Date: Mon, 30 Nov 1998 13:23:39 -0800 (PST) From: Ted Tarbell To: mdiers@mdisas.nascom.nasa.gov, ecs@soc.nascom.nasa.gov Subject: MDI Jitter Test Results SOHO Pointing Stability as Measured by MDI MDI measured the SOHO pointing using its high-speed jitter mode during a very quiescent period on 12-Nov-98, 21:33 - 21:44. In this time period, the rms pointing jitter seen by MDI was 0.042 and 0.041 in the X and Y axes respectively. The MDI ISS X-Axis = MDI yaw = rotation about the S/C Yb axis = E-W on the Sun, I think; the MDI ISS Y-Axis = MDI pitch = rotation about the S/C Zb axis = N/S on the Sun. Many reaction wheel resonances were detectable, all at very low levels similar to those seen earlier in the mission; these are given in the table below. Overall, the pointing stability continues to be excellent. On 23 October, 22:38 to 23:20, MDI happened to collect jitter data during some roll maneuvers by UVCS. When UVCS was not moving, the quiescent pointing stability was 0.050 and 0.043 arcsec in X & Y, similar to that of 12 Nov. During the UVCS roll, these increased to 0.054 and 0.053 arcsec, with most of the added motion at the 10 Hz stepping rate. When the MDI image stabilization is on, these were reduced to about 0.01 arcsec. The transients, which last for a few seconds at the start and end of the maneuver, had peak deviations of about 1/3 of an arcsecond from the average pointing, without image stabilization; they are less than 0.03 arcsec with stabilization on. Thus, the jitter caused by UVCS roll is very small and should not be a problem for the ACS or for any instruments. The web page summaries of jitter test results have been (or soon will be) updated with results of the last three tests: see http://www.lmsal.com/MDI/cal/jitter.html. Reaction Wheel Resonances on 12-Nov-98 Frequency X-Amplitude Y-Amplitude Harmonic* Cause** Hz milli-arcsec milli-arcsec (rms) (rms) 21.38 < 0.4 0.6 RW1 RU 33.54 2.5 2.6 RW3 RU 39.55 0.8 < 0.4 RW2 x 2 BE 42.67 2.1*** 2.9*** RW1 x 2 BE 57.10 < 0.4 1.0 RW2 x 2.89 BORR 61.59 1.8 1.2 RW1 x 2.89 BORR 77.01 < 0.4 1.5 RW2 x 3.89 BORR 79.18 < 0.4 2.3 RW2 x 3.99 BE? 82.91 1.9 5.6 RW1 x 1.89 BORA 85.33 3.2 10.4 RW1 x 4.00 BE? 96.76 1.2 2.0 RW3 x 2.89 BORR 98.85 1.4 2.1 RW3 x 2.95 BORR 104.25 1.4 1.6 RW1 x 4.89 BORR 106.66 0.8 0.8 RW1 x 5.00 ? 120.85 1.8 4.0 RW2 x 6.10 BIR 124.26 1.0 1.2 RW1 x 5.83 BORA 130.21 < 0.4 4.3 RW1 x 6.11 BIR * The FOT provided reaction wheel speeds at the time of the test as follows: RW1 = 1280.0 RPM = 21.33 Hz; RW2 = 1189.3 RPM = 19.82 Hz; RW3 = 2010.7 RPM = 33.51 Hz. ** The causes are taken from an MMS listing and have been abbreviated as follows: BE Bearing Eccentricity BIR Bearing Inner Race BORA Bearing Outer Race, Axial BORR Bearing Outer Race, Radial RU Rotor Unbalance *** The 42 Hz amplitude is somewhat variable in time; the others are steady.