By Neil English (auth.)
In the 1980’s, at the sidewalks of San Francisco, beginner astronomer John Dobson all started displaying throngs of individuals how you can construct and use huge aperture scopes, frequently from scraps. The Dobsonian,‘Dobs,’ are actually the best-selling huge telescopes on the planet. There are an excellent number of varied Dob kinds, starting from intricate and ornamental creations to basic mass marketplace designs, and new versions look for all time. during this identify, Neil English provides the last word consultant to purchasing and utilizing a advertisement Dobsonian for leisure astronomy. He offers in-depth debts of a few of the versions (plus components) out there – either economic system and top rate – including describing the wealth of strategies that amateurs have made to their Dobs to optimize their functionality within the box.
Even after thirty years of innovation, the Dobsonian Revolution indicates no symptoms of abating. discover the place the long run lies for those huge aperture ‘scopes and the intriguing avenues John Dobson’s imaginative and prescient will take us down within the coming years.
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Additional resources for Choosing and Using a Dobsonian Telescope
Daniel Observatory near Greenville, South Carolina. 5-inch Star Blast telescope, I shrugged it off as just a kid’s toy. 5-inch f/8 ‘scope gives respectable images, but an f/4 focal ratio demands both an excellent parabolic figure and careful collimation, or images would be mush. For the price, I doubted that either was true. Boy, was I wrong! One night, a couple of years ago, I loaded up my old Celestron C8 Ultima, a bunch of eyepieces, charts, and a red lamp and headed for our dark sky site. 5 on a modified bar stool.
Surface accuracy is all well and good, but it doesn’t tell the whole story. Errors in the figure of the mirror surface making up the objective can lead to increased spherical aberration, coma, distortion, field curvature, and astigmatism (the five Seidel errors). To this end, optical engineers have devised an even better way of expressing optical quality – enter the Strehl ratio. 24 Choosing and Using a Dobsonian Telescope To understand this quantity better, picture again the image of a tightly focused star seen at high power through the telescope.
An OK mirror will be figured to an accuracy of ¼ of a wave, that is, the microscopic irregularities in the shape of the mirror cannot be more than about 140 nanometers in order for it to operate satisfactorily under most conditions. Such a mirror is said to be diffraction limited, which means that the optics are constrained by the wave nature of light itself and not by any flaws in its optical figuring. Who gave us that idea? That honor goes to the nineteenth-century physicist, Lord Rayleigh, who reckoned that an image distorted by anything more than ¼ wave of yellow-green light would appear obviously degraded to the eye.
Choosing and Using a Dobsonian Telescope by Neil English (auth.)