Download PDF by Allyn J. Thompson: Making Your Own Telescope
By Allyn J. Thompson
Entire, specified directions and various diagrams for developing a home made telescope. No complex arithmetic are concerned, and no past wisdom of optics or astronomy is required to persist with the text's step by step instructions. Contents hide, between different themes, fabrics and kit; tube components and alignment; eyepieces, and comparable difficulties; atmosphere circles; and optical ideas. 6 plates. a hundred figures.
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But on account of the unequal coefficients of expansion of crown and flint glass, lenses above 3" in diameter are seldom cemented; thus an additional toll of light is taken at the two inner surfaces. STORY OF THE TELESCOPE 23 surfaces. The cost of application of the coatings is low, and thus improved, the refractor again moves up ahead of the mirror in light-transmitting qualities, a lens of about 5%-inch aperture being the equal, in this respect, of a 6-inch aluminized mirror. In the matter of resolving power, the 6-inch mirror will excel the 5Vz-inch lens, although diffraction caused by the ohstructive secondary mirror and supporting vanes of the reflector has a moreor-less deleterious effect on the image.
The result of roughreached the proper depth, the sur- ing out the curve with a long faces are then brought into contact stroke: mirror hyperbolic, tool oblate. and made spherical by wearing back the edges of the mirror. This is accomplished by means of a short stroke, in the early use of which the radius lengthens slightly. The reason for this is apparent from a study of the diagram. It does not matter that the pyrex mirror blank and the plateglass tool are of unequal hardness, and of different expansion co- 42 MAKING YOUR OWN TELESCOPE efficients.
About five minutes is enough for each charge, but if one is unable to maintain heavy pressure a charge should be worked for a longer period. The purpose of each successively finer grade is to remove the pits left by the preceding grade, and to be assured of this not less than 40 minutes should be spent on each. Before leaving No. 120, work the last charge down well, then follow with a charge of No. 400 (worked with the mirror on top), and again test for focal length. This time we have a surface which, when wet, wiII be capable of producing a fairly sharp image of the window of the Foucault lamp.