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DigiCULT
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Info
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Carte du Ciel plates the digitised mono-
chrome heliographic atlas images can be
used for this purpose.
A
erial photographs need to be digitised
as raster images, requiring an accurate
stepping in both the X and Y directions.
The digital image can then be stored as
one large image, or in the form of a tiled-
TIFF file containing the adjacent non-
overlapping images of the individual
footprints.
T
he D4A pilot project will develop the
necessary hardware and software to
digitise the plates by the end of 2005.The
real production phase should be realised
through a future follow-up project.When
the D4A high-resolution scanner is avail-
able, this project will hold an international
workshop based on the methods and chal-
lenges of digitising collections. A digital
catalogue is under construction that will
be integrated into a user-friendly database
accessible from the Internet to allow public
access to this important scientific and his-
toric information.
telecentric objective used will be limited
to its central part where the distortion is
less than a pre-defined maximum. In this
way an `optical' contact copy of the origi-
nal image onto the digital detector will be
achieved. In order to be able to reach and
maintain a high geometric and radiometric
accuracy, the digitiser will be placed in a
clean room, at a temperature of 18C
0.1C (1 sigma) and a relative humidity
of 50% RH 1%RH (1 sigma).These
conditions will be obtained by circulating
air over a chilled water radiator in order to
produce a stable laminar flow.To obtain a
sharp image, the air between the photo-
graphic emulsion and the objective should,
however, be kept at rest.The exact techni-
cal details are still under study.
I
n order to save the historic notes on the
glass side of the hand-measured
astrophotographic plates, a graphical scan-
ner will be used to produce low-resolution
`pre-scan' digital images.These will be
coupled to the database and used to re-
determine the position of the plate centre
and the magnitude range of stars, etc.
measurable/visible on the plate. For the
polyester sheet), the optical quality of the
telescope used and the exposure time.
Photographic plates containing stellar
images can have a density range of 5 (i.e. a
grey scale ranging from 0 to 100,000) and
a sub-micron stellar positional accuracy.
The photographic image is made up of an
irregular distribution of developed grains
of varying sizes whereas a digital image
consists of equally spaced and sized square
or rectangular pixels. In order to capture
the level of accuracy of the analogue pho-
tographic images as closely as possible, a
digital detector is needed with at least a
10-bit ADU (Analogue-to-Digital convert-
er Unit) read-out and a pixel size of about
5 micrometres.We will mount the digital
camera above the plate, perpendicular to its
surface, and use an air-bearing open frame
XY table to allow us to position the plate
with a geometric accuracy of some ten
nanometres.We will use a two-sided tele-
centric lens; this is to ensure that, if the
original image is not perfectly flat, the
introduced error will only slightly enlarge
the projected image of a point source,
while keeping it isotropic and without dis-
placing it.The part of the footprint of the
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The Schmidt telescope building at the KSB in Ukkel (Brussels) that will house in its base-
ments the D4A digitiser in a climate controlled clean room with adjacent plate archive.
KSB
,
2003