Imaging Observations with ACIS

ACIS at a Glance

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.

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 seleted the aimpoint is on chip 7. Prior to Cycle 9, the RPS software offered the choice of "default" chip selections depending on which ACIS mode was selected. Proposers are now required to specify chip selections for each target. Proposers can select up to 6 chips, and in addition can designate up to 5 CCDs as "optional". When the observation is being scheduled, the mission planning team may turn off one or more of the optional chips to prevent the ACIS Power Supply and Mechanism Controller (PSMC) from overheating (see the Optional Chips Page)

Proposers can designate any combination of up to 6 chips as "on" and any combination of 5 chips as "optional". If it is necessary to turn chips off to prevent PSMC over-heating, the chip designated O1 will be turned off first, followed by O2-05. The following recommended chip selections are likely to be suitable for many science projects:

• ACIS-I Imaging: chips I0-I3 and S2 and S3 are on, with S3 designated as O1 and S2 as O2. In the event chips need to be turned off S3 will be the first chip to be switched off, followed by S2. The ACIS-I imaging chips will not be turned off.
• ACIS-S Imaging: S1-S4 plus chips I2 and I3 will be turned on, with I2 designated as O1 and I3 as O2. If the PSMC is in danger of overheating, I2 will be switched off first, followed by I3
• ACIS-S Spectroscopy: chips S0-S5 will be turned on, with S0 as first optional chip and S5 as second optional chip.

---

ACIS Background Issues

Estimates of ACIS background rates can be found in Section 6.16.2 of the POG.

• Table 6.9 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 6.10 should be used for the total background count rate when calculating the Total Field Count Rate. The numbers in Table 6.10 are much larger than in Table 6.9 because they reflect ALL background counts that are telemetered. Again, the numbers in Table 6.10 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.

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, data is constantly moved through the array to the framestore region, 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 background per pixel 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. 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.