Tag Archives: gaps in 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.

Why North?

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Published: October 11, 2010
Last Updated: April 26, 2018

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Trouble with Long Exposures – Part 2 of 2

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In the previous article I discussed 4 of the 6 most common problems that occur with long exposures. Those problems are:

Poor Focus
Dim Stars (low contrast)
Strange Colors
Purple or Pink Glow

In this installment we tackle these two issues

Gaps in Star Trails
Lots of Noise (Colored Speckles)

Gaps in Star Trails

To oversimplify a bit there are four causes for gaps in star trails created from successive exposures:

Camera limitations
Camera or intervalometer misconfiguration
Processing choices
Weather conditions

Camera limitations: I described this issue in my article “How long does a 30 second exposure take?” All the Canon cameras I own – including the top of the line 5D Mark II require 32.8 seconds to complete a single 30 second exposure. Well there you go: almost 3 seconds of time where there is no exposure. This problem can be compounded by two common misconfiguration blunders:

Failing to allow enough time between exposures when using an intervalometer. Or using the wrong drive mode on the camera.
Failing to turn off long exposure noise reduction.

To avoid intervalometer misconfiguration I operate in either continuous exposure mode or bulb mode. I use continuous exposure mode when my exposures will be many and a maximum of 30 seconds – e.g. when trying to capture meteors or planning for a time-lapse animation. In continuous exposure mode I set my intervalometer with a start delay and then program an exposure time of several hours… AND I put my camera in Manual, high-speed continuous exposure mode with a typical exposure of 30 seconds. You do not really need an intervalometer for this – a locking cable release is sufficient.

When I operate in bulb mode, I try to get a moderately long exposure. Usually in the 4 to 10 minute range depending on the sky conditions. In this setup it is very important to put the camera in Bulb exposure and program the intervalometer to leave a 3 second gap between one exposure and the next. I have recently discovered, however that the Canon 5D Mark II will work with my intervalometer set to 1 second intervals. That’s goodness. I am still trying to work out whether the problem is due more to the timer or the camera. I do know that in continuous exposure mode all my cameras require 32.8 seconds per each 30 second exposure. Failure to allow a long enough pause between exposures can cause unexpected results.

Photo 1: For the first half of the evening I mistakenly left long exposure noise reduction on. The result was that half of my shots occured at every-other eight minute intervals.

The “dotted lines” in the circle above were caused by leaving on long exposure noise reduction. The result was that the intervalometer timed an 8 minute exposure, waited three seconds and then pressed the shutter for the next 8 minute exposure. However 3 seconds after the exposure completed it was still doing long exposure noise reduction so that cycle was skipped until the intervalometer released the shutter for the next 3 second “off” interval.

I have gotten into the habit of setting my exposure length to 3 seconds less than what I want… e.g. 9:57 for a 10 minute exposure. I then set a 3 second inter-shot interval. I used to set a 10 minute exposure plus a 3 second gap – but the predictability of starting a new exposure every 10 minutes makes it easier to monitor what is going on.

Another cause for gaps: changing the battery. I can offer the following important tidbits when you need to change the battery.

Do not wait for your battery to be exhausted. A partial exposure may not stack well or be completely written to your card. Battery exhaustion will likely occur at an inopportune time.
Have everything at hand in advance of the change. For example, keep the battery in your front pocket where your body heat will keep it warm.
Practice a battery change BEFORE you start your exposures. Only by practice beforehand will you be able to discover that the battery compartment is blocked by your tripod, or impossible to reach, etc.
When you DO change batteries beware! Your camera settings may change dramatically!

Processing choices you make when stacking the star trails also affect whether your gaps will be inconspicuous. Do not do any sharpening until you complete your stacking – and even then avoid sharpening the star trails themselves. The method used to stack trails is significant. However, I have observed that people do not notice gaps even in this image of 19 8-minute exposures printed out at 20×30 inches.

Photo 2: Even though it is composed of 19 eight minute exposures the gaps are never noticed even when printed at 20x30.

Weather conditions can also introduce gaps. In a truly dark sky where clouds are not lit by city glow, moonlight or twilight, clouds become “black holes” and block starlight. Low or fast moving clouds can obscure some, most or all of one or more images in the set. This can be perplexing if you happen to be sleeping during exposures which started and ended with clear skies. Another problem is dew which may form a fog that diminishes or eliminates some or all of the exposures. Vigilance with a rag, the use of a hood or a dew heater are your only weapons against dew.

Lots of Noise (Speckled Colors)

I purposefully left the noise in Photo 1. It’s quite noticeable in the rock silhouette at the lower right and appears mostly as red specs. Annoying? Well, yes, but it is not the end of the world. In order of effectiveness here are your best approaches to keep the noise manageable:

Shoot at a lower ISO (100 or 200)
Shoot and stack shorter exposures – longer exposures generate more noise.
Capture the foreground and the star trails separately. A better lit foreground will exhibit less noise.
Shoot during colder seasons – lower temperatures result in lower noise.
Control stray light with a lens hood – and close or cover your viewfinder while exposing.
Use high ISO noise reduction
Use noise reduction post processing tools. Chrominance noise is usually most in need of correction.
Use long exposure noise reduction.

Hopefully you noticed that long exposure noise reduction (LENR) is last on the list. If you are trying to stack star trails it is impossible to get continuous trails with LENR on. It is also the least effective unless you are only going to shoot one shot.

Before we go much further, it is worthwhile to note that there are 4 causes of “noise” and each has a different source. The random speckles are usually what is meant by noise. Those random speckles are created by heat, limitations in the electronics, and things as bizarre as electromagnetic phenomenon like sunspots. No kidding. True noise is by nature random and LENR can not do a thing to combat random noise except to diminish it by reducing the luminance of the offending pixels – which also reduces the sharpness of your image. But there are 3 other kinds of noise that are not random though often lumped into the same general category: hot/stuck or degraded pixels, local heat noise (sometimes called amp glow), and high ISO noise. LENR is effective for these because they are not random.

Hot or stuck pixels usually appear as bright pink, red, blue, green, white or purple spots. They are caused by either electronic problems on the sensor chip or by the dyes used to detect the color. A pixel detects the intensity of the color red by use of a red dye (inkjet droplet) over a sensor site. If that red dye is insufficiently thick, or missing altogether then that pixel location will always read hot if there is any light falling on it – and if the problem is electronic it may read hot even if no light is striking it. Dead or degraded pixels are just the opposite. Too much dye or dead electronics at a pixel site. Degraded pixels are stuck black or darker than the surrounding pixels and are seldom if ever noticed in night photography.

Locally caused heat noise is noticeable in some cameras and is due to the heat of electronics in proximity to the sensor. In my opinion this problem is a design flaw in the camera. However this kind of noise is repeatable so LENR can help correct it. The “Pink or Purple Glow” that results from this flaw was discussed in Part 1.

High ISO noise has an understandable parallel in the world of audio. Take nearly any cheap radio. Turn it up. At some point the sound will become distorted and harsh. This harshness is because there are limitations in the signal, the amplifier circuitry and the speaker used to produce the sound. Increasing the ISO in your camera is the photographic equivalent of the audio scenario. At some point amplifying the light measurements made at each pixel makes the noise more obvious.

Trouble with Long Exposures – Part 1 of 2.

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I administer a group on Flickr called “Star Trails” and moderate a group called “Best of Star Trails“. The good news is there is a constant source of new exciting photography there… and a fair number of beginners facing some common problems. Some of the problems are due to limitations in the camera, and some are due to the selection of exposure time, ISO, f-stop or focus. Some are due to cockpit errors of the kind I described in my August 13th article: Many Paths to Failure regarding unattended shooting with an intervalometer. This list is in addition to those problems and in a way is a bit more fundamental.

Common problems are:

Poor Focus
Dim Stars (low contrast)
Strange Colors
Purple or Pink Glow
Gaps in Star Trails – see part 2.
Lots of Noise (Colored Speckles) – see part 2.

Let’s tackle those one at a time.

Focus is Poor

Poor focus is a topic unto itself which I covered in My Camera Can Not Focus in the Dark – And Neither Can I! But there are a few other causes besides having an incorrect focus. Additional problems that may create noticeable lack of sharpness:

An unsteady tripod (often noticeable when there is wind). And it may not just be the tripod. Check the quick mount plate and the tension on the knobs.
Condensation (that is dew) on the lens. Use a lens hood (helps), and if really bad a lens heater.

Stars Are Not Very Bright

Often the lack of stars is due to an unnecessarily small aperture. Selecting a smaller aperture can help with your image, too. Here are some examples. First is an example from Miguel Leiva:

Photo 1: f/18, ISO 100 for 30 minutes.

???? Trails of Moon, Venus & Jupiter over the Nepean River 30/11/08

Photo 2: f/20 ISO 400.

In Photo 1 a small aperture allows greater depth of field so that focus is sharp from the foreground to infinity but that small aperture also diminishes the contrast in the stars. Taken to an extreme a high f-stop (tiny aperture) with stars can produce an effect like that in Photo 2 by Vincent Miu which was a runner up in the 2009 Astronomy Photographer of the Year contest. The very small aperture, f/20, eliminates all but the brightest elements from the night sky.

While a tiny aperture reduces the number of stars captured, a large aperture (small f-stop number) and/or a high ISO results in many more visible stars especially when the sky is dark. Compare these shots:

Photo 3: f/3.5 at ISO 640: A lot of stars make for a pleasantly dizzying image.

Photo 4: f/4 ISO 100

Photo 3 was shot at f/3.5 ISO 640, while Photo 4 was f/4, ISO 100. Both include about the the same star field but many more stars are present in the higher ISO shot. Even if you are not trying to reduce the number of stars in the field, you might be forced to use a smaller aperture to get more depth of field. Another common problem that causes reduction in contrast is sky glow. When the sky itself begins to lighten you can be sure that the stars will not contrast well. The best way to control this is to take shorter exposures and later at night – or on a clearer night (cold winter nights produce the clearest skies). The moon is also a huge source of glow. Treat the glowing moon just as you do artificial light glow – reduce your exposure length (and ISO) to take pictures when the moon is strong. But do not give up just because you can barely make out stars in your night sky – the camera can see them better than you can! Photo 6 is a perfect example. The city glow made it impossible to see more than 8 or 9 stars toward the north and yet the star trails are quite present.

Colors are Strange

Many people are surprised to see that the stars in their photos are different colors: red, orange, yellow, blue and white. Those are the natural colors of the stars. People are also surprised to see a blue sky however even modest amounts of moonlight or a very long enough exposure will result in blue sky! Unfortunately sometimes the stars or the sky are unnaturally colored. Usually the culprit is one or more of these factors:

Incorrect white balance setting (I recommend “Daylight”)
The presence of artificial light.

Photo 5: White balance problem due to different types of light. In this image I compromised to keep the colors on the land as natural as possible.

Getting the white balance right is not hard except when there is lot of artificial light – streetlights, city glow, etc. Unfortunately there are many different types of lights each with their own color characteristics. The popular low pressure sodium vapor lights are nearly monochromatic yellow-brown in color. There is really no way to get a naturally colored look when sodium lights predominate the scene. Florescent, tungsten, LED, and other light types all differ in their color profiles and when several different sources are in play for a scene it gets harder to keep a natural looking scene.

Sometimes when handed lemons you can make lemonade as in Photo 6. I could not correct for the predominate sodium vapor lights so instead of fighting I adjusted the color temperature to make the foreground elements look as natural as I could and did not worry that the stars became white – most people think of them as white anyway. It certainly helps that the image also includes a portion of twilight illumination to help keep the scene realistic looking.

A City and A Mountain. Part A [5_024371-434g]

Photo 6: When corrected for the sodium vapor lights the mountain looks almost natural, but the stars have lost their color.

And there is yet one more way to solve the color problem; but you will have to do some editing. To fix different color lights you can color balance each element separately and then combine the elements into one image. For example using “Daylight” white balance for the star trails and “Tungsten” for the street scene may produce a natural and pleasing looking photograph. Photo 6, above was manipulated in a similar way. Once it became completely dark the glow from the city lights caused flaring and ghosting. The solution was to choose one properly exposed frame from twilight and layer that on top. Layering like this is easier if you have an overexposed daylight shot that you can use as a mask. More on that in the Night Photography Workshop

There is Pink or Purple at the Edges

Some cameras, particularly older models may suffer from “amp noise”. The glow or noise is usually visible at the corners or edges of the photograph and usually only with longish exposures (over 8 minutes). Here is an example from Ethan Doerr of what “amp noise” may look like.

Photo 7: Amp noise is prominent in this photo taken on a Nikon D80 with a 572 minute exposure at 100 ISO. Nikon: D80, D90, D40, D200, D3000, and possibly other cameras may exhibit similar anomalies. Photo by Ethan Doerr – used with permission

If your camera is subject to amp glow there are some tactics you can try. The simplest is to keep your exposures short and stack them. Or perhaps allow the camera to cool down from time to time. Or only shoot in Antarctica ;-).

For more Trouble with Long Exposures see Part 2.

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Tag Archives: gaps in star trails

Why Aim North?

The Advantages of Shooting To the North

The Disadvantages

Another Northern View