Not Your Everyday Banana

by Bart Arondson

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Many photos of Milky-way show it in nice colors instead of just the black of sky and white of the stars. Is the key to having those colors in the capturing of the image or in the post processing, or both? Or am I just doing it in a wrong time of year when the visible part of Milky-way is simply not showing color?

Here is a sample photo of how Milky-way looks like in Finland in September:

image

APS-C sensor camera, 18 mm, 30 sec, f/3.5, ISO 3200 - captured in pitch-black darkness of Finnish countryside, far far away from any citylights, at time when moon was still below visible horizon.

What I'd like to achieve is something like the photo in Vivek's answer to How do I capture the Milky-way?

I've just bought a wireless shutter release for longer bulb exposures, and have plans to build a barn-door tracking mount. But is longer exposure the needed trick, or what is?

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Different parts of it appear to be much more or less colorful than others. Take a look at this image of it that allows you to freely move around it: galaxy.phy.cmich.edu/~axel/mwpan2/krpano –  dpollitt Sep 29 '13 at 23:04
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All other things being equal, longer exposures will make the stars appear whiter as more and more of them blow out all three color channels. –  Michael Clark Sep 29 '13 at 23:34
    
@dpollitt - I just got to view the rotating Milky-way panorama and it sure is not very exiting section of the Milky-way there near Andromeda. Not the dullest part either, but it really is nothing compared to the brightest section. I guess I'm trying this during wrong season. Thanks for the link, it was helpful. –  Esa Paulasto Sep 30 '13 at 4:14
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4 Answers 4

up vote 21 down vote accepted

The vast majority of night sky photos have been boosted in post to achieve their brightness. This is more true for cameras with smaller sensors than for cameras with larger sensors, but in general, even if you shoot the night sky at ISO 3200, you are going to need to boost exposure to get one of those nice, bright single-frame Milky Way shots.

There are a few things you can do to increase the brightness of your night sky shots.

First and foremost, don't be afraid to push ISO. I own a Canon 7D, not particularly great at high ISO, and I usually use ISO 1600 and 3200 for my night skies. If you are using ISO 100, 200, or 400, your too low (unless you are also using an f/1.4 or faster lens, and even then, I would still recommend using ISO 1600 at least!)

Second, find the darkest skies you possibly can. This can often be difficult, especially in densely populated areas. As an example, almost the entire eastern half of the United States is riddled with light pollution, as can be seen by this view of Dark Sky Finder. On this map, blue and green are acceptable for night sky photography, but still prone to issues with light pollution. In my milky way shot here, I was 50 miles from Denver, in a blue zone, and I still had considerable problems with the metro area light bubble over the horizon (note, this image is heavily processed, original below):

enter image description here

The darker the skies, the brighter night sky objects can be made (key point here..."made"...there is a post processing component that I'll discuss later.) An additional note here...another key factor in achieving dark skies is finding moonless skies. New moons are the best time, and you usually have several days on either side of a new moon where you basically have no moon at all (it either rises or sets with the sun). Even when the moon is out, so long as you time your trip during a period when the moon is not actually in the sky should be fine. That usually means later (i.e. some time after midnight) for waxing moons, and earlier for waning moons. Around full moon, night sky photography is usually out, unless you want the moon to be involved for some reason.

Third, use the longest exposure you can. In the past, this has largely been guided by the 600 Rule, which says divide 600 by the effective focal length of the lens to determine the longest exposure that will not produce star trailing. Today, with ever smaller pixels, the 500 Rule seems more effective. Here is a table for some common wide-field focal lengths:

 Focal Length |   FF   | APS-C (1.5x) | APS-C (1.6x) 
=====================================================
    10mm      |   --   |     33s      |     31s
    14mm      |   35s  |     23s      |     22s
    16mm      |   31s  |     21s      |     19s
    18mm      |   27s  |     18s      |     17s
    24mm      |   20s  |     14s      |     13s
    35mm      |   14s  |     10s      |      9s
    50mm      |   10s  |      6s      |      6s

In general, wider lenses allow longer exposures, as the angular movement of the sky covers a smaller portion of a wider frame for any given unit time. On APS-C, a 10mm lens will offer the longest exposure time, while on FF a 14/16mm lens will offer the longest exposure time. (Note that 16mm on FF is the same as 10mm on APS-C, so the option of a 14mm lens on FF is an added benefit.) Also don't rule out fish-eye lenses, which offer a full 180° field of view, and therefor an even longer potential exposure time, albeit with a warped projection. It should also be noted that, while a 50mm lens in the table above allows only a very short exposure, it is also often possible to get a 50mm lens with a one-stop, sometimes an even two-stop advantage over other lenses. The 50mm f/1.8 lens is usually the cheapest lens in a lineup, and finding a 50mm f/1.4 lens is often not that hard. That is like exposing for one or two stops longer with any other lens, so the short exposure time is often still workable with a 50mm lens.

It should be noted that the 500/600 Rule assumes identical output magnification. With night sky photography, that is a pretty good assumption, but not necessarily always true. If you intend to crop for any reason (i.e. to blow up a nebula or galaxy), you should be applying your additional crop factor as well. Using a longer focal length is usually a better alternative, however longer focal lengths quickly run into exposure length issues anyway without further measures.

Fourth, if you have the option, get a camera with the biggest pixels and lowest high-ISO read noise you can get your hands on. Technically speaking, the Canon 1D X would be the best astrophotography camera on the market today. The Canon 5D III is a far more accessible alternative, and still offers pixels considerably larger than any APS-C part on the market. Larger pixels not only increase the amount of exposure time you have before star trails start occurring, but they gather more light in total during that time as well, so its a double benefit.

Fifth, photograph on nights that have good seeing, low atmospheric distortion, etc. The amount of stellar light that reaches an itty bitty camera on the surface of the earth is often dictated by how much of that light is scattered by the atmosphere. Even outside of urban light pollution bubbles, skies full of dust or moisture will warp and scatter a lot of the light reaching the atmosphere. The clearer and crisper the skies, the better your exposure will be. There are various sites on the internet that can probably help you find clear, dark skies with good seeing.

Finally, remember to post process. Even an ISO 3200 shot under decently dark skies is going to be fairly dim at the maximum 500 Rule exposure time. A sky devoid of dust or significant moisture, with good seeing, will produce some beautiful exposures. Mountainous regions, particularly above 11,000 feet, offer this kind of sky in spades, however are less accessible. For any other area, including blue areas on the Dark Sky Finder site, your night sky photos will require some exposure boost and tone mapping in post to fully bring out the detail you are looking for. As an example of how extreme edits may need to be, here is the original version of my shot above...still riddled with light pollution from a city fifty miles away:

enter image description here

Despite the sky clarity problems, you can bring out a lot of detail and color with some processing. You will usually end up with very bright stars, which can be a bit unappealing, and the only way to really fix that is to find darker skies.

One final option, for those who have the money, is to get a tracking mount. Good telescopes usually come with an equatorial tracking mount which will nicely track with the sky. This is only really an option if you are just photographing the sky...any included landscape/foreground will blur as the camera tracks. If you buy a tracking mount or better yet a decent telescope, that opens the door to deep sky astrophotography, which is complimentary to the kind of wide-field astrophotography I've discussed so far.

Photo Statistics

Camera

  • Canon EOS 7D
  • Canon EF 16-35mm f/2.8 L II
  • Gitzo GT3532LS Tripod
  • Gitzo GH1780QR Ball Head

Exposure

  • Focal Length: 16mm
  • Shutter: 25s
  • Aperture: f/2.8
  • ISO: 3200

Processing

  • White Balance: 3590 (5250)
  • Tone:
    • Contrast: +45
    • Highlights: -100
    • Shadows: -30
    • Whites: +71
    • Blacks: -25
  • Presence:
    • Clarity: +35
  • Tone Curve:
    • Highlights: +20
    • Lights: +20
    • Darks: -25
    • Shadows: -60
  • HSL/Color/B&W:
    • Saturation:
      • Red: -35
      • Orange: -16
      • Yellow: -5
      • Aqua: +20
      • Blue: +45
      • Purple: -5
      • Magenta: -35
    • Luminance:
      • Red: -65
      • Orange: -20
      • Blue: +70
      • Purple: +100
      • Magenta: +35
  • Sharpening:
    • Amount: +40
    • Radius: 0.5
    • Detail: 20 (25)
    • Masking: 70 [Causes blurring of smooth areas, which is what I wanted]
  • Noise Reduction:
    • Luminance: 80

(Note: Original values in parentheses when difference is important.)

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Impressive answer, thank you for taking the time to provide examples and the great text as well. –  dpollitt Oct 7 '13 at 22:41
    
+1 from me, well detailed. –  John Cavan Oct 8 '13 at 0:18
    
Very good, detailed answer for how to make dark sky photos more colorful. But as your example demonstrates, when you deal with light pollution the cast of the polluting light will minimize the differences between the colors of the stars. The only way to allow each source of light from the sky above the atmosphere to be differentiated in terms of color is to minimize the influence of atmospheric light: Shoot in the darkest location possible at the highest altitude possible through the driest air possible when the moon is not in the sky. –  Michael Clark Oct 8 '13 at 1:48
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@MichaelClark: I think thats pretty much exactly what I said...no? –  jrista Oct 8 '13 at 7:26
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@drfrogsplat: It really doesn't matter the generation of the sensor. Any technology that can be applied to smaller pixels can be applied to larger pixels, so there will NEVER be an SNR advantage to smaller pixels in the grand scheme of things. Greater area per pixel, stronger signal, greater SNR, lower noise. That's just the fact of things, at least when it comes to astrophotography. Regular photography may be different, as what often matters most is total light per sensor area, rather than per pixel area. In AP, the total light per pixel is really the most important thing. –  jrista Feb 27 at 0:25
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There are several things you can try.

  • A full frame sensor will allow you to saturate the colors more in post processing before noise becomes an issue.
  • Stacking multiple images will also allow you to increase saturation because stacking images also reduces random noise.
  • Use the formula in this answer to calculate your ISO setting. In your example above it calculates to about ISO 2500. The higher the exposure, the whiter the stars will appear. Even if a star is, for example, more green than red or blue, if you blow out all three color channels it will appear white.
  • Set White Balance using color temperature and experiment with different values to bring out cooler or warmer colors.
  • Shoot from the darkest sky possible (away from light pollution), at the highest altitude possible, through the driest air possible, when the Moon is not in the sky.
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Thanks. Can't get a ff-camera, so that leaves me with stacking at lower ISO. You suggested 5200 K in your answer to How to set White balance in a photo of stars? and it was a good starting point, I did not move far from it. –  Esa Paulasto Sep 30 '13 at 0:01
    
5200K is a color temperature, not an ISO. That aside, the other question made no mention of maximizing color saturation in astrophotography. –  Michael Clark Sep 30 '13 at 0:53
    
Hehe, of course 5200 K is not ISO, K stands for Kelvin and measures temperature, in this case color temp. And that's what the other question was all about, nothing more to it. Your answer here is very enlightening, and I will do a stacking job at lower ISO and give it more saturation. My blowing all color channels seems the likely reason to missing colors, if there ever was those colors to begin with. –  Esa Paulasto Sep 30 '13 at 1:19
    
a full frame sensor isn't inherently less prone to noise. –  Agos Sep 30 '13 at 9:14
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It is if the amount of light falling on the lens from the field of view projecting on the lens is the same and the pixels are significantly larger than those on the 7D, which is true of all Canon FF cameras on the market at this time. The 7D's pixels are 4.3µm wide. The 1D X, 5D3, and 6D respectively are 6.9µm, 6.25µm, and 6.54µm wide. Thus each pixel on the FF models has over twice the surface area with which to catch photons per pixel as the 7D does. –  Michael Clark Sep 30 '13 at 20:49
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Time of year plays a big part in capturing a nice view into Milky way. So does the location on Earth where you are with your camera.

It appears the sample photo (in the question) above is of a not-so-exciting part of Milky-way. The widest and most colorful part of Milky-way is where the direction is towards the center of our galaxy. Because earth's axis is tilted, and the planet earth travels around the sun, this direction changes, not only by time of day, but also by time of year. Unfortunately the center of our galaxy is not well in view for photographing it in Finland (North of 60° latitude) at any time of year, except in daytime during summer, which is impossible time for photographing it.

Here is two screenshots of different sections of Milky-way:

Milky way with Andromeda in sight
^^ This is the same view as in the sample photo in the question.

Milky way center mass
^^ Here is a wider and brighter section of Milky-way.

Easy to see how time of year/day makes a big difference in which part of the Milky-way is visible. A nice photo of the center mass of our galaxy is shown in another question "What kind of camera.." photographed by Bala Sivakumar. AFAIK that view is not possible to photograph in Finland, and I don't mean the building in the photo.

You should use Stellarium software to find a good time for photography. First take a look at a page suggested by @dpollitt: galaxy.phy.cmich.edu/~axel/mwpan2/krpano and find a view you'd like to try to capture. When found, zoom in to see the star/galaxy designators on the image. Pick one of them.

In my case I chose to focus on M16, the Eagle Nebula, so I put that into the search field in Stellarium and set it to keep M16 centered on screen. A focus object is needed because we can't tell the program to "keep the nice part of Milky-way in center". Then I started playing with date and time values.

Stellarium screenshot
^^ A screenshot of Stellarium software.

This way I found out that to catch a good view on the wider portion of Milky-way I should wait till the week after easter 2014, and get out with a camera each morning between 3 and 4 a.m. For the screenshot I adjusted Stellarium settings to increase the brightness value of Milky-way and lower the Light-pollution level.

The rest of the actual photographing of colorful Milky-way goes as suggested in other answers to this question, and generally as in about every question/answer concerning Milky-way photography.

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Long exposures, fast lenses, low background light, clear skies and having the milkyway overhead at the time. This typically takes very long exposures. In this photo, it was likely a 30 second exposure with a flash used to expose the couple on the bench quickly.

If there is too much background light from nearby cities or the moon, then it won't be possible as it will be washed out. Similarly, if the milky way isn't overhead at the time, you will get a mostly black star field.

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