29 research outputs found
Measurement of the absorptance-emittance of black paint surfaces at temperatures from 10 to 120 [K]
Developments done at CSL for the testing of Herschel Telescope and Instruments of the Planck Reflectors and Spacecraft
A Target Projector for Videogrammetry under Vacuum Conditions
Videogrammetry is a 3-dimensional co-ordinate
measuring technique that (now) uses digital image
capture as the recording method. Images are taken from
at least two different locations and the light-rays from
the camera to the measurement object are intersected by
triangulation into 3D point coordinates. With a large
number of images, the camera orientation and 3D point
locations are accurately determined with the use of a
full mathematical model (bundle adjustment). Adhesive
targets are commonly used for point materialization.
Those targets consist of a retro-reflective material, with
an adhesive backing for sticking to the structure under
investigation. A drawback of these types of targets is
that they can lose their adhesion and shape during
thermal vacuum cycling, especially when cryogenic
temperatures are involved. In addition the operation of
placing and removing targets is a critical procedure that
can lead to undesirable contamination and damage to
the test item. Because they require to be physically
attached to the surface to be measured, this can also
compromise the quality assurance of the test object.
Such problems were encountered during cryogenic
thermal vacuum qualification testing of the ESA Planck
Surveyor mission telescope reflectors.
In the development described here the aim was to
replace the use of adhesive targets by projected dots.
The idea is not fundamentally new. Indeed a US
company, Geodetic Systems Inc. (GSI) proposes
already a commercial target projector for
videogrammetry which uses a flashlamp and is adequate
for workshop and laboratory applications. Dot
projection videogrammetry is also suggested as a shape
measurement method of Gossamer structures,
membrane reflectors, etc…Note that there are fundamental differences in use and
applications of retro-reflective targets and dot
projection:
• Retro-reflective targets are materialized on the test
article. They appear as fiducials attached to the test
article. Any relative distortion of the materialized point
can be tracked with respect to the test article itself.
• Projected dots are not materialized on the object.
Practically a specific point cannot be tracked by this
method. The dense cloud of projected dots allows
sampling the surface or the shape in an arbitrary way.The Target Projector System (TPS), described here
(Fig.1), was designed and manufactured to operate in
the Large Space Simulator (LSS) of ESAof ESA under
thermal vacuum conditions and to have a minimized
temperature exchange with its environment. It is
operational over a temperature range of 90K to 350 K,
and has a wavelength of 808 nm.Qualification tests and results under vacuum, along with
an assessment of the videogrammetric accuracy
achievable for various configurations of this unique
device are presented