Sunday, April 15, 2012

Once (twice?) in a Lifetime Photo Op: The 2012 Transit of Venus

On June 5-6 2012, a *very* rare astronomical event will occur: the planet Venus will cross between the earth and the sun on its orbit, visible as a "transit" of Venus across the face of the sun. Venus transits occur in pairs, 8 years apart -- with more than one hundred years between pairs. The 2012 transit is the second in the most recent pair. Here is some information on photographing this very rare event...

I was fortunate enough to be able to travel to England for the first transit in the most recent pair, in June 2004. That transit wasn't visible from North America (except for the very end of it on the eastern US coast), so Europe was the place to be. The image above was taken mid-transit from just outside Reading, England, where the entire transit was visible. The second transit in the pairing will occur June 5th or 6th (depending on your location), and then we won't have another one until December this one is pretty much your only chance!

The first thing to do is find out whether you'll be able to see the transit from where you live, and if so how much of it. The map above ( from The Transit of Venus Project shows roughly where the 2012 transit will be visible; the Pacific Ocean Basin, Eastern Australia, and Eastern Asia get to see the entire transit, while most of North America, Europe, and Central Asia get to see most of it but will be cut off at either sunrise or sunset. Most of Africa and South America won't be able to see any of this year's transit. Where I live (Southern California), I'll be able to see all of it except the last 20 minutes or so -- not bad for not having to travel at all! For exact timing at your particular location, visit the Local Transit Times page of The Transit of Venus Project.

*WARNING* Exercise caution looking at and photographing the sun! Never do so without a safe filter for your eyes, camera, or telescope! Serious eye or equipment damage can result from not using safe filtration!
I'm very serious about the warning above. Below you'll find information about safe ways to filter your camera lenses or a telescope -- please don't ignore this information. And don't stare at the sun with your naked eyes, or through an unfiltered telescope, camera lens, or can cause yourself serious eye damage. Please be safe!

You can photograph the 2012 transit with a "normal" camera (DSLR) and telephoto lens -- but long focal lengths will make the sun larger on your camera's sensor, and provide the most detail possible. My image at the top of this post was made with a Canon 300D (original "Rebel" DSLR) at 800mm focal length, which is a good focal length to use. How long of a focal length is best? The two images below will give you a good idea of how large the sun will be on your imaging sensor for APS-C sized sensors, and for 35mm/full-frame sensors:

As you can see from the image size charts above, a 200mm lens on an APS-C DSLR produces a fairly small sun image -- about 12% of your vertical frame height. A 1200mm focal length (common on inexpensive 6-inch f/8 reflector telescopes) is an ideal size, filling up 71% of the frame vertically; big enough for lots of detail, but with enough leeway to not require critical centering. If you don't have a telescope, use what you've got -- the charts above will show you about how big the sun will be in your images.

As for the *very* important solar filter: once again, do NOT photograph the sun, or look through your camera viewfinder/telescope without a *SAFE* solar filter in place. What's a safe solar filter? There are lots of options:

Simple Solar Filters

For about $35US, you can get a Sheet of Baader Planetarium AstroSolar Film (from Oceanside Photo and Telescope). This safe, easy to use material can be used to make an inexpensive but optically excellent solar filter for your camera lenses, or telescopes up to 8" in diameter. Making your own filter using the film is simple -- instructions are at the "Baader Planetarium Web Site. For most people, this is the option I would recommend. One sheet of the AstroSolar film will make filters for 4-8 normal telephoto lenses, so if you have some photo friends who might be interested, get a sheet and split the cost with them to save even more.

Commercial Solar Filters

The image above is of my setup (and me!) at the 2004 Transit in England. I used a commercially available glass solar filter, like this Full Aperture Solar Filter available from Amazon. They're available in a variety of sizes to fit most telescopes or large telephoto lenses, and fit snugly and securely. They're also a bit more durable than home-made versions as above. Of course, they cost a bit more -- but if you get interested in solar photography (not just for Venus transits, but for sunspots, eclipses, etc.) then they're a good investment.

*DO NOT* use "secondary" filters that fit over the back of telescope eyepieces -- the "primary" filter types above will all keep most of the sun's heat out of your telescope or camera lens so they won't overheat -- the "secondary" types of filters will not, and the inside of your telescope or camera lens will get very hot, and possibly cause the filter to crack. I can't stress the safety issue enough -- do this right!

All of the filter types above block between 95% and 99% of the sun's light...that might seem like a lot, but the sun is so bright you'll still be able to do short exposures (around 1/500th sec.) at ISO 100 or 200, so you won't have to worry about tracking the sun as it moves across the sky. A standard tripod or telescope mount without tracking motors will suffice very nicely.

Pick the best focal length you can, focus carefully (at infinity for camera lenses), securely mount your solar filter, and be prepared for some amazing shots of something your parents and probably grandparents couldn't see, and your children and grandchildren also probably won't see. You're luck to be around for this event, so take advantage of it! Much more information about safely observing and photographing the 2012 Venus transit can be found at the Transit of Venus Project web site.

This last image is a composite of shots I did during the 2004 transit, showing how Venus moved across the face of the sun from our point of view. The variation in the position, size, and shape of Venus was due to early-morning mist and poor atmospheric conditions at the beginning of the transit (which began just after sunrise from England), to the better conditions as the transit progressed.
I hope you'll get excited about taking photos of this fun and very rare event!

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