Requirements for the Star Tracker Parallel Science Programme

Transcription

1 Requirements for the Star Tracker Parallel Science Programme Rømer System Definition Phase 2000/2001 Document no.: MONS/IFA/PL/RS/0003(1) Date: Prepared by: Hans Kjeldsen and Tim Bedding Checked by: Jørgen Christensen-Dalsgaard Authorized by: Jørgen Christensen-Dalsgaard Classification: Open: The document is unclassified and there are no restrictions in circulation.

2 Requirements for the Star Tracker Parallel Science Programme Teoretisk Astrofysik Center, Institut for Fysik og Astronomi, Aarhus Universitet This document may only be reproduced with permission of TAC/IFA, except within the Rømer project where any type of reproduction is allowed. MONS/IFA/PL/RS/0003(1) 2

3 DISTRIBUTION This document is for internal use by the Rømer project. All Rømer key persons will be able to access the document via the DSRI webpage: MONS/IFA/PL/RS/0003(1) 3

4 Contents 1. SCOPE 5 2. APPLICABLE DOCUMENTS 6 3. STAR TRACKER PARALLEL SCIENCE REQUIREMENTS Assumed detector properties Assumed read-out electronics Assumed optical system Required readout procedure for parallel science Output data produced Data volume and rate 12 MONS/IFA/PL/RS/0003(1) 4

5 1. SCOPE This document specifies the requirements for the Parallel Science Programme on the Rømer Star Trackers. The document has been prepared by the Theoretical Astrophysics Center and the Institute of Physics and Astronomy at Aarhus University as a contribution to the Rømer System Definition Phase (The Danish Small Satellite Programme). MONS/IFA/PL/RS/0003(1) 5

6 2. APPLICABLE DOCUMENTS AD1: Rømer Science Mission Specification MONS/IFA/MIS/RS/0001(1) AD2: MONS Payload Requirements Specification MONS/IFA/PL/RS/0001(2) AD3: MONS Field Monitor Requirements Specification and Parallel Science MONS/IFA/PL/RS/0002(1) AD4: MONS Payload Electronics Requirement Specification TERMA # DT AD5: MONS Field Monitor System Definition Phase Design Report MONS/AUS/PL/RP/0002(1) MONS/IFA/PL/RS/0003(1) 6

7 3. STAR TRACKER PARALLEL SCIENCE REQUIREMENTS The Attitude Control Subsystem on the Rømer platform will include two Star Trackers in order to provide the needed attitude information for calibrating the ACS. The ACS update frequency will be 1 mhz. In order to increase the science output of the Rømer mission, we are planning a parallel science programme aiming at surveying the field of view for variable stars. We also plan to collect integrated images of the whole field of view in order to allow more general programmes in cosmology, solar system work and stellar physics. The baseline for the Star Tracker parallel science programme is a standard TERMA Star Tracker, which consists of the following major parts: Optics Mounting interface to the spacecraft Baffle Radiator for cooling the detector CCD Detector Unit Read-out Electronics Unit Data Processing Unit (Rømer main computer - CDH) 3.1 Assumed detector properties CCD chip: Image: Image Area Size: Frame transfer Operation: Pixel Size: CCD Operating Temperature: CCD Temperature Stability: CCD Temperature Measurement Accuracy (Telemetry): Marconi Applied Technologies CCD Front Illuminated (AIMO CCD Sensor) 1024 x 1024 pixels 13.3 x 13.3 mm x 1024 pixels Storage area 13 um square -10 degc 5 degc (RMS) 1 degc (RMS) MONS/IFA/PL/RS/0003(1) 7

8 3.2 Assumed read-out electronics Saturation Level: e/pix A/D-Conversion: 8 bits or better Conversion factor: 35 e/adu (soft saturation: 8960 e) Readout noise: 20 e/pix Readout Frequency 1 MHz Integration Time: 25 msec - 2 sec programmable Accuracy of Integration time: 1 usec (RMS) Read-out Modes: Full frame and window read-out Read-out time: 1 usec/pix (1 MHz) Full frame: 1049 msec Vertical shift time: 6.0 usec (0.166 MHz) Horizontal shift time: 380 nsec/pix (2.6 MHz) Full frame: 398 msec Read-out per field: 7 x 13 x 13 pixels: 42 msec 28 x 13 x 13 pixels: 149 msec 64 x 13 x 13 pixels: 332 msec 630 x 630 pixels: 494 msec 1024 x 1024 pixels: 1049 msec 3.3 Assumed optical system Lens Aperture, D 24 mm Focal Length, f 35 mm f-ratio f/1.46 Image scale at detector: 0.61 mm/deg Field of view: 22 deg x 22 deg Pixel-Size: 77 arcsec/pix FWHM (star) 2.5 arcmin (2 pix) Transmission Efficiency: 50 % (no filter) incl. CCD: 20 % Optical Bandpass: 400 nm nm (no filter) No. of photons V=5 36,000 e/sec Saturation V=5: exp=1 sec Saturation 43 msec: V = msec: V = msec: V = msec: V = msec: V = 4.2 MONS/IFA/PL/RS/0003(1) 8

9 3.4 Required readout procedure for parallel science 5 sec procedure: Time: (msec) EXPOSURE: READ OUT: msec (STR1) 42 msec - 7 stars (43 msec exp) msec transfer msec (1) 332 msec - 64 stars (150 msec exp) msec transfer msec (2) 494 msec x 630 pix (332 msec exp) msec transfer msec (STR2) 149 msec - 28 stars (497 msec exp) msec transfer msec (3) 332 msec - 64 stars (150 msec exp) msec transfer msec (4) 494 msec x 630 pix (332 msec exp) msec transfer msec (STR3) 149 msec - 28 stars (497 msec exp) msec transfer msec (5) 332 msec - 64 stars (150 msec exp) msec transfer msec (6) 494 msec x 630 pix (332 msec exp) msec transfer msec (STR4) 149 msec - 28 stars (497 msec exp) msec transfer msec (7) 332 msec - 64 stars (150 msec exp) msec transfer msec (8) 42 msec 7 stars (336 msec exp) msec transfer x 43 msec (9-17) + 9 x 7 msec transfer 9 x 42 msec 9 x 7 stars (9 x 43 msec exp) msec (STR5) 42 msec - 7 stars (43 msec exp) msec transfer msec (18) 332 msec - 64 stars (150 msec exp) msec transfer msec (19) 42 msec 7 stars (336 msec exp) msec transfer x 43 msec (20-28) + 9 x 7 msec transfer 9 x 42 msec 9 x 7 stars (9 x 43 msec exp) MONS/IFA/PL/RS/0003(1) 9

10 3.5 Output data produced 5 sec. procedure: 5 x 150 msec: STR-frames: 64 stars (NO SCIENCE) 3 x 332 msec: 630 x 630 pix IMAGE: 235 stars (V > 3.8) 3 x 497 msec: 28 stars: (V > 4.3) 2 x 336 msec: 7 stars: (V > 3.8) 20 x 43 msec: 7 stars: (V > 1.6) 33 exposures Stellar classes: Magnitude limit Exposures: Number of stars: Class-I: V > x 43 msec 7 Class-II V > x x 332 msec 7 Class-III V > x x 332 msec 28 Class-IV V > x 332 msec 200 Photometric precision per minute for the star trackers used for parallel science. Curves for the four different stellar classes are show. Saturation for class I is at V=1.6, for class II and IV at V=3.8 and for class III at V=4.3. In order to estimate amplitudes for coherent oscillations that can be detected at S/N=4 using the Star Tracker, one should multiply the scatter by We will therefore be able to detect oscillations with amplitudes below 30 ppm for the brightest candidate stars. MONS/IFA/PL/RS/0003(1) 10

11 The following table shows the noise for stars of different magnitude observed using the four different exposure times. Stellar magnitude 20 x 43 msec 3 x 332 msec 2 x 336 msec 3 x 497 msec V = V = V = V = V = V = V = V = V = V = V = V = Based on this table we can estimate the noise per min for the four different types of exposure sequences (stellar classes). Stellar magnitude Class I (7 stars) Class II (7 stars) Class III (28 stars) Class IV (200 stars) V = V = V = V = V = V = V = V = V = V = V = V = Finally we may estimate the amplitude of modes that can be detected by the Star Tracker after 30 days of observing (duty cycle = 85 %). The table below indicate amplitudes that can be detected at S/N=4. Stellar magnitude Class I (7 stars) Class II (7 stars) Class III (28 stars) Class IV (200 stars) V = ppm V = ppm V = ppm V = ppm 33 ppm 43 ppm V = ppm 50 ppm V = ppm 36 ppm 61 ppm V = ppm 60 ppm 100 ppm V = ppm 130 ppm 240 ppm V = ppm 320 ppm 570 ppm V = % 780 ppm 0.14 % V = % 0.19 % 0.35 % V = % 0.48 % 0.85 % Milli-magnitude oscillations can be detected in stars down to magnitude V=8. MONS/IFA/PL/RS/0003(1) 11

12 3.5.1 Data volume and rate We will produce data for 242 stars per minute (each in 2 apertures), plus housekeeping (e.g., pitch, yaw, roll and background estimates). The total data rate will be 1050 bytes per minute. This means 636 kbytes per orbit and 1.27 Mbytes per 24 hr. + 25% margin: 1.6 Mbytes/24 hr. MONS/IFA/PL/RS/0003(1) 12