University of Leicester
PLUME
Ref: PLM-CAM-SRD-706-1
Date: 08/11/2007
Science Requirements Document for Optical Camera
O. Littlejohns
Date
Updated Reference Number
change
08/11/2007
24/11/2007
PLM-CAM-SRD-706-1
PLM-CAM-SRD-706-2
first version issued
second version issued
The secondary payload proposed is an optical Complimentary Metal Oxide Semiconductor
(C.M.O.S.) sensor camera. This camera aims to take images of the Earth from a Low Earth
Orbit (L.E.O.) and supply the data to the On Board Data Handling (O.B.D.H.) subsystem of
the Cubesat for storage and eventual transmission to the ground station.
Whilst this payload has a low science output, it increases payload density without drawing
significantly on the available resources and provides excellent outreach material, especially
with the aim of encouraging interest from school children.
The camera will consist of four main components; a C.M.O.S. detector, filters, a lens optic
and shielding. It has been decided that the C.M.O.S. detector will not have to be ‘rad hard’,
that is being able to withstand a total dosage of above 250kRad, but could be either of the
categories ‘rad tolerant’ or ‘commercial’, each able to withstand a total dosage 20-50kRad
and 2-10 kRad respectively. It is believed that by increasing the shielding of the device by
introducing thicker Aluminium plate, the total dosage over the duration of the mission can be
kept to below an amount that would significantly degrade the quality of the optical system.
For most C.M.O.S. detectors, an Infrared (I.R.) cut off filter is required. This is because the
detector will still register the arrival of near I.R. photons, but will consider them to be photons
of the same frequency as red light. This means that the images produced, without a filter,
would suffer from reddening, and so reduce the overall image quality. Whether or not this
would be required would depend on the final detector that is sourced, however, it would be
best to include such a filter in the design for the camera as it is likely to be needed, and they
are relatively cheap and easy to acquire.
The optics of the proposed system will be a glass harsh environment lens. There are several
available from the Edmund catalogue. More research into the performance and
specifications of these would be required before selecting the most suitable for the required
application. Such a lens would illuminate an area of less than 10 millimetres diameter and so
the detector array area would be more suitable if it were all illuminated by the inbound flux
from the lens. A lens optic offers the highest image quality output for a C.M.O.S. detector,
whereas a pinhole optic, as previously modelled, would not provide sufficient image quality
given the capabilities of the detector array. A lens optic offers the best image quality output
for a given mass, volume and power input.
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University of Leicester
PLUME
Ref: PLM-CAM-SRD-706-1
Date: 08/11/2007
The camera must be sensitive to an optical photon flux of approximately 2.5 x 10 20
Photons/s/m2, which is that expected to be reflected from the Earth’s albedo at an altitude of
500km. It is assumed that in this waveband, the contribution to the flux is negligible from the
Earth’s blackbody spectrum.
The camera is designed with the view of taking images of the Earth from L.E.O. To achieve
this, the detector and aperture have a pointing requirement facing towards the surface of the
Earth when an image is taken. For this to occur, the satellite must be two axis stabilised.
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