Tag Archives: noise reduction

Exposing for Stars

Original Publication: October 20, 2010
Last Updated: November 2, 2017

In two previous articles I covered the most common problems that face anyone doing long exposures. In part 1 of 2 I discussed: Poor Focus, Dim Stars (low contrast), Strange Colors and Pink or Purple glow. In part 2 of 2, I tackled gaps in star trails, and noise.

It seems like I have omitted a rather important element: how to choose your exposure settings in the first place.  An astute Flickr user asked What is the Best ISO for Stacking Startrails. Good question and I realize I have not approached the question from that point of view… starting from the beginning, that is. So pay a little attention here and I will hopefully demystify that question for you.

First, the answer will always be “it depends”.  Just as with daylight or indoor exposures the settings to pick depend on the conditions. Is there a bright moon? Is there a strong sky glow from light pollution? How well does your camera manage noise? How cold (or warm) is it? Are there any bright light sources that need to be managed?  What is the intended result?

Most people attempt to approach a star trail the way they would expose any low light scene. But that approach is flawed.  Let me illustrate the germaine elements with a story about my friend, the little star named “Drizzle”.

Drizzle Drizzle Little Star

The film in a film camera, or the sensor in a digital camera can be thought of as a piece of absorbent canvas. Imagine that the light (photons) of a single star are a stream of tiny little droplets of star juice. The brightest star – Sirius – can spurt 200 droplets per second. The faintest stars visible to the eye, like Drizzle, only manage a single drop per second.  Moreover to be able to notice anything at all, we require a minimum of 20 droplets. Doing a tiny bit of math we realize that we have to collect droplets from Drizzle for at least 20 seconds. But for Drizzle to stand out in any noticeable way we need to collect Drizzle juice for about a minute – 20 was only enough for Drizzle to be discernable.

But wait. We have a collector in front of our canvas that grabs the incoming droplets and shrinks them. That collector is our lens and the shrinkage is related to the size of the iris (aperture). Even when we make our collector opening as big as we can (f/2.8) it will take two collected droplets to equal up to one direct droplet. In fact, if we make our aperture even smaller (f/16) we shrink all the incoming  droplets so that they are only 1/100th of their original size. If we set our collector on f/16 we will have to collect two hundred minutes worth of Drizzle’s meager output to be able to see Drizzle’s shine.

But wait… we forgot about something else much more important! Drizzle and his companions are continuously moving across the sky and each droplet of their juice passes through our collector and falls on an ever-changing place on our canvas. After perhaps 15 seconds the juice of all of our stars will fall noticeably further away – on the next pixel. Poor Drizzle stands no chance of making an appearance because he cannot spew enough juice in his brief time over any part of our canvas to leave a noticeable mark.
How then can we help little Drizzle make an appearance? Well, we can collect more of Drizzle’s juice by using a bigger collector (lens). With a bigger lens instead of collecting one droplet at a time, we can collect two or with a really, really big lens perhaps 4 droplets at a time. After all, a bathtub in the rain collects more water in a minute than a thimble will! Sadly we already have the biggest collector we could afford, and opened it as wide as it will go: so how else can we help Drizzle? Answer: We will employ the last trick our camera can muster: we can change our canvas so that one droplet leaves a mark as noticeable as though it were 10 times its size. This is what is happening when we change our ISO from 100 to 1000.

Oh, I forgot to mention that while we are collecting juice from our starry friends, truly random events occur that cause droplets to appear out of nowhere and plop onto our canvas. We call this noise. The longer we allow our canvas to collect juice, the more random droplets there will be scattered all over our image – unwelcome intrusion by the sprinkler from hell. Unfortunately our sky is not completely black.  Dust and moisture in the air grab light pollution from distant city lights and make the sky rain droplets everywhere. The stronger the light pollution is the harder it will be for Drizzle to stand out. In fact, when the sky becomes as bright as Drizzle we will not be able to see Drizzle at all.

What did we learn from Drizzle, besides sympathy for his plight?

  1. If we narrow our aperture we will get fewer noticeable stars. And the stars we do get will stand out less (lower contrast).
  2. If we use a bigger (larger diameter) lens we can collect more light and thus see more stars.
  3. If we increase our ISO we get more stars, and unfortunately more noticeable noise.
  4. The longer we expose the more noise there will be.
  5. As the background sky glow increases the dimmer stars will be overwhelmed and there will be insufficient contrast to see them.
  6. Eventually an exposure that is too long will wash out the sky and stars.

And the most important take away:  The number of stars we can capture in an image is unrelated to the length of the exposure because the stars are moving.

That last one surprises most people. That is why I highlighted it. I see many folks trying hard to work out the right exposure based on the ambient light. But that is not the correct starting point.

And Now… The RIGHT Exposure

I hear you: “We just want to know what the correct exposure is! We really did not need to hear about your constipated little star.” True, perhaps I spilled more than you wanted to know but my little drizzle buddy hopefully made it clear that only the aperture and the ISO settings have a significant effect on how many stars will be observable in the image.  Now that we know that a really small aperture will eliminate most of the stars, that a long exposure will invite more noise, and that a higher ISO will both allow dimmer stars to be seen AND increase the effect of any noise we hopefully can use those parameters to narrow in on what we need to control. Here are our goals:

  • Use shorter exposures for less noise, better contrast, and less interference from background glow
  • Select moderately open apertures for more light and better contrast
  • If possible shoot in cooler ambient temperatures.
  • Where possible select locations with darker skies.

I hear you oh impatient one. You wanted to know WHAT EXACT SETTINGS you should use. Try this:

f/4, ISO 200, 4 minutes.

That should be about right on most nights where there is half or less moon and not too much sky glow. But remember that exposure triplet is for the sky not the foreground. We can refine those settings by answering the following questions and adjusting the exposure, ISO and aperture as indicated.  Adjustments to larger ISO are optional. Adjustments to a lower ISO are not… except to try and see that failing to reduce ISO or exposure time produces lots o’ noise.

  • What is the air temperature (Fahrenheit)?
    • Greater than 80 degrees (ISO ÷ 4)
    • Between 60 and 80 (ISO ÷ 3)
    • Between between 38 and 60 (no change)
    • Below 32 (ISO × 2)
    • Below Zero (ISO × 4)
  • How bright is my sky (background glow)?
    • About what you would expect less than 20 miles from a large city:  ISO ÷ 2 and exposure ÷ 4, f-stop +1 (1 minute at ISO 100, f/5.6 or f/7.1).
    • It is quite dark but the moon is full (ISO ÷ 2) – exposure ÷ 2
    • It is dark, but the moon is 1/2 full (no change)
    • Some noticeable sky glow, but I can see the Milky Way (ISO x 2)
    • I sat in the dark for 30 minutes and I literally cannot see my hand in front of my face. I can only tell it is there because it blocks out the star light and I still have sensation in my fingers. (ISO × 3) or f/stop + 1.
  • How many stars do we want in the image?
    • As many as I see with my eyes – aperture at f/4 or f/5.6 or even f/7.1
    • Plenty (ISO × 2)  – an aperture at f/4 to f/5.6
    • A sky full (ISO × 4)  – aperture at f/2.8
  • How good is my camera at managing noise?
    • Really Terrible: ISO  ÷ 4 and exposure ÷ 2.
    • Terrible (ISO ÷ 2)
    • OK (no change)
    • Good to Great (ISO × 2) and increase exposure by double

Suppose we end up computing a value of f/5.6, ISO 25 and exposure 2 minutes. What does that mean?  It means something is far less than ideal and throwing in the towel is in order – especially if the minimum ISO on the camera is 200.  If on the other hand the series of multiplications and divisions nets an ISO greater 1600 it is better to dial the ISO down a bit (unless you have a newer generation, highly  capable camera).  The above is pseudo scientific and adapted from observations and exposures – both successes and failures. Actual results may vary.

Ok, now I am sure an explanation is in order. Let me start with the one thing I think causes the most variability – the ambient temperature. The hotter it is, the stronger the noise. That is simply a fact of the electronic world. Film shooters have an advantage here. Film does not become noisier as it is exposed longer – it does have a different problem though. The longer film is exposed, the less sensitive it becomes.  Scientists and astronomers are well aware of the temperature problem. In fact, the really serious image makers super-cool their sensors to remove as much noise as possible.  How much difference does temperature make?  Well, a guy named Gary Honis built a miniature refrigerator to cool his camera.  He is very happy if the mini-fridge gets the temperature of his camera down to 5 degrees! Why? Well one can take a look at his charts.  But I can summarize: a 5 minute exposure at 77 degrees fahrenheit had more than 2,500 noisy pixels while at 5 degrees, less than 100 pixels displayed noise! Twenty five times LESS noise!  When it is exceptionally cold outside I smile because I know that if my battery survives my image is going to be that much better!

The next issue is the camera itself. The smaller and denser the sensor (the higher the megapixels) the more prone it is to noise. Sure some cameras have sophisticated processing to reduce the noise but most of the algorithms also reduce the contrast and thus the sharpness as well. A big sensor with big pixels is better for lower noise performance.  Another issue has to do with the design of the sensor – the method used to collect and read out the count of photons in each of its buckets (pixels). Some methods are just better than others.

Look at the histogram. If the shots are even close to being over exposed reduce the exposure time. If things are too dark, and it is miserably cold out increase the ISO or open up the aperture – or both. If it is warm only open the aperture because a higher ISO or longer exposures will result in more noise.

To get a well lit foreground requires one of the following:

  • Shooting when there is more moon (and get fewer stars)
  • Painting the foreground with artificial light
  • Shooting for a much longer time (and live with the noise).
  • Shoot some shots at twilight to get a nice foreground and layer in the star trails.

Can I Reduce Noise by Averaging?

In a word, no. The normal method of stacking selects the brightest pixel from each shot… and hopefully those are the stars. But those bright pixels can also be noise. If there is noticeable random noise in 10 images then there will be 10 times as much noise in the finished image! It is important to keep the noise as low as possible in the first place!  Averaging can be done. Something undesirable then happens: the foreground image and the overall sky will look much smoother but the star trails will lose contrast. Why is that?  What is the average of 100 + 1 + 1 + 1 + 1?  Answer: twenty one. What did I just do… I took 5 shots with nearly black pixels  (1’s) and one shot with a bright star (100) and effectively I reduced the brightness of the star from 100 to 21.   Here are examples. First is stack which selects the brighest pixel from each of the 11 images. Next the same 11 shots are averaged. The red light on the foreground is from one of the frames. Averaging the star trails clearly reduce the contrast of the stars (and the Saguaro) very significantly.

11 Images stacked using “Brighten” (lighten) mode. One of the images has the Saguaro lit by a brake light of a nearby car.

11 images averaged. Notice the big difference in the contrast of the startrails and the muted light on the Saguaro. On the other hand the sky is “smooth” since averaging also averages out random noise.

If it is not possible to reduce noise by averaging, then what is LENR (Long Exposure Noise Reduction)? Many DSLRs can be configured to enable or disable LENR. Usually I leave it off because using LENR makes your shot take from 50% to 100% longer. Why? Well the camera is effectively doing what the photographer can do by putting a lens cap on. It is taking a dark frame. It closes the shutter and lets the pixel counts accumulate.  It then uses those accumulated pixels to subtract them from the image it just captured. If you have amp glow (multi-pixel hot spots that appear pink or purple) the subtraction will remove the glow.  If you have hot pixels – areas that always read out as bright, the subtraction will eliminate those as well since they will also be in the dark frame.  Since the performance of the sensor changes rather dramatically with increased or decreased heat taking the dark frame immediately after the exposure works best. Of course the camera imaging chips may also do things like sharpen or blur pixels that it thinks are out of range with surrounding pixels. Blurring may improve the appearance overall but it also has an effect very similar to the average stack we looked at earlier.  It is much better to do what we can to keep the noise low in the first place.

What about simply increasing the exposure with the exposure (or lightness) control? There is no magic in that slider. If you do nothing to “increase the exposure” the pixel values are left alone. If, however you increase the exposure by 1 stop, the effect is to double every value. A 10 becomes a 20, a 50 becomes a 100. This, of course, makes those pixels brighter – but it also makes every other pixel brighter, too, including the mild noise. A slightly noisy unnoticeable “3” value becomes a quite noticeable “24” when you increase the exposure by 3 f-stops – an eight-fold increase.

**NOTE: In the earlier Drizzle discussion the word droplet and photon appear to be used interchangeably. But photons are very tiny things and in reality Drizzle’s one droplet per second is really about 2,000 photons per second in case more scientific numbers are desired.

 

***Extra NOTE: There *is* a way to reduce the noise by averaging, even if you didn’t take Dark Frames.

Why North?

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

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Trouble with Long Exposures – Part 2 of 2

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

  1. Poor Focus
  2. Dim Stars (low contrast)
  3. Strange Colors
  4. Purple or Pink Glow

In this installment we tackle these two issues

  1. Gaps in Star Trails
  2. 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:

  1. Camera limitations
  2. Camera or intervalometer misconfiguration
  3. Processing choices
  4. 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:

  1. Shoot at a lower ISO (100 or 200)
  2. Shoot and stack shorter exposures – longer exposures generate more noise.
  3. Capture the foreground and the star trails separately. A better lit foreground will exhibit less noise.
  4. Shoot during colder seasons – lower temperatures result in lower noise.
  5. Control stray light with a lens hood – and close or cover your viewfinder while exposing.
  6. Use high ISO noise reduction
  7. Use noise reduction post processing tools. Chrominance noise is usually most in need of correction.
  8. 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.

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:

  1. Poor Focus
  2. Dim Stars (low contrast)
  3. Strange Colors
  4. Purple or Pink Glow
  5. Gaps in Star Trails – see part 2.
  6. 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:

Cone Heads STILL in Awe [22012-2362]

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

(son of) Bristlecone Pine Star Circle

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:

  1. Incorrect white balance setting (I recommend “Daylight”)
  2. The presence of artificial light.
Pleasanton Circular File [5_018700-20]

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.

star trails

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.