The Colour of Dark


Colors of the Dark Sky Panorama

What colour is the dark night sky? Depending on conditions, it can be any colour you want.

I shot this 360° panorama last night from my backyard under what looked like a clear and fairly dark, moonless sky. Looks can certainly be deceiving. The camera picked up all kinds of colours the eye couldn’t see.

Let’s review what’s causing the colours:

• To the north just left of  centre the horizon is rimmed with a bright yellow glow from all-night perpetual twilight present around summer solstice at my mid-northern latitude.

• Above that shines a green and magenta band from a low-level aurora just visible to the naked eye.

• Much of the sky is tinted with bands of green from ever-present airglow, caused by oxygen atoms at the top of the atmosphere giving off at night some of the energy they absorbed by day. I had thought the sky would look blue from the perpetual twilight but the airglow seems to overwhelm that.

• Yellow glows around the horizon at left (west) and right (southeast) are from urban light pollution from towns 50 km away.

• Some strands of remaining cloud from a departing thunderstorm add streams of brown as they reflect lights from below.

• Finally, the Milky Way shows up in shades of yellow and pale blue, punctuated here and there by red patches of glowing hydrogen hundreds of light years away.

The only thing missing this night was a display of electric blue noctilucent clouds.

The sources of most of these colours are an anathema to observers of faint deep-sky objects. Aurora, airglow and certainly light pollution just get in the way and hide the light from the distant deep sky.

A word on technique:
I shot this panorama using an 8mm fish-lens to shoot 8 segments at 45° spacings. I used the excellent software PTGui to stitch the segments together, which it did seamlessly and flawlessly. Each segment was an untracked 1 minute exposure at ISO 3200 and f/3.5. The panorama covers 360° horizontally and nearly 180° vertically, from the ground below to the zenith above. It takes in everything except the tripod and me!

– Alan, June 8, 2013 / © Alan Dyer

All-Night Satellites


ISS Pass #2 (June 4-5, 2013)

It was a marvellous night for Space Station watching.

Right now those of us at northern latitudes in North America are enjoying the opportunity to see the International Space Station come over not once but often 2 or 3 times a night, as it is now lit by the Sun all night long (on our nights down here on Earth, that is).

Here are two shots from the night of June 4-5, 2013 taken from my home in Alberta at a latitude of 51° North.

My featured image above is from the ISS pass that began at 1:55 am, and is a stack of 4 tracked 2.5-minute exposures, so the stars are not trailed, but the ground is! On this pass, the ISS came overhead. This view is looking north, toward the all-night perpetual twilight we see on the Canadian Prairies around summer solstice. There’s also a low band of green aurora on the northern horizon.

I shot the image below on the ISS’s pass one orbit earlier at 12:18 am. This image is looking south to the ISS’s high pass across the south. It’s a composite of 4 untracked 2-minute exposures –  thus the stars are now trailing in circles around Polaris at the top of the frame.

ISS Pass #1 (June 4-5, 2013)

Both shots are horizon-to-horizon all-sky views with an 8mm fish-eye lens.

The sky isn’t dark, even in the shot taken at 2 am. At this time of year around summer solstice at northern latitudes, the sky never gets astronomically dark but is lit a deep blue by sunlight still streaming over the pole and bathing the night in a glow of perpetual twilight.

– Alan, June 5, 2013 / © 2013 Alan Dyer

 

Time-Lapse Techniques — Creating Star Trails


Dinosaur Park Star Trails (May 26, 2013)

The stars wheel above the Cretaceous-age sediments of Dinosaur Provincial Park.

One of the most powerful techniques in the nightscape photographer’s arsenal is to stack lots of short-exposure images together to create the equivalent of one long exposure showing the motion of the stars. A creative tool to do this in Photoshop is the “Advanced Stacking Actions” from Steven Christenson who maintains a blog and eStore called Star Circle Academy.

I used one of his Actions to create the feature image above. Unlike more run-of-the-mill stacking procedures, Christenson’s nifty Actions can create star trails that look like comets or streaks fading off into the sky at their tail end. It’s a clever bit of Photoshop work achieved by stacking each successive image at slightly lower opacity.

You can use his Actions to create a single composite still image, as above, or to create a set of “intermediate” frames that can be turned into a time-lapse movie with stars turning across the sky and drawing trails behind them. My movie shows several variations. Click the Expand button on the movie to have it fill the screen and reveal the sub-titles.

In Clip #1 I stacked the original set of 360 images without any trailing, using the original frames that came from the camera, albeit with each frame processed to enhance contrast and colour.

In Clip #2 I stacked the images using the “Comet Trails” Action, one that produces very short comet-like streaks.

In Clip #3 I used the “Long Streak” Action to produce longer star trails, but the process also creates unusual cloud streaks as well. Rather neat.

In Clip #4 I used the more conventional “Lighten Mode” to create trails that accumulate over the entire sequence and never fade out. The result on this night was pretty wild and excessive, with the twilight and moonlight adding other-worldly colours.

I certainly recommend the Star Circle Academy Photoshop Actions. While there is a basic Test Set available for free, the full Advanced set is well worth the $30.

– Alan, June 1, 2013 / © 2013 Alan Dyer