When a budding astroimager decides they’re ready to move past their DSLR and buy a dedicated astro camera, a hard question arises: Should they choose a one-shot color or monochrome camera? It’s a difficult decision because both have advantages and disadvantages.
One-shot color cameras (OSC) capture all three color bands (red, green, and blue) in one image, just like a DSLR or smartphone. They have a grid of tiny color filters arranged in 2×2 squares on top of the sensor. A common type of array is the Bayer matrix, named for Kodak scientist Bryce Bayer, its inventor. In each group of four pixels, one is red, two are green, and one is blue. (Green is doubled because our eyes are most sensitive to the green part of the spectrum, the peak of the Sun’s light emission.)
In the raw version of OSC images, each pixel records the intensity of the red, green, or blue light as a number, and the image will appear monochrome with a checkered pattern. To make a color image, the raw image is “debayered,” a process that interpolates the surrounding similar pixels to estimate the specific color in each pixel. DSLRs and phone cameras do this automatically, but astrophotographers need to use image-processing software to debayer their images after calibrating them, before they register or align them. There are several different algorithms for accomplishing this.
Monochrome cameras, on the other hand, do not have a Bayer matrix; each pixel records the intensity of the light that falls on it. Red, green, and blue filters are placed in front of the camera to capture the intensity of each color, and then those are combined into a single RGB image at a later point during processing.
So which kind of camera should you get? There are several factors to consider. Here’s my list of the winners for different categories.
True resolution: Mono
This is the winner because there is no interpolation to determine the color of a pixel.
Light transmission: Mono
The tiny color filters of the Bayer matrix can have lower light transmission efficiency than filters of monochrome cameras. It’s a little more complicated than this, but users of both kinds of cameras will notice a difference, particularly in narrowband imaging.
Total frames needed: OSC
Since OSC cameras capture all bands of optical light in one image, fewer total frames are needed to make a complete color image than with monochrome cameras. This is advantageous when you have little time under a good sky, such as at a star party.
Ease of preprocessing: OSC
In this case, you only have to calibrate, register, and stack one set of images rather than three to make an RGB image. Just don’t forget to debayer!
Processing flexibility: Mono
Mono offers more ways to combine the different color channels, particularly if you’re doing narrowband imaging. You can also take luminance frames, which capture the intensity of all the optical wavelengths of light at once, making a higher signal-to-noise ratio image that can showcase more detail.
Cost: OSC
Not only are OSC cameras cheaper than mono (due to economy of scale with all the DSLRs out there), but when you have a mono camera, you also need color filters and a filter wheel.
So who wins the battle? It all depends on your needs. If you have a little more money and more clear nights, monochrome cameras can produce higher-quality images but also require more processing. OSC cameras, on the other hand, are cheaper and require fewer total frames to make a complete image, maximizing time under good skies. I like to have both!
