FOR THE amateur's first telescope a focal ratio of f/8 is recommended because that ratio is an optimum compromise between the long focus types best suited to study of the planets and other extended objects, such as diffuse nebulae, and the short focus types best suited to galactic star uses. The same compromise is recommended because of mirror-making reasons. Long or short focus mirrors are no place for the tyro to get first experience, each giving its own peculiar kind of extra headache, mainly having to do with the testing during the work.

For second and third telescopes the current style trend is toward the two types mentioned. First came the RFT and became well established. More recently amateurs have discovered the value of ratios like f/12 or f/15 for the planets. Experience of Robert E. Smith, D.D.S., Medico-Dental Building Sacramento, California, with the 5" f/15 shown in Figure 1 at the left compared with the 10" f/8 at the right, is of pointed interest. He made the 10" f/8 but with it he failed to find the canals of Mars. So he wrote Professor Clyde W. Tombaugh of the Lowell Observatory for advice and was told this: "Perhaps you have not succeeded in getting steady enough atmospheric conditions. Under very steady atmospheric conditions, I have seen the main canals around the Solis Lacus with a 5" f/15 reflector at X150, and with my 12" f/12 I have seen about 15 or 20 canals.

Dr. Smith, with C. A. Fogus, respectively president and director of the Sacramento Valley Astronomical Society, set to work to build an f/15 and made it an 8". This gave it a 10' tube but the long-focus mirror wasn't itself so easy. "Don't let anyone tell you it is easy to bring a long-focus mirror to a perfect sphere," Dr. Smith shouts. "The slightest zone in a 'longie' shows up decidedly, owing to magnification, where the same zone in a short focus mirror would not be so noticeable. However, Carl E. Wells of Roseville, California, an authority on things telescopic, pronounced my mirror OK and its performance proved he was right. Not only is planetary detail wonderful but the Orion Nebula is a better sight, while definition on the moon is really out of this world. But Mars was too distant and as yet I've seen only the snow cap, no canals."

Dr. Smith gives us the word "longie," so why not "shorty" also. Longies and shorties.

Professor Tombaugh also advised parabolizing the long focus mirrors if as large as 10" in ratios like f/10 or f/12, but suggested that an 8" f/15 might work well if left spherical (Parenthetically, he took the occasion to urge beginners to cold press, saying:

"When I learned to do this, my zone troubles practically disappeared. I always cold press with extra weight-two to three times that of the mirror for at least 15-30 minutes." "ATM." urges this often but the evidence is that many beginners and a few others skip it. Cold pressing is basic.)

Smith's 8" f/'15 seemed somehow familiar. Some other telescope, somewhere. Milwaukee! Digging down into one of our numerous towers of papers and files we come up with Figure 2, a telescope built some years ago by the Milwaukee Astronomical Society and it, too, proved, as suspected, to be an 8" f/15. Twin brothers. E. A. Halbach, president of that organization says this reflector, which they call their planetary telescope, gives much better planetary images than the larger (13") f/8.3 within the dome in the background. More evidence.

This is a good occasion to publish the hitherto unpublished photographs of the unusually well built Milwaukee dome ofwelded steel construction. These show its detail better than many words.

Figure 3 shows the one-piece shutter and its simple track extending to the right, a piece of angle iron. No complication here.

Figure 4 details the solid dome ring of curved steel channel, with one of the rollers and a retainer. Again simple, effective. Worth copying.

Figure 5 shows in the background the framework of the shutter, which is in closed position. Extending to the left is a single straight length of angle iron acting as the track on which the a top of the shutter rolls to one side on the two rollers visible at the top. Two bent fingers of iron keep the shutter from lifting off in the wind.

Figure 6 is the bottom corner of the same shutter, with one of the two rolls running on the same angle iron track seen in Figure 3.

"The whole dome turns easily," Halbach states, "and a good wind will turn it when the shutter is open and acting as a sail."

No blueprints are available. None really needed. Study the illustrations, sketch up your own, proceed to make it.

HOUSTON'S BEAM SHIFTER is a simple auxiliary to the knife-edge as commonly used. It was devised by Walter Houston,

R.F.D. 10, Box 323 Cincinnati 27, Ohio, and its purpose is to provide a kind of slow-motion control to the knife-edge, more delicate than ordinary hand control-not, however, by means of screws.

In the photograph (Figure 7) a telescope maker is shown testing a mirror.

At left is a housing for the testing lamp. The little projection on its right side is a right-angle prism with a slit (or could be a pinhole).

At right is the tester's hand on the base of the knife-edge.

Between the two objects just named is a loose cylindrical disk of wood. Mounted vertically on the top of this disk is a slip of plate glass-say a couple of inches or less in width and 3" or so in height. In the half-tone these details do not show clearly but their exact shape does not matter greatly. This wood-plus-glass unit may be slid around the table at will. Its slip of glass functions as a planeparallel (sufficiently so for the purpose needed).

We now put it in use.

The mirror is set for the familiar Foucault test.

Then the plane-parallel is brought up and slid into the returning cone of rays and is turned so that they strike it perpendicularly.

Next, final adjustment of the knife-edge is made and the Foucault shadows are picked up-through the plane-parallel-perpendicularly.

Then the knife-edge is withdrawn a little to the user's left so that the mirror will be illuminated all over and is left there.

Finally, the plane-parallel is rotated so that the cone of rays no longer strikes it at right angles. Back come the shadows.

Here we have a delicate control. The underlying principle is simple. If a beam of light passes through a plane-parallel at normal incidence, no deviation of the beam will occur (although if it is a converging beam-and in this case it is-it will shorten the focal length about 2/3 of the thickness of the glass but this is a minor effect in this test). If, however, the plane-parallel is rotated, so that the beam is no longer normal to its face, it then is shifted slightly-it sidesteps. It is offset a little.

Normally, in the Foucault test, we move the knife-edge into the beam. Here we move the beam into a stationary knife-edge.

"The rig," Houston reports, "works very well in testing by rolling the shadows to their crests. Its sensitivity is amazing, though it requires no delicate movements of the operator. I used it in teaching a class of beginners and some of them preferred it to the other method. It is the same principle as used in the line-shifter of the Hale spectrohelioscope. ('A.T.M.' page 196. at bottom) to bring the desired line on the slit. Advanced workers and sharks at theoretical optics will probably find in this arrangement an error of a millionth of a wavelength. Basically, the stunt is for helping tyros."

Others may like it, too.

WALKDEN'S various richest-field telescopes are designed from formulas based on several fixed fundamentals, as described in "A.T.M.A.," and one of these fundamentals is, or was, Chapman and Melotte's table of star densities. With these density data the richest RFT for refractors proved to be a 2-3/4" f/6 and for reflectors a 4" f/4. These are the figures in heavy type in the table on page 636 of "A.TM.A."

But the Chapman and Melotte table were for photographic magnitudes and more recent tables by Seares and van Rhijn are for visual magnitudes. This makes a difference in the richest RPT. Walkden has recently worked out the new data. The richest RFT for refractors now is a 5" instead of a 2-3/4" and for reflectors it is a 7" instead of a 4".

Better fish out your "A.T.M A." now while you're reading this, and pencil in some new data.

Page 624, column 7, opposite 10, 11, 12,13,14,15, write in: 7.85, 21.09, 55.03 138.2, 334.5, 775.7, respectively. Column 8, for same numbers: 531, 571, 594, 593, 572, 524.

"The new maximum," Walkden writes, "is evidently at about mag. 12.4 of column 1, so a 5" instead of a 2-1/2" RRFT is now indicated for column 3.

Now, on page 636, interpolate a new column between 6" and 8" reflectors and mark it the new reflector RRFT. The column: 7", 29-12", 4.2, x24.5, 0.85", 1.20", 1.61", 0.40", 0.96", 1.07", 0.61", 0.45", 3.50", 1.57", 2.22", 35", 12.7", 107, 225.

Next, in same tables, bottom line, change 244, 243, 241, 228, 215, 203, to 206, 217, 223, 224, 223, 221, respectively. These small changes will be made in the next printing of "A.T.M A."

Must you now toss out your present RRFT and commit suicide? A little study of the chapter, such as you probably have already made, will show that the old RRFT isn't so far off the bull's-eye as the above alterations may seem to imply. And, if making a new one, a 7" mirror blank will be unobtainable, at least in Pyrex unless you pay (plenty) to have an 8" blank ground down or trepan it yourself. However, fairly reasonable ATMs probably would settle for an 8" or a 6" RRFT as a pretty good approximation.

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