RINGLESS STARS
( ABOUT APODIZATION )
ANECDOTE #. 1
In
Year1958 (international geophysical year), the Sun was full of enormous spots, than I could not still see, and the idea I had of the telescope was that one of a tube full of lenses.
I remember of being remained enough amazed when I discovered that it could be constructed with only two lenses, and began immediately
to construct one.
Although it was better than that one of Galileo, it was really enough modest, and thought to buy a better one. I was young and without a money in my pocket, but my collection of stamps was worth as a 120 mm refractor, and I decided to buy it !
Meantime, I had learned from the books that the image of a point formed from an objective is not a mathematical point, but a point of ended dimensions, known as "diffraction image".
This it is formed of two parts: a central one (a false disc) caused from the surface of the objective, and a peripheral one (diffraction rings) caused from the edge of the objective.
After the buy of the telescope, the first what I made was to control if the star images looked like as the books descripted them. And I stated that many double stars were not so easy to resolve, because of the presence of those rings. The intuition was therefore immediate: "how nice would it be, if the stars could be seen without those rings…"
But how to make it? Almost
impossible, of course:
how can I imagine a lens without edge?
But… thinking it well, it would be possible :
if I succeeded to find or to make to construct a degrading filter, more transparent in its center and darker in its edges…
I had made the invention of the century!
And as if it were not
enough... I found immediately a second way to resolve the problem:
if I constructed a lens with a transparent glass (the convergent one) and another of black glass (the divergent one), making so that the divergent has
a minimal thickness near the center (= maximum transparency) and a much greater thickness to the edges (transparency = zero) I would obtain the same result. With a remarkable loss of brightness, obviously; but this would not be a problem for the observation of the Moon and, above all, of the Sun.
Very convinced, as I am still today, 50 years after, than it is much more difficult to patent an invention then to make it, I held it secret for some decade…until I turned up in the library of the Astronomical Observatory of La Plata, Argentina, the town where I had been destined as employer of the Italian Foreign Office.
I found a technical book
(*) where a technique was explained, called "apodization" (from Greek
α-ποδος = without feet), for the aspect of the graphic representation of the diffraction image produced from an apodized lens.
Curious thing: the book in which this technique was explained was right of year 1958 !!
Enough curious is
also to state as this technique is so little diffused.
In the common lenses it would be enough difficult to apply, due to the presence of the diaphragm, but in those astronomical ones, that are practically always used to full opening, would have to be more simple.
Perhaps you will say:
it would be more opportune to make a larger telescope. And it can be true.
But it is also true that a small, perfect telescope is
ever better then a larger and imperfecter one. And the presence of rings is an imperfection…
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(*)"Concepts of classical optics" by John Strong - John Opkins University
-
Ed. W.H. Freeman & Co. - San Francisco - 1958.
Page 410 and followings., by Pierre Jacquinot
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