Slawomir Kruczkowski
Introduction
The purpose of my speech is a demonstration of possibility of application the
Fresnel diffraction phenomena in amateur astronomy.
Some about the history… The first reference to diffraction phenomena appears
in the work of Leonardo da Vinci but first accurately described by Grimaldi
in a book, published in 1665. Huygens, the first proponent of the wave
theory, seems to have been unaware of Grimaldi’s discoveries. The possibility
of explaining diffraction effects on the basis of a wave theory was not noticed
until about 1818. In that year there appeared the celebrated memoir of Fresnel
in which we showed that the diffraction can be explained by the application
of Huygens construction together with the principle of interference.
In 1909 P.A. MacMahon has been described first, in the note “On the determination
of the apparent diameter of a fixed stars” (1908, MNRAS, 69, p. 126), application
of the Fresnel diffraction phenomena in the astronomy. The works of A.E. Whitford’s
(1939, ApJ, 89, p. 472) and J.D. Williams have been suggested that angular diameter
of stars may be registered by photoelectric equipment.
The great evolution in photoelectrical registration astronomical phenomena was
in the years that followed the Second World War. In the years 60’s and 70’s
was observed great value of the lunar occultation, which had in view to calculate
diameters of the stars.
At the beginning years 90’s to notice a disappearance of interest of using diffraction
phenomena in astronomical observations. I suppose so, development HST, ESO observatories
caused change of trends.
Well… I hope that the growing interest of dynamics small bodies the Solar System
and possibility of collision will cause that we don’t forget about the Fresnel
diffraction phenomena.
Phenomena
Fresnel diffraction is a special case of diffraction on any obstacles – describe
diffraction on a straight edges. Fig. 1 show at one with the situation, when
light source is a star and straight edge is edge of Moon disc.
Left side of the drawing show an area with filling in 40% of gray (not a white-ness
of sheet) because is impossible to show bright-ness of light on right side.
It is important that actual time of occultation is, when density of light source
amount 25% normal density. Often instead units of distance – d use be units
of time. In case lunar occultation 1 m correspond with 1.1 ms.
Moreover, the observa-tions of the phenomena are better quality when light from
the star will filtered, particularly by narrow-band filter.
Applications
In practice of amateur astronomy may be to apply the Fresnel diffraction
phenomena to:
a) central or graze occultation – improving of lunar limb profile by calculating
of degree of the slope lunar limb profile. Fig. 2 show mechanics of phenomena,
which consist in extended time of visibility the Fresnel diffraction on factor
1/cos ?,
b) central and graze occultation – calculating diameter of the stars, which
have angular diameter between 0”.002 and 0”.020 (fig. 3),
c) asteroidal occultation – showing a problem of varying complexity. Fig. 4
present an example occultation: disappearance ? Geminorum by 381 Myrrha (1993,
AJ, 105, p. 1553),
d) central and graze occultation – detecting double and multiple systems of
stars. Fig. 5 present an example: double star, when elements have similarly
brightness (1970, AJ, 75, p.575).
Fig. 2 Fig. 3
Fig. 4 Fig. 5
A way of development
Today haven’t technical obstacles, so that to execute showing earlier aims.
The astronomical kit telescope – recorder (especially when recorder is
a photomultiplier) allows to recording the phenomena about brightness 12m –
13m.
Remain important question, yet:
a) wont the amateurs to go in this direction?
b) how much have an influence on this decision good intensions and how much
– cost?
We remember, source of success in observation projects is a teamwork. If we
wont to accomplish good results, many amateurs should be equipped with good
quality telescope – recorder kits. I now, that a lot of numbers of amateur
astronomers haven’t so much money but my second speech has purpose to present
solution of this problem.
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