MONS Field Monitor. System Definition Phase. Design Report

Size: px

Start display at page:

Download "MONS Field Monitor. System Definition Phase. Design Report"

Jeffery Blankenship
5 years ago
Views:

1 Field Monitor System Definition Phase Design Report _AUS_PL_RP_0002(1) Issue 1 11 April 2001 Prepared by Date11 April 2001 Chris Boshuizen and Leigh Pfitzner Checked by Date11 April 2001 Tim Bedding Approved by Date11 April 2001 Leigh Pfitzner Auspace Limited A.C.N PO Box 17 Mitchell ACT 2911 Australia. Telephone: (+61 2) Facsimile: (+61 2) admin@auspace.com.au

2 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. Distribution Copy Number Recipient Location Original Auspace files 1 L Pfitzner Auspace 2 T. Bedding USYD 3 F. Hansen DSRI (ROEMER Web site) Document Issue Status Revision Record Date Description of Change Document Change Number

3 Page: i Table of Contents 1 Scope Applicable and Reference Documents Applicable Documents Reference Documents Technical requirements Optical Design Lenses Filters Mechanical Design Detector CCD Mounting CHU Housing Baffle Thermal Design Budgets Mass Budget Power Budget Assembly Integration and Test Interface Control Documents...13

4 Page: 1 1 Scope This document is the 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 FM Requirements document (RD5). The work package is managed by Auspace Limited and performed in conjunction with the University of Sydney.

5 Page: 2 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) RØMER Technical Description. RD2. roemer/dssp/sci/rs/0003(1) RØMER Science Mission Specification RD3. RD4. RD5. RD6. OMC Integral: OMC/INT/20000/ICD/001 OMC Optical System Mechanical ICD: OM-CSL ICD-001 Field Monitor Requirements: /MFM/SPEC/2001/001 Terma A/S - MFE ICD ZD

6 Page: 3 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 Measuring Oscillations in Nearby Stars FM 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

7 Page: 4 3 Technical requirements The design specifications are based on the FM Requirements document (RD5). The Field Monitor is needed because of the possibility of light from neighbouring stars, particularly variables, entering the field stop of the 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 Field Monitor, which will obtain in-focus observations of the field simultaneously with the 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 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 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 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 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.

8 Page: 5 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 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 FM. The baseline is therefore to use the lens barrel from the Integral OMC for the 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: 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:

9 Page: 6 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 FM with the Telescope would be relaxed.

10 Page: 7 5 Mechanical Design Figures 5.1 to 5.4 show the proposed 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 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 ±15ìm. A similar approach is proposed for the 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 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 FM. The baffle length may be up to that of the 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 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 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 FM is 376mm.

11 Page: 8 Figure 5.1 Field Monitor Design Concept

12 Page: 9 Figure 5.2 FM Sectioned View

13 Page: 10 Figure 5.3 Mons FM - Top Isometric View Figure 5.4 FM Bottom Isometric View

14 Page: 11 6 Thermal Design The proposed thermal interface from a small, dedicated, radiator to the 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 FM radiator and the baffle aperture. The temperature of the MFM CCD can be controlled to 10 o C 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 10 o C will be reached. Platform Mounting Panel temperature 20 o C max Radiator area 0.005m2 Baffle aperture area 0.003m2 Radiator & Baffle emittance 0.9 CHU Housing to Mounting Panel Conductance W/ o C CHU electronics power dissipation 0 W If then the Mounting Panel temperature were to drop to 0 o C, for example the steady state radiator and CCD temperatures would drop to about -24 o C. If this is too cold then a heater could be provided, with an input of about 3.3W, to achieve the -10 o C requirement. Clearly, the various parameters can be optimised in the Detail Design Phase.

15 Page: 12 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 % margin 0.92 Total 4.60 Table 5.1 Mass Breakdown 7.2 Power Budget 0.3 W normal, 4 W peak

16 Page: 13 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.

MONS Telescope Optical and Mechanical Sub-System System Definition Phase. Design Report

MONS Telescope Optical and Mechanical Sub-System System Definition Phase. Design Report Telescope Optical and Mechanical Sub-System System Definition Phase Design Report _AUS_PL_RP_0001(2) 17 April 2001 Prepared by Date 17 April 2001 M.L. Pfitzner Checked by Date 17 April 2001 T. Bedding