Sunday, March 22, 2015

More astrophotography

Yesterday marked the 14th rendition of the annual Shreve Migration Sensation. I was flattered to be asked back to speak; this go-round my topic was the wetlands of Wayne County. The SMS takes place in the village of Shreve, population about 1,500. During SMS, that number swells to about 3,000. It's a big event, and great for the local economy. Thanks to everyone who organizes the Sensation, and puts in the hard work to make it happen.

As you may have learned, predicting weather and atmospheric phenomena is hardly an exact science. Knowing that I'd be in Wayne County until late in the day, I kept an eye on various forecasts, which were calling for a clear, cloud-free Saturday evening. That was good news, as I could turn my camera to the stars once again. Didn't happen, at least initially - clouds rolled in and blanked out the evening's stars. After shooting waterfowl (with camera) in one of the marshes until dusk settled thoroughly in, I headed for home. About half an hour into the trip, the cloud cover was swept aside and the night sky glittered with stars. I turned around, and headed quickly back to one of the most remote, darkest corners of the massive Killbuck Marsh that I knew.


By the time I got there, rigged the camera, donned boots, and waded back to a good spot, it was thoroughly dark. To my south, the sky seemed plenty dark, but for star shooting one generally wants to point the camera north, towards the North Star, or Polaris. This celestial body is in line with the axis of Earth, and does not appear to rotate and thus provides an excellent reference for dramatic long exposure night shots.

What I did not anticipate was the high degree of light pollution, even in this seemingly remote spot. Even though it is about eight miles to the north, the city of Wooster put up such a glare of night light such as to ruin any astrophotography opportunities. For this shot, the camera was set to take continuous 30-second exposures, and 84 of those are stitched together to make the image. As the camera labored away, and I kicked around the darkened marsh, I realized that the light was probably going to kill the shot. And it did. Very few stars are visible, and the light makes it appear is if the sun is rising. There wasn't much I could to do to mitigate for the light by adjusting camera settings, or in post-processing.

I figured I'd let the shot play out, though, as even if bad it would be a learning experience. After the 45 minute exposure was complete, I packed everything up and headed for another, darker spot that was (sort of) on the way home.

Ah! Much better! This hour-long series of 108 30-second exposures was made from a remote spot near Mohican State Forest in Ashland County. Even though it is far darker than the previous locale, plenty of light still bleeds through courtesy of the distant cities of Loudonville, and probably Ashland. However, the distant glow creates an interesting backdrop that I don't altogether mind.

While this image is a composite of 108 shots stitched together with editing software, I have done very little else post-processing other than to slightly lighten the image to allow the trees in the foreground to show better. The dotted line going through the shot is from an airplane passing through; yet another form of nighttime clutter that the astrophotographer must deal with.

Other than finding a dark corner of the world, the most challenging aspect of making star trail images is setting up the shot. I have learned that in light-plagued Ohio, using a single very long (half hour to an hour) exposure will not work. Such a lengthy exposure lets in far too much light. Short 30-second bursts is the way to go, and to do that one must mount the camera on a sturdy tripod and activate the shutter with an inexpensive remote cable release. It must have a locking mechanism, so that when the button is depressed and locked, the camera will keep firing shots every 30 seconds until told to quit, or the battery runs out. The lens choice is important. I used Canon's fabulous new 16-35mm f/4 ultrawide angle to make these images. It was set at its widest, 16mm, and focused to infinity. Camera settings are fairly straightforward and the following seems to work pretty well in general. Aperture should be wide open, which as noted is f/4 with this lens. Shutter speed is set to 30 seconds, which is the longest my (most, probably) camera will go without resorting to the "bulb" setting. ISO is critical, and the setting that will require the most onsite tweaking. An ISO between 500 and 800 will probably suit many conditions, and for the image above I settled on ISO 500. It's always best to take several practice shots and assess the quality of star shine on the camera's view finder to dial in ISO.

Probably the trickiest part of setting up a star shot is composing the image. It goes without saying that you're working in very dark conditions, and that makes it tough to accurately assess one's surroundings. I like placing Polaris in or near the left top corner of the image, as the stars appear to rotate around that fixed point. Polaris is clearly visible in this image, framed (almost) perfectly through a gap in some foreground trees. That's one of the major tricks - getting Polaris exactly where you want it in the shot. Looking through the camera's viewfinder and finding an individual star is not easy; in fact it can be impossible to tell which is which. A hugely helpful aid is a "star finder" app for a smart phone. I have one called Sky Guide on my iPhone and it makes life much easier. When held to the sky, it shows a map of the celestial objects in view with prominent ones labeled. By holding that over the top of my camera like a gun sight, I can place Polaris in my camera's field of view with fairly good confidence. It's also important to carefully assess the terrestrial objects and place them in the bottom of the field of view, unless you wish to have a shot that only shows stars. In this case, I liked the way that a large white pine provided a border on the left, and the manner in which the deciduous trees sloped to the right. I did try and pinhole Polaris in that little gap between the pine and other trees, and nearly nailed that. To better assess the location of terrestrial objects and place them appropriately in the image, a flashlight can be used to illuminate things making them easier to find in the camera's view finder.

Once all of this has been done (and you've checked to see that the battery is charged and space is on the memory card) all that remains is to activate the camera shutter. And wait an hour or so, fingers crossed and hoping everything was well set up. If so, the end result can be extraordinary. I look forward to shooting astrophotography in a truly dark environment someday.

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1 comment:

Anonymous said...

Hi Jim. Lot of hard work. Rewarding shots, I especially like the second. I too have Sky Guide on my iPad and have enjoyed it a lot. Fun reading post. Gary Wayne