ASTRONOMICAL TECHNOLOGY — MEINEL 297 



accurately. In practice, the photoelectric photometer isolates a single 

 small region of the sky at the focus of the telescope. The size of the 

 diaphragm of the photometer is kept small in order to reduce the 

 noise signal from the background of the sky, but large enough to 

 permit the blurred image of the star to pass completely througli the 

 hole and to allow for inaccuracies in the guiding of the telescope. 

 The usual size of the diaphragm is 1 to 2 mm. diameter. 



The high efficiency of the photoelectric surface has led to efforts to 

 construct an imaging system where the astronomer could take a "pic- 

 ture" of many stars at a time rather than one by one. While the 

 theory of an image tube is very simple — one needs only to electron- 

 ically accelerate and focus the electrons emitted from the cathode upon 

 a fluorescent screen or directly upon a photographic emulsion — ^the 

 practical attainment of an image tube proved full of technical diffi- 

 culties. The earliest use of an image tube in astronomy was by 

 Krassovsky (U.S.S.R.) who adapted an infrared snooperscope tube 

 to photograph the infrared airglow spectrum, a task not possible at all 

 with the photographic emulsion since it is not sensitive to the infrared 

 beyond 1 micron wavelength. The first image tubes to rival and 

 exceed the direct photograph were made in France by Lallemande. 

 While the use of these in astronomy has permitted unusual observation 

 such as the rotational velocity of the nuclear regions of the Androm- 

 eda nebula by Walker (U.S.A.), each tube must be made minutes 

 before use — hardly like taking a photographic plate out of a box 

 purchased months beforehand. 



The adaptation of the television tube method to astronomy has 

 recently become possible by the development of tubes with high 

 sensitivity to low levels of light and with integrating properties. 

 Many astronomers in the U.S.A. are now experimenting with systems 

 using commercial tubes with good success. The promise of this type 

 of image tube is foreshadowed by the time in the near future when 

 astronomers will want to have their "photograph" taken from a space 

 telescope transmitted back to the earth. 



A vast technology has developed for infrared detection in the region 

 from 1 to 12 microns and which has only recently been applied to 

 astronomy. The principal reasons for the lack of development of 

 infrared astronomy are that the atmosphere transmission is highly 

 variable in these wavelengths and detectors are still senstitive enough 

 only to permit one to reach the brightest stars with a large telescope. 

 To illustrate the problem of background noise in the case of an infra- 

 red telescope it is only necessary to remember that the maximum of 

 the infrared emission from material at room temperature is at 10 

 microns. The detector therefore looks at the star through a telescope 

 that is literally glowing with its own light even though it is night and 



