Fresnel Diffraction during Lunar Grazing Occultations

Diffraction of the star's light at the edge of the Moon causes the contacts during a graze to last even more than a second, appearing as pronounced gradual events

Limovie analysis of Tom Campbell's tau Scorpii graze tape shows that the gradual events were due to diffraction, not stellar duplicity

New: 2007 May 4 UT, 2h UT

     Fresnel diffraction at the edge of the Moon during a lunar 
occultation is described quite well by Michael Richmond here.  Part 
way down, about the 7th figure is a graph showing the intensity of 
monochromatic light at the edge of the Moon as a function of 
distance above it. Above the Moon's surface (to the right in that 
graph) you see the decreasing waves.  In the other direction (to the 
left, under the Moon's surface) you can see the light of the star 
but ever fainter with an exponential decay. I call that the "toe" of 
the diffraction pattern. During an occultation of a bright star, you 
can see quite far down the "toe" of the curve. During a total 
occultation, the diffraction pattern sweeps across the observer 
quickly and he does not notice the "toe" - the occultation appears 
instantaneous. Video recordings only slightly start to resolve the 
"toe"; they usually show the decrease occurring over 2 or 3 frames. 

     But during a grazing occultation, the contacts of the star with 
the Moon's surface occur usually at very shallow angles so that the 
time scale of the diffraction pattern is greatly increased relative 
to that of total occultations.  For a bright star, the "toe" can then 
be seen as a gradual fading or brightening of the star over a period 
of a few to several tenths of a second, even more than a second in 
extreme cases with the right lunar slope and a bright star such that 
one can see very far down the "toe".  A good example was a grazing 
occultation of the 2.8-mag. star tau Scorpii observed by several 
observers in Florida in March; the visual observers noticed the 
gradual events and some of them concluded that the star was double.  
But one of the observers, Tom Campbell, obtained a good video 
recording of the graze.  The observations are described briefly here 
and a 24-megabyte .mpg file of Campbell's video is there so you can 
see the graze.  Tom created an .avi version of the video and 
processed it with the Limovie software which can make a photometric 
plot of the intensity of the star's light; all of the gradual events 
that he recorded were well-fit with the Fresnel diffraction pattern, 
as you can see in this Word file that Tom prepared, a classical 
example of Fresnel diffraction during a graze. 

     If the star were really double, instead of a smooth diffraction 
curve, there would be two of them, one for each component.  And 
especially for separations greater than 0.02 seconds of arc or so, 
the events, instead of appearing gradual, would appear as step 
events.  So if, during a graze, one sees only gradual events, 
without any steps, or maybe just one event might appear like a step 
due to a small cliff on the Moon, then one can conclude that the 
star is single and that diffraction effects have been seen.  But if, 
during a graze, nearly every event is not gradual, but appears to 
occur in quick steps, then the star is probably double.  If the 
separation is 0.01 arc second or less, the diffraction patterns of 
the two components are intermeshed with each other and the events 
still look gradual, but perhaps more gradual than the case for 
single stars; one can not tell then from visual observations if the 
star is a very close double or not.  But in this case, an analysis 
like Tom Campbell did with a good-quality video recording might 
reveal the two diffraction patterns. 

David