Adjusting White balance for Astrophotography
If you are new to the world of photography, you may not know exactly what the term white balance is referring to. When it comes to astrophotography, this camera setting still has its place but becomes less important as you learn how to capture RAW images and edit your images in post.
In photography, white balance is referring to color temperature. It is measured in degrees Kelvin (K). White balance is an adjustment in the camera to ensure that the colors in your photos are accurate to how they appear in real life.
White balancing is the removal of unrealistic color casts in the image. White balancing means adjusting the color balance in an image to compensate for the color temperature of the illumination source. This can be done automatically in the camera, or manually in subsequent image processing.
Adjusting the white balance of a RAW image in Adobe Camera Raw.
Adjusting White Balance for Astrophotography
If an object appears white in real life but has a red or purple tint in your photo, the white balance needs to be adjusted in order to render the white as white within the photo. This process will also calibrate the colors of the entire picture.
To achieve the correct white balance, one has to take into account the color temperature of the light source illuminating the scene, for example, the warmth or coolness of the light illuminating the scene.
What this means for wide-field astrophotography is that shoot location and sky conditions will dictate the white balance selection in your images, which it’s why it’s essential to know how to set the white balance manually. Cameras come with various WB presets, which are only fine to use when shooting RAW.
For example, I typically leave the white balance setting on “auto” when taking images in RAW file format for a deep-sky image.
The white balance can easily be changed in post-production if you shoot RAW
If you’re shooting in JPEG instead (which I don’t recommend for astrophotography), you must select the correct white balance while shooting.
The original image shows the frame out of the camera vs. the stacked, color adjusted final.
White Balancing for Night Photography:
Daylight (~5200K) is nearly always too warm for night sky photos, but I often find this to be this most natural “general” setting to use when shooting in RAW.
Generally, cooler settings of between 3200-4800 are used for astrophotos, depending on location and environment. I enjoy my photos of the night sky to have a cooler blue look, rather than a red/brown feel to it.
Here is an example of a wide-angle astrophotography shot using a cooler white balance. The photo was shot using the daylight white balance, but was “cooled-down” in post.
This photo was shot at dusk using daylight white balance, but was adjusted to look cooler in post.
In light-polluted skies, the excess ambient light reflecting off the atmosphere creates a warm glow to the sky, therefore in urban areas a lower (colder) setting around 3,400-3,900K would be more suitable. In darker skies, settings of 4,000+ will yield better results.
The image above was shot from a Bortle Scale Class 4 site, which is why it appeared naturally cool, straight out of the camera. From my light-polluted backyard in the city, shots appear much warmer with reddish-brown hue to them. This is not a great look, and I always adjust the color balance of the wide-angle shots I take from home.
The light pollution from the city gives all of my shots from home a warm feel.
To find the best WB for your photo, start with the suggestions above and shoot test photos in increments of 100. Adjust your white balance to a cooler or warmer setting, depending on your location and the desired look of the image.
In a stock DSLR camera, the sensor, Bayer filters, and processing algorithms have all been finely tuned by the manufacturer’s engineers and color scientists to produce accurate color with the original long-wavelength filter in place.
Modifying the filter by removing it, or substituting another filter with different passband characteristics changes this situation. The automatic color balancing algorithms, and the built-in preset color balances like daylight, won’t work correctly with a modified camera.
The above photo was captured using a (full spectrum) modified DSLR camera and appeared very red before adjusting the white balance.
For astrophotography, white balancing a DSLR camera that has had the manufacturer’s low-pass long-wavelength filter removed or modified can be accomplished by Using a Custom White Balance in the camera.
Raw linear data does not have a white balance adjustment applied to it in the camera like JPEG images. However, there are notations, called tags, that are included in the raw files, that describe what white balance the camera was set to when the image was taken.
When a raw image is opened in the camera manufacturer’s dedicated image processing program, such as Canon’s Digital Photo Professional, or Nikon View, the embedded tag is usually applied as the default white balance setting.
This setting can easily be changed for the raw file in this software or if the raw file is opened in Photoshop. Standard white balances, such as daylight, can be applied. With a stock, non-modified camera, all we need to do is simply select daylight as the white balance for the deep-sky image.
For my final image of the Andromeda Galaxy, I left all of the subframes as “auto” white balance.
Daylight White Balance will not work however for modified cameras because their settings are configured with the manufacturer’s long-wavelength filter in place in front of the sensor. With a modified camera, we can shoot a custom white balance as a workaround.
Shooting a Custom White Balance
Jerry Lodriguss has discussed this subject in detail on his website. He is a professional photographer who understands the importance of white balance, and its relevance in astrophotography today.
- Use a digital gray card directly illuminated by sunshine on a clear day at noon with the Sun high in the sky. You can also use a white piece of paper.
- Fill the frame with the card or piece of paper.
- Ensure that there are zero shadows on the card.
- Use raw file format.
- Use auto exposure and take a picture of the card.
- Select this card in your camera’s menu to use as a custom white balance. Read your camera’s manual to find out exactly how to do this as it is different for different cameras.
Use this custom white balance to shoot another picture of your gray card or white piece of paper, again on auto-exposure, as a test to be sure your custom white balance is working. It should come out gray.
For the obsessive-compulsive, you should use a spectrally uniform gray card to shoot a custom white balance. White paper can have ultra-violet brighteners in it that may throw off the custom white balance.
When you use auto exposure on a white piece of paper, it will not be white because it will be underexposed. This is correct. On a modified camera, it could be red, which is also correct. This red color is what the custom white balance will correct.
It is acceptable to use a 50mm lens to shoot the custom white balance. Then use the custom white balance when you shoot through the scope, or with another lens if you intend to use that same filter.
For example, if you plan to shoot with a light-pollution filter, you should, therefore, use it on the camera when you shoot the custom white balance.
The white balance that you use in the camera, whether stock or modified, to shoot the gray card in the sunshine will not matter. This is because the camera will set the white balance on the raw data.
Store the picture file of your gray card on an old memory card and keep it with you. That way, if you change the custom white balance, you can re-apply it at night at the telescope without having to shoot it again in the sunshine.
Setting the White Balance on a Star like our Sun
You can create a natural color balance in your image by setting the “white point” of your image on a G-type main-sequence star (G2V) in Adobe Photoshop. This is because the way we perceive white light is based on the spectral characteristics of the Sun.
In Adobe Photoshop, you have the option of using the “set white point” eyedropper in the Levels dialog box. The difficult thing about this method is actually confirming that you are sampling an actual G-type main-sequence star (that is not oversaturated).
Software such as PixInsight has features to identify and adjust your images from a database of star types, but you can manually adjust your images if you can confirm that you’ve found a yellow dwarf like our Sun. Jerry Lodriguss has listed a link to suitable G2V stars for this technique on his website, although I have never gone to these links myself.
Setting the white balance of a deep-sky image using a G2V star.