Controlling the ACIS FP Temperature Turn Off the ACIS DH Heater Turn Off the SIM FA6 Heater

Chandra X-Ray Observatory CXC

ACIS Ops team September 6, 20071

Controlling the ACIS FP Controlling the ACIS FP Temperature Temperature

I.I. Turn Off the ACIS DH HeaterTurn Off the ACIS DH HeaterII.II. Turn Off the SIM FA6 HeaterTurn Off the SIM FA6 Heater

ACIS Ops TeamACIS Ops Team



ACIS FP Temperature over the MissionACIS FP Temperature over the MissionBissell (NGST), 8 hr averages of FP temperature, 30 day running average



ACIS FP Temperature over the MissionACIS FP Temperature over the Mission

Grant

(MIT)



ACIS FP Temperature over the MissionACIS FP Temperature over the Mission

Grant

(MIT)



Average ACIS FP Temperature vs. Pitch Angle 2005Average ACIS FP Temperature vs. Pitch Angle 2005

DePasquale

(SAO)

Filtering out times with Earth in the radiator FOV and perigee passages

Peak around a pitch angle of 100 deg is affected by some calibration observations at warmer temperatures which have not been completely filtered out



Average ACIS FP Temperature vs. Pitch Angle 2006Average ACIS FP Temperature vs. Pitch Angle 2006

DePasquale

(SAO)

Filtering out times with Earth in the radiator FOV and perigee passages



ACIS FP Temp vs Pitch Angle, DH Heater Power, Earth Solid AngleACIS FP Temp vs Pitch Angle, DH Heater Power, Earth Solid Angle

DePasquale

(SAO)



ACIS DH Temperature over the Course of the MissionACIS DH Temperature over the Course of the Mission

DePasquale

(SAO)

Side A has warmed to -59.9 C later in the mission



Possible Responses to a Warmer FP TemperaturePossible Responses to a Warmer FP Temperature

1) Turn off the ACIS DH heater and the SIM FA6 heater

PRO: maintains calibration and best performance of the CCDs

CON: DH temperature is unregulated

2) Impose new MP constraints on duration of time spent at pitch angles >120

PRO: stable DH temperature for Fid Lights

CON: adds yet another constraint to an already tightly constrained schedule

3) Operate at a higher FP temperature

PRO: easy for operations and MP

CON: degrades CCD performance, requires a lengthy recalibration

Option #1 is the preferred option

NOTE ** ACIS FP temperature set point will remain at -119.7 C **

Today we only consider turning off the ACIS DH heater (SIM FA6 to be discussed at a future meeting)



Thermal Modeling with the ACIS DH and SIM FA6 OffThermal Modeling with the ACIS DH and SIM FA6 Off

Analysis by Joe Vogrin (NGST) and Neil Tice (LMA):

“Chandra ISIM Thermal Study to Lower ACIS Focal Plane Temperature”

- the ACIS and SIM temperatures were modeled for a range of pitch angles

between 45 and 172 in 10 degree steps

- study indicates that the ACIS DH temperature side A/B will range from

-71/-69 C at pitch=45 to -67/-66 C at pitch=145 (current temperatures are

-62.4/-59.9 C)

- steady-state solution, these are equilibrium temperatures, whether they will be reached on-orbit depends on the time constants (which can be long) and the duration of time the heaters are off

- study indicates that the FP temperature will be up to 3.0 degrees colder at a pitch angle of 145. This is a large enough difference to significantly improve the performance of the CCDs.

- study indicates that ~2.5 degrees of the improvement comes from a lower DH temperature



Rationale for Regulating the ACIS DH Temperature Rationale for Regulating the ACIS DH Temperature

1) Provides a stable thermal environment for volume enclosing the FP

2) Provides a stable surface on which the Fid Lights are mounted

-- Slow fluctuations in the DH temperature from -60 C to -70 C will not cause significant fluctuations in the ACIS FP temperature

-- The contraction/expansion of the ACIS DH will produce small but measurable changes in the positions of the Fid Lights which we will need to account for (more on this later from Tom Aldcroft)















Risks of Turning Off the ACIS DH Heater

``MIT Assessment of Turning DA Heater Off’’ from Bill Mayer (MIT)

1) FP gets too cold - unlikely, requires 2 failures, in flight the FP has been as cold as -123.0 C, could reach -127.0 C with DH heater off, engineering unit tested to -148 C

2) Focus changes as DH contracts/expands - expected motion of 7 um is small compared to 80 um depth of focus (motion is ~25 um if both the DH heater and the SIM FA6 heater are turned off)

3) Interface connectors get too cold - connectors were tested down to -72 C

4) DH heater power surge upon turnon - not a concern, sufficient capacity in PSMC

5) Reliability of turning DH heater on and off - no relays are used

MIT and PSU teams believe there is negligible risk in turning off the DH heater

6) Accumulation rate of contaminant increases to unacceptable levels - change in temperature from -60 C to -70 C is unlikely to make a significant difference



Ground Qualification Tests of ACIS DH

``NGST IOC: Chandra ISIM TV Test Temperatures and Associated Data in Support of Decreasing ACIS FP Temperature’’ from Joe Vogrin (NGST)

Date Test Description

Time on Primary Survival Heaters

at ~73.1ºC as Measured by

1CBAT (Hours)

Time on Red. Survival Heaters

at ~75.6ºC as Measured by

1CBAT (Hours)

Total Time at Survival

Temperatures (Hours)

6/08/98 Observatory TV Test 13 3 16 6/09/98 Same as above 23.5 0.5 24 6/10/98 Same as above 2 0 2 8/17/98 ISIM TV Re-test 17 6 23 8/18/98 ISIM TV Re-test 24 0 24 8/19/98 ISIM TV Re-test 24 0 24 8/20/98 ISIM TV Re-test 24 0 24 8/21/98 ISIM TV Re-test 24 0 24 8/22/98 ISIM TV Re-test 20 0 20 Tota l Time 171.5 9.5 181



ACIS Detector Housing Survival Heater Geometry

•HR7, HR8: 22 W @ 29 V• Prime survival -71.1 °C• Rdnt survival -73.6 °C

•Better than 1.19 °C accuracy (@ -70 °C ) for 1YA94 SSTS that controls survival heater

•Located on +Z face of Detector Housing (side A, 1CBAT)

•Closest telemetry thermistor is 1CBAT (FM07)

Heater description

Pr or Red

Htr Bus

Enable Cmd

Disable Cmd

Heater Flight Telemetry Mnemonic

Heater Power at 29 volts (watts)

Heater Resistance (ohms)

Heater Close Set Point Temp (°C)

Heater Open Set Point Temp (°C)

Primary Heater Monitor Thermistor Mnemonic

Backup Heater Monitor Thermistor Mnemonic

ACIS Det. Hsg. Survival

PR 2 NA NA NA 22 38.2 -71.1 -69.8 1CBAT 1CBBT

ACIS Det. Hsg. Survival

RD 7 NA NA NA 22 38.2 -73.6 -72.5 1CBAT 1CBBT

Trinh(NGST)



Risks to the Fid Lights (Tom Aldcroft)



Implementation: DH Cooling RateDePasquale

(SAO)

Beginning of life, forward-Sun, it would take ~40 ks to go from -60 C to -70 C

Vogrin predicts -71/-69 C for asymptotic value at this point in the mission for forward Sun and -67/-66 C for tail Sun

1CB

AT



Implementation: Commanding-- very simple, one ACIS HW command to turn off the heater ``1HHTRBOF’’

-- two ACIS HW commands to turn the heater on ``1HHTRBEN’’ and ``1HHTRBON’’

Telemetry Verification

1) Heater state is reported by ``On/Off’’ (1DAHTBON) and ``Enb/Dis” (1DAHTBEN)

2) Heater current (1DAHBCU) and voltage (1DAHBVO) also reported, both will trip Yellow Low limits (0.20 A for 1DAHBCU and 3.50 V for 1DAHBVO)

3) Camera Body temperature currently is 1CBAT/1CBBT = -62.4/-59.9 C, digitization is ~2.47 C, may trip Yellow Low limit of -70.5 C, Red Lower Limit is -75.5 C

-59.9 C

-62.4 C

-64.8 C

-67.3 C

-69.8 C

-72.2 C

-74.7 C

1CBAT/1CBBT would need to be monitored during any test, and the heater turned back on if the temperature reaches -72.2 C

1CB[AB]T



Three Proposed Tests

1) Turn ACIS DH heater off and on during a realtime COM during a perigee passage, use a CAP for this test, verify that heater responds as expected, (as early as SEP1007 week)

2) Turn off the ACIS DH heater during a perigee passage, turn off as soon as last science observation ends and turn on 2 hr before first outbound science observation starts, execute in the stored command load (55-60 ks to cool), (as early as SEP1707 week)

3) Turn off the ACIS DH heater during an 80 ks observation (split into a 47 ks and 45 ks piece) of the CDF-South, (as early as SEP1707 week)

-- select one of the two CDF-S observations and allow the DH to cool for 35 ks and turn the heater back on 10 ks before the observation ends

-- Tom will analyze the results and attempt to reconstruct the aspect

-- commanded in the stored command load, monitored during realtime COMs

-- have CAP ready to execute SOP_61014_DAHTR_B_ON to turn the heater back on if temperature gets too low

-- pitch angle is expected to be ~120 +/- 3 degrees, other CDF-S observations in coming weeks will have higher values of pitch and should have longer exposure times

ALL tests scheduled for tail-Sun orientations in which 1CB[AB]T should not drop below -67/-66 C



Implementation: Future Plans

-- If the test during the CDF-S test is successful, success criteria:

-- If the verification test and the test during a perigee passage are successful, we would like to turn off the DH heater for all perigee passages, success criteria:

1) Schedule additional tests during remaining CDF-South observations

2) Proceed with implementation of DS SW to reconstruct aspect with a variable DH temperature

3) Schedule tests at different pitch angles and work gradually to more forward-Sun attitudes to characterize cooling rates in different attitudes

4) Return to the FDB to ask to turn off the DH heater for good once the SW is available

-- Continue to investigate the option of turning off the SIM FA6 heater to gain more control on the ACIS FP temperature

Verification test - receive expected TLM verifiers that heater turns off and on

Perigee test - DH cools below -60 C but not down to -72 C & warms to -60 C in 2 hr

Absolute astrometry - 0.4 arcsec error

Image reconstruction - demonstrate that Enclosed Counts Fractions are statisically identical to previous observations

Documents

Controlling the ACIS FP Temperature Turn Off the ACIS DH Heater Turn Off the SIM FA6 Heater