Wolfgang Rothe
Berlin, Germany
The observation of bright limb events is always a challenge for the
observer and the used equipment. A pre-diction of such event starts very
often again discussions whether an observation can be successful or will
be hopeless even under best conditions.
A thesis, a calculation tool and technical guidelines for a successful
bright limb observation are given in this contribution in order to support
active observers and to avoid frustrating failures.
Working hypothesis for a successful bright limb event observation:
The focal illumination intensity of the star image must be equal (or
higher) than the focal il-lumination intensity of the bright limb of the
occulting celestial body.
The intensity of the star image adds to the intensity of the bright
limb image when star is in the immediate vicinity of the limb immediately
before the disappearance. It results in the double intensity (200%) of
the con-tact spot in this case. The intensity of this spot of bright limb
jumps from 200% down to the formal value 100% (or 0,75 mag) at the moment
of occultation. This jump should be observable both visual and using video
if the contact spot is observed with full concentration. The thesis above
is valid for the faintest star component in case of stepwise occultations.
Spectral class of the star will have a certain influence, it is not considered
here.
May be, that a very experienced visual observer or an observer with
video equipment is able to recognise smaller jumps of contact spot brightness
and consequently fainter star contacts at the bright limb. On the other
hand an inexperienced observer may recognise a larger jump only.
Focal illumination intensity of the star image
A star is a practical pointshaped object but its image is a little
circular disk. The diameter of this disk is theo-retically estimated by
diffraction of light waves at the rim of the telescope aperture. However
we have to con-sider additional effects in practice which significantly
enlarge the diffraction disk: mainly the atmospheric szintillation,
but also defocusing due to imperfection of telescope and added equipment.
Then the diffraction disk has to be replaced by the overall scattering
disk. So we can calculate
with
BF = focal illumination intensity of the image
Bx = illumination intensity of the object with brightness
x magnitudes in aperture plane
? = transmission efficiency of telescope
D = diameter of (unobstructed) telescope aperture
f = equivalent focal length of the telescope
ds = focal diameter of overall scattering disk (mainly
influence of diffraction and seeing)
?s = angular diameter of overall scattering disk
The main point is here the influence of defocusing effects.
Focal illumination intensity of larger objects
Our large occulting objects as moon and the large planets up to Saturn
can resolved into their real shape even by small telescopes. Then we have
to modify the formula (1) into
with the new parameters
dH = linear diameter of the large object (here is assumed as
a full circular disk) in the focal plane
?H = angular diameter of the large object in nature (here assumed
as a full circular disk)
No significant influence of defocusing effects for larger objects is
given when their natural angular diameter is much larger than the overall
scattering disk.
It was created a calculation tool in order to simplify the calculations
above and to have a overview (Excel sheet, runs also with Lotus-1-2-3),
available from the author by request. A screen shoot is shown below:
Recommendations for more successful bright limb observations
Make the star image as sharp as possible, that means: avoid bad
seeing places, plan enough time for temperature adaptation of telescope,
test the use of colourfilter in case of chromatic focus differences, adjust
to best focus with care. This is the most important conclusion from calculations
above.
Use a large magnification (or equivalent focal length) that allows
to recognise the scattering disk of the star easily. The scattering disk
should cover several elements (2x2 at least) of the light sensitive receiving
device ( i.e. sensitive cells of the retina or pixels of a CCD chip) in
order to reach an averaging over several elements. Individual elements
may have different sensitivity or malfunction (ill or dead retina cells,
hot or black pixels) which can result in an unstable star image.
Avoid any overload of video camera or eye. A saturation prevents the
recognition of small differences or jumps of brightness. Use limiting aperture
diaphragm (eccentric for most mirror telescopes) or neutral filters to
ensure linear operation of camera or eye.
Avoid disturbing light due to internal reflexes when bright objects
are in or near the field of view. Such interference can occur as small
but be very bright spots at the image. Check carefully your equipment for
such effects, cover critical components with black matt painting, install
additional diaphragm.
A good experience
The grazing lunar occultation of Regulus, mag 1,4, spec B8, on 24th
April 1999, moon illumination 72%, first contacts at dark limb, later at
bright limb:
This event was observed by 3 visual and 6 video observers in Germany
near Berlin. One experienced visual observer in the more total region could
reliably time a bright limb disappearance. Video observers recognised some
bright limb events in real time at the monitor, later were recognised 20
reliable events which were com-patible to each other. This video analysis
was a hard job, special adjustments for brightness and contrast of the
monitor were required to identify all events surely. Some other as suspect
reported bright limb events were completely incompatible, they must be
excluded. It seems that Regulus gives only a small margin for video recording
at the bright limb, regardless of its 1,4 mag brightness, due to the nearly
blue spectrum.
Other expedition in Czechia, Germany, Poland and Slovakia observed
this event with similar results.
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