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HRC Science and Calibration - S. Murray
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The background is reduced by about a factor of 2 from the raw rates by the
application of the pipeline screening procedures developed by the HRC
Team and the CXC LETGS team.
HRC and ACIS are cross calibrated often. Several calibration targets are
observed by ACIS and HRC over each AO. On average I think these happen every
three to six months.
The HRC QE appears to be stable at the few percent level since
launch. We have not been able to see any significant change in the HRC
count rate from calibration targets, though there may be a slight
indication of a trend in the direction of decreasing count rate
although not statistically significant at this time. The current
calibration program adequately monitors the HRC QE. However, there may
be some spatial variation in HRC QE (particularly at low energy) which
has not be well mapped, and whose time history is not well known.
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HRC Imaging and Deconvolution - M. Karovska
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Yes, for bright sources.
Yes.
At this point we do not have an error map for ChaRT.
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The HRC Degapping Procedure - A. Kenter
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Presentation
Ringing is caused in the on-board hardware in the
HRC coarse position determination.
Event position is a localized centroiding algorithm that uses
three taps of the HRC electronics centered on the
tap with the maximum collected charge. Prior to reducing the
number of taps to three the HRC starts with four taps; one
of the center two is the actual tap with maximum about which
the centroiding will take place. The reduction of 4 to 3 taps
starts out with a default choice of which of the two
center taps is the actual maximum.
If this choice is correct, there is no ringing. If this choice
is incorrect there is a switch and this switching introduces
electronic noise which causes the ringing. (More in-depth
explanations are available with an apprenticeship in the EE dept.)
The latest and greatest degap is presently implemented.
The degap algorithm assumes that any structure is due to
non-linearity. All dead areas, hot pixels, and data with
structure are avoided: hot/dead pixels are eliminated, and data where
the UVIS structure is evident are avoided (e.g. do not
use carbon X-ray flat fields in areas of detector which are seriously
modulated by the UVIS structure).
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Imaging Characteristics Affecting the LETGS - V. Kashyap
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With MARX, "unity" is the sum total of all collected photons
within a given wavelength region because there is no background
to confuse the issue. With flight data, background is a big
problem, and must be modeled out before the total number of
source photons can be determined. This is subject to significant
systematic uncertainties. Thus, both flight data and MARX simulations
must be used in conjunction to arrive at the correct answer.
Indeed, more photons from strong emission lines will be scattered
outside the bow-tie, but this makes no difference when fractional
enclosed energies are computed.
The wavelengths are calculated using the CIAO routine tg_resolve_events.
And yes, we believe that the pixel shifts and differences between +ve and -ve
orders arise due to different effects.
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Non-Linearities in the HRC-S Detector - R. van der Meer
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Presentation
No, there is no significant irregularity in ACIS-S, as can be seen in
the updated slide 15 of the proceedings of my presentation.
Yes, as is shown in the updated slides 9 and 10 of the proceedings of
my presentation, the effect is about 5 HRC-S pixels large. I
understood from the other talks that this is about the size of image
blurring.
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Characteristics of the HRC Background - M. Juda
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Presentation
The MCP gain is higher toward the center of the
MCPs; as a result, more background signals exceed the trigger
threshold and are registered as events.
There are two different ways that the observing modes can
effect the QE (and consequently the EA) for modes used to date:
i)Modes may select specific areas of the detector as active. Ignoring or
rejecting events from regions on-board will make the effective QE of
those regions zero.
ii) For the high-precision timing mode, the
trigger level is raised above the default level. This might result in
a small decrease in the QE at low energies by clipping the low
pulse-height tail. A measurement of PSR B0540-69 during the
timing-mode configuration test did not indicate a significant change
(at ~1% statistics) in the observed rate from the default
configuration.
No, the default QE is assumed for the EA used in PIMMS.
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The Decrease in ACIS Low-Energy Sensitivity - P. Plucinsky
ACISABS Parameters:
par1 = Days between Chandra launch and ACIS observation
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Presentation
George Chartas is the expert on this tool since he wrote it.
However, one can go to the web
site and the parameters are listed:
par2 = norm parameter in decay rate equation
par3 = tauinf parameter in decay rate equation
par4 = tefold parameter in decay rate equation
par5 = Number of carbon atoms in hydrocarbon
par6 = Number of hydrogen atoms in hydrocarbon
par7 = Number of oxygen atoms in hydrocarbon
par8 = Number of nitrogen atoms in hydrocarbon
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ACIS Background - M. Markevitch
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The dark Moon observations (compared to the Histogram Mode
data) showed that the background at the stowed position
inside the SIM and at the aimpoint are similar. They have
served their purpose and are now superseded by a longer
exposure with ACIS stowed, which does not include the
variable soft X-ray background component.
The same power-law model continues to lower energies (to 0.3
keV at least).
One should check a ligth curve for chip S3 in the 2.5-7 kev
band (or in the 2.5-6 kev band for chip S1 if S3 is
completely covered by the target emission). Even if it looks
"constant", compare the rate with the nominal rate from the
corresponding blank-sky background dataset. If the BI chips
show a flare, but the FI chips do not, then it is a soft
flare, whose spectrum can be modeled as shown.
Nonuniform; the spatial distribution is under investigation.
Dick Edgar and Shanil Virani have a while ago, and found a
rather strong correlation. But it wasn't a 100% correlation
so its usefulness for flare prediction appeared to be
limited (although there must be some interesting physics in
it).
The VF mode filtering should not be applied to bright
objects at or near pileup, because it will result in loss of
good events. The 5x5 pixel event islands may start to
overlap even when the true pileup (when the 3x3 pixel
islands overlap) is weak. Most if not all clusters are below
that brightness. To be sure, one can make an image using
only the VF-rejected events and see if any obvious sources
are seen. Note that there is no intrinsic loss of events in
the data because of the use of VF mode (provided the
telemetry did not saturate).
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Evolution of ACIS Charge Transfer Inefficiency - C. Grant
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The BI CCDs have always had serial CTI due to defects introduced
in the manufacturing process, while the FI CCDs to this day do not
have detectable serial CTI. Serial CTI can be increased from
radiation damage to the serial register, but the particles would
have to be of a high enough energy to pass through the framestore
shield. The standard ACIS protection scheme (moving ACIS out of
the focal plane during radiation belt passages and solar storms)
will help reduce radiation damage to the detectors, however there
is very little that can be done to prevent damage
from very high energy particles that pass through the
SIM itself. That said, there does not appear to be any
appreciable increase in serial CTI since launch.
CTI is a function of the history of the charge traps encountered by
each charge packet as it is transfer to the readout. If an X-ray
event is preceeded (along the readout direction) by another event,
it will lose less charge during transfer than if the preceeding
event was removed. The preceeding event is called "sacrificial
charge" and can reduce the measured CTI.
In the case of ACIS CCDs, most of the sacrificial charge is due
to cosmic ray background events. The bumps and wiggles are primarily
due to changing background rates.
The results shown here are, to the best of my knowledge, completely
consistent with other similar measurements. I agree with Paul's
statement that the BI CCDs show smaller changes with time than FI
CCDs.
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Including CTI Time Dependence in the PSU CTI Corrector -
L. Townsley
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The CTI corrector corrects ACIS events for CTI. It includes the
long-term time dependence measured and parameterized as linear changes
by Catherine Grant. In that sense, it accounts for long-term changes
in the background. The user could attempt to account for day-to-day
changes by CTI-correcting the External Calibration Source data taken
close in time to the celestial data in question, then adjust the
results based on residual CTI effects seen in the calibration data.
Only background effects that carried over from the calibration data to
the celestial data could be accomodated this way, of course. For long
enough observations, the user can check the fidelity of the CTI correction
by examining the spatial dependence of the gain (after CTI correction)
via the instrumental Ni K-alpha line at 7.47 keV. This line is faint,
though, so a long observation is required to use it as a diagnostic.
It takes a very large amount of calibration data gathered over many
orbits to tune the CTI corrector, especially to learn about column-to-column
variations (what I call the deviation map, similar to what MIT calls
the trap map). So by its nature the tuning of the CTI corrector
involves some averaging over time-dependent quantities. We have attempted
to remove the largest of these by incorporating a linear parameterization
of the CTI time dependence into the corrector. This regularizes the
gain so that it is not seen to change substantially as a function of
time. The FWHM is increasing as CTI increases, but so far this is
not a large effect. It is certainly the case that the CTI corrector
could be re-tuned as the characteristics of CTI change over the life
of the mission. The trade-off is between having enough data to do
a sensible tuning and combining so much data that CTI effects evolve
throughout the dataset.
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ACIS Response - R. Edgar
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No. They exist only for the extended imaging array FI chips (I0, I1,
I2, I3, and S2). We have response products (FEFs) for these chips
with the CTI corrector, only for T = -120 C and only for the nominal
frame time (3.2 s). CTI is a function of trap filling, and so depends
on temperature, frame time, and a host of other variables.
I'm not sure what the question was, but there are gain nonlinearities
at low energies on S3. We find (again at -120 C) our EO102 fits have
residuals consistent with zero down to about 700 eV, but there are
systematic residuals at Oxygen VIII (lyman alpha line at 653.6 eV) and
Oxygen VII (Helium-like triplet at 561.0, 568.5, 574.0 eV).
We have some off-axis observations of the QSO PKS 2155-whatever with the
LETG/ACIS combination. These were done with the source off axis by
various amounts in the dispersion direction, thus dragging the low energy
portion of the spectrum across S3 (and the other S chips). We're looking
at these data to try to improve the S3 response matrix products, especially
at low energies.
Gordon Garmire expressed interest in the S3 response down to even lower
energies (300 eV or the carbon edge at 284 eV). We are interested in
such things, but the calibration data are sparse.
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ACIS Spectral Response - A. Vikhlinin
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Yes, but some modifications in the ACIS simulator are required. We
need to have an access to the PHA distribution before any
modifications to the CTI effects.
No.
They are identical for practical purposes.
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Performance Tests and Verification of Current ACIS FEFs - N. Schulz
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Yes, they account for CTI. The FEFs in the BI chips are using
an advanced ACIS model for back-illuminated devices provided
by the MIT ACIS IPI teams. This model is adjusted to every chip
and node using the on-orbit external calibration source and
LETG/ACIS-S data. So far we have verified that the model represents
the redistribution function very well above 1 keV. Below 0.8 keV
there are still some problems with redistribution tails. We also
currently address the low energy gain and time drifts of the gain.
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Future ACIS Calibration Work - D. Schwartz Glenn Allen answers:
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This question must be answered by the data system.
It is my very strong opinion and understanding that the cosmic ray
dead time is not accounted for anywhere in the CXC data system.
The effect of the dead area is something that affects the flux (i.e. the
ARF). The pieces involved in computing the ARF are GTIs, DTCOR, QE,
and QEU.
The GTIs are only for occasions when no data is available due to a dropped
frame, a flush, bad aspect, etc. The DTCOR compensates for flushes and the
41 ms readout time. The QE is from a model which does not include the
cosmic-ray background. The QEU is relative to the readout, which is defined
to be 1. Therefore, the dead area is not included for ACIS data. (i.e.
ACIS flux numbers are systematically low by the average fraction of the dead
area.)
I think it would not be too difficult to modify the various pieces of the
puzzle to handle a dead area correction. I will not go into the details of
such a change. What may require more work is quantifying the correction
(either average values for the BI and FI CCDs or as functions of a
telemetered quantity?).
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Exploring the Limits of the ACIS Pile-up Model - J. Davis
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Yes. The pile-up model assumes that the photon arrival times are
Poisson distributed. I took this into account by filtering on times
where the light curves looked flat.
Strictly speaking, it works only for plus and minus one orders. It
can be applied to third order at long wavelengths if the first order
flux at 3*lambda is small enough that it does not affect the third
order.
It should, but I have not investigated it.
I believe so, but this needs to be investigated in more
detail.
I used Herman Marshall's QE tweaks, found here.
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On the use of HETGS higher order spectra for science analysis -
N. Schulz
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LETG/ACIS data won't help here. Ideal observations would include
either a very bright source where the intrinsic source spectrum
is very well known or a bright continuum source that is not
piled in the first orders and which is observed for a very
long exposure.
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Absolute Time Calibration - A. Rots
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The question would probably be better answered by the instrument
teams. My understanding is that there was a calibration done but not
a complete end-to-end measurement with sufficient accuracy. The delay
is fixed. For clarity, I should add that the following two effects
are not considered included in this delay: the HRC time stamp
assignment error and the time offset calculated for ACIS CC mode
events.
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Computation of Times of Arrival for CC Mode Events -
G. Allen
These questions are very closely related, so I will answer them
jointly.
Before using acis_process_events to compute the times of arrival of
continuous clocking mode event data, the values of RA_TARG and DEC_TARG
should be modified to be as accurate as possible (i.e. < 0.5 arcsec).
Otherwise the inaccuracy of the source position will result in a
corresponding inaccuracy of the times of arrival. The best values to use
for RA_TARG and DEC_TARG are the reconstructed values instead of the actual
source coordinates. For example, if the continuous clocking mode
observation is IMMEDIATELY proceeded by a timed mode observation to
determine the coordinates of the source, then use the RA and DEC coordinates
of the source, as determined using the timed mode observation, should be
used for RA_TARG and DEC_TARG. Use of the reconstructed position of the
source instead of the actual position will help avoid inaccuracies in the
times of arrival due to inaccuracies in the aspect solution. However, in
some cases it is not possible to use the reconstructed position. In these
cases, use the actual RA and DEC coordinates for RA_TARG and DEC_TARG.
Since the aspect solution is accurate to one pixel or less most of the time,
use of the actual position of the source should be fine most of the
time.
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Presentation
The times of arrival are computed using the values of RA_TARG and
DEC_TARG, the measured pointing direction of the telescope, and the measured
position of ACIS relative to the telescope. The times are independent of
the measured CHIPX coordinates of the events. Therefore, disabling the
pixel randomization does not affect the times of arrival. (The times of
arrival are not randomized within a pixel.)
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