![]() In practise it can in some cases be useful to go beyond the upper magnification limit, for example to split a narrow double star. The associated dawes limit of resolution is approximately 0.45 arc seconds, which means that smaller details can not be observed. As a rule of thumb, the maximum useful magnification is about 2 times the aperture in mm, which in my case results to 500. The size of this disk is inversely proportional to the aperture size. The limited aperture size causes the image of a point source (like a star) to be spread out into an image that shows like a disk with a set of rings around it. The upper magnification limit is determined by diffraction effects. For a focal length of 1600mm such eyepiece assumes a primary focal plane diameter of 40mm, which is impossible to obtain with a 32mm focuser! A remedy for this would be to shorten focal length, but this in turn makes high power harder to reach. For example a plossl eyepiece used with 35 power will have a true field of view of 1.4°. At such low magnification the true field of view my become larger than admitted by the combination primary focal length and field lense diameter. Note that another factor playing a role in low magnification is vignetting at the eyepiece fieldlense. In the case of a 250mm primary and 7mm pupil diameter, the ratio becomes approximately 35. This vignetting is most visible when the exit and eye pupil are equal in size. magnification too small) the eye pupil will cut-off the exess, effectively causing vignetting. The bundle of light cut off by the primary mirror aperture is projected into a smaller bundle of light leaving the eyepiece, by ratio of the magnification. The range of useful magnifications is determined by the diameter of the eye pupil in dark and the diameter of the primary mirror. The optical performance parameters addressed in this paragraph are: First some optical performance considerations are given, and subsequently some thoughts about mechanical issues. The design of my telescope, presented in the succeeding section, is based on this theoretical background. This section deals with the theoretical basis for some design choices, and has a somewhat generally applicable character. ![]()
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