I always wanted an OSC camera to play and experiment with. However, I could never justify spending a lots of money on a second camera. CCD OSC cameras are quite expensive and, until recently, CMOS cameras were mostly good for planetary imaging only.

But lately, a number of affordable cooled CMOS cameras has appeared that promise to be very good at deep sky imaging also. So, I finally decided to get one. My choice fell on ZWO ASI178MC Cool, because of its smaller 2.4um pixels, 14 bit A/D converter and high resolution of 3096 X 2080 pixels. The camera is based on OSC variant of Sony CMOS IMX178 sensor with dimensions of 7.4mm X 5mm. Smaller sensor dimensions mean that you don't have to worry much about field curvature and illumination much, and that translates to simpler imaging setups.

Sony IMX178 sensor is back-illuminated, improving its sensitivity. With 2.4um pixels ASI178MC is really not a speed demon, but it is more sensitive than one would expect, and very useful for deep sky imaging. My camera is an earlier version, with a guider port, later versions have USB hub instead of it. The camera is delivered without 12V power adapter. 12V is used only for supplying power to regulated Peltier cooler that can cool the camera down to 35 - 40 degrees C bellow the ambient temperature. Camera electronics are powered via USB port. Stable power supply is very important for the best results with the camera. I strongly recommend you to connect the camera to USB3.0 port, not only because it is much faster, but it can also supply the camera with 900mA of current (compared to 500mA that USB2.0 port can provide).


With its sensor diagonal of 8.92mm and high resolution ASI178MC - Cool coupled with smaller telescope like AT65EDQ gives nice image scale useful for imaging of many popular deep space objects. For imaging with the camera's small pixels proper focus is very important. For best results with AT65EDQ I have to refocus several times during the night as the temperature changes. As you can see on the image, AT65EDQ + ASI178MC - Cool setup is so light that I had to add an extra1kg weight on top of the telescope in order to be able to balance my mount properly.

For DSO imaging I get the best results with camera working in highest dynamic range mode (HDR) and sub frames exposed from 8 to 16 minutes. 4 minutes sub frames may be sufficient for brighter clusters. For long term exposures subtracting of a properly exposed dark frame (the same duration and temperature as lights, 30 sub exposures or more) is a necessity. The camera has a weird looking dark frame, but it subtracts well. Only one note: I didn't have much luck scaling darks using the bias frame, so I'll stick to using darks of the same exposure as light frames.

By using photographic lenses with ASI178MC imaging of more extended deep sky objects is also possible (despite of its smaller sensor size). Here you can see ASI178MC connected to Nikon Micro-Nikkor AF 200mm f/4 D ED telephoto lens. ZWO has lens adapters available for its cameras for Canon, Nikon and Sony lenses. I found out that my QSI Nikon adapter also works quite well with ASI178MC (with 15mm T2 spacer added).

Focusing lenses can be quite tricky, and steady hand plus some patience is definitely needed. Closing the lens one stop usually helps a lot. On the other hand, it seems that photographic lenses tend to keep their focus better as temperature changes, so refocusing is rarely needed.

ZWO also has a monochromatic version of this camera: ASI178MM - Cool. With that camera you can get color image by taking multiple exposures through different filters. I opted for OSC (One Shot Color) version because it is much simpler to image with (and I already have monochromatic QSI583). OSC camera can get color image in a single exposure, because of RG&B filters that are integrated in the sensor in a pattern called "Bayer Matrix". For that simplicity you pay with somewhat diminished sensitivity and resolution.