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Dear CHANDRA Observer,
A CXC scientific staff member will be assigned to each scheduled
observation to aid the observer in planning and executing the
observation.
[contact scientist] (xx@cfa.harvard.edu; Phone: (617)495-xxxx; Fax:
(617)495-xxxx) will be your contact for your ACIS observation:
[Object] Seq: xxxxxx ObsId: xxxxx Title: YOUR TITLE
until the observational data have been received by the CXC.
(***In addition to your scientific contact, please COPY ALL
CORRESPONDENCE TO cus@head-cfa.harvard.edu***)
Your observation is currently scheduled to be observed during the week
of Day-Month-Year.
The current schedules can be found at
http://cxc.harvard.edu/target_lists/longsched.html
In order to ensure the success of your program, your participation in
the planning process is essential. Please review and confirm the your
instrumental setup as it appears in our data base.
You may link to your current observational set-up by going to the
following URLs:
http://cxc.harvard.edu/cgi-gen/mp/target.cgi?(seq_nbr)
http://cxc.harvard.edu/cgi-gen/target_param.cgi?(obsID)
Alternatively, you can go to
http://cxc.harvard.edu/targets/
and enter the appropriate sequence number in the search page, and then
click on "Obscat parameters."
When reviewing your setup, please consider the several important
issues discussed below.
In particular:
* the target name,
* the target coordinates (in decimal degrees),
* the exposure time (in kilo-seconds),
* the target offset (in arcminutes) if applicable,
* the defocus (in mm) if applicable,
* the SIM-Z offset (in mm) if applicable,
* the roll constraint (in degrees) if applicable,
* the time constraint if applicable.
To view the placement of the detector on the optical (DSS) or X-ray
sky (if this region was observed by ROSAT), go to
http://cxc.harvard.edu/targets
Find your proposal using the form provided by filling in the "PI Name"
"Sequence Number", etc. Then click "Sequence Number". (Or click on
"sequence number" at the bottom of the form and locate and click your
sequence number.) Finally, click the "DSS" or "ROSAT" buttons on the
"Summary of Sequence Number" page.
If you find errors in your instrumental set up, please inform your CXC
contact scientist of the appropriate changes. Even if everything is
correct, please inform the contact scientist that you are satisfied with
the current set up. Please always copy email to cus@head-cfa.harvard.edu .
***Please return any corrections or the confirmation of satisfaction by
day-month-year.
(If we do not have your confirmation in sufficient time for scheduling, the
observation will have to be deferred, possibly as long as one year.)
- -------------------------------------------------------------------
Important issues to consider:
IS SENSITIVITY BELOW 1.0 keV IMPORTANT TO YOUR SCIENCE?
It was discovered during AO-3 that the low-energy sensitivity of the
ACIS instrument has been decreasing throughout the life of the
mission. For more detail on this effect, please consult the web page
at:
http://cxc.harvard.edu/cal/Acis/acis/Cal_prods/qeDeg/index.html
The decrease in efficiency is presumably due to a layer of
contamination on the filters and/or CCDs. The GO should make use of
the tools referenced on this web page to evaluate the impact of this
decreased sensitivity on their science. Since this effect was not
known at the proposal time, the choice of ACIS-S vs. ACIS-I detector
may need to be re-examined. The effect is still under investigation,
so GOs are advised to check the "What's New" link on the CXC web
page frequently.
IS ENERGY RESOLUTION CRITICAL TO YOUR SCIENTIFIC GOALS?
Because of radiation damage to the FI devices, their effective energy
resolution has degraded, and is strongly dependent on the distance of
the event from the readout node. Because of the focal plane geometry,
the detected energy resolution is worst near the aim point of the
imaging array, where the mirror performance is best. The performance
of the FI devices is considerably better at -120 C than at -110 C.
The ACIS CCDs have been operated at -120 C since January 2000. In
addition, the spectral resolution of the FI data can be improved by
applying a CTI correction to the data. GOs are advised to examine the
CTI correction SW available on the contributed SW page:
http://asc.harvard.edu/cont-soft/software/ACISCtiCorrector.1.37.html
GOs are also advised to check the "What's New" link on the CXC web
page frequently as the CXC expects to release a version of the CTI
corrector during AO-4.
For a general discussion of the ACIS energy resolution the GO is
advised to review the information linked to main CXC page:
http://cxc.harvard.edu
(click "Instruments & Calibration", then "ACIS", then "Energy
Resolution")
For further improvement in effective energy resolution, you may
consider moving your target closer to the readout nodes on the imaging
array by offset pointing (at the expense of spatial resolution).
Targets can be imaged closer to the readout nodes on theS-array by a
SIM-Z translation, without compromising image quality.
IS YOUR TARGET AN EXTENDED X-RAY SOURCE?
If your target is spatially extended, and your science depends on the
correct background subtraction, consider the following:
(1) The back illuminated (BI) devices (S1 and S3) may be affected by
the particle background flares. On average, their background rate
(after the standard event screening) is greater than twice the
quiescent rate for 20-30% of the time. This fraction is considerably
lower for the front illuminated (FI) devices. Note that a
reobservation request will be considered only in the most extreme
cases (if the background rate was greater than five times the
quiescent rate for more than half of the exposure).
(2) The quiescent background rate, after the standard event
screening, is larger by a factor of 1.5-3 for the BI devices compared
to the FI devices depending upon the bandpass of interest. The
quiescent background rate has decreased by about 25% over the mission
but it may be increasing slightly for AO-4. For more details, check
the relevant section of the calibration web page (go to
http://cxc.harvard.edu, click "Instruments & Calibration", then
"ACIS", then "ACIS Background", then "General discussion").
If your source substantially fills S3, you may want to consider
turning on S1 in order to get an indication of background changes.
In choosing the appropriate device, consider the relative target count
rates using the FI and BI chips (the latter have significantly higher
low energy sensitivity), the relative background rates, the risk of
flares, the spectrum of your source relative to the background
spectrum, and the desired spectral resolution. If S3 will be the main
chip and the target fills that chip, consider using S1 for flare
detection.
USE OF VERY FAINT MODE
Studies of ACIS background (see
http://cxc.harvard.edu/cal/Acis/Cal_prods/vfbkgrnd) have shown that for
weak or extended sources a significant reduction of background at low and
high energies may be made by using the information from 5 x 5 pixel
islands, i.e. Very Faint mode, instead of the Faint mode 3 x 3
island. This screening results in a 1-2% loss of good events. CIAO
2.2 and later provides a tool to utilize the VF mode for screening
background events. See the thread
http://cxc.harvard.edu/ciao/threads/aciscleanvf/ . (See also
http://cxc.harvard.edu/cont-soft/soft-exchange.html, or
http://cxc.harvard.edu/cont-soft/software/clean55.1.0.html, for
contributed software to perform this screening.) The rmf generation
is the same for Very Faint mode as it is for Faint mode.
Please note that VF mode uses more telemetry; the limit is ~68 cts/s,
which includes the target flux and the full background from all
chips. Check the relevant section of the calibration web page
(http://cxc.harvard.edu, click "Instruments & Calibration", then
"ACIS", then "ACIS Background", then "General discussion"; see
Sec. 1.3) for discussion of background flares and the telemetry limit.
If the expected count rate for an observation is within ~20% of the
saturation limit, consider applying an upper energy cutoff (12-13 keV)
and/or using fewer than six chips, in order to reduce the total
background rate and the likelihood of saturation.
The above background screening can amplify the pileup effect.
Therefore, this mode cannot be used to reduce background in the
region of bright, i.e. piled-up, sources. However, there is no
intrinsic increase of pileup in the data, and the software can be
applied to selected regions excluding bright sources.
IS YOUR TARGET AN X-RAY POINT SOURCE?
If your target is a point source, be sure you have estimated your
count rate correctly. Take appropriate measures to avoid event pileup
at levels that may compromise your scientific goals. Because of the
Poisson nature of pileup, its effects can compromise your observations
with even a small event flux per frame. For example, roughly 4% of
your events will experience pileup for a count rate of 0.1 events per
readout frame. Pileup can affect both the shape of the PSF and the
observed spectral energy distribution of your source. See the
calibration webpage for information regarding pileup, and the
Proposers Guide section 6.16 for steps that can be taken to mitigate
the effects of pileup.
The GO is also advised to become familiar with the SW which models the
effect of pileup on the observed spectrum:
http://space.mit.edu/CXC/analysis/davis/head2002/index.html .
HIGH TELEMETRY RATES
Beware of high telemetry rates. The limiting telemetry rate that
avoids data loss (saturation) is a function of ACIS mode. Estimate
the count rate of all sources in your field (using the PIMMS software,
for example) and the potential increase in count rate due to
background flares on the BI devices. If you do not need the BI
device, and you are approaching telemetry saturation, consider turning
off the BI device(s). See Table 6.10 in December 2001 AXAF Proposers'
Guide for telemetry saturation rates for the available ACIS modes.
OPTIONAL CONSIDERATIONS
If your observation does not use gratings, is greater than 30 ks, at a
high Galactic latitude and free of large extended sources, it may be
useful for background calibration and monitoring. Provided it does
not interfere with your science goals, consider the following:
(1) Use Very Faint mode, telemetry capacity permitting. VF is a
superset of Faint mode and all existing calibration products apply
when you use default processing, which only considers the 3 x 3 island
data (i.e., when you do not use the special tool to screen the 5 x 5
island to reduce background).
(2) If your main chip is S3, consider also using S1 to help background
modeling.
CALIBRATION
Please consult the calibration webpage for additional information.
http://cxc.harvard.edu
(click "Instruments & Calibration", then "ACIS")
CHANGES NOT PERMITTED
Be advised, the following changes in your observing program are not
usually permitted:
a) Adding constraints (e.g. roll angle or a time constraint).
b) A change in exposure time or target, or instrument switches that would
deviate from, or be inconsistent with, your proposed scientific goals.
Last modified: 05/02/03
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