Important Updates

Documents and Tools for Proposers

• Please see the Proposer's page for detailed information on how to submit a proposal.
• The Proposers Observatory Guide (POG) is the primary HRC reference manual. Basic HRC principles can be reviewed in Section 7.2 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.

HRC Observing Strategies

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

• Bright sources could possibly exceed the total count or count rate limits diminishing the scientific quality of the observations. Exceeding the limits could affect the event timing information or yield a lower than expected count rate. Read about HRC linearity issues here. Section 7.13 of the POG discusses considerations to make when observing bright objects with HRC.

Total Count Limits
During observations, the spacecraft is dithered in a Lissajous figure to average out pixel-to-pixel response variations and to eliminate the gaps in the spectral coverage when using the LETG/HRC-S combination (see the three HRC-S segments here). A 450,000 count limit for an on-axis source has been imposed over the course of the dither pattern to limit the adverse affects on gain and QE due to the total amount of charge extracted from the MCP at the point of extraction.

If a target is thought to exceed this value, make a note in the comments section of the RPS form. The CXC may offset the pointing to limit the accumulated count to a given region of the MCP.

Count Rate Limits
The count rate limits vary the total count rate over the full FOV (telemetry limit) and the count rate for a single source within the frame (linearity limit). The two are discussed below.

Telemetry Limits
Over the whole FOV, the maximum telemetered count rate is 184 cts s^-1. It is possible to exceed this limit and use the valid rate (the rate of triggers that pass on-board validity tests) to correct for primary rate saturation.




Linearity Limits
The linearity limits refer to the limited count rate for an on-axis point source. This means that avoiding the telemetry limit does not guarantee the linearity limits are not exceeded. The HRC-I linearity is verified in the laboratory at ~5 cts s^-1 and is ≈10% at 20 counts s^1. The HRC-S limit is ~25 cts s^-1.

A few methods are listed below to eliminate the effects of count rate limits:

1. Inserting a transmission grating (LETG or HETG) to reduce flux and analyze zeroth-order image.

2. Offsetting the aimpoint will smear the image out, thereby reducing the number of counts for one part of the detector. To avoid the telemetry limit, significant offsetting is a possibilty which will greatly reduce the spatial resolution.

To avoid telemetry limits specifically, windowing is available to reduce the flux from nearby bright objects.
• Background Issues The background event rate is very important for observing extended low surface brightness objects. The HRC-S has considerably higher background than HRC-I, making HRC-I the instrument of choice for high resolution imaging. Background issues are discussed in the POG.

HRC-I
The HRC-I uses an anti-coincidence shield to reduce the detector's exposure to cosmic rays and lowers the valid event rate to ~100 cts s^1. After standard processing, this rate drops to 1.7x10^-5 cts s^-1 arcsec^-2. For point source observations of 100 ks or less the background is negligible, but the low rate can become significant for extended low surface brightness objects.

HRC-S
The anti-coincidence shield for the HRC-S does not work due to an uncorrectable timing error. This leads to a background rate that exceeds the telemetry rate limit of 184 cts s^-1. To overcome this issue, a "spectroscopy region" as been designated using the edge blanking feature, which like windowing, ignores the events occuring outside ofthe designated edges. The new region extends the full length of the detector but only 3.4 arcmin in the cross dispersion direction. This arrangement creates a quiescent background rate of 120 cts s^1. More can be read about the spectroscopy region in Sec 9.3.6 of the POG.

Background Fluctuations
As with most all detectors on space-based telescopess, the HRC-I and HRC-S experience background fluctuations due to charged particles. These occurrences will usually last from 3-10 minutes and can range from a factor of 2 to 10 over the quiescent background rates. These events affect about 20% of observing time and can be easily recognized and removed from observations.

• When to use the HRC. The two HRC detectors, HRC-I and HRC-S, were optmized to fulfill imaging and spectroscopic missions, repectively. But the detectors can be used for a wide array of observations depending on the proposer's science goals.
Spectroscopy
Even though the HRC-S is optimized for spectroscopy, both the HRC-S and HRC-I can be used with the LETG for spectroscopic studies. Read about LETG detector choices here.

HRC-S
The HRC-S is primarily designed for spectroscopy. Also, the QE is smaller than ACIS-S in the 1.2-20 Å range, but is larger at longer wavelengths. The HRC-S can also be configured to have the highest time resolution of any detector on Chandra. Therefore, observations that are interested in spectral information longer than 25 Å or require high time resolution should utilize the LETG/HRC-S combination.


HRC-I
Although the HRC-I is optimized for imaging, it can still be used for spectroscopy. Coupled with the LETG, the HRC-I detector provides wavelength coverage from 1.2-73 Å. Also, because of its size (30x30 arcmin), it can be used if a large detector in the cross-dispersion direction is required. Therefore, observations of very weak sources with interesting spectral features beyond the limit of LETG/ACIS-S or observations requiring a detector with a very large cross-dispersion area should utilize the LETG/HRC-I.

Be sure to check if the ACIS-S detector can better fulfill specific spectroscopic goals.

Imaging
The best HRC imaging is done with the HRC-I due to its lower background (Sec 7.10 of the POG) and large field of view (30x30 arcmin). The instrinsic PSF FWHM is ~0.4 arcsec but the resolution degrades as the target moves off-axis due to the increasing width of the HRMA PSF and the increasing deviation between the curved HRMA focal surface and the flat HRC-I detector. The center of the HRC-I FOV yields the best focus and the best image quality.

Timing Studies
The HRC-S has three segments that are normally used for spectroscopy. But when put into timing mode, only the middle segment of the MCP is used to generate event triggers. The detector is not edge-blanked like with normal spectroscopic studies. This way, even though the active detector area is decreased to 6x30 arcmin, every event is telemetered (assuming that the count rate is below the telemetry saturation). This yields the fastest time resolution aboard Chandra of 1.6 ms.
• HRC-S in Timing Mode
  A wiring error in the HRC causes incorrect times to be telemetered to the ground. High resolution (16 microseconds) can be achieved using HRC timing mode. This mode uses the central segment of HRC-S, giving a field of view of 6x30 arcmin. Read more about timing in the POG.
• Telemetry Saturation
  It is important to ensure that events from sources and background do not exceed the telemetry limit. Proposers and observers need to estimate the total count rate very carefully. It is possible to reduce the total count rate by "blanking" regions of the array. For example, it is possible to telemeter events only from a region of interest thus discarding background photons. It is also possible to "blank out" a very bright source. For more information please see the POG.