Entering the world of astrophotography cameras

I once was astrophotographing with two other guys who used monochrome cameras. The cameras had active cooling and were attached to motor-driven filter wheels. At this time I already heard of such cameras but still did not consider them as an upgrade, since the benefits did not seem one the same scale with their high price. I also heard that monochrome cameras require more time in acquisition and processing than “one-short color” (OSC) cameras. On this nigh, one of those guys and me were targeting the same nebula. His total exposure on all separate channels was the same as on my Canon 800Da DSLR, and the final result was still better by dimensions. After I while, I decided to upgrade my Canon DSLR not a cooled OSC camera, but to a monochrome camera. At the end it made up a point of no return for me. There are no alternatives for monochrome cameras in Deep Sky astrophotography, in my option. The only problem is a higher price and the need for filters and a filter wheel which cause additional expenses.

Arguing against and pro OSC and monochrome camera is one of the never stopping topics in the astrophotography community. It as as host as the discussion of a reflecting telescope is better than a refractor. I still cannot resist and bring my points.

Quantum efficiency (QE)

The upgrade camera, ZWO ASI294MM Pro (https://astronomy-imaging-camera.com/product/asi294mm-pro), has QE of around 90% on the specification. This means that around 90% of photons are converted in a signal. Canon 800D achieves 54% (https://astrophotography.app/EOS.php). Even assuming that ZWO “pushes” the spec a bit, the difference is very remarkable.

Decomposing vs native capturing

All DSLR, mirror-less, and specialized OSC cameras have so-called Bayer matrix filter (https://en.wikipedia.org/wiki/Bayer_filter) in the front of the sensor. This filter is used to filter RGB color to dedicated cells in the sensor. There are many technologies how the resulting signal from the sensor is processed and how the RGB cells are arranged in the filter. The result is the same: the physical sensor resolution only partially contributes to the final image. The monochrome ZWO ASI294MM Pro and OSC ZWO ASI294MC Pro use the same sensor, but only in the OSC version it is shared by all RGB colors (with green used twice) resulting in a loss of effective resolution

Capturing per color channel natively

In post processing, an RGB colors are processed separately in one or another form to achieve the best possible results. Images from an OSC camera are decomposed into separate red, green and blue channel images. A monochrome camera delivers separate images for all three basic colors in the full native resolution of its sensor

Native narrow-band capturing

Alone with RGB filters, narrow-band filters are used with monochrome cameras to capture this parts of the spectrum. With OCS cameras, this is achieved with multi-band filters which try to pass the specific parts of the spectrum all on once for one shot of an OSC camera. This is a compromise with results pretty much proportional to the price of these filters. Even at maximum price, the result cannot compete with native capturing with a monochrome camera by nature

Cooling

Active cooling is missing in DSLR and mirror-less cameras. Most deep sky specialized OSC cameras have the same active cooling as their monochrome pedants. My Canon 800D reports the sensor temperature of up to 30° Celsius while the outside is around 0° during a long series of shorts. The cooling in the ZWO ASI294MM Pro can keep the sensor temperature at -15° Celsius even at 20° Celsius outside temperature. This is a big difference resulting is a much lower noise and so better SNR.

What are the downsides?

Compared to DSLR, you need some additional computer hardware to store the images and control a monochrome camera. Compared to OSC, separate images from RGB channels need to be combined to provide a quick result. Narrow-band images need to be mapped to real colors, whereas this is a part of creativity and might nor purely count as a downside 😉 As mentioned above, you need more filters and with this a filter wheel to handle them. In most cases, the rotation of the filter wheel is automatically controlled by the software. This is an additional component which is present is most integrated packages like N.I.N.A. or in Ekos INDI Library (used in StellarMate OS), and certainly in the ZWO ASIAIR.