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MONS Field Monitor
System Definition Phase
Design Report
MONS_AUS_PL_RP_0002(1)
Issue 1
11 April 2001
Prepared by
Date11 April 2001
Chris Boshuizen and Leigh Pfitzner
Checked by
Date11
April
Tim Bedding
Approved by
Date11 April 2001
Leigh Pfitzner
Auspace Limited
A.C.N. 008 576 645
PO Box 17
Mitchell ACT 2911
Australia.
Telephone: (+61 2) 6242 2611
Facsimile: (+61 2) 6241 6664
[email protected]
www.auspace.com.au
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2001
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MONS/AUS/PL/RP/0002(1)
Issue No: 1
Date: 11 April 2001
MONS
Field Monitor Definition Report
Copyright
Auspace Limited
The copyright in this document is the property of Auspace Limited. This document is
supplied by Auspace Limited on the express terms that it shall be treated as confidential and
that it may not be copied, used or disclosed to others for any purpose except as authorised in
writing by this Company.
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Auspace files
1
L Pfitzner
Auspace
2
T. Bedding
USYD
3
F. Hansen
DSRI (ROEMER Web site)
Revision Record
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MONS
Field Monitor Definition Report
MONS/AUS/PL/RP/0002(1)
Issue No: 1
Date: 11 April 2001
Page: i
Table of Contents
1
Scope ............................................................................................................................................................. 1
2
Applicable and Reference Documents ....................................................................................................... 2
2.1
2.2
Applicable Documents .............................................................................................................................. 2
Reference Documents ............................................................................................................................... 2
3
Technical requirements ............................................................................................................................... 4
4
Optical Design .............................................................................................................................................. 5
4.1
4.2
5
Lenses ....................................................................................................................................................... 5
Filters ........................................................................................................................................................ 5
Mechanical Design ....................................................................................................................................... 7
5.1
5.2
5.3
5.4
Detector .................................................................................................................................................... 7
CCD Mounting ......................................................................................................................................... 7
CHU Housing ........................................................................................................................................... 7
Baffle ........................................................................................................................................................ 7
6
Thermal Design .......................................................................................................................................... 11
7
Budgets ....................................................................................................................................................... 12
7.1
7.2
Mass Budget ........................................................................................................................................... 12
Power Budget ......................................................................................................................................... 12
8
Assembly Integration and Test ................................................................................................................. 13
9
Interface Control Documents ................................................................................................................... 13
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Field Monitor Definition Report
MONS/AUS/PL/RP/0002(1)
Issue No: 1
Date: 11 April 2001
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1 Scope
This document is the MONS Field Monitor Sub-System design report. It describes the current
state of the design evolution carried out under WPD 4110 during the System Definition
Phase. The design specifications are based on the MONS FM Requirements document
(RD5).
The work package is managed by Auspace Limited and performed in conjunction with the
University of Sydney.
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2 Applicable and Reference Documents
2.1 Applicable Documents
AD1.
gen/dssp/man/pln/0009(1)
DSRI Project Management Plan for System Definition
Phase of Next Danish Satellite Mission.
2.2 Reference Documents
RD1.
roemer/dssp/mis/tn/0002(3)
RD2.
roemer/dssp/sci/rs/0003(1)
RD3.
OMC – Integral: OMC/INT/20000/ICD/001
RD4.
OMC Optical System Mechanical ICD: OM-CSL-31000-ICD-001
RD5.
MONS Field Monitor Requirements: MONS/MFM/SPEC/2001/001
RD6.
Terma A/S - MFE ICD 255505 ZD
RØMER Technical Description.
RØMER Science Mission Specification
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Abbreviations and Acronyms
AIT
Assembly, Integration and Test
CCD
Charged Couple Device (Detector)
CHU
Camera Head Unit
DSRI
Danish Space Research Institute
FOV
Field of View
ID
Inner Diameter
MONS
Measuring Oscillations in Nearby Stars
MONS FM
MONS Field Monitor
OD
Outer Diameter
OMC
Optical Monitoring Camera
PCB
Printed Circuit Board
PSF
Point Spread Function
REO
Read-out Electronics
RD
Reference Document
TBC
To Be Confirmed
TBD
To Be Determined
TERMA
TERMA Elektronik AS Space Division
WPD
Work package Description
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3 Technical requirements
The design specifications are based on the MONS FM Requirements document (RD5).
The MONS Field Monitor is needed because of the possibility of light from neighbouring
stars, particularly variables, entering the field stop of the MONS Telescope and affecting the
photometry. Although ground-based observations can provide a snapshot of the field of the
target stars, they will not (except through unrealistic efforts) be able to detect faint, largeamplitude variable stars that would confuse the detection of the very low amplitude
oscillations in the target stars. Detailed simulations have confirmed the need for on-board
monitoring of these stars. Given the importance of this issue, it was decided to include an
extra camera on the satellite, the MONS Field Monitor, which will obtain in-focus
observations of the field simultaneously with the MONS Telescope. The Field Monitor could
essentially be a copy of the Star Trackers, but with a longer focal length lens to give a field of
view of at least 1 degree. The need for intensive ground-based checkouts before launch is
therefore greatly reduced.
There are two further objectives of the MONS FM. The first is to obtain additional data for
the primary science mission by observing brightness oscillation in the target star, thereby
acting as a back-up if the main telescope fails. To achieve this, the MONS FM should be
filtered since the target star is bright. The filter should be blue because the amplitude of
oscillation is greater in the blue than in other visible parts of the spectrum.
The second objective is to do parallel science by detecting oscillations and other types of
variability in neighbouring stars in the field of view. Given that these stars will be much
dimmer than the star being observed by the MONS Telescope, increased sensitivity is
required.
There are two ways to deal with this issue of dynamic range. The first is to use exposure
bracketing, with the first bracket will consist of a short exposure of the CCD area containing
the target star. The exposures should be short to prevent detector saturation. In the second
bracket long exposures and read-out of the whole field are taken. The long exposures are
necessary to allow sufficient photons to be collected from the fainter stars, but then the main
star will saturate on the detector. While the region of the CCD illuminated by the main star in
this bracket may be ignored in the read-out, it is expected that some charge bleeding between
neighbouring pixels will occur. The extent of this effect should be determined. If effect of
bleeding is too great, the bracketing procedure may not be a suitable solution.
The second approach is to add a filter on or above the centre of the CCD that will act to dim
the target star brightness by a factor of about 10. It is proposed that both approaches be used.
3.1 Integral OMC
An offer has been made to provide the Integral Optical Monitoring Camera (OMC) for use as
the MONS FM (RD3, RD4). The OMC consists of a high quality optical system, a baffle and
lid/sun-shield assembly, and a CCD. The OMC has a 5° FOV, and is designed for use with
the same CCD as TERMA can provide.
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The Baffle assembly of the OMC is considered to be too heavy, and a read out computer will
not be supplied. On the other hand, the optics are considered to be very good for the purposes
of the MONS FM, and may be used even if the other components of the OMC are not. The
OMC provides a field of view of 5 degrees, much larger than the minimum 1-degree required
by the MONS FM. The baseline is therefore to use the lens barrel from the Integral OMC for
the MONS FM. A baffle, CCD and the Read Out Electronics will need to be obtained from
other sources Also, a spacer connecting the lens assembly and the CCD will be required.
4 Optical Design
4.1 Lenses
The baseline is to use the lens barrel from the Integral OMC. Its properties are summarised in
Table 1. The two OMC filters and possibly also the radiation-resistant glass shield would be
discarded.
Optical system:
The Integral OMC optics
Field of view:
5 deg x 5 deg
Aperture:
50 mm diameter
Focal length:
153.7 mm (f/3.1)
Optical throughput:
> 70 % at 550 nm
PSF:
> 70 % of energy within 1 pixel
Angular pixel size:
17.6 x 17.6 arcsec
Table 1. Parameters of the Integral OMC optics.
4.2 Filters
A filter is needed to mask the centre of the CCD, to reduce transmission of light from target
star to avoid saturation. The requirement on the filter bandpass is 400 to 435 nm, although
can be modified if needed (RD5).
There is no constraint on the mask shape or thickness, but its area should be sufficient to dim
the target star and as few as possible neighbouring stars. The requirement is for a diameter of
40 arcmin (1.8 mm), although this can be increased by a factor of two or perhaps more if
needed (RD5).
There are several design suggestions for the design of the filter at this stage. The filter could
be:
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1. A spot of interference coating on the centre of clear glass, and placed over the CCD.
Because coloured glass would not be used, there would be leakage (transmission outside
the required bandpass).
2. A small piece of filter attached to the surface of a supportive piece of glass. The effects
of, and problems with, any adhesives that may be used need to be considered
3. A cylindrical (or rectangular) plug inserted into a hole cut into a supportive glass sheet.
This solution is good in that the adhesives and glass edges can be placed away from the
image area of the target star, and with the edges parallel to the ray path, deviation of the
rays will be minimised. Some ghosting is then to be expected on the area of the CCD
below the edges of the plug.
4. A cylinder or other shape wedged between two supportive sheets of glass. A score or
recess could be put into the glass sheets to hold the filter in place.
There are concerns about the practicality of manufacturing and using the filter required for
options 2 and 3. This needs to be studied in the Detailed Design Phase. One immediate
solution is to increase the diameter of the filter. The only trade off with doing so is the loss of
parallel science, but it is considered that having the filter up to one-ninth of the total CCD
area will still leave sufficient detector area for parallel science. There is a side-effect gain in
that tolerances on aligning the MONS FM with the MONS Telescope would be relaxed.
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5 Mechanical Design
Figures 5.1 to 5.4 show the proposed MONS FM design concept.
5.1 Detector
The baseline is to use a CCD provided by TERMA, which will be the same as used in the Star
Trackers and MONS Telescope. It is assumed that the CCD is bonded to a plate of copper or
other high conductivity material, attached to the PCB.
5.2 CCD Mounting
The TERMA Star Tracker PCB has 3 holes around the CCD area (RD6) which align with
three holes in the Star Tracker housing and it is assumed that these are used to accurately
position the CCD relative to the focal plane. The CCD will be required to be positioned and
stable in the image plane to ±15μm. A similar approach is proposed for the MONS FM. A
spacer approximately 50mm in height will be provided to position the detector relative to the
lens assembly. This spacer should not obstruct the field of view
5.3 CHU Housing
The TERMA Star Tracker CCD housing is designed for a shorter focal length lens assembly
than the Integral OMC one. It is proposed that, rather than try to adapt the TERMA Star
Tracker housing, a new one is designed to provide both the MONS FM to platform mounting
function as well as housing the CCD and optimally interfacing the CCD to the lens assembly
and baffle.
5.4 Baffle
A baffle will be required to reduce stray light entering the MONS FM. The baffle length may
be up to that of the MONS telescope. Since the OMC optics have a 5° FOV, the baffle will
need an inner diameter of up to 78mm at the opening, or 55x55mm, if square. Neither a sun
shield nor a cutaway design is considered necessary. A cover for cleanliness control should
be removed just before launch.
A baffle with square cross-section is proposed because this matches the detector shape. The
internal fins should be black painted to reduce stray light reflections. The overall length
should be less than about 450 mm to allow the entire MONS FM assembly to fit within an
envelope of 552 mm. The baffle may be shortened to a minimum length that excludes light
from Earth incident at 30°. This shortening will then place the opening of the MONS FM
below inside the walls of the spacecraft, reducing the chance of sunlight entering the baffle.
With the concept shown in Figure 5.1 the overall length of the MONS FM is 376mm.
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Figure 5.1 MONS Field Monitor Design Concept
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Figure 5.2 MONS FM –Sectioned View
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Figure 5.3 Mons FM - Top Isometric View
Figure 5.4 MONS FM – Bottom Isometric View
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6 Thermal Design
The proposed thermal interface from a small, dedicated, radiator to the MONS FM is direct to
the CHU mounting flange via a simple aluminium bracket. If the CCD is closely coupled to
the CHU housing, thermally, then the CCD temperature will be close to the radiator
temperature
Heat will be lost radiatively both from the MONS FM radiator and the baffle aperture.
The temperature of the MFM CCD can be controlled to –10oC by correctly sizing both the
radiator and thermal stand-offs between the platform Mounting Panel and the CHU housing
mounting flange.
Assuming the following parameters, a steady state radiator temperature of approximately
–10oC will be reached.

Platform Mounting Panel temperature
20 oC max

Radiator area
0.005m2

Baffle aperture area
0.003m2

Radiator & Baffle emittance
0.9

CHU Housing to Mounting Panel Conductance
0.165 W/oC

CHU electronics power dissipation
0W
If then the Mounting Panel temperature were to drop to 0oC, for example the steady state
radiator and CCD temperatures would drop to about -24oC. If this is too cold then a heater
could be provided, with an input of about 3.3W, to achieve the -10oC requirement.
Clearly, the various parameters can be optimised in the Detail Design Phase.
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Date: 11 April 2001
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7 Budgets
7.1 Mass Budget
ITEM
Mass (kg)
Lens Barrel
1.46
CHU PCB Assembly
0.30
CCD Support Plate
0.07
CCD Locator
0.11
CHU Housing
0.93
Baffle
0.47
Baffle Support
0.34
Sub total
25% margin
3.68
0.92
Total
Table 5.1 Mass Breakdown
7.2 Power Budget
0.3 W normal , 4 W peak
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Date: 11 April 2001
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8 Assembly Integration and Test
Key issues to be addressed in the Detailed Design Phase include:

Model philosophy.

Procurement philosophy.

Cleanliness control.

Optical, mechanical and thermal testing

AIT documentation.
9 Interface Control Documents
Mechanical and Electrical ICDs will be developed in the Detail Design Phase:
For the purposes of the System Definition Phase, the overall envelope and foot print is
indicated in Figure 5.1.
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