The Cadence of the Moving Sky


Saturn, Mars and the Milky Way over the Bow River

Saturn, Mars and the Milky Way appeared in the twilight over the Bow River.

I shot this scene on August 24 from the viewpoint at Blackfoot Crossing Historical Park, overlooking the Bow River. Mars appears between Saturn above and Antares below, in a line of objects west of the Milky Way.

The valley below is the traditional meeting place of the Blackfoot Nation, and the site of the signing of Treaty Seven between Chief Crowfoot and Colonel MacLeod of the North West Mounted Police in 1877.

The image is a panorama of two images, each 20-second exposures at f/2 and ISO 1600 with the 24mm lens. I shot them just prior to shooting time-lapses of the moving sky, using two cameras to create a comparison pair of videos, to illustrate the choices in setting the cadence when shooting time-lapses.

The movies, embedded here, will be in the next edition of my Nightscapes and Time-Lapse ebook, with the current version linked to below. The text explains what the videos are showing.

 

Choose Your Style

When shooting frames destined for a time-lapse movie we have a choice:

  • Shoot fewer but longer exposures at slower ISOs and/or smaller apertures.

OR …

  • Shoot lots of short exposures at high ISOs and/or wide apertures.

 

The former yields greater depth of field; the latter produces more noise. But with time-lapses, the variations also affect the mood of a movie in playback.

This comparison shows a pair of movies, both rendered at 30 frames per second:

Clip #1 was taken over 2 hours using 20-second exposures, all at ISO 2000 and f/2 with 1-second intervals. The result was 300 frames.

Clip #2 was taken over 1 hour using 5-second exposures also at f/2 and 1-second intervals, but at ISO 8000. The result was 600 frames: twice as many frames in half the time.

Clip #1 shows fast sky motion. Clip #2 shows slow motion.

Clip #2 exhibits enough noise that I couldn’t bring out the dark foreground as well as in Clip #1. Clip 2 exhibits a slower, more graceful motion. And it better “time-resolves” fast-moving content such as cars and aircraft.

Which is better? It depends …

Long = Fast

The movie taken at a longer, slower cadence (using longer exposures) and requiring 2 hours to capture 300 frames resulted in fast, dramatic sky motion when played back. Two hours of sky motion are being compressed into 10 seconds of playback at 30 frames per second. You might like that if you want a dramatic, high-energy feel.

Short = Slow

By comparison, the movie that packed 600 frames into just an hour of shooting (by using short exposures taken at fast apertures or fast ISOs) produced a movie where the sky moves very slowly during its 10 seconds of playback, also at 30 frames per second. You might like that if you want a slow, peaceful mood to your movies.

So, if you want your movie to have a slow, quiet feel, shoot lots of short exposures. But, if you want your movie to have a fast, high-energy feel, shoot long exposures.

As an aside – all purchasers of the current edition of my ebook will get the updated version free of charge via the iBooks Store once it is published later this year. 

— Alan, August 26, 2016 / © 2016 Alan Dyer / AmazingSky.com

 

The Moving Stars of the Northern Hemisphere


Arizona Star Trails - Circumpolar Looking North

I present a montage of time-lapses illustrating the motion of the sky in the Northern Hemisphere. 

Any stargazer should be familiar with how the sky moves, with stars rising in the east and setting in the west.

From the northern hemisphere, when we look north we see the sky rotating counter-clockwise around the North Celestial Pole, near Polaris. As you’ll see in the video, even Polaris moves, though not much over the night. The stars that never set, but just move across the northern horizon, are the circumpolar stars.

When we look south we see the seasonal constellations, the ones that rise and set, and change over the seasons.

I shot the images for these sequences from southern Arizona, in early December 2015.

So the night starts with the summer stars setting in the west and the autumn stars dominating the sky. But then Orion and the winter stars rise and march across the sky over the night, setting before dawn, as the spring stars rise.

The south-looking movie is a dusk-to-dawn sequence. Note the Zodiacal Light in the west at right in the early evening, then reappearing in the east at left before dawn brightens the sky, and as Venus and the Moon rises.

Also note the moving bands of red and green airglow, a natural phenomenon of the upper atmosphere.

 

I posted a matching set of movies in my previous blog post, shot from the Southern Hemisphere. But here’s the link to the movie.

 

Both sets of movies were shot from nearly identical latitudes – about 31°, but 31° N for Portal, Arizona and 31° S for Coonabarabran, Australia.

As such the Celestial Poles appear at equal altitudes above the horizon. And the angles that the stars rise and set at in relation to the horizon are the same.

But the direction they move is opposite. When looking 180° away from the Pole, the seasonal stars move from left to right in the Northern Hemisphere, but from right to left in the Southern Hemisphere.

Visitors from one hemisphere to the other are bound to get turned around!

— Alan, August 25, 2016 / © 2016 Alan Dyer / AmazingSky.com

 

The Moving Stars of the Southern Hemisphere


Southern Sky Star Trails - OzSky Looking South

Nothing amazes even the most inveterate skywatcher more than traveling to another hemisphere and seeing sky move. It moves the wrong way!

Whether you are from the southern hemisphere traveling north, or as I do, travel south from the Northern Hemisphere, watching how the sky moves can be disorienting.

Here I present a video montage of time-lapses shot last April in Australia, at the annual OzSky Star Party near Coonabarabran in New South Wales.

Select HD and Enlarge button to view at full screen at best quality.

You’ll see the sky set in the west but traveling in arcs from right to left, then in the next clip, rise in the east, again moving from right to left. That’s the wrong angle for us northerners.

Looking north you see the seasonal constellations, the ones that rise and set over a night and that change with the seasons. In this case, the night starts with Orion (upside-down!) to the north but setting over in the west, followed by Leo and bright Jupiter. The sky is moving from east to west, but that’s from right to left here. The austral Sun does the same thing by day.

Looking south, we see the circumpolar constellations, the ones that circle the South Celestial Pole. Only there’s no bright “South Star” to mark the pole.

The sky, including the two Magellanic Clouds (satellite galaxies to the Milky Way) and the spectacular Milky Way itself, turns around the blank pole, moving clockwise – the opposite direction to what we see up north.

I shot the sequences over four nights in early April, as several dozen stargazers from around the world revelled under the southern stars, using an array of impressive telescopes supplied by the Three Rivers Foundation, Australia, for us to explore the southern sky.

I’ll be back next year!

– Alan, August 19, 2016 / © 2016 Alan Dyer / www.amazingsky.com

 

The Perseids Perform


Radiant of the Perseid Meteor Shower (2016)

It was a great night for shooting meteors as the annual Perseids put on a show.

For the Perseid meteor shower I went to one of the darkest sites in Canada, Grasslands National Park in southern Saskatchewan, a dark sky preserve and home to several rare species requiring dark nights to flourish – similar to astronomers!

This year a boost in activity was predicted and the predictions seemed to hold true. The lead image records 33 meteors in a series of stacked 30-second exposures taken over an hour.

It shows only one area of sky, looking east toward the radiant point in the constellation Perseus – thus the name of the shower.

Extrapolating the count to the whole sky, I think it’s safe to say there would have been 100 or more meteors an hour zipping about, not bad for my latitude of 49° North.

Lone Perseid in the Moonlight
A lone Perseid meteor streaking down below the radiant point in Perseus, with the sky and landscape lit by the waxing gibbous Moon, August 11, 2016. Perseus is rising in the northeast, Andromeda is at right, with the Andromeda Galaxy right of centre. Cassiopeia is at top. Taken from the 70 Mile Butte trailhead in Grasslands National Park, Saskatchewan.

The early part of the evening was lit by moonlight, which lent itself to some nice nightscapes scenes but fewer meteors.

Perseid Meteor Shower Looking North (2016)
The 2016 Perseid meteor shower, in a view looking north to the Big Dipper and with the radiant point in Perseus at upper right, the point where the meteors appear to be streaking from. This is a stack of 10 frames, shot over one hour from 1:38 a.m. to 2:37 a.m. CST. The camera was on the Star Adventurer tracker so all the sky frames aligned. The ground is from a stack of four frames, mean combined to smooth noise, and taken with the tracker motor off to minimize ground blurring, and taken at the start of the sequence. All exposures 40 seconds at f/3.2 with the 16-35mm lens and Canon 6D at ISO 6400.

But once the Moon set and the sky darkened the show really began. Competing with the meteors was some dim aurora, but also the brightest display of airglow I have even seen.

It was bright enough to be visible to the eye as grey bands, unusual. Airglow is normally sub-visual.

But the camera revealed the airglow bands as green, red, and yellow, from fluorescing oxygen and sodium atoms. The bands slowly rippled across the sky from south to north.

Airglow is something you can see only from dark sites. It is one of the wonders of the night sky, that can make a dark sky not dark!

TECHNICAL:

Meteor Composite Screen ShotThe lead image is stack of 31 frames containing meteors (two frames had 2 meteors), shot from 1:13 am to 2:08 a.m. CST, so over 55 minutes. The camera was not tracking the sky but was on a fixed tripod. I choose one frame with the best visibility of the airglow as the base layer. For every other meteor layer, I used Free Transform to rotate each frame around a point far off frame at upper left, close to where the celestial pole would be and then nudged each frame to bring the stars into close alignment with the base layer, especially near the meteor being layered in.

This placed each meteor in its correct position in the sky in relation to the stars, essential for showing the effect of the radiant point accurately.

Each layer above the base sky layer is masked to show just the meteor and is blended with Lighten mode. If I had not manually aligned the sky for each frame, the meteors would have ended up positioned where they appeared in relation to the ground but the radiant point would have been smeared — the meteors would have been in the wrong place.

Unfortunately, it’s what I see in a lot of composited meteor shower shots.

It would have been much easier if I had had this camera on a tracker so all frames would have been aligned coming out of the camera. But the other camera was on the tracker! It took the other composite image, the one looking north.

The ground is a mean combined stack of 4 frames to smooth noise in the ground. Each frame is 30 seconds at f/2 with the wonderful Sigma 20mm Art lens and Nikon D750 at ISO 5000. The waxing Moon had set by the time this sequence started, leaving the sky dark and the airglow much more visible.

— Alan, August 13, 2016 / © 2016 Alan Dyer / AmazingSky.com