Our calculator, at typical seeing of 2-4”, uses the Nyquist formula of 1/2 and the 1/3 to stop stars becoming square so the optimal range is between 0.67” and 2”. It is better then to image with a resolution 1/3 of the analog signal, doing this will ensure a star will always fall on multiple pixels so remain circular. Using typical seeing at 4” FWHM, Nyquist’s formula would suggest each pixel has 2” resolution which would mean a star could fall on just one pixel, or it might illuminate a 2x2 array, so be captured as a square. There is some debate around using this for modern CCD sensors because they use square pixels, and we want to image round stars. So, if OK seeing is between 2-4” FWHM then the sampling rate, according to Nyquist, should be 1-2”. Nyquist’s formula suggests the sampling rate should be double the frequency of the analog signal. In the 1920s Harold Nyquist developed a theorem for digital sampling of analog signals. In affect, over-sampling reduces field of view. Over-sampled images look rather nice because the stars are round with smooth edges but if you have more pixels than are necessary why not use a reducer to reduce the telescope’s effective focal length, which makes the image brighter and enables you to fit more sky on your sensor. For a smoother more natural look more pixels are required, but not too many because if you use more pixels than are necessary to achieve round stars the image is 'over-sampled’.
Too few and the image will be 'under-sampled’, the stars will appear blocky and angular'. Short focal length telescopes and ideal seeing conditions provide the smallest stars, longer focal lengths and less favourable skies produce larger stars.įor a star to retain it's round shape when viewed on your screen or photograph it’s diameter must cover a sufficient number of pixels. Assuming high quality optics, the diameter of the point of light is determined by the telescope’s focal length (longer focal lengths result in larger star diameters) and the sky's ‘seeing’ conditions (atmospheric dispersion spreads the point of light, making it larger). A telescope focuses a star as a round point of light.
0 kommentar(er)
