Mono or Color Camera?

In astro photography two quite different approaches for taking images exist. On one hand there is the classic workflow using a monochrome camera along with filters, and then there is the apparently more modern use of one shot color resp. OSC cameras.

If you consider starting astro photography, I and most others will recommend using a color camera. If you already own a digital system camera which can be adapted to a telescope, that's fine. If you prefer to adjust for a specialized astro camera, please also focus on a color camera first. After getting comfortable with that workflow, obtaining some decent results from this setup and finally realize that you are effectively limited by not using a mono camera, you may consider an upgrade.

From a physical view a monochrome camera using the same sensor as its color twin has the potential to produce sharper images, as long as the telescope and visible conditions are capable enough. As a consequence a monochrome camera is more demanding on other parts of the setup to produce its full potential. You do not automagically produce better astro images just after such an upgrade.

First of all a color camera is much simpler to use and creates color images right out of the box. Due to its Bayer Filter its effective resolution is only a quarter compared to its monochromatic counterpart. When processing the raw sensor data the obtained color information is spread across neighboring pixels so the resulting pixel quantity looks identical. In fact the color information is upscaled by a factor of two and the luminance information averaged, resulting in a less sharp image.

Regarding color purity such a color camera has some disadvantages in astro photography as well, since its RGB filter is optimized for daylight photography and modeled to resemble the sensitivity of the human eye. Filters used with monochrome cameras are optimized with astro photography in mind and may even reject typical light emitted by terrestrial city illumination. To counteract light pollution there are filters for color cameras as well including, so called, dual narrowband filters which only forward most common wavelengths of hydrogen (H-Alpha) and oxygen (O3) to the sensor.

A color camera  may capture images for RGB, H-alpha and O3 in exactly two series. Trying the same using a mono camera will require five sequences, namely red, green, blue, H-alpha and O3. If you add luminance (to enhance dark nebulae) and sulphur (S2), that increases to seven. In addition to your light frames, you do need matching flat frames for each filter as well (check out my astro glossary for further information).

The images produced from a monochrome camera then needs to be processed, which needs to be taken into account regarding storage space and your computer's performance. Even if only red, green and blue channels are used to create a RGB image,  the lights need to be processed separately and combined afterwards. The potentially higher flexibility provided by a monochrome camera require more effords in processing for each single image.

In contrast to an increase in amount of processing time, you can obtain similar or even slightly better images from same total exposure time when using a monochrome camera. But be warned, you probably will challenge yourself with more demanding targets which require a significant higher exposure time.

Let's not forget the cost. Not only that monochrome cameras are considerably more expensive compared to their color counterparts, a filterwheel along with a set of filters is required as well. Depending on the preferred filter quality this easily adds up to some USD 1500-3000.

Even more points need to be considered. For which purpose do you intend to do astrophotography? If your target is a computer monitor or smartphone, a modern color camera already provides sufficient resolution for a 4k resolution.

And what about viewing conditions? A monochrome camera images color channels separately over a lengthy period of time. In order to combine these channels later without color fringes or similar artefacts, visible conditions need to be constant during this session. Since the earth is rotating and you point through different layers of air during the night, this is obviously difficult to accomplish. Under uncertain conditions you may take images using a color camera as long the sky is clear. When using a monochrome camera you may end the night with one complete channel missing.

If you prefer hunting galaxies using larger focal lengths, you may better pick a color camera. These comparatively bright objects do not take much benefit from monochrome cameras. Better invest your money into a more capable mount and optics.

To take color images from planets or the moon a color camera is highly recommended. The separate color channels of a monochrome camera can hardly be combined to obtain a nice image. On the other hand for black and white images using an infrared filter a monochrome camera obviously can provide a higher image resolution, as long you have perfect seeing conditions.

However, imaging emission and dark nebulae will highly benefit from using a monochrome camera. These collect more light through narrowband filters and create somewhat finer noise. Only monochrome cameras are able to collect a real luminance signal (full visible light) to enhance overall image contrast.

These two images visualize some differences quite obvious:

One shot color camera (30x180s, 1,5h)

Monochrome camera (3x17x120s, 1,7h)

Both images were taken with nearly the same exposure time, the same sensor (Sony IMX571, with Bayer mask on the OSC), the same optics (TSQ-100ED, 580mm, f/5.8 APO refractor) and was processed with the same workflow. Since the subs from the OSC had a 50% longer exposure time, the brighter stars came out slightly larger.

One obvious difference are the colors. Since the Bayer mask of a color camera should reproduce terrestial photography as our eyes will see, the filters in a monochrome setup will differentiate light by its wavelength without overlap. So the green channel of a color camera still gets some amout of Hydrogen Alpha photons so that these areas look less intense red as they should be. If the target contains some oxygen as well, like the center area of this target, the result is kind of neutral grey, while the RGB image from the mono camera is more violet in this area, the addidive color mix of blue and red.

The magnified area in the top right visualizes the difference in resolution. While the tiny stars look kind of blurred in the image from the color camera, they are pretty sharp in the image produced by the mono camera. Similar is true when looking at the image noise. This is blurred as well in the OSC image but not in the RGB. This is a sideeffect from the algorithm which converts the four physical R/G/G/B pixel from the Bayer pattern into four RGB pixels in the image and not to avoid using a color sensor.

And finally the full version of the Soul Nebula IC1848 along with data from narrowband filters which likely only can be reproduced using a monochrome setup: