Tag Archives: Star Trails

Why Aim North?

The Advantages of Shooting To the North

Curvature of the star trails is strongest around the north star. Exposures of about 6 hours will appear to be full circles (24 hours of exposures are actually needed to make complete circles and that is not possible in one night except near the North Pole!).
The moon will never intervene into the shot because the moon never passes through the northern sky. [NOTE: those in the southern hemisphere still have to worry about a moon in their Southern shots]
Cassiopeia and Ursa Major (the Big Dipper) are bright constellations that can always be found in the Northern Sky – so there is always some interesting sweep of stars possible. The region immediately around the North Star, however has dimmer stars which may only be captured through long exposures. [The southern hemisphere suffers for lack of many bright constellations near the southern celestial equator]
With just a smidgen of star hopping skill it is easy to find the north star which, weather permitting, is always visible in the night sky.
The moon sweeps east to west giving long shadows from the right or left of the subject. And when the moon is highest in the sky it can cast strong face light.
The sun also never appears in the northern sky so it is safe to leave a camera running from before sundown to after sun up. Camera damage can result from a long exposure pointed at the sun. [NOTE: those in the southern hemisphere still have to worry about a sun in their Southern shots]
Since the moon cannot enter into a northern shot a photo can be made regardless of the moon’s phase and for as long as I choose. For shots toward the East, South or West it is important to know the moon phase and location during the hours of shooting to prevent problems from flare or washout. [NOTE: those in the southern hemisphere still have to worry about a moon in their Southern shots].
The stars in the north move the slowest through the field of view which allows them to be brighter and reduces inter-exposure gaps in the trails.
If I know my latitude I know how high to point the camera and be guaranteed to get a circle in the view.
I do not need to know what constellations will be visible in the direction I will shoot.
Two major meteor showers (the Perseids and Quadrantids) and 3 periodic meteor showers (the Giacobinids or Draconids, the Ursids and the Andromedids) are well placed in the northern sky.

The Disadvantages

There are a few detriments to pointing north, however:

Not every situation lends itself to a view from the south.
It takes a longer exposure to form a pleasing arc.
To get a circular arc, I must include at least 10 degrees or so above and below the North Star. The farther north you are, the higher in the sky the center of rotation. Those at more northerly latitudes will be more constrained in their choices.
The altitude (degrees above the horizon) of the north celestial pole may constrain the choice of lenses to very wide-angle – and may force you to use portrait mode. Or you can create your exposure by stitching together foreground and sky shots.

Another Northern View

Looking North from Grandview Campground, White Mountains, Bishop, California. Shot at ISO 800, f/3.2 for 6 minutes each. Began at 10:11 PM and ended at 5:35 AM. That is 75 shots x 6 minutes = seven and a half hours. Grandview is at latitude 37 degrees North, so the center of the circular pattern is 37 degrees above the horizon.

What About OTHER Directions?

Southern View

Woosh: 19 images. Each image: 6 minutes, ISO 800, f/3.2, Canon 5D Mark II, 16mm; Patriarch Grove, White Mountain, California

Eastern View

Western View

Notes

Contrary to popular belief, Polaris, the North Star, is not the brightest star in the sky. Sirius is the brightest star. The brightest objects in the night sky are the moon, and the planets Venus, Mars, Jupiter and Saturn. Also while Polaris is quite NEAR to the North Celestial Pole, it’s not exactly there so even Polaris will make a trail.

This is a reissue of an article originally written in October 11, 2010; Thoroughly revised and updated.

The Revenge of Lens Correction

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There are plenty of ways to make your images look weird. Some of the perturbations are due to sneaky little things that Adobe Photoshop, Adobe Lightroom and/or Adobe Camera RAW might be doing to the data. We already talked about the “cooking” that is applied by default to RAW images and why letting that cooking stand unchallenged may be a bad thing. We’ve even warned you about Blur and Jaggies that may NOT in fact be in your images.

Recently Dan Barr asked us what we thought was causing a problem in his stacked star trails. If you read the title you’ve probably already figured out the culprit… Lens Correction! You may not notice anything weird if you process only a single image, but what if Star Trails, or image stacking are what floats your boat?

Notice the strange pattern in the upper left. This image is cropped from a larger image. Image by Timbo2013

Look in the upper left of the image above. That cross hatching is one possible artifact.

Why does this happen? The lens correction is a mathematical model that moves pixels around. Not surprisingly, since the images change – even if slightly, the results vary slightly, too.

How do you fix the problem? Don’t mess with your images before you stack them. Save the lens correction, contrast adjustments and other tweaks for after you’ve finished stacking.

Here is a before and after comparison:

Notice the odd “Moire” like pattern above and to the right of the mountain? (Image courtesy of Dan Barr)

It’s a little subtle. Here is the weird part close up – notice the vertical undulations? The oddness somewhat resembles sensor banding noise except when you look at a larger scale, the lines are concentric.

By redoing the stacking operation without performing lens correction, Dan was able to get an image without the waves:

Stacked first, then adjusted – no moire!

With the strangeness vanquished, Dan was able to improve the brightness and contrast as well.

Intervalometer Tricks

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Scratching your head and wondering what an Intervalometer is? We’ve covered that in this article, and talked about some super fancy Intervalometers in this article.

Before we launch into the tricks, let’s first get some terminology straight.

Long Exposure – in my vernacular this is an exposure over 30 seconds – the limit of most DSLR cameras.
TimeLapse – a series of photos taken over time that compresses (or expands) the actual time when made into a movie. Usually all the exposures use the same settings. An event that takes 3 hours can be distilled into a 30 second video. An event that takes fractional seconds – like a balloon popping – can be shot at high speed and expanded into a movie that lasts much longer. Usually expanding the time is called “Slow Motion”.
StarTrail – like its timelapse brethren, implies a series of shots taken over time and combined into one exposure to show the star motion OR a StarTrail can be created from a SINGLE very Long Exposure.
Bramping (aka Bulb Ramping) – a timelapse techinque in which the length of the exposure is changed over time to accommodate the setting sun, rising moon – anything that involves a gradual change in the ambient light.

How is a Timelapse different from a Star Trail?

The two are not different, except that by intention a Star Trail created from multiple exposures requires a minimal interval between one shot and the next or gaps result. For a timelapse – which can be taken at night or day – the key is having a regular interval between each shot. Changing the interval between shots has the effect of warping time.

Ok, Got it. Tell Us About the Tricks

Sub-Second Intervals

Why would you want sub-second intervals? For one, to catch as many meteors as possible. The second or so that the camera spends with it’s shutter closed is a second you might miss that brilliant fireball. Another reason to keep the interval REALLY short is to reduce or eliminate gaps in star trails. But sub second intervals are the hardest trick of all. There are almost no intervalometers that allow setting an interval shorter than one second, and even if it’s possible many cameras can not handle sub-second intervals. However, there are a few devices that can do sub-second intervals: Trigger Trap for one. The best way to find the shortest possible interval is to set up the camera and try! Set the interval to say 700 ms and see if your camera can run off a sequence of 15 to twenty 30-second shots without missing a beat. If that works, set the interval to say 500ms. Note that the minimum interval will depend on the camera, as well as the size of the image, and speed of the memory card. Once you find the minimum, leave a little extra time and use that. My Canon 5D Mk II was happy with 450 ms intervals between shots. That’s HALF of the waiting time of one-second intervals.

Shake Reduction – Mirror Lock Up

Many people worry about mirror slap. Mirror slap occurs when the little mass of the mirror “wiggles” the camera enough to blur long-ish exposure shots. Mirror slap is particularly pernicious in the 1/4 to 2 second exposure length. It may also be a problem if you have your camera attached to a delicately balanced telescope at high magnification. How do you solve the problem? It depends on your camera, but there are several approaches to try:

Leave live-view on (which will eat batteries and may result in excessive warming of the sensor)
Use the camera self-timer in mirror lock-up shooting mode. Most cameras will behave properly if your exposure length is not bulb. That is, they will move the shutter, wait for the delay to expire and then take the shot. Remember to allow a delay that is at least one second longer than the shot length plus the self timer delay. For example, let’s say you want to take as many 24 second exposures as possible but you need at least 8 seconds for mirror slap to stabilize. Set the camera to 24 second exposures with a 10 second self-timer. Then set the intervalometer to take a 1 second(!) exposure every 36 seconds. The reason for the 1 second exposure is to allow enough time for the shutter release to be recognized while the 36 second delay allows for 10 seconds of timer, 24 seconds of shot and a 2 second safety buffer.
See the Maximum Shots, minimum interval trick. But instead of 1 second delays, change the length of the exposure to the amount of time you need for camera stability + 1 second.
Want to do shake reduction in BULB mode and without a self-timer? Set the exposure length to the desired amount of time and use a short interval. With mirror lock-up on, you’ll get every-other exposure at the desired length. Note: this is the most “iffy”mode as it depends on your camera behavior.
Finally for shake reduction in BULB mode WITH a selftimer, set the intervalometer as normal, but set the length of exposure longer and include the self-timer interval. For example to take 60 second exposures with a 10-second self timer, set the exposure length on the intervalometer to 70 seconds.

Variable Length Shots

While this technique seldom works well, you can allow the camera to determine the exposure length via metering. All you have to do to make this work is to have the camera take 1 second exposures (as before) no more frequently than the longest exposure you expect to take. Some fancier devices, like the Trigger Trap and the CamRanger can even be configured to change the exposure length over time. This feature is called “Bulb Ramping”.

Extended Self Timer

Got a big group shot and no wireless remote. Not a problem. Set the camera to a short (e.g. 2 second) self-timer delay, and set the shot delay to say 20 seconds – or as long as you need to safely climb on top of the human pyramid to get that perfect shot. Since you can allow multiple shots, you’ve all got plenty of time to change your poses, or re-architect your human pyramid. We use this trick all the time when we’re conducting workshops. It allows us to set up our camera and walk away while we instruct. We leave enough time to set up for the shot. Most cameras will blink or flash to let you know they are about to take a picture so everyone can time that cheesy fake smile. Note: If using a Flash, you can lengthen the interval between shots to give the flash extra time to recycle.

Maximum Shots, Minimum Interval

When not in Bulb mode, it can be maddening to not have the Intervalometer and the camera in sync. Set the camera to 20 seconds and the intervalometer to 19 and you’ll miss about every other shot. Bummer. Here is a trick to maximize the number of shots and not care much about the actual exposure time. Set the intervalometer to take one second shots with one-second intervals between each. The maximum shot-to-shot delay will be two seconds that way and it doesn’t matter what your exposure length is on the camera if it’s NOT bulb.

What if you want BULB mode? How do you configure that?

Answer: (Select the text below to reveal)
That’s the normal intervalometer configuration mode. Camera is bulb, length of exposure is whatever you need and the interval should either be 1 second, or exposure length PLUS one second depending on the intervalometer.

Blobulous Revisited – Part 2

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Last installment we covered the basic idea behind creating a star trail where a foreground element is moving. In this case the moving element is a radio telescope peering into the sky to discover planets and black holes. A normal “lighten mode” stack produces the image at the left, below, while with just a little bit of work we can get the image on the right.

To recap, we use a single frame from the sequence and some careful masking to remove the blurred part of the image. For effect, we also don’t use the stack in 100% mode. See the prior article for details.

In this article we’ll show you step-by-step how we achieved the total look.

Of course we start with the stack.

Then we layer in a copy of the stack and a single frame (the last or nearly last frame).

Here we have set the background (stack) to 36% Opacity, effectively darkening it. The single frame is 100% opacity and in Lighten blend mode. What we want to do is to remove the blurred part by replacing it with the unblurred single image. It’s easy to do. Select the STACK, create a “Reveal All” mask, and then go to town painting black on it (be sure to select the MASK, not the image). When done, the mask we create will look something like this:

Notice how we used a slightly soft brush to “blend” the background and the still frame. We could stop right there, but I notice that the ground and the telescopes are a bit too bright, and I’d like to make the stars pop out. So the next course of action is to apply a curve, select the “Increase Contrast” option. Here I’ve adjusted the result just slightly.

Next we want to tone down the bright stuff. We add another adjustment layer, and a “Hide All” mask and then paint white back on the mask to tone down what we want. You’ll notice that in the process, the colors intensify a bit.

The next step we’ll want to take is to reduce the saturation – our radio telescope is moving from white to yellow. So the next step is to add a Hue and Saturation layer. As before we mask off everything, and then paint in only the radio telescope. We could cheat and use the same mask from the Brighten stack layer – and just invert it, but it’s not a complicated thing like a tree, so it’s pretty easy to change the mask to only operate on the radio dish and pedestal. At the left, you can see how strongly we moved the saturation – and we upped the brightness a bit, too.

If we didn’t mask off the telescope and instead applied the saturation adjustments globally, we’d see this – not what we want. (Shift-Click on the mask turns it on or off – in this case we see that the mask is off by the red X through it)

You won’t notice in the small size, but the large image has a number of Hot Pixels (red, and blue) that stand out. To solve this problem we use “Alt-Ctl-Shift-E” (Command-Option-Shift E for you Mac-o-philes) to make a copy of the layer. I named the layer “Heal” because I then used the spot healing tool to fix up those little problems. I recommend making the healing tool diameter just slightly larger than the area to be healed.

To make the Radio dish pop just a little more, a little sharpening is in order. In fact, sharpening the ground will work, too. Duplicate the Healed layer (Ctl-J or Command-J). Name it Sharpen then use Filter -> Sharpen -> Smart Sharpen. However I don’t like sharpening my stars, they look harsh. As before we’ll create a Hide-All mask for our Sharpened layer and use a white brush to reveal the areas we want to have sharpened. This is called selective sharpening. In the small image here, the effect is not as obvious as in the larger image.

To get just a bit more pop, a little more contrast is in order. I created a curve and pulled up the midtones a fair amount while making minor negative adjustments to the highlights and the darks. But, that adjustment brightened some areas a bit too much so I created a reveal-all layer mask and painted black on the areas that were then too bright.

Et Voila, we’re done!

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Tag Archives: Star Trails

Why Aim North?

The Advantages of Shooting To the North

The Disadvantages

Another Northern View