In a set of three previous postings (part 1, part 2, part 3) I assessed the sensitivity of the camera I have been using, my “super” camera, the ZWO ASI6200MC, and the very similar monochrome camera, the ZWO ASI6200MM.
This posting is intended to review some basics of the light we are receiving from the stars and how we perceive this light. We start with the emission from our local star, the sun.
Figure 1 shows the radiation emitted by the sun and that arriving at the Earth’s surface.
The black line is a theoretical curve describing the radiation we’d expect from a body at the sun’s temperature (black-body radiation). The yellow region, which tracks the black line fairly closely, is the actual emission from the sun. This radiation has to travel through the Earth’s atmosphere and the radiation that results at the Earth’s surface is shown in the red regions.
The x-axis is the wavelength in nm and can be compared with the wavelengths previously presented in analysing the cameras.
Other stars will have similar profiles of radiation but will vary somewhat because stars are of differing sizes and have different surface temperatures. A red giant star will have the spectrum more in the red colours, which means that the surface of such a star is slightly cooler than our sun (beneath the surface it may be hotter, though). Red photons have lower energy than violet – it is the UV that causes sunburn.
That gives us a good sense of the light arriving at our eye from the sun and stars. Our eyes will have a certain spectral response, though. What does that look like?
Figure 2 shows the sensitivity of the human eye as a function of wavelength.
The human eye is similar to the colour camera discussed above, where the different colour receptors overlap. As with Sony cameras a lot of computing must take place in our perception to separate the colours. The curves are certainly different from those of the camera. Overall we are sensitive to wavelengths from approximately 400nm to 700nm which roughly matches the peak of the sun’s radiation output – unlikely to be a coincidence.
The ZWO ASI6200MC colour camera comes with a built-in UV and IR cut filter that blocks all radiation below 400nm and above 700nm. The Bayer colour filters are not very sensitive outside this range in any case. By doing this the camera captures only the range of wavelengths that humans can perceive.
The ZWO ASI6200MM monochrome camera does not have a UV/IR block filter built in. Standard observations would normally require that a separate filter be placed in front of the camera which would typically remove these wavelengths. However, it is possible to observe frequencies spanning a wider range than humans perceive to improve sensitivity. Alternatively, we may block some visible light and use the camera to sense UV or IR light, as is done for lunar observations.
The James Webb Space Telescope is sensitive to a range of frequencies from the red to the infra-red. It is not sensitive to human-visible blues or greens.
The CMOS colour camera on my iPhone 7 is more sensitive to some frequencies than my eye and I use the camera in some situations to take advantage of this fact, It is a similar colour camera to the one evaluated so will have a Bayer matrix to produce colour images, with Bayer filters. My main use of the iPhone in this fashion is to see a very dim red LED temperature display on my oven that is nearly invisible to the human eye. The dimness comes from a faulty electronic component that could be replaced if I chose to do so.
Clear weather to all.
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