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DigiCULT
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obtained recently, very accurate stellar
proper motions can be calculated for stars
as faint as magnitude 15.This goes deeper
than the ones obtained from the HIPPAR-
COS astrometric satellite. Digitising these
plates in an optimal way (using a very pre-
cise XY table, the appropriate optics, and a
stable environment) is crucial to the accu-
racy of the data. Furthermore, the data
reduction software used for calculating the
stellar positions should use an appropriate
fitting profile.
D
igitising a photographic image can be
done `on the fly' (using a digital
detector moving with constant speed in
one direction) or `on the step' (using a digi-
tal detector at rest).The way traditional
scanners work means some of the finer
details of the image can be smeared out
over neighbouring pixels, creating a soft-
looking image. However, certain pho-
togrammetric scanners create a `dead' time
between integration intervals in order to
produce a hard(er) image.The level of
detail in the photographic plates requires a
very high optical resolution to produce
high geometric and radiometric accuracy
in the digital copy, and precludes the use of
sharpening filters in the scanning process.
The D4A digitiser that will be used for this
project is able to digitise photographic
images and spectra on glass plates and poly-
ester sheets as well as on polyester film rolls
to an extremely high level of precision.
T
his project will develop a two-dimen-
sional plate digitiser that will operate
on the step in order to create a precise
digital optical copy of the original image.
For most astronomical applications, over-
lapping digital sub-images can be used.
Bright stars in the overlaps are used to tie
up the whole image and to transform
measured X and Y positions on the image
into celestial `alpha' and `delta' co-ordi-
nates.The accuracy of the photographs
depends on the type of emulsion used, the
type of supporting layer (glass plate or
with the light of the indi-
vidual stars split up in
wavelength). Later on,
emulsions on polyester
sheets were used.The
largest collection consists
of the images taken for the
discovery of comets and
small planets. Usage of
photographic techniques
for astronomical purposes
gradually stopped after the
introduction of the CCD
detector in the 1980s.
P
lates consist of a distri-
bution of silver grains
embedded in a gelatine layer fixed on a
glass plate or polyester sheet. As such, they
are very sensitive to changes in tempera-
ture, relative humidity (RH), and chemi-
cals, and are at great risk of degradation,
such as chemical reactions from finger-
prints, humidity causing destructive fungi
and so on. Most photographic collections
were/are stored in conditions that are far
from being ideal. In order to improve the
lifetime of its astrophotographic plates, the
KSB is constructing an acclimatised plate
archive that will be kept at 18C and 50%
RH. As the plates have been stored in
more humid conditions up till now, a
gradual change of the relative humidity
will have to be applied.The photochemical
re-bathing of old plates, in order to remove
remains of historic fixing agents (which
cause a gradual fading of the image and
deposit a purple shade), dust and fungi, is
currently being studied in order to deter-
mine possible distortions and to find out
what effect this bathing has on the lifetime
of the plates.
T
hese old photographic plates contain
valuable information about the posi-
tion and the brightness of heavenly objects
that cannot be reproduced. By comparing
stellar positions obtained from these histor-
ical photographic plates with positions
More recently, colour film has been used;
however, for photogrammetric purposes
greyscale film is still being used by the
NGI.
T
he KSB archives contain 30,000 pho-
tographic images of the sky mostly
on glass plates, some on polyester sheets,
with dimensions ranging between 16 cm x
16 cm and 30 cm x 30 cm, medium to
high resolution spectra of individual stars
and low resolution field spectra used for
star classification.The oldest collection is
formed by the plates taken around 1900
for the Carte du Ciel international project,
which aimed to conserve an image of the
sky at that time.Two series of plates were
taken. Long exposures, containing three
exposures of about twenty minutes on
each plate, were used to make an atlas of
the heavens using newly developed helio-
graphic technology.These plates used fine-
grained emulsions in order to obtain the
best possible geometric (astrometric) preci-
sion. A second series of short-time multiple
exposure plates were taken to produce the
first astrographic catalogue.To photograph
fainter stars and to take shorter exposures,
coarser grained emulsions were used.
Photographic plates were also used to take
spectra of individual stars or to make low
resolution field spectra (i.e. a sky image
A plate from the Carte du Ciel collection, enlarged to
show triple exposure.
KSB
,
2003