![]() The loss in surface contrast for the instrument with all the structures Heating cable for the secondary and some screws (bottom). The optical path, like a focuser, vane mountings, mirror clamps, a Theoretical instrument with several additional structures reaching into Into the optical path is presented, together with the influence on theĨa: an instrument with spider and secondary (top), compared with a In Figureħa a theoretical instrument with many constructive structures reaching Sometimes at the cost of a higher weight of the instrument. Optical path, or if we should spend some effort to prevent this, Like mirror clamps or the focuser should be allowed to reach into the In a somewhat lower conrast, due to the higher amount of diffractedīuilding optical instruments, we often have to decide if structures Observations - that is a much higher value than normallyĪssumed! For a four-vane spider, this value should be lower than 3%.ħa: a four vane spider of 5% clearly has more influence on theĭiffraction pattern than a three vane spider of 5%.įour supporting structures of the same size as in Figure 6a will result Three-vane spider with vanes with diameter under 5% of the linearĭiameter of the mirror is uncritical for the contrast in planetary 7a would be for a tetrapod respectively.įollowing the simulations, a rule of thumb for the vanes would be: a Secondary optical component) is supported by a tripod that reachesĭirectly from the cell of the main mirror to the first focus. The geometry of FigureĦa is often found in lightweight (robotic) telescopes, that carry anĮlectronic camera instead of a secondary mirror. Have very little influence on planetary conrast. Really massive supporting "vanes", that reach into the optical path, Has a three-vane spider of 1.5% mirror diameter. Instrument with 20% central obstruction (left) compared with an Thick, combined with a central obstruction of 20%. Three vane spider that is 1.5% of the diameter of the main mirror Thickness on a 400mm instrument (that is 1.5%) is simulated. In the Figures 5a and 5b the influence of vanes of 6mm Rather often, is the influence of the vanes that carry the secondary Interesting than the central obstruction, that has been discussed (right) again compared to the image with an unobstructed instrument Temperature aclimatisation) are not identical.ĥ0% central obstruction and the resulting diffraction patternĤb: An instrument with 50% central obstruction Should be difficult to figure out at a side by side test, if theĬonditions of the instruments under test (state of collimation, On the other hand, one can imagine that even this difference The apparent diameter of Mars was assumed to be 20" (arcseconds).ĥ0% central obstruction (Fig 4a and Fig. Simulated view of Mars with the unobstructed optical system from Fig.1aĪt a diameter of the entrance pupil (=objective diameter) ofĤ00mm. Test-object, since its surface with the many low contrast features ofĭifferent size is very sensible to a decrease in image contrast of the ![]() I used Mars (an image from the Hubble Space telescope) as Obstructions on the image quality bare from different opticalĬonditions of the instruments used normally for such side by side ![]() This gives us the opportunity to compare the influence of With exactly this entrance pupil but an otherwise perfect optical The image that would result from imaging an object with an instrument TheĮnergy distribution in the diffraction pattern, we are able to simulate Diffraction Pattern (Airy disk, right) of an unobstructed circularĮntrance pupil (left), like from a Refractor or a Schiefspiegler. ![]()
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