"The difficulty of making an optical flat or an accurate spherical mirror for testing purposes only, need no longer deter the amateur who has the ambition to make a compound telescope; these accessories are not absolutely necessary, as once believed. Kirkham's method of testing Gregorians from foci is entirely practical and within the capacity of anyone who can make a good Newtonian telescope.

"My first telescope, illustrated in the December, 1933, number of SCIENTIFIC AMERICAN, was a six-inch Newtonian. I afterward made an eight-inch, f/9, Newtonian which proved so much better than the first that I had no further use for the six-inch. It then occurred to me that I might use the six-inch tube for a small Gregorian.

"I decided, in the first instance, on a 6-inch f/5 primary, with a 1.75-inch secondary, making p=7.5 inches. These values are in proportion to Hindle's example in 'Amateur Telescope Making' and fit into my 54-inch tube as though it were made for them. It took just one month of my spare time to make the change, and I now have a good compound telescope of convenient dimensions and considerable magnifying power. Because of the small size of my primary, which was limited by the size of my old tube, I did not seek a very high magnifying power; but the new telescope is very satisfactory, and I feel amply repaid for the effort.

"My Gregorian is of the non-perforate type. (See drawing.) I use a one-inch prism located about a foot from the primary mirror. This comes near enough to the declination axis to make the eyepiece always convenient. Instead of an adjustable adapter tube, I slide the block which carries the eyepiece tube between two of the strips of my wooden telescope tube, and pick up the focus wherever it may be. I have a thumb screw adjustment for accurate focusing.

"While it is necessary to keep rather closely to the values of f, p and p' employed in determining the RC of the secondary, the latter can easily be figured by Kirkham's method, even though f and p' vary slightly from assumed values. This is fortunate, for it is extremely difficult to keep these dimensions from varying slightly during final polishing and figuring.

"It is better, in this case, to keep the value of p constant, and absolutely necessary to figure the mirrors accurately.

"The primary must be a true paraboloid and the secondary a true ellipsoid. It bothered me at first to know how I was to determine when the secondary was correctly figured; but I found it was only necessary to apply the test for a perfect spherical surface: in other words, an apparently flat surface, with the pin hole at one focus and the knife edge at the other. In such a test an ellipsoidal surface will appear perfectly flat at the outer focus. This test is really more delicate than Foucault's center of curvature test, for the reason that conditions are precisely the same as when a star is under observation-which is not true in the other case.

"Any variation of f, p or p', of course, varies the amplifying power slightly, but this is relatively unimportant.

"It took me several hours to line up the optical elements properly, as this is tedious, but only slight adjustments were afterwards necessary.

"I secure the telescope to the saddle by means of leather straps, and take it into the house when not in use. My long focus Newtonian fits into the same saddle."

Figures 1, 2, and 3, show the optical layout and details.

SO much for Mr. Decker's Gregorian. From another worker who is making a Greg we have received the following comments, suggesting that the Greg is not a suitable job for a beginner (we constantly receive inquiries from beginners who want to tackle one as their maiden job):

"The biggest job about the Gregorian is the mounting. I find that the positions of the Greg mirrors are very critical. Unless they are exactly right, you get blurred images with tails. It is a tough job to line up a system of this sort, and it is highly important that provision be made for adjustment in every direction-lengthwise, crosswise, tilting, and so on-and for locking the parts firmly after they are once set correctly. A Greg is not a job for a beginner -it's a difficult job for any one. The high magnification of the Greg results in a small field, and it is hard to keep objects in view. Jupiter sails across it like an airplane."

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