"THE mirror of the 20-1/2 inch Newtonian-Cassegrainian shown above is mounted in a cast iron cell with adjustable edge supports and resting on a mechanical flotation system giving 18 equally loaded symmetrical points, emanating from three adjusting screws. The tube is of timber, built up on aluminium rings. The fine adjustment in declination is operated by a hand wheel and screw from the Newtonian end, and more slowly by means of an extension shaft operated through bevel gears, from the Cassegrain end. The polar axis and fork are cast in one piece, and rotate in ball bearings, the top ball-bearing; housing being adjustable. The worm wheel is of large diameter and the worm is provided with slow motion gearing which operates by a hand rod and hook joint, while the clock is running. The drive is by gramophone motor.

"The mirror is of 20.2 inches aperture, is of eight foot focal length, and has a five inch hole through the center. Interchangeable spiders for the top end of the tube carry, respectively, the diagonal plane mirror and the Cassegrain convex mirror. The latter is mounted in a cell about six inches outside diameter, and provides an equivalent focal length of 30 feet.

"The performance of this telescope, which is shown in actual use, will probably be fully described in an astronomical journal by Dr. W. H. Steavenson, F. R. A. S., one of the foremost amateur observers in England, who has erected it in his observatory."

The advantage of an entirely enclosed observatory, which can be heated sufficiently for bodily comfort without spoiling the definition of the mirrors, has been somewhat extensively discussed in the book "Amateur Telescope Making," pages 50-51. Mr.

Hindle is responsible for a particular type of polar observatory of which several illustrations are reproduced. One photo-graph reveals the interior, showing the comfortable method of observing.

"The actual site is N. latitude 53-3/4 degrees," Mr. Hindle writes. "The primary mirror is paraboloidal, mounted in a cast iron cell, looking face downward from the top of the hut. Actually this mirror is a disk 25-1/2 inches in diameter; the excavation being only 20 inches aperture and the outer portion providing a suitable support. The focal length is eight feet and the cone of rays is turned horizontally into the hut by means of a slightly elliptical mirror adjustably mounted in an 'A' bracket, which performs the function of the spider in the orthodox telescope tube. The only moving portion is the coelostat, which has a plane mirror of 25-1/2 inches aperture, counterbalanced and pivoted on a surface diameter for declination, and revolving on a polar axis for right ascension.

"Electrically driven reversible variable speed motions are applied to declination, and right ascension, controlled from inside the observatory. An additional elliptical flat, deflecting the rays directly from the coelostat, permits the use of a short finder telescope if required. (This flat shows in one of the photographs, below the bracket. -Editor.) A sextant with reading microscope is fixed on the declination axis, and the polar axis reads zero hour when pointing due south. A concrete foundation about a foot thick supports the entire observatory, which is very rigidly constructed and braced, two light I-section beams being provided for sliding the primary mirror up or down in its cell.

"The adjustment of the mirrors to the polar axis is facilitated considerably by the optical combinations that can be obtained in a very interesting manner. The construction of the hut itself insures that the primary mirror and elliptical flat roughly approximate to the polar angle. They are set optically in line with each other, after which the coelostat mirror is moved in declination until it is precisely parallel with the parabolic mirror which can be observed from the eye position. lt is now rotated in R. A. and if the parallelism is lost, then the coelostat base (and polar axis) is adjusted by the base screws until parallelism with rotation are secured. The three mirrors are now on one optical axis and the sextant is set to read 90 degrees. The coelostat mirror, at zero hour R. A. is now brought perfectly level, and the sextant should read 180-2 X latitude; in this instance 180-107-1/2 = 72-1/2 degrees from the pole, or 17-1/2 degrees N. declination.

"In case of an error a fresh optical axis nearer the correct position is selected for the first two mirrors, and the coelostat adjustments repeated. If an inclinometer is available, then the angle of the plane mirror can be tested right away when the two large mirrors are parallel with each other.

"Silvering the mirrors need not upset the adjustment. The coelostat mirror is readily replaced in the same plane. The other two are removed one at a time, and each brought to a correct position by reference to the other. The adjustment to the meridian is checked by the southing of some prominent object at the precise moment indicated in the N. A."

WELL, how many of you amateurs are now going to make a Hindle comfort coop for cozy constellation conning? No frozen fingers, no summer mosquitoes to slap and cuss at. It looks mighty good to ye--ed-and inexpensive, too. In Yankee latitudes the southern face of the hut will be raked back at a lower angle-just the thing, for this will provide more room at the bottom to sprawl out long legs and big feet in comfort. A fellow ought to be really comfortable in this kind of observatory.

Next comes Mr. Hindle's third contribution, a brand new test for the Cassegrain (and Gregorian, too). This is being tried out at Mount Wilson and may be used on the 200-inch. We quote verbatim Mr. Hindle's explanation of the discovery, as transmitted to the Royal Astronomical Society and published in the Monthly Notices (March, 1931) of that distinguished body, of which he is a member.

"The figuring of the secondary mirrors for compound reflectors has always been considered a difficult problem. The comparatively recent revival of the Cassegrain is undoubtedly due to the 'parallel ray' system of testing adopted by Professor Ritchey, and illustrated and described on page 39 of his work, 'The Modern Reflecting Telescope.' It may be noted that the concave secondary mirror for a Gregorian can equally well be tested by the same method (see Figure 1).

"That test leaves much to be desired. No matter how carefully the mirrors are collimated. the convex does not appear a perfect surface of revolution when examined under the knife-edge, probably be cause the illuminated pin-hole and the eye cannot simultaneously be on the optical axis. There is a large blind spot in the center of the convex, due to its interposition in the parallel rays returning from the plane mirror. The area visible is that due to point illumination only. The supports for the convex obstruct the view to some extent, in addition to which there are diffraction effects around all obstructions. The plane mirror used must be at least as large as the paraboloid, and requires first a spherical mirror from which it is derived with diminished accuracy. The five reflections are objectionable; to a certain extent they drown that figure of the secondary mirror which we wish to see.

"By the remarkably simple device of substituting a slightly larger spherical mirror in place of the paraboloid with a radius of curvature approximately equal to the focal length of the latter (Figure 2), we immediately dispense with the parallel rays, and reduce the number of reflections to three. The secondary mirror is then seen under the shadow test in no uncertain manner, It can be correctly figured over a Iarger diameter. The blind spot in the center is much smaller, and the supports for the secondary mirror do not obstruct the view. Diffraction effects are therefore negligible.

"The circles to the left of Figures 1 and 2 show respectively the appearance of the small mirrors from the secondary focus.

"The mirrors are set up at approximately the required distance apart and squarely facing each other by reflection. The exact value of the radius of curvature of the spherical mirror is not of any importance: it is only necessary that its center of curvature and the shorter conjugate focus of the secondary mirror should coincide when testing.

"If the image of the pin-hole is examined with an eyepiece before the secondary mirror is corrected, there is such a considerable difference of focus that two distinct images may be found along the line of aberration, with much dispersion of light. When the secondary is correctly figured to look perfectly flat, all the light is concentrated within the image of the pin-hole. The details of which are plainly visible. The expansion of the image is the same on both sides of focus; in fact, the test is precisely similar to that of a spherical mirror at its center of curvature. It therefore follows that the surface of the secondary mirror must be accurate to within a very small fraction of a wavelength.

"It is obviously better to refer secondary mirrors to a spherical mirror, whose accuracy can be tested at any time by visual inspection, rather than to a combination of mirrors derived from the same source, with diminishing accuracy. It is likewise of the utmost importance to be able to produce secondary mirrors without reference to the paraboloidal mirrors with which they have to work.

"The uncorrected secondary mirrors for Cassegrain and Gregorian telescopes show diametrically opposite appearances under the knife-edged test. The former has a protuberant, the latter a depressed intermediate zone, at the average focus. The hyperboloid is therefore more difficult to produce, having a depression in the convex spherical surface, reaching a maximum depth at the intermediate zone, and diminishing to nothing at the edges and center. A corrected convex, if resting inside a concave spherical surface of suitable curvature would make contact on the edge and center only. Such a figure cannot be produced haphazard, and if the depressed zone is unsymmetrical, an astigmatic image results.

"The Gregorian concave, like the paraboloid, is corrected by excavating the center more deeply, the excavation diminishing to nothing at the edge. It can therefore, more easily than the Cassegrain, be corrected by star tests if workshop tests are unavailable, gradually making the ellipsoid deeper until full correction is attained. This probably explains the predominance of the Gregorian before workshop tests were devised."

Now there ought to be an enhanced interest in the Cassegrain, and a few of the all-but-extinct Gregorian may be attempted too. Mr. Hindle has written for us a compact treatise on these types, and we have made three carbon copies of it to lend out for limited periods to bona fide Cassegrain-Gregorian workers who will swear with one hand on "A. T. M." to return them promptly for the next fellow's use.

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