Update on ACIS Operations

1. Update on ACIS Operations • New Patch for ACIS Flight SW • Control of the ACIS FP temperature

2. Update to the ACIS Flight SW • ACIS flight SW has been updated three times during the mission in 2000, 2004, & 2008 • We are preparing the fourth patch for uplink in Spring 2010 • This new patch contains two new items: Fixes a bug in a subarray mode – the list of bad pixels and columns onboard is misinterpreted if the subarray starts in a row other than row 1 Prevents Back End Processor (BEP) from crashing when one of the Front End Processors (FEPs) is powered down - there have been several occurrences when radiation levels have been high, that ACIS was commanded by the stored command load to start the first science observation of the orbit, it started collecting the bias data, but then received a high radiation shutdown signal from the satellite, but ACIS waits for the bias maps to be collected before stopping the science run, but before the bias maps are collected a command is sent which powers down the FEPs, this caused the BEP to crash in the past

3. Control of the ACIS FP Temperature • Desired temperature -119.7 C, FI/BI CCDs go out of gain spec at -119.2C/-118.2 C • Mean T of science observations • ACIS DH heater turned off in April 2008 • SIM FA6 heater turned off in August 2009

4. Controlling the ACIS FP Temperature • Camera Body used to be held at -60 C, it now floats between -65 and -70 C FP is contained in the “Camera Body” DH heater is on the “Camera Body”

5. Exploded View of ACIS HW

6. Restrictions on Spacecraft Attitude • unfortunately there are no other heaters we can turn off which would help to keep the FP cold • must now consider imposing restrictions on the satellite’s orientation during observations (which is already done for several other systems on Chandra)

7. Impact of Current Restrictions on Spacecraft Pitch Angle • restrictions imposed in 2005 significantly increased the amount of time spent at tail-Sun attitudes, pitch>115 • unfortunately this is a hot attitude for the ACIS FP temperature

8. Chandra in the Shuttle Bay Before Launch • thermal properties of the back surfaces of SIM are changing with time

9. Chandra in the Shuttle Bay Before Launch • heating on the back surface of the SIM, heats the ACIS Camera Body • heating of the shade for the radiator, heats the radiator

10. Earth in the ACIS Radiator FOV • Earth in the ACIS radiator FOV has a significant impact on the FP temperature • The complex shape of the shade has been approximated by a simpler shape, we are working on a more realistic model • we recently included reflections off of the interior surfaces of the shades

11. ACIS FP Temperature vs. Earth Solid Angle Without Reflections Adams-Wolk (SAO)

12. ACIS FP Temperature vs. Earth Solid Angle With Reflections Adams-Wolk (SAO)

13. Future Strategies to Keep the ACIS FP Cold There are two reasons the ACIS FP temperature gets warm now: Earth in the ACIS radiator FOV Sun on the back surfaces of the SIM and the radiator shade Our only option now is to impose restrictions on the spacecraft attitude, but that will be difficult because:

14. Backup Material

15. Chandra’s Changing Orbit and Implications for the FP Temperature • Chandra’s orbit is changing such that in 2012 the inclination will be at its highest value in the mission and the perigee will be at it lowest • the solid angle of the Earth in the ACIS radiator FOV will also reach missions highs in 2012 • other mission constraints such as momentum management and thermal issues frequently conflict with keeping the ACIS FP cold during perigee passages Davis (Chandra FOT)

16. The FP Temperature during Recent Perigee Passages Adams-Wolk (SAO) Example of a good perigee passage science science cal cal perigee

17. The FP Temperature during Recent Perigee Passages Adams-Wolk (SAO) Example of a bad perigee passage science science cal cal perigee