Astrophotography beginners often ask, “Should I get a monochrome or a one-shot color camera?” There are advantages and disadvantages to each. This article will compare the ZWO ASI1600MM Pro monochrome camera with the ZWO ASI294MC Pro camera.
One-shot color cameras, or OSC for short, are what one typically thinks of when thinking of a camera. Just like a DSLR or smartphone, all visible wavelengths are captured at once, through the red, green, or blue filters imprinted on the camera sensor. In a color image, each image pixel consists of four pixels grouped together: one red, two green, and one blue. These pixels are combined to create a single RGB color pixel, then interpolated to regain the full resolution of the image. The micro-filter array on the sensor is called the Bayer matrix.
For the astrophotographer, this means that you only need to take one image to get all colors, which saves time compared to monochrome cameras. For those with few clear nights, this is especially advantageous. In addition, OSC images are easier to process – no need to combine multiple monochrome image filters.
Monochrome cameras, on the other hand, do not have the Bayer matrix; every pixel captures any wavelength of visible (and near-infrared) light. To create a color image, you must use filters that pass the specific wavelengths of light desired. The most commonly used are LRGB filters; luminance, red, green, and blue. Imaging this way allows every pixel to count for a particular color, instead of only one-quarter or one-half of the pixels on the sensor. LRGB filters lead to higher resolution and a higher signal-to-noise ratio (SNR).
Using the luminance filter also allows all visible wavelengths to pass, which, while not necessary to create a color image, can greatly increase the detail and signal-to-noise ratio. In addition, the large LRGB filters used with monochrome cameras transmit more light than the Bayer filters do; 90 percent to 99.9 percent transmission figures are common. An article in Digital Photography Review1 says that as much as 1 EV (stop) of light is lost through the Bayer filter. Monochrome cameras can also be used for narrowband imaging with artificial color palettes, as well as ultraviolet and infrared imaging for objects such as planets.
Because one must use multiple filters to create a color image, it takes three to four times longer to create a complete image, depending on if a luminance filter is used. Because of the need to use multiple filters to create a color image, the processing is also more challenging and time-consuming.
Let’s compare three images of the Orion Nebula (M42): one in monochrome LRGB on the 1600, one in OSC on the 294, and one in narrowband (Ha+OIII) on the 1600.
While this is not perhaps the most equal comparison of the two cameras, it gives an example of the differences between imaging with each. OSC cameras can do narrowband imaging with multi-bandpass narrowband filters, although the SNR is significantly lower than doing narrowband imaging with a monochrome camera.
Both the ZWO ASI1600MM Pro and the ASI294MC Pro can take amazing images. Monochrome cameras have better signal-to-noise ratio and resolution, but require much more time to create a complete image. One-shot cameras capture complete images more quickly and are easier to process, but suffer some transmission and resolution losses due to the Bayer matrix. Many astrophotographers, the author included, end up having one of each.
1Butler, Richard. “Resolution, aliasing, and light loss – why we love Bryce Bayer’s baby anyway.” Digital Photography Review, March 29, 2017. Accessed May 7, 2021. https://www.dpreview.com/articles/3560214217/resolution-aliasing-and-light-loss-why-we-love-bryce-bayers-baby-anyway
Website: ZWO Optics