White Balance

White balance settings adjust the overall color of a scene appropriately for the color temperature of the ambient light.

Indoor tungsten lighting is much more red than outside in open shade, which is more blue. Red light sources are called "warm" and bluer light sources are called "cool". You can see this if you set your camera to daylight white balance and then shoot a shot indoors lit by normal tungsten light bulbs, and then go outdoors and shoot in open shade on a sunny day with the same daylight white balance. The source of illumination inside is a glowing tungsten filament in a light bulb which is very warm. Therefore the overall image will have a red bias. The source of illumination in open shade is the blue sky. Therefore the light from it is very blue.

White balance settings attempt to adjust for the differences in the color of the light source. Auto-white balance usually works well with daylight and more blue-illuminated scenes and not quite that well with warm light sources.

Daylight White Balance	Tungsten White Balance	Auto White Balance

For astronomical use, if you are shooting raw file format, the white balance setting on your camera doesn't really matter. You can change the white balance later in software. However, if you are shooting JPEG the white balance setting does matter because this adjustment is applied to the image in the camera before it is written to the JPEG file. The white balance setting in the camera will also be applied to the small thumbnail image that is displayed on the LCD screen on the back of the camera during a preview when shooting raw.

Even though the white balance setting is not applied to the raw file, it is usually saved in the IPTC header information that accompanies the raw file. When the image is opened in software such as Photoshop or Canon Digital Photo Professional or Nikon View, this white balance is applied to the image by default or by selecting the "as shot" option.

Because auto white balance usually does not work that well with warm illumination sources, such as the sodium vapor lighting that now predominates most urban areas, it usually doesn't work that well for use for deep-sky astrophotos.

On the other hand, if you just want a quick and dirty JPEG image in the camera, shooting a custom white balance set on the sky itself in a long exposure can work fairly well.

White Balancing Astronomical Images

To record the correct color of the stars with a stock, unmodified camera, the white balance should be set to daylight. Our eye evolved to see daylight from the Sun as neutral. Setting the color balance to daylight in the camera, or color balancing the white point to daylight white balance in subsequent image processing ensures that all of the star colors we see in an image are correct for our visual perception.

To achieve correct color balance we can set the white balance to daylight in the camera, or shoot a white or gray card in sunshine, and use it to determine the white point adjustments needed to color balance the image to daylight white balance.

If we have a modified camera with the long-wavelength filter removed or replaced with a different glass, the built-in color balance settings of the camera will not work correctly. In this case we can use a white sheet of paper, or an 18 percent gray card to set a custom white balance and apply it to our images.

Place the blank white sheet of paper, or the gray card in direct sunshine at noon on a clear day. Move close to fill the frame with the paper or card. Be careful not to cast your shadow on it. Take an exposure of it with the camera set to the ISO that you will use for your long-exposure deep-sky images and shoot with the camera on auto-exposure. In both cases, the camera should expose so that both the white paper and the gray card are exposed for a middle gray. This is correct. Do not overexpose the white paper to make it white, we want it to be gray. The camera's meter will naturally underexpose it, making it gray, because the camera's meter thinks everything it sees is 18 percent gray.

This gray image will be used for setting the white balance to daylight when we do a white point adjustment in later image processing. Since the image was shot in daylight, adjusting the white point to neutral will, by definition, set the white balance of the image to daylight. This same adjustment can then be applied to the deep-sky images to assure correct white balance.

This technique of shooting a white sheet of paper, or a gray card, can also be used with unmodified cameras when raw file format is used. The same white point adjustments applied to neutralize the gray image are applied to the raw image.

Shooting a gray card in sunshine can also be used to set a custom white balance in the camera for astronomical images taken later at night. If this is done before a deep-sky imaging session, this same custom white balance can be applied to the raw files of the long-exposure deep-sky images to achieve correct white balance.

If you use light pollution filters with either a stock or modified camera, you should shoot a custom white balance with the filter on the camera. For narrowband filters, a custom white balance will not work.

I keep a series of custom white balance frames of a gray card stored on an extra memory card. Before an imaging session with my modified camera, I pop it in the camera and choose the correct white balance for the filter I plan to use. If no filter is used, I simply pick the custom white balance gray card that was shot in sunshine with no filter. I keep this memory card write protected by sliding the write protect tab down on the left side of the SD memory card. After setting the custom white balance, I swap out that card for a larger memory card that I use to record the long-exposure deep-sky images.

The Color of the Sky

Don't be surprised if the sky does not turn out a pleasing color on the display on the back of the camera with a long exposure if the in-camera white balance is set to Auto, or Daylight, especially if the site has any man-made light pollution at all. Also, the night sky, even at the darkest locations, has color in it from the natural sky glow and possible faint auroral activity. This color is usually a dark brown. This sky glow is comprised of four components:

1. Airglow is the brightest component and is caused by oxygen atoms glowing in the upper atmosphere which are excited by solar ultraviolet radiation. Airglow gets worse at solar maximum. Airglow can add a faint green or red color to the sky background. The color may be vivid if there is a strong aurora occurring.

2. Interplanetary dust particles reflect and scatter sunlight and make up the zodiacal light and Gegenschein. This light is primarily the color of sunlight.

3. At night starlight is scattered by the atmosphere, just as sunlight is during the daytime. Air molecules scatter short blue wavelengths more, which is why the daytime sky is blue. The night sky also has a very faint blue component from scattered starlight.

4. Countless stars and nebulae in our own galaxy also contribute to the brightness of the night sky, most easily seen in the form of the Milky Way.

Some people prefer to make the sky background black on the theory that this is how the object would appear if you could view it from space. However, while it is possible to make the sky black by underexposing it, this is not a good way to do it. This will put any faint detail down in the noise of the camera. You can also make the sky black in subsequent digital processing, but this is not a good thing to do either because faint detail will be difficult to distinguish, and this detail will be difficult to reproduce.

Other people prefer to filter out the light pollution and natural airglow, either in the camera or in subsequent processing, and make the sky background a neutral gray color lighter than black. This can be accomplished in camera by setting a custom white balance on the sky background itself, neutralizing the sky background with a levels adjustment, or by subtracting the sky background during processing. The first two methods are not as accurate as the last, but depending on your needs and how critical you want to be, they may work just fine for you.

Advanced Techniques

• Dithering - Dithering is changing the position of the scope so that the objects moves very slightly between each frame. This helps reduce pattern noise and streaking that can sometimes result from less than perfect dark-frame subtraction. It also helps with the removal of hot and/or dead pixels in the image. Dithering is an essential technique for advanced imaging.

  When dithered images are combined by adding or averaging, the pattern noise is moved around and it tends to average out, making it less visible.

  If an autoguider is used, tracking must be turned off between each sub-exposure and the scope moved so that the object and stars fall on different pixels. This can be difficult to accomplish because it takes the automated process of gathering sub-exposures and inserts a manual operation between each frame. If you are attempting to capture 20 to 30 sub-exposures this can present quite a challenge.

  Some of the latest DSLR control programs, such as Nebulosity, Images Plus, Backyard EOS, and AstroPhotography Tool, can automate dithering with an autoguider. Dithermaster will work with Canon's EOS Utility. Standalone autoguiders that incorporate a remote interval timer for camera control, such as the Lacerta MGEN and LVI Smartguider 2, can also dither.

• Drizzling - Drizzling also moves the position of the telescope slightly, but with the intent of improving the spatial resolution of the final image if the image is undersampled. This is a technique that the Hubble Space Telescope uses as well as other professional observatories. A more detailed explanation can be found here. Drizzling is usually only helpful for images that are undersampled.

• Scalable Dark Frames - It is possible to shoot dark frames at different temperatures and exposures, and then scale the dark frames to match the temperature and exposure of the light frames. You will also need to shoot separate bias frames if you want to scale your darks.

• LRGB - Incorporate a separate high resolution luminance image into the RGB data by converting to Lab mode and substituting the Luminance channel. This is really a technique for monochrome CCD Cameras. It doesn't really work with a DSLR because of the RGB filters over each individual pixel in the DSLR prevent you recording a high-resolution luminance-only image.

• HaRGB - It is possible to shoot through narrow bandpass filters, such as hydrogen-alpha (Ha), to increase the contrast between hydrogen-alpha emission objects and the sky background. These narrow-band images can then be blended into normal RGB images to improve the final image. A modified camera is necessary for this technique.

Controlling and Automating the Imaging Session

If you are going to be shooting multiple sub-exposures for hours at a time, doing it by hand every 3 to 5 minutes can become extremely tiring and demanding. Luckily there exists hardware and software tools that allow complete automation of the imaging session with a DSLR camera.

Although it is possible to automate several hours of imaging, it is not always a good idea to just let the camera run unattended for this length of time. Things can, and do, go wrong for no apparent reason. The camera battery or power supply can die. Guide stars can be lost due to passing clouds. Memory cards can become full.

It is a good idea to periodically check on the progress of an imaging session by examining an actual image. This can be done on the LCD on the back of the camera by magnifying the image, or by opening the image in an image processing program and enlarging the stars at least to 100 percent enlargement. Focus and tracking can be checked in this manner.

Focus can change during the night as temperatures fall, especially in fast optical systems, so re-focusing may be necessary.

If you are going to gather multiple hours worth of data on a single object, and are not controlling your imaging session and automatically dithering between frames with a computer, it is also a good idea to pause the image gathering sequence at intervals, perhaps every half hour or so to dither. Focus and tracking can be checked, and the scope's autoguider turned of briefly while the scope is moved slightly. The guidestar can then be re-acquired by the autoguider, and tracking and exposures started again. Although not as effective as dithering after each individual exposure, dithering after a group of exposures, when several groups are taken, is better at reducing pattern noise than no dithering at all.

• Hardware - The Remote Interval Timer

  The Canon TC-80N3 Timer (left) and the JJC Timer Remote Control (right)

  The Canon TC-80N3 and its Chinese clones, such as the JJC Timer Remote Control 4, are very powerful tools for automating an imaging session.

  They offer the ability to control the total number of exposures, the exposure time, the time interval between exposures, and the delay before an exposure. They are battery powered and also have a built-in light so you can see the settings in the dark.

  You can set one of these timers to take a series of, say, 20 five-minute exposures, and then go off and observe or even take a nap while the scope tracks the sky and the camera takes pictures.

  For Bulb Exposures longer than 30 seconds: Set the camera to single shot mode. Turn off autofocus on the lens or camera if using a camera lens. Set the exposure mode to bulb on the camera. Set the total number of exposures desired on the remote timer. Set the interval time between exposures to 10 seconds to give the camera time to download the raw file to the in-camera memory card or to the computer. Set the exposure time on the remote timer to the desired exposure. Start the exposure sequence by pressing the START button.

  
  

  Mirror Lock-Up Trick

  If you want to use the mirror lock up with a remote interval timer you have to use a trick. Put the camera in self-timer mode and set the mirror lock-up with the camera controls. Then program the remote interval timer as normal but add the length of the camera's self-timer time to the exposure time. The remote interval timer will then signal the camera to start the process and the mirror will lock up, the self timer will count down, and the shutter will open.

  Unfortunately, with this method of using the self-timer on some Canon DSLRs, the white focus assist light will shine very brightly while counting down the time until the start of the exposure. This can irritate not only you, but also any other astrophotographers or observers around you who are dark adapted. It is not recommended for this reason unless you are working alone. Or, you can simply place a piece of black tape over the light. You may only have to put up with a blinking red LED on other cameras.

  For long time exposures on a solid mount, you generally don't need to use the mirror lock up anyway. The vibration from mirror slap will be minimal and usually won't be recorded amidst degradation from seeing and tracking errors anyway. For high-resolution planetary work, using the mirror lock up can be beneficial.

• Software
  • Astro Photography Tool (APT)
  • Backyard EOS (BYE)
  • Canon EOS Utility
  • Images Plus
  • Maxim DSLR
  • Nebulosity
  • AstroArt

  These software packages all support automated image acquisition with DSLR cameras. The down side compared to using the remote interval timer is that you need a computer to run the software, and power supply for the computer in the field.

  Canon's EOS Utility software is included in the purchase price of Canon DSLR cameras and works very well. Nikon's Camera Control Pro software costs extra.

When automating an exposure sequence, whether using with a remote interval timer or software in a computer, it is important to program in enough time between exposures to allow the camera to process the image and download it to the memory card or computer. If the automation process tries to start the exposure before the download is complete, the camera may just ignore the command. This is especially important in older cameras that use USB1 for camera control where the downloads may be quite long. You should do some tests and time how long the downloads take and check to see exactly what happens with your methodology. Newer cameras usually have quite fast download times and the pause can usually be set to about 10 seconds without any problems.