In a previous posting I assessed the sensitivity of the camera I have been using, my “super” ZWO ASI6200MC camera. It is an extremely high quality camera, and users rave about it. Nevertheless the separation of colours and overall sensitivity is certainly not perfect, as we observed in that analysis.
This second post will look at the equivalent monochrome camera, the ZWO ASI6200MM camera.
Figure 1 is the efficiency (percent of photons that should produce signal in the CMOS sensor) of the ZWO ASI6200MM monochrome camera, shown with no filter and with RGB filters from a company called Astronomik. The passband with a Hydrogen-Alpha filter is also shown which is only 4nm wide and appears as just a spike here (@656nm).
The monochrome camera has no Bayer filter/matrix. There is no interpolation to determine colour: The photons are captured pixel by pixel. To capture the red spectrum one uses a filter in front of the camera which rejects all wavelengths except red. One then has to separately deploy green and blue filters. Finally the three are combined to create a colour image.
For the monochrome camera we need to look at three factors:
- Sensor quantum efficiency as a function of wavelength.
- The protective window’s spectral response (percentage of light passed as a function of wavelength).
- The passbands of typical RGB filters.
Figure 2 shows the quantum efficiency of the sensor found in the ZWO ASI6200MM Pro monochrome and ZWO ASI6200MC Pro colour cameras.
In these cameras there is a piece of glass sealing off the sensor. It keeps the sensor clean and is used to filter some unwanted light. For the monochrome camera the “protective window” excludes far less light, which is both good and bad. It does provide the photographer with more control but those frequencies extend outside the visual spectrum and may produce images that don’t match what the eye would see.
Figure 3 shows the spectral response of the protective window used in the monochrome camera.
The Astronomik Deep-Sky RBG filter set seems to be quite popular so we use it to illustrate typical filters used with a monochrome camera.
Figure 4 shows the RGB frequency response of the Astronomik Deep-Sky RGB filters.
Armed with all this information we can determine the probability of a photon being registered by this camera. This time there is very little overlap. The quantum efficiency does vary widely across the visible spectrum: Lots of photons are lost and don’t register at all. There is a gap here between green and red, near 600nm. This gap is apparently by design, to avoid light pollution. The curve does reach nearly 90%.
Figure 5 is the efficiency (percent of photons that should produce signal in the CMOS sensor) of the ZWO ASI6200MM monochrome camera, shown with no filter and with RGB filters from a company called Astronomik. The passband with a Hydrogen-Alpha filter is also shown which is only 4nm wide and appears as just a spike here.
This graph was produced by MS Excel, and the line colours are not the spectral colours the represent. On the left is blue, green is in the middle and red is on the right. The yellow line is with no filter in place, something that is done fairly frequently.
It’s certainly not perfect, mainly attributable to the Sony sensor’s quantum efficiency variation with wavelength. Comparatively it is much neater, with a more reasonable balance between the three channels and less overlap between channels.
A third and final posting will compare the two cameras and discuss the implications of these results. That posting will discuss observations of the “luminosity” channel and how that can be combined with RGB imaging.
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