoid this is to shorten your exposure time multiplied by the focal length of your lens to less than 400. For example, if you have a 50mm lens, you can take about an 8 second exposure without noticeable star trails. Some people call this the rule of 600, but in my experience, the stars exhibit too much streaking when using this target.

For capturing the comet, the ideal exposure length will vary based on the sky conditions. Shorter exposures when the sky is still light, longer as the sky darkens. Make use of your camera’s review feature and experiment! If you’re camera can show a histogram of the image you took, try to make sure the peak isn’t bumping up against either side of the graph. (If the histogram is flat against the right side, try a shorter exposure. If it’s flat against the left side, try a longer exposure.)

Update 3/17 – The photos above were taken at a 250mm focal length with a 6 second exposure. This definitely adds some drift, but also helps an otherwise faint comet stand out a little better.

3. High ISO

A higher ISO setting allows the camera to capture more detail, but also more noise. Since Pan-STARRS is going to be a faint object, you want to use a setting on the higher side without introducing too much noise to make sure you capture the comet and its tail. Figure out the ISO range for your camera, and set it to one or two steps down from the highest setting. For instance, I have a Canon SD780 with a range of 100 – 1600. For this camera, I’d set the ISO to 800, which is one step down from the highest. For my Canon T3i DSLR, I’d set the ISO to 1600. (This camera extends up to 6400, and 1600 is two steps down.)

4. Faster is Better (f-stop)

The one thing to know about F-stops is, the faster (smaller the number), the better. Simply put, a smaller F-stop means more light gets to the image sensor. If you can set the f-stop lower, you’ll capture more detail in the same exposure time.

If you have a zoom lens, even on your point and shoot, typically zooming out (shorter focal length) also results in a shorter f-stop. You capture a larger area of sky, with more starlight hitting the sensor, and because of the 400 rule, you can also take a longer exposure.

5. Minimize the Shakes

Don’t even think of trying to take a long exposure photo while holding the camera – your hands just aren’t that steady. In fact, just the vibration caused by pressing the shutter button can cause streaks instead of stars, and the only streak you want to see is the tail on the comet.

Fortunately just about every digital camera includes a 2 second or 10 second delay timer. Look in the camera manual to learn how to set this. Another option for some cameras is a remote shutter release. If you have one, use it. If you don’t, use the delay timer.

If you have a DSLR, you also need to worry about mirror shake. Typically when you snap a photo with a DSLR, the mirror that sits in front of the sensor to give you the preview in the finder quickly flips up, which can cause the camera to shudder briefly. You can eliminate this by using the Live View mode or by setting the mirror lockup feature. Again, you’ll need to look in the manual for how to set this on your camera model.

6. Lenses Like Hoodies

A lens hood is a simple and cheap way to help prevent stray light. The importance of this is magnified significantly when taking long exposures of the stars. If you are anywhere near a city, or just a stray light source, a lens hood will help cut down the bright background and minimize any particular hot spots on the image from a nearby streetlight or passing car.

If you don’t have a lens hood, or you have a point and shoot that won’t accept one, you can also just shield the camera from any specific light source with a hat, blanket, or your body. A lens hood also makes a fun and simple DIY project. Just remember that you want to make the interior of the hood as dark and non-reflective as possible.

Finding Pan-STARRS

• Sky & Telescope’s Updates on Comet PanSTARRS
• Seattle Astronomy: PanSTARRS peeks through Seattle clouds

Good luck comet spotting!

If you do capture an image of Pan-STARRS, please share it with me on Facebook or Twitter!