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ACIS Observing Strategies

    Important Updates

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Split Obsids

    Due to the thermal budget of each orbit, some obsids are split into multiple obsids during the 32 days leading up to the observation. The policy is that when an approved observation is split, both ends are approved so long as they are being done in the same week or are pool targets. Observers will only be notified if both portions of the split do not end up in the same week and the target was not a pool target.

Optional CCDs

    In Cycle 8, the policy of selecting optional CCDs was implemented. Because of changes in the Chandra thermal environment, the ACIS Power Supply and Mechanism Controller (PSMC) has been steadily warming over the course of the mission. Under current thermal conditions and assuming an initial PSMC temperature of less than +30~C, observations at pitch angles less than 60 degrees which are longer than ~50 ks and which consume maximum power within ACIS (6 CCDs clocking) are likely to approach or exceed the Yellow High thermal limit for the PSMC.

    To counter this, all observers are asked to review their CCD selections and determine which, if any, of their CCDs can be turned off to prevent the PSMC from overheating.

    If no optional CCDs are selected, six CCDs are being clocked and the observation MUST be scheduled at a pitch angle less than 60 degrees, then the observation is likely to be split into two (or more) observations.

    If no optional CCDs are selected, six CCDs are being clocked and the observation is not constrained in such a way as to prohibit it, the observation is likely to be scheduled at a time for which the pitch angle is greater than 60 degrees. If the observation is assigned a time in the Long Term Schedule for which the pitch angle is smaller than 60 degrees, the observation may be rescheduled for a later date when the pitch angle is larger based on the detailed scheduling of the time slot for that observation.

    While these restrictions apply to all observations, those most likely to be affected are those for which the approved observing time is larger than 25 ks and at least five required CCDs. Note that even if your observation is very short, it may occur in a sequence of observations at bad pitch angles, and it is therefore important that you consider whether any chips are optional unless your target is at an ecliptic latitude greater than 60 degrees.

Energy Resolution Considerations

    Because of radiation damage to the FI devices, their effective energy resolution has degraded, and is strongly dependent on the distance of the event from the readout node. Because of the focal plane geometry, the detected energy resolution is worst near the aim point of the imaging array, where the mirror performance is best. The performance of the FI devices is considerably better at -120 C than at -110 C. The ACIS CCDs have been operated at -120 C since January 2000. In addition, the spectral resolution of the FI data can be improved by applying a CTI correction to the data. The CTI correction is applied by default in the standard processing of the FI CCD data.

    For a general discussion of the ACIS energy resolution the GO is advised to review the information on the ACIS Energy Resolution page.

    For further improvement in effective energy resolution, you may consider moving your target closer to the readout nodes on the imaging array by offset pointing (at the expense of spatial resolution). Targets can be imaged closer to the readout nodes on the S-array by a SIM-Z translation, without compromising image quality.

Observing Below 1keV

    It was discovered during Cycle 3 that the low-energy sensitivity of the ACIS instrument has been decreasing throughout the life of the mission. For more detail on this effect, please consult the ACIS QE Degradation web site. The decrease in efficiency is presumably due to a layer of contamination on the filters and/or CCDs. The GO should make use of the tools referenced on this web page to evaluate the impact of this decreased sensitivity on their science.

Observing Extended Sources

    If your target is spatially extended, and your science depends on the correct background subtraction, consider the following:
    (1) The back illuminated (BI) devices (S1 and S3) may be affected by the particle background flares. In the past, the BI background rate (after the standard event screening) was greater than twice the quiescent rate for up to 30% of the time. This flare fraction has declined with time, and is practically zero at present. We do not know if/when the flares will return. The front illuminated (FI) devices have always been little affected by flares. Note that a re-observation request will be considered only in the most extreme cases (if the background rate was greater than five times the quiescent rate for more than half of the exposure).
    (2) The quiescent background rate, after the standard event screening, is larger by a factor of 1.5-3 for the BI devices compared to the FI devices depending upon the bandpass of interest. The quiescent background rate has been increasing since 2004. It has now reached (for FI CCDs) or surpassed (for the BI CCDs) the rates observed at launch. For more details, check the ACIS Background Page.

    In choosing the appropriate device, consider the relative target count rates using the FI and BI chips (the latter have significantly higher low energy sensitivity), the relative background rates, the risk of flares, the spectrum of your source relative to the background spectrum, and the desired spectral resolution. If S3 will be the main chip and the target substantially fills that chip, consider using S1 for background flare detection.

Observing Bright X-ray Sources

    If your target is a point source, be sure you have estimated your count rate correctly. Take appropriate measures to avoid event pileup at levels that may compromise your scientific goals. Because of the Poisson nature of pileup, its effects can compromise your observations with even a small event flux per frame. For example, roughly 4% of your events will experience pileup for a count rate of 0.1 events per readout frame. Pileup can affect both the shape of the PSF and the observed spectral energy distribution of your source. See the calibration webpage for information regarding pileup and the Proposers' Guide Section 6.15 for steps that can be taken to mitigate the effects of pileup. The GO is also advised to become familiar with the SW which models the effect of pileup on the observed spectrum.

    High Telemetry Rates
    Beware of high telemetry rates. The limiting telemetry rate that avoids data loss (saturation) is a function of ACIS mode. Estimate the count rate of all sources in your field (using the PIMMS software, for example) and the potential increase in count rate due to background flares and the BI devices. If you do not need the BI device and you are approaching telemetry saturation, consider turning of the BI device(s). See Tables 6.7 and 6.10 in the December 2005 AXAF Proposers' Guide for telemetry saturation rates for the available ACIS modes.

    Optional Considerations If your observation does not use gratings, is greater than 30 ks, at a high Galactic latitude and free of large extended sources, it may be useful for background calibration and monitoring. Provided it does not interfere with your science goals, consider the following:
    1.Use Very Faint mode, telemetry capacity permitting. VF is a superset of Faint mode and all existing calibration products apply when you use default processing, which only considers the 3x3 island data (i.e., when you do not use the special tool to screen the 5x5 island to reduce background.
    2.If your main chip is S3, consider also using S1 to help background modeling.