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Last modified: 14 December 2015


Proposing Imaging Observations with ACIS

Proposer Threads (Cycle 18)


ACIS at a Glance

These notes provide an overview of the issues to consider when planning a Chandra proposal that uses ACIS. For more comprehensive information, please refer to the references listed below.

The ACIS array consists of a total of 10 CCD chips each with an 8' x 8' field of view on the sky. The layout is shown in the POG Figure 6.2. ACIS-I (the imaging array) consists of a 2x2 array of Front Illuminated Chips and ACIS-S (the spectroscopy array) consists of a 1x6 array of 4 Front Illuminated Chips and 2 Back Illuminated chips. A maximum of 6 chips can be turned on at any given time.

Observers are encouraged to use 4 or fewer CCDs if their science objectives are not significantly affected by turning one or more CCDs off. If the observer's science objectives require 6 CCDs, the observer should set 5 CCDs to "Y" and the sixth CCD to "Off1" in RPS and if the proposal is accepted, the Oberver may work with their User Uplink Support Scientist to change the 6th CCD to "Y". But observers should be aware that 6 CCD observations will be increasingly more difficult to schedule as the mission progresses.

The Chandra Remote Proposal Submission (RPS) software requires proposers to specify which instrument is to be at the aimpoint for a given target. If ACIS-I is selected, the aimpoint is on chip 3. If ACIS-S is selected, the aimpoint is on chip 7. Proposers are also required to submit specific chip selections for each target. In order to prevent the ACIS Power Supply and Mechanism Controller (PSMC) from overheating, the mission planning team may turn off one or more of the chips that the proposer designates as "optional".

If it is necessary to turn chips off due to thermal reasons, the chip designated Off1 will be turned off first, followed by Off2-Off5. Observers should specify the chip set that is best for their primary science. The document "Selecting Required and Optional CCDs" lists and explains in detail recommended chips selections that have proven to be popular, and would facilitate a more useful and homogeneous archive

ACIS Background Issues

Estimates of ACIS background rates can be found in Section 6.16.2 of the POG.
  • Table 6.10 gives background rates for individual chips (in counts/sec/chip) for ACIS-I and ACIS-S at the aimpoint.
These numbers should be used for signal-to-noise calculations. They reflect the typical background count rate in an events file that has been screened to include only ASCA grade g02346 events (see POG Section 6.14 for description of event grades), and to exclude bad pixels and bad columns. A typical observation will have many more background counts in the "raw" data files (since some ASCA g157 events and some bad pixels/columns are included), but they will be removed in standard processing. The background count rate may be higher during a flare. Flares are discussed in Section 6.16.3 of the POG, "Background Variability".

The Chandra Remote Proposal Submission (RPS) software requires an estimate of the Total Field Count Rate. This is the sum of all sources in the field of view plus the TOTAL background count rate from all chips that are turned on. Table 6.11 should be used for the total background count rate when calculating the Total Field Count Rate. The numbers in Table 6.11 are much larger than in Table 6.10 because they reflect ALL background counts that are telemetered. Again, the numbers in Table 6.11 are for quiescent periods. Flares will increase the background count rate. The Total Field Count Rate is used to check that the telemetry stream is unlikely to saturate in a proposed observation.

Telemetry Format and Observing Mode

The Telemetry Format determines what information about each detected event is telemetered to the ground. ACIS Telemetry Formats are described in the POG Section 6.14.2. For ACIS, the choices are:
  • Faint (F) Format. Gives position, time, and total pulse height for each detected event, plus pixel values in a 3x3 region surrounding the event that characterizes the event grade.
  • Very Faint (VF) Format. Same as for Faint, but pixel values in a 5x5 region surrounding the event are telemetered.
  • Graded Format. Gives position, time, total pulse height, and grade of the event. Pixel values are not telemetered.
VF format is the choice for many imaging observations. VF format gives the maximum amount of information possible about each detected event. In addition, information in the 5x5 pixel region can be used to reject more background photons than is possible for other Telemetry Formats. This is very useful for reducing background in extended sources (see POG 6.16.2), such as clusters of galaxies, but is unsuitable for bright sources. This is because the telemetry saturation limit is lower in VF format than for other formats.

Proposers should be aware that telemetry can saturate in VF observations during a background flare. This possibility can be minimized by utilizing additional filtering on-board (for example, reducing the upper energy cutoff and/or raising the lower energy cutoff) and/or reducing the number of CCDs used in the observation. When applying an energy filter in VF mode, a lower energy of 0.1 and a range of 12.0 is strongly suggested for most observations.

In general, the optimal format is that which telemeters the maximum information without saturation.

Two Observing modes are available:
  • Timed Exposure (TE) mode. Here the array integrates photons for a fixed period of time (Frame Time) before being read out. The total sky exposure time closely equals the sum of all frames. The result is a two-dimensional image with each event tagged by position, energy etc, depending on the telemetry format chosen. In TE mode, the available Telemetry Formats are Very Faint, Faint, and Graded. It is possible to vary the Frame Time, for example to mitigate pileup (see POG Section 6.15.3).
  • Continuous Clocking (CC) mode. In CC mode, rows from the imaging array are continuously clocked into the framestore array, giving a 1x1024 pixel image. One spatial dimension is lost, but the timing resolution is considerably improved (3 msec). In CC mode, the available Telemetry Formats are Faint and Graded. CC mode should only be used for bright sources, and can be useful for mitigating pileup. It should also be noted that the effective background in CC mode is increased by a factor of ~1000!
The vast majority of observations are carried out in TE mode. CC mode is useful to mitigate pileup in bright sources and to obtain higher timing resolution. The High Resolution Camera may be the instrument of choice if timing is the primary observational goal.

Choice of Chips in Imaging Mode

The following points are relevant when deciding which chips to turn on:
  • If ACIS-I is selected, then the aimpoint is on the Front Illuminated (FI) chip I3. The FI chips exhibit degraded energy resolution near the aimpoint due to Charge Transfer Inefficiency (CTI). This effect can be mitigated somewhat in data processing. The ACIS-I array has a large (16x16 arcminute) field of view.
  • If ACIS-S is selected, then the aimpoint is on the Back Illuminated (BI) chip S3. The BI chips are more sensitive at lower energies, and have a more uniform spectral resolution as a function of position on the CCD. The field of view of the ACIS-S array is smaller, and the background in the S3 chip is higher than the FI chips.
  • It may be desirable to utilize more restrictive energy filtering than the default and/or to turn off a chip to avoid telemetry saturation in Very Faint mode.
  • A restricted region of the chip, or subarray, can be selected to reduce the frametime and provide a faster readout. This can help to avoid telemetry saturation and mitigate pileup.
  • The BI chip S1 can be used to monitor flares during an observation and for long-term analysis of background variations. Select to turn on this chip if it isn't detrimental to the science.
  • Non-standard chips may be selected, for example to optimally cover an extended object.

Last modified: 14 December 2015
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