![]() ![]() The same effect is seen with narrowband filters for CCD imaging. Therefore the filter is receiving a parallel beam of light (directly from the Sun) and the telescope itself can then operate at any focal ratio. The newer Coronado filters place the primary filter on the front of the scope. With the older DayStar filters, the primary filter was located at the back of the scope and required the telescope to operate at a very slow focal ratio (greater than f/30) to have an approximately parallel beam of light entering the filter. An example of this effect is the need for solar H-alpha filters to have a roughly parallel beam of entering light. In other words, a steep light cone entering a narrowband filter can actually change the bandpass wavelength. In contrast, a typical narrowband filter has a bandpass of just 3-5nm (see following pages for specifics).Ībove: Some of the more common narrowband filters, with RGB filters in the background for comparisonĪn interesting effect of narrowband filters is that the bandpass is a function of incident light angle. Therefore, a typical RGB filter might have a bandpass of 100nm. The entire visual spectrum runs, approximately, from a wavelength of 400nm (blue) to 700nm (red). The bandpass is simply how much of the spectrum the filter allows to pass. Narrowband filters instead capture only a very small part of the spectrum. (There is sometimes a gap between the green and red filters to block a prominent light pollution emission line, as in the diagram below.) Each of the RGB filters covers approximately one third of the visual spectrum and the filters overlap slightly so that the whole spectrum is detected by the CCD. Red, green, and blue (RGB) filters are designed to approximate the color sensitivity of the human eye, so that the resulting image is true color. ![]() In normal color imaging, three filters (red, green, and blue) are used to separate the primary colors of the visual spectrum. ![]()
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