Important Updates

• New ACIS contaminant model, updated 2010.

Documents and Tools for Proposers

• Please see the Proposers page for detailed information on how to submit a proposal.
• The Proposers Observatory Guide (POG) is the primary ACIS reference manual. Basic observing principles can be reviewed in Section 6.2 of the POG.
• ProVis is the Chandra target visibility tool. It gives target visibility, roll and pitch angle as a function of time. Pitch angle can be important for a time constrained observation (see notes on time constrained observations and POG chapter on visibility and constraints)
• PIMMS is the Chandra count rate simulator
• ObsVis allows Chandra instrument fields-of-view to be overlaid on sky images.

ACIS Observing Strategies

Before submitting a proposal, ACIS proposers need to be aware of the following issues:

• Bright sources (additional information) might be affected by pileup. Pileup results when the count rate is so high that two or more photons are detected as a single event. PIMMS can be used to estimate pileup while pileup and pileup mitigation are discussed in Section 6.15 of the POG. The point source thread discusses pileup. A detailed description of pileup, how to mitigate it and how to deal with it is given in the Chandra ABC guide to pileup.

• 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.

• Observing Extended Sources (additional information) Background rates can be very important when estimating the exposure times for extended objects (see proposal thread on Groups and Clusters). The ACIS on-orbit background is composed of cosmic X-ray background, charged particles and the "trailing" of the target image during readout. Significant background flares can occur during an observation. Background issues are discussed in the POG.

• 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.

• Choice of ACIS chips (additional information). ACIS can read out a maximum of 6 chips. Proposers must select which chips are required to be ON, and also designate chips that are OPTIONAL (can be turned off by the Mission Planning team when thermal issues dictate). Read more about ACIS parameters and priorities on the RPS page. Also the document Specifying Optional CCDs for Cycle 14 and the Future describes in detail the thermal limitations and gives recommendation about operating and 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. Likewise observations at pitch angles larger than about 130 degrees, longer than about 50 ks, and with 6 CCDs operating are likely to approach or exceed the Digital Processing Assembly (DPA) and Detector Electronics Assembly (DEA) thermal limits. Each of these temperatures can be reduced by reducing the number of operating CCDs.

  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, DEA and DPA from overheating. The document Specifying Optional CCDs for Cycle 14 and the Future describes in detail the thermal limitations and gives recommendation about operating and optional CCDs.

  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 or larger than 130 degrees. Then the observation is likely to be split into two (or more) observations.

  If no optional CCDs are selected, and six CCDs are on 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 and/or less than 130 degrees. The user should be aware that such an observation may be listed in the long term schedule (LTS) at a date for which the pitch angle for this target is less than 60 degrees or larger than 130 degrees. However, when the observation finally appears in the short term schedule, it may be at a date for which the pitch angle will be greater than 60 degrees and/or less than 130 degrees, or may be segmented.

  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.

• Choice of Back vs. Front Illuminated Chips (additional information). The four ACIS-I chips offer the largest field of view and lowest background. However, the FI chips suffered radiation damage which degraded their spectral response. Because of this, many observers choose the S3 chip for high resolution studies where a large field of view is not required.

• 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.