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10th Inkaba yeAfrica/!Khure Africa (AEON) Conference/Workshop
Lord Milner Hotel, Matjiesfontein - Karoo
29 September – 3 October 2014
Optical configuration and optical
tests of the HartRAO Lunar Laser
Ranger
N. Nkosi1, 2, L. Combrinck1, 2 , M. Akombelwa2
1. Hartebeesthoek Radio Astronomy Observatory (HartRAO),
South Africa.
2. Programme of Land Surveying (Geomatics) School of
Engineering, University of KwaZulu- Natal, South Africa.
OUTLINE
•Introduction
•LLR telescope
•Telescope Mirrors: Cleaning and coating
•Coude path
•Measurements, tests and simulations
INTRODUCTION
•The HartRAO LLR will be a highly accurate system, consisting
of integrated optical, mechanical, electronic and software units
• Requires the development of several new subsystems,
including characterization and constant monitoring of the optical
systems
•Characteristics under investigation includes:
-Various reflection coefficients
-Optical path environmental conditions (temperature, pressure,
humidity)
-Overall system parameters
•Output data will aid system monitoring, validation and quality
checks
LLR TELESCOPE
Figure 1: 1-m Classical
Cassegrain telescope
Figure 2: Convex hyperbolic
secondary mirror
Figure 3: Parabolic primary mirror
LLR TELESCOPE….cont
•Both the primary and secondary mirrors are of a honeycomb design
Figure 4: Honeycomb mirror designFigure 5: Whiffle-tree support system
•The substrates are made from Schott Zerodur ceramic glass
material through a technique of fusing glass plates and a core
together at high temperatures.
•The primary mirror is held in place by an 18 point whiffle-tree
floatation cell( Figure 5)
TELESCOPE MIRRORS:
CLEANING AND COATING
During laser ranging, the telescope is exposed to dust and other
particles. This reduces the reflectivity of the mirrors, hence
limiting the capability of observations
Studies have revealed a direct relationship between the detection
capability of a telescope and the cleanliness and efficiency of the
coatings (Dierickx, 1992).
To ensure maximum reflectivity, mirror surfaces have to cleaned
and re-aluminized from time to time
Mirror cleaning is a delicate process and one should avoid
damaging the optics during this process.
TELESCOPE MIRRORS:
CLEANING AND COATING..cont
A visual inspection of the primary mirror’s coating was done by
illuminating the mirror from behind with an LED light source. This
revealed major degradation of the coating
Attempts to remove coating off primary mirror failed.
• Potassium hydroxide solution
• Ferric Chloride
• Hydrochloric acid & Copper Sulphate
• Nitric acid
Figure 6: Coating
degradation on primary
Figure 7: Mirror surface after
failed attempts to remove
coating
Coudé Path
Dielectric coudé mirrors
Diameter = 50 mm
Flatness = λ/10
Reflectivity=99% @ 532 nm(λ)
Capability ≥ 1. 25 GW
Dry Nitrogen
gas
Mirror Mounts
• Kinematic precision
• Angular resolution = 0.1 Arcseconds
• Removes
dust and
contaminants
Path purge adapters
• Join delivery tubes
• Purge dry nitrogen gas into system
Coudé Path
Insulation
Polyethylene pipe insulation
Operating temperature= -40˚C to 90˚C
Thickness = 15 mm
Conductivity = 0.031 to 0.043 W/mK
Protective cover
Galvanised steel
Delivery tubes
OD= 65mm
Black anodized to prevent reflection
Measurements, Tests &
Simulations
Acknowledgements
We appreciate support towards this project from
the Inkaba yeAfrica project, the NRF and HartRAO