WebPIMMS CHANDRA Help File
Instrument Combination and Model Parameters
Information on the CHANDRA Effective areas
ACIS Pileup Predictor for CHANDRA
INSTRUMENT COMBINATION AND MODEL PARAMETERS
ACIS-I - ACIS, spatial imaging
ACIS-S - The normal readout instrument for HETG spectra
HRC-I - HRC, spatial imaging
HRC-S - The normal readout instrument for LETG spectra
In WebPIMMS this has been simplified into the 4 base detectors. The output will reflect all of the possible grating combinations.
PIMMS PREDICTION - The predicted count rate (in the absence of pileup for ACIS), in counts / s
For PILEUP parameters, please see Pileup Help Section
Information on the CHANDRA Effective areasThe CHANDRA effective area files were generated using the current versions of the calibration files as provided by the IPI teams. These have been updated based on new results from in flight calibration data. All effective areas assume observation of an on-axis point source and use the prediction of the in-flight mirror (HRMA) effective area integrated over the full point spread function (PSF).
Generated using the current FI-chip QE which is appropriate for all the FI chips on ACIS. The resulting effective area does not account for the problem of pulse pile-up, which will impact any observation that will generate more than a few hundred counts in a 10 ksec exposure. This corresponds to a photon rate above about 0.03 count/sec/pixel. The PIMMS output includes an estimate of the fraction of counts which are piled up based on current models. Please see the CHANDRA Proposers' Guide for more information on pileup.
High and Low Energy Transmission Gratings (HETG/LETG)
All effective areas are for both positive and negative orders summed together. Currently no non-standard instrument/grating combinations are supported by PIMMS.
ACIS-S will be the normal readout instrument for HETG spectra. The curves use the flight instrument chip layout properly accounting for FI chips. A single observation provides spectra from both grating assemblies (HEG and MEG) simultaneously, in a cross-shaped orientation. PIMMS provides count rates for both HEG and MEG separately, or for their sum. Since the energy response and spectral resolution of the two grating assemblies differ, separation of the output may be important for some observers. In all cases the output is for isolated first order. The energy resolution of the ACIS instrument will allow the user to separate this from the higher order light. See the CHANDRA Proposers' Guide for more information.
HRC-S is the normal readout instrument for LETG spectra. Since the HRC has no energy resolution, order separation will be an issue for this instrument combination. The HRC-S includes three alternative "filters" to provide the observer with alternative ways to isolate the 1st order light: normal, LESF (Low Energy Suppression Filter), HESF (High ESF). Please refer to the CHANDRA Proposer's Guide for more details on the HRC and its filters.
PIMMS allows determination of the following count rates:
0th order (undispersed) with NORMAL (HESF is identical to NORMAL at 0th order) 1st order through NORMAL, LESF and HESF High (2-9 summed) orders through NORMAL and LESF (letghi) Since the physical position vs energy relation differs for each order, the summed high order light can be calculated with PIMMS only for a single energy band per area file. That chosen gives the sum of all the light in the full energy range 0.08-10keV which physically falls on the detector. The letghi curves are not valid for any other energy range.
Note: When HESF is in use, 3 spectra will be incident on the HRC-S. 2 are HESF spectra and are summed in the PIMMS count rate, the 3rd contains 1/3 of the light, is (otherwise) identical to a NORMAL spectrum and is not included in the PIMMS count rate.
It is recommended that the observer first assess the amount of high
order contamination for the target using the NORMAL options (LETG1,
LETGHI). If high order contamination is significant, the observer
can then use PIMMS and MARX to investigate the options of LESF (for
estimating the high-order spectrum) and HESF (for suppressing the
high-order contamination (0.08-0.4 keV)). The CHANDRA Proposer's Guide
provides reference information for using PIMMS in the investigation.
For guidance with simulating the spectra using MARX, please refer to
the MARX User Guide.
This version of Pimms only calculates pileup for the following instruments:
FRAME TIME - Through the "Pileup Parameters" section of the PIMMS GUI, enter the frame time (the fundamental unit of exposure for ACIS). The valid range is from 0.2 to 10.0 seconds, in 0.1 second increments for simplicity. The default and maximum value is 3.2 seconds.
Please refer to the Specific Help for CHANDRA RPS (Target Form -> ACIS
Parameters -> Parameters that affect PILEUP -> Subarray: Frame Time)
and the Proposer's Guide (ACIS chapter) for more information
regarding the selection of frame time and how its value affects
% PILEUP - The estimated pileup percentage for the predicted count rate with ACIS. The ACIS spectrum will be "piled up" by this amount.
CTS/FRAME AFTER PILEUP - The estimated number of counts per frame after ACIS pileup is taken into account.
CTS/S AFTER PILEUP - The estimated count rate (number of counts per second) after ACIS pileup is taken into account. This quantity is the "cts/frame after pileup" divided by the input "frame time". NOTES
The current pileup model begins to break down and becomes increasingly invalid as a predictor for pileup fractions greater than about 75%. 75% pileup is approximately reached when the PIMMS output is equivalent to about 3 counts/frame or higher. Because the model does break down above 75%, the "cts/frame" and "cts/s" results may appear anomalous when the pileup percentage is this high. Please refer to the Proposer's Guide for information on mitigating the pileup effect.
For feedback or questions on WebPIMMS, please contact Michael Arida. email@example.com