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From cosmoquest In a Newtonian telescope, a parabolic mirror is usually used, since that surface focuses light from infinity to a nearly perfect on-axis image. Some telescope manufacturers (and a few amateurs) substitute longer f/ratio spherical mirrors, since they may sometimes be easier for some people to figure and test rapidly. However, if the f/ratio isn't long enough, the performance (especially at high power and for planetary viewing) may suffer. One way to rate telescope mirrors is by seeing how much their surfaces deviate from a perfect parabolic shape. One common rule of thumb states that the telescope's optics must not produce a wavefront error of more than 1/4 wave in order to prevent optical degradation. This requirement is sometimes extended somewhat to require that the mirror's surface must not deviate from a "perfect" paraboloid surface by more than an eighth wave (approximately 2.71 millionths of an inch) in order for the mirror

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Substructured Ronchi gratings are used to sharpen and increase the number of fringes in Ronchigrams, thereby increasing their spatial resolution and allowing greater accuracy in the evaluation of a surface under test. This work presents a simple method for generating substructured Ronchi gratings and for calculating the intensity pattern produced by this type of grating. For this, we propose the generation of this kind of grating from the linear combination of classical gratings; the pattern of irradiance produced by these Ronchi gratings will be a linear combination of the intensity patterns produced by each combined classical grating. A comparison between theoretical and experimental Ronchigrams was obtained by analyzing a parabolic mirror.

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into a CNC machine: tinkernut stepper motor on the linear slide: 18 degree step, or 20 step per revolution each revolution is around 3 mm, 3 / 20 , = 0.15 mm linearly per  step from hypertextbook CD track is 1.6 um track to track distance, seems to be quite good from repairfaq and here three beam optical pickup system: Arduino and DVD Bipolar Mini Stepper Motor driven by a L293D Arduino next next    from homofacien: plotter CDROM  

The Mobsby null test     Martin Trittelvitz 2001   Today, there are essentially two types of optical tests to determine the surface aberration of telescope mirrors:   1.) The shadow sample by Foucault and its derivatives such as Zone Test by Couder.   2.) The Ronchi, and the quantitative Ronchi by Mel Bartels.  http://www.efn.org/~mbartels/tm/software.html (See also Foucault and Caustiktest) Both have a common major drawback: For perfect spherical mirrors, both zero tests, that the shade sample after Foucault  and the lines in Ronchigram , are straight and parallel. There have since taken no deviation, to be corrected or recalculated,  to a perfect sphere. The evaluation is done by simple rvisual observation.  If the image from Foucault test,  just complete dimmed,  then the mirror should be spherical. If reading from the lines at Ronchi are straight, it is a spherical mirror!   Everything changes when you want to make a parabolic mirror. The lines at Ronchi test are  b