HUGE TELESCOPE

Sun (Auckland), Volume I, Issue 288, 25 February 1928, Page 10

HUGE TELESCOPE

REFLECTOR CAST THIRD EVER PRODUCED A notable scientific achievement was realised, recently by the American Bureau of Standards when it completed the test in making a huge disc of optii cal glass for use as a mirror in a re- ! fleeting telescope, writes the New j York “Times.” | When the glass experts of the bureau ; uncovered a furnace, which they closed i more than eight months ago. they disI covered to their satisfaction a perfect I mass of optical glass, 70in in diameter. | llin thick and weighing 3,5001 b. The furnace was covered on May 7, I 1927, and ever since that date the j molten glass has been cooling. Bej cause of numerous difficulties and failI ures that had attended such efforts i heretofore those in charge had been in j suspense for weeks as to the outcome. I The disc will make possible the first “all-American” telescope ever manufactured. Large discs of optical glass of the size required not only had never been made in America, but only twice elsewhere. and those that had been completed are said to be imperfect. After four unsuccesstul attempts in the Bureau of Standards to obtain a disc of the size required, a unique method was developed by Mr. Finn. One thousand pounds of cullet (broken glass of the same composition as the glass to be made) and 4,600 pounds of sand and chemicals were melted in a single large pot in a gas fired furnace. Molten Glass Poured Into Mould The molten glass was stirred by hand for six hours and at the proper time, or on May 7, the pot was tapped. The Molten glass flowed into a mould of the required size, which was specially designed for the purpose. This mould was a carefully insulated annealing furnace, provided with electrical .heating elements by means of which temperature could be adjusted and controlled to within one degree. The temperature of the glass when poured was about 2,400 degrees Fahrenheit. For one week the temperature was slowly lowered until it reached 1,112 degrees. The glass was held at this point for about four days to allow the temperature of the glass and furnace to become uniform throughout. At 1,112 degrees this particular kind of glass (borosilicate crown) is quite rigid and yet sufficiently viscous to yield to cooling stresses without danger of cracking.

Beginning on May 8 the glass was allowed to cool slowly at an average rate of four and a-half degrees a day till 860 degress was reached. It was then annealed at this temperature for six weeks, during which time no variation greater than two degrees Fahrenheit was permitted. Final cooling was started on August 30 and room temperature was attained on January 16. During all these months there was no assurance as to what would be found in the annealing furnace when the final “unveiling” took place, because the slightest bit of foreign matter in the glass might start radial cracks which would split the glass into fragments. Too sudden changes in temperature would be equally serious. The next step toward completion of this reflector is to drill a hole at its optical axis. Although the majority of opticians would refuse to grind a hole in a piece of glass as large as this except at the owner’s risk on account ©f the possibility of breaking it, the experts at the Bureau of Standards feel so confident that this can be done without any difficulty that they will do it at their own risk.

After this the disc will be sent to an optician to be ground, polished and “figured” on one surface until it becomes parabolic. Such a surface has the property of reflecting parallel rays of light to a focus. It is the curve used in automobile headlight reflectors, only in the case of a telescope mirror the curve is so slight that it is hardly noticeable.