IN REFLECTING telescopes, seeing is often hampered by "temperature effects" within the tube. In the valuable contribution that follows, one amateur telescope maker, F. N. Hibbard, of the U. S. Weather Bureau Office at Richmond, Va., describes experiments on various types of tubes, made with J. C. Vaughan of Petersburg, Va.

"Bell's book, 'The Telescope,' 'A.T.M.,' and many other books tell what a stellar image may look like and what it should look like. A good image of a star, under a power of 40 or 50 times the telescope aperture in inches, and under good seeing conditions, will be a perfectly round disk, of a size varying inversely as the diameter of the objective according to the Dawes formula, with the disk surrounded by a series of concentric rings of graded intensity. On rare occasions of exceptional seeing, the rings will be needle fine like the web of a spider.

"Deviations from this perfect image, assuming the mirror to be well figured and supported by ample leverage, can usually be traced to a pinched or imperfect prism, improper alinement of the optical train, a poor eyepiece, a possible warping of the mirror by changing temperatures, unsteady air imperfect eyesight, or temperature differentials in the telescope tube.

''These defects, their images and their remedies, have been well covered by writers-with the exception of temperature differentials. The latter, otherwise known as 'tube effects,' have been cussed and discussed for years, without any final agreement as to remedy, and, curiously, without exactly spotting the underlying cause. One writer says he likes a metal tube; another says a better image is possible with a lined tube; a third suggests the use of a fan on the tube to mix the tube air into uniformity of temperature, another mentions the superior performance of a wooden tube, particularly after the panels had been removed.

"It is not generally known, outside the Weather Service, that various substances or materials exposed in the open air under a clear night attain temperatures different from that of the surrounding atmosphere. Repeated experiments prolonged for months show that thermometers so exposed consistently register lower-than-air temperatures, differences amounting generally to several degrees and occasionally as much as 8 degrees. They further show that the differences persist for hours at a time. In other words, objects exposed to the clear night sky seldom attain the temperature of the air, but differ from it and from one another.

"To conceive the process by which two contiguous substances, such as air and metal, can attain and maintain such an anomalous thermal condition for hours together is difficult. It depends, however, principally upon the individual absorption and radiation properties of the substance.

"With this background, we are ready to see what happens in a telescope tube.

"My first experiment was to use an 8" mirror in a solid metal tube open at the upper end. (Figure 1). The air next to the metal of the tube chilled and drained down the tube, producing differential refraction of light and a poor image.

"The second experiment consisted of lining the tube with cork, asbestos, and paper. This left the air in the tube warmer than the outside air. As the chill of evening advanced, a mixing of this warm air with the cooler outside air at the mouth of the tube again produced images distorted to some extent.

"The third experiment consisted of boring holes in the cork and metal of the upper part of the tube to afford drainage of the tube air to the outside before it reached the mouth of the tube. This experiment showed that cool air from the outside drifted through the holes, and each wisp of cool air caused a streak across the stellar image.

"The fourth experiment was made with the same mirror placed in a skeleton tube built of six angle irons. Theoretically, this tube should have given a splendid image, as the air inside and outside should have had the same temperature. But, alas for science, the more sense I used the less she worked. Air drifting through the tube between the metal work became chilled by the metal, produced a different refractive index, and again left streaks across the image.

"The fifth trial was carried out by wrapping the same metal with thick paper (Figure 2) and painting the paper a dead black. This improved the image to some extent but bad images still resulted. In fact, the images some nights with this skeleton tube were quite intolerable, and well-known features of the lunar topography at times were obliterated. The effect of the chill of the metal was proved by placing a heavy cardboard lining within the tube, shutting off the wisps of air chilled by the iron. The moment the cardboard was in place, the images entirely cleared themselves of streaks and became passably good. This experiment was tried over and over until the evidence of differential temperature was proved beyond question. The conclusion was inescapable that the ironwork had an entirely different temperature from that of the air, and that the difference persisted hour after hour.

"The final experiment was to use a solid metal tube, line it throughout with -a quarter-inch thickness of cork, and cut a large hole of about 20 square inches in the tube just above the mirror to ventilate and cool the mirror. This gave excellent results, particularly when the hole was covered before observation to steady the inside air. The mirror was a wonderful piece of work and it cut doubles down to the Dawes limit time after time.

"By this time my friend, James C. Vaughan (Figure 3) of nearby Petersburg, Va., and I had decided that a larger mirror would be a fine thing to have, particularly for nebulae and clusters. So we prevailed on J. W. Fecker to fix us up with a mirror 12-3/8'' in diameter, which proved to have a magnificent figure. We decided to continue the temperature control experiments with this larger mirror, and built for it a skeleton tube (Figure 4) of duralumin, designed so that it could be converted easily into a closed tube when so desired. Observational results were identical, except that the larger tube, which was 15" or 16" in diameter and had relatively less metal, showed relatively smaller chilling effects. Differential chilling and differential refraction were slightly less, but were not eliminated until this larger tube was lined completely.

''In constructing this large lining, it was found that even a small crack or hole would produce a tiny ribbon of light across the image, the ribbon shifting erratically as the wisp of air drifted bout within the tube. Spaces between the ends of the wooden lining and the metal work had to be calked with cotton or wool. Ample ventilation for the tube was provided underneath the mirror, but the mirror itself was protected from the direct draft. When we finally cut the vents through the cork lining of the cell, cold air from the cell felt like the discharge from an opened refrigerator.

"These experiments show us rather clearly what the 'tube problem' is, why the tube walls should be well separated from the cylinder of light entering the tube; why a skeleton tube does frequently produce unsatisfactory streaky images; why a tube must be ventilated; why a lined tube gives better results than a bare metal tube, and why a wooden tube, particularly if well ventilated and held solidly in alinement, does and should give excellent images These deductions probably do not apply to telescopes housed in a dome but may apply to some extent to the dome itself and its surroundings.

Several kinds of wood may be used to advantage in making tube linings, but my preference is Douglas fir or balsa, which are light in weight and have low thermal capacity and high insulating properties."

Cogitating on these matters, as a question in physics, independent of telescoptics, your scribe wondered if, then, a given object would not take on the temperature of the surrounding air if given plenty of time, and asked Hibbard, whether (to choose an extreme case) Tutenkhamon's toes after 3000 years in his tomb wouldn't assume the exact temperature of the air in the coffin. His reply:

"In a practical way, I should expect enclosed objects to have about the same temperature, such as Toot's toes. But I could arrange Toot in the open air so that each foot would have a separate temperature and neither foot exactly that of the air. Apparently, radiation of heat to a clear cloudless sky at night by suitable materials gives the maximum favorable conditions for temperature differentials. There is a rich field for quantitative determinations of variation, which has not yet been explored.

''Experiments in this field should bring out rather interesting relations between color, material, condition of surface, whether matte or polished, and changes that occur when night finally becomes day. I should expect most objects, particularly with a black matte surface, to become warmer than the air under daylight sunshine. The extreme heat of railroad rails on a hot day seems to point to an actually high temperature and not entirely to rapid transmission of heat from rail to finger when the rail is touched by finger.

''The lower temperature of metal at right under a clear sky would not be expected to obtain during the day. The situation would be reversed. And this brings us back to the original statement that the temperature-of a body depends on the relative rates of its absorption of heat from surroundings and its radiation of heat to its surroundings, principally the sky. There necessarily would be moments, if not longer periods, when metal and air were at the same temperature."

A noted meteorologist to whom the above discussion was shown added the following: "A material object will not necessarily reach the temperature of the surrounding medium, no matter how much time is allowed. It would provided molecular conduction were the only influence acting to transfer heat, but in nature this is never the case. Among other influences, one that is always operating is radiation.

"Any object is always continually gaining heat by at least two processes radiation and conduction, and also continually losing heat by both of these processes. An equilibrium temperature is reached when the rate of gain from the combined action of both radiation and conduction equals the rate of loss In general, this equilibrium temperature will not be the temperature of the surrounding medium.

"Within an enclosure, however, with uniformly heated walls, the temperature would become uniform."

Tutenkhamon was "within an en closure," hence not an ideal illustration since a telescope mirror is partly enclosed, partly open.

IN HIS letter Hibbard makes other comments. He votes for a gear drive with a 359-tooth gear on the polar axis which makes possible a simple get combination for sidereal time, a rotating tube, or eyepiece end rotating about the tube axis, which is easy to make and greatly adds to convenience in observing; four small holes in either end of the tube for the occasional use of cross-hairs, or threads, in checking alinement; and solid eyepieces, which give brilliant images and no ghosts despite their small field. He has a 1/2'' triplet aplanat eyepiece that gives absolutely colorless images of such difficult objects as Venus.

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