"Although I have almost learned the SCIENTIFIC AMERICAN'S instruction book 'Amateur Telescope Making' by heart, and have recommended it to many others," says Mr. Goddard, "I must admit that I bought my mirror. It was made by John Mellish of St. Charles, Illinois, the same person who figured the 30-inch for the University of Illinois.

"I began about 20 years ago experimenting on mirrors, liquid objectives-that is, filling the space between two ground surfaces with silicate of soda-and buying small inexpensive telescopes and objectives, making an extension on my tube. I became rather discouraged with my mirrors because I would try them once, obtaining a good image, then the next time they would be distorted. I did not know at that time that the temperature was affecting my glass. I finally decided on a mirror made by someone who knew how, hence the 16-inch. I found out afterward that I was bothered just as much by temperature with the new glass, until I learned to give it time to cool, and then later to use a fan [explained below-Ed.].

"As for results on the 16-inch, I have seen seven satellites of Saturn and of course all of the rings. At very rare intervals I have seen the companion of Sirius and also the companion of Antares. The climate here at Portland is not good enough for much power and I use a one-inch eyepiece more than any other. However, I have been able to use up to 640 diameters magnification quite successfully. I can see the surface markings in the lunar crater Plato quite clearly when the light is right.

"The small refractor in the photograph is a four-inch Gall and Lembke with a John Berni objective. It is very good and I use it mostly on the sun. It comes in handy to have an extra glass when a class or group of people come.

"The tube and mounting of the 16-inch weigh about 750 pounds, so I have a ball thrust bearing around the two-inch shaft of the polar axis. This also makes the R. A. motion very steady. The concrete base is 20 feet in diameter and two feet thick in the center around the 12-inch pillar. (I wheeled all the concrete, too, a sidelight on astronomy.) The pillar is solid concrete with a six-inch pipe set in the top and a six-inch cast iron 45-degree ell threaded on to the pipe. I am fortunate in that Portland is on the 45th meridian and the ell was just right.

"The declination axis is a two-inch cold-rolled shaft set in boxes on a steel plate which has been welded to a heavy collar set on the end of the polar axis. The circles are cast brass disks ten inches in diameter, with the hours and degrees ruled on the perimeters. The divisions are down, respectively, to four minutes and three degrees, which are in turn divided on the verniers.

"The tube is heavy-gage galvanized iron and is 17 inches in diameter and 128 inches long. The finder is a two-inch glass with cross-hairs. I plan to have an observatory later. The dome will be 20 feet in diameter."

IN his letter, just quoted, Mr. Goddard referred to a fan. The use of an ordinary electric fan for maintaining a circulation of air through the tube of a telescope was mentioned on page 405 of our number for last May. All astronomers, professional and amateur, know that star twinkling, unsteadiness of image, "boiling," and sundry other optical nuisances are caused by atmospheric conditions, mainly the irregular distribution of temperature in the air, which produces differential refractive effects; and that a night with misty, hazy air usually will provide better seeing than a still, sharp night, in which the stars stand out against a crystal clear sky. It usually is assumed that the whole of this effect takes place aloft but some workers have questioned whether half of it, at least, is not caused actually within the tube of the telescope itself. Here, too, there often are temperature effects, due no doubt to the stratification of air in the tube. Accordingly, last May, it was suggested that open-sided-that is, latticed-tube might prove to be superior to the type of tube made of solid sheet metal or wood.

Bearing on this point Mr. Goddard adds the following comment to his letter: "I have the 16-inch reflector but as yet no observatory, and I found the open or latticed variety of tube was unsatisfactory in my case because of the interference of street lights, so I finally settled on the present closed tube with an open door just above the large mirror. This worked better, but often I would have to wait an hour or more for the mirror temperature to settle. I read of some experiments by Professor W. H. Pickering with a fan so I decided to give it a trial.

"I purchased a small six-inch electric fan and attached it direct to the tube so it would blow in through the door. But the slight vibration of the fan was so magnified in the eyepiece that the idea seemed impracticable. I then secured the standard for a music rack and made a separate mounting for my fan, also in order that I could raise it or lower it and set it at any angle. When I first tried it, a friend who was not accustomed to looking through a telescope happened to be calling, so I asked him to look at Jupiter. Only about three of Jupiter's belts were visible-until I turned on the fan. Then he said 'Why, that makes a great difference. I can see 100 percent better.'

"The effect was like blowing away a fog, and the detail, even with 600 diameters, was very clear. Since then I have found the fan so far ahead of any other method that I always use it."

WHAT was it that Professor Pickering said about the use of a fan in the article Mr. Goddard read? We quote it from Popular Astronomy for last March, where it was published: "We all know when first starting a furnace in the autumn that the hot air does not at once pour out of the flue, but is for a time blocked by the cold air already there, and which floats on top of the hotter, lighter medium; later gradually mixing with it, and finally being forced out into the room. If we could look through those mixing masses of air in the flue, we should find the seeing intolerable. It occurred to me while using my reflector that that was just what was occurring within the iron tube of my telescope. When the shelter is rolled away, and the instrument is exposed to the outside air, the upper end of the tube is cooled by radiation to the sky, while the lower end, not so completely exposed, retains some of its original warmth. We accordingly have within the tube a mass of cold air resting upon a mass of warmer material. Since the quality of the seeing is not influenced by the velocity of the air currents, whether within the tube or in the sky, no matter how high that velocity may be, but is strongly influenced by the least difference of temperature, I determined to go to the bottom of the matter and collect the fundamental facts."

PROFESSOR Pickering next describes experiments he made, ascertaining actual temperatures in various parts of the tube, and in the surrounding air, by means of thermometers, and concludes: "The obvious remedy to employ is to use an electric fan, forcing a current of air from the window near the mirror up through the tube: Care must be taken to support the fan in such a manner that it shall not jar the telescope. A six-inch fan is plenty large enough. With poor seeing, due mainly to currents in the upper air, the improvement resulting is not marked, but with good seeing it is most striking. It is very nicely illustrated by throwing a bright star out of focus before turning on the fan. The image usually rotates, sometimes in one direction, sometimes in the other. The rings always visible in such an image constantly vary in shape, with an appearance that we may describe as 'moulding.' The instant the fan is turned on all is instantly changed, the rings become circular, and all rotation ceases. Mr. Phillips in a recent paper ( Journ. Brit. Astron. Assoc., 1929, 39, 113) speaks of having tried an 'electric blower' but does not seem to have been quite satisfied with the result, because trouble began 'as soon as the blower is stopped.' In my original paper (Report on Mars, No. 41 Popular Astronomy, 1928, 451) describing the use of a fan, I state that an 'electric fan sending a current of air through the window near the mirror, and up through the tube, was later found to give greatly improved results.' It did not at that time seem necessary to me to state that the fan should be kept going. In order to avoid jar I always support it on a cushion.

"Since the opinions as to the relative advantages of open versus closed tubes are so divergent, it appears to me likely that with neither form are the mixing air masses entirely avoided, and that each observer should settle which is the best form for his own individual case. However, when an electric current is available I feel that a closed tube with a fan entirely avoids the whole difficulty, and is therefore better than any open tube can be."

WHETHER to use the conventional closed tube, a latticed tube, or a closed tube with a fan driving air through an opening near the base ought, it would seem, be determinable once and for all by a simple experiment. The case is not, however, that simple-not by a long shot. No two telescopes, no two situations, no two nights are alike, and therefore the decision will not easily be arrived at in many instances. At least, no opinion ought to be formed until, like Mr. Goddard, the user has observed conditions over quite a period of time. Workers who have formed opinions are invited to submit them for publication for the benefit of others. Some have had trouble with the latticed tube due to dewing of the mirror. Sometimes this may be remedied by wrapping the lower part of the tube with cloth-or making it solid in the first place.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.