background image
Carte du Ciel plates the digitised mono-
chrome heliographic atlas images can be
used for this purpose.
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
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.
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
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.