The ACIS Bake-out has been indefinitely postponed.

See Update on ACIS Bakeout for more information.

Notes: We already have enough CTI data in hand to measure the total optical depth at 670 eV to 5% in 1 arcminute strips on the ACIS chips, so CTI data in addition to the nominal operating procedures is not required before the ACIS bake-out. However, the optical depth at the O-K edge is not as well determined. With a 50 ksec observation of PKS2155-304, we can determine the optical depth at the O K-edge to 10%. At present, the total optical depth of the contaminant on ACIS-S decreases by 40% between chipy=1 and chipy=512, and then increases by 10% between chipy=512 and chipy=1024. Thus, to measure the chipy dependence of the optical depth at the O K-edge to better than 10% requires five 50 ksec observations of PKS2155-304 at different SIM_Z offsets. Abell 1795 has been observed twice in the past and is a good continuum source to measure the spatial variation of the contaminant on ACIS-S and ACIS-I in the 0.5 - 1.0 keV band.

Note added on May 7, 2004: The above set of 5 ACIS observations of Abell 1795 were completed in Jan. 2004. These observations will not be repeated until after the ACIS bake-out.

Notes: The primary objectives of the observations taken during the first orbit after bake-out are to get a quick estimate on the amount of material removed and determine if there are significant gain changes induced by an increase in CTI. The count rate in the L-complex of the ECS on S3 after bake out will be somewhere between 0.070 cts/s (0% removed) and 0.12 cts/s (100% removed). Based on the observed scatter in the K-L ratios measured in previous CTI observations, we expect a 1 sigma uncertainty of approximately 5% in the total detected counts within the L-complex in a 30 ksec CTI observation. We will thus be able to measure the depth of the contaminant after bake-out with a 1 sigma error corresponding to 10% of its depth before bake-out. Each E0102-72 observation is 8 ksec long. The centroid of the OVIII Ka line can be determined to 0.2% in an 8 ksec observation near the middle of I3. Thus, these observations will be able to measure a 1% gain degradation at 5 sigma.

One of the greatest uncertainties regarding the bake-out process, is wether some of the contaminant will condense back onto ACIS, and the timescale of such a process. We do not want to start mapping the contamination until most of the recently baked-out material has settled back onto ACIS and the rate of increase is back to the ambient level before bake-out. We therefore delay the PKS2155, Abell 1795, and long CTI measurements until orbits 2-5.

Assuming the spatial variations in the contamination after bake-out are similar to those before bake-out, i.e., 10% variations on scales of 1 arcminute, our goal is to measure the optical depth of the contamination to 5% on scales of 1 arcminute. To achieve this accuracy, we need 400 ksec of CTI data. The above plan includes 400 ksec on S2 and S3, 335 ksec on I3, and lesser amounts on the remaining chips. The long CTI exposures during orbits 2 and 5 will measure the optical depth with an accuracy of 3% on the I3 and S3 chips, so these observations will determine if the contamination is increasing by more than a few percent over the timescale of a week.

All PKS2155 and Abell 1795 observations are taken with the same set-up before and after bake-out. In case the contamination is still increasing after orbit 5, there will be a one week period of HRC-I and LETG/HRC-S observations. The standard CTI measurements will be taken during this time. GO observations with ACIS will then resume around orbit 9.

Notes added on May 7, 2004: The after bake-out calibration plan above now contains a "priority" column. The calibration observations are assigned either a "M" for mandatory or a "F" for flexible. The mandatory observations include all CTI measurements, the E0102 raster scan to measure the low energy gain of the CCDs, the LETG/ACIS-S observation of PKS2155-304 at the nominal gratings position, and the Abell 1795 observations at the ACIS-I and ACIS-S aimpoints. This suite of observations will determine if ACIS has experienced a significant increase in CTI, the total optical depth and approximate spatial varition of the contaminant, and the depth of the C, O, and F edges at the nominal gratings position.

The observations assiged an "F" may be postponed to a later time if the initial CTI and E0102 observations show that the bake-out process was sucessful in removing most of the contaminant. If there is still significant contamination after bake-out, then it will be necessary to more accurately map out the total optical depth and the depth at the atomic edges with the full set of PKS2155-304 and Abell 1795 observations. Delaying the "F" observations will allow the GO observations to begin 245 ksec sooner. However, if several months pass before these observations are performed, then the comparison observations at the nominal pointings will have to be repeated, which will add 80 ksec to the calibration plan.

Related Memos: Composition of the Chandra ACIS Contaminant

Analysis of ACIS Data Affected by the Low Energy QE Degradation

Spatial Distribution of the Contaminant on ACIS

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