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. NOTE THAT THIS LETTER IS UPDATED FROM PREVIOUS CYCLES. _______ will be your contact for your ACIS observation: _________ Seq: ####### ObsId: ##### 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 DD-MMM-YYYY. In order to ensure the success of your program, your participation in the planning process is essential. Please review your instrumental setup as it appears in our data base and confirm it, or request changes, by reply email to me, no later than DD-MMM-YYYY. You may link to your current observational setup by going to the following URLs: http://cxc.harvard.edu/cgi-gen/mp/target.cgi?##### http://cxc.harvard.edu/cgi-gen/target_param.cgi?##### 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 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, * optional CCDs. Observers MUST verify that the target coordinates are correct. 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.) Click the "Obscat parameter for obsid xxxxx" to view all of the parameters for your observation. Finally, click the "DSS" or "ROSAT" buttons on the "Summary of Sequence Number" page. These plots may not be immediately available in the case of a short TOO or DDT observation. ------------------------------------------------------------------- ***New for Cycle 12:*** PLEASE REVIEW THIS SECTION RESPONSE TO USINT SUPPORT: As Chandra ages, the thermal constraints on scheduling observations increase. Observations that are approved for flight without constraints can be pulled into a schedule at anytime. Please help our planning team by PROMPTLY responding to your USINT contact to confirm or change parameters as this will ensure that your observation is approved for flight as soon as possible. This allows our planning team to have a large pool of observations to schedule (and you might get your observations completed sooner!). For Cycle 12, Observers are reminded to review their CCD selections carefully. As in the past, we strongly recommend that all programs select one or more CCDs to be "Optional" which could be turned off in cases where the thermal environment requires. Please review the section below on Optional CCDs. If you have difficulty in assessing the best CCD to select as an optional, please contact your USINT contact person. Reducing the number of operational CCDs results in lower temperatures for the ACIS electronics and FP. If GOs do not have a compelling scientific reason to have 6 CCDs on for an observation, they may wish to specify only 5 or fewer CCDs to increase the probability that the FP temperature will be cold and stable for their observation. GOs may still require that 6 CCDs be on for their observation, but this may require that the observation be scheduled at a different time during which the thermal environment is more favorable or that the observation be split into multiple pieces. In general, GOs should specify 5 or fewer CCDs unless the science objectives require 6 CCDs. For observations using the VF mode, observers should take note that as the Solar activity is increasing for Solar Cycle 24, the Chandra background rates are decreasing. This means that the risk of telemetry saturation is decreasing. It is suggested to use a background count rate of ~42 cts/s using an energy filter of 0.1-12.1 keV (that is energy filter lower=0.1 keV and energy filter range=12.0keV) to calculate the risk of telemetry saturation. Please refer to the POG or contact your USINT personnel for estimating your risk of telemetry saturation. ***End of new for cycle 12*** ------------------------------------------------------------------------ You should have already been contacted by the Chandra Director's office concerning the key parameters: Instrument Grating Coordinates Optional Chips Preferences Constraints Comments given to CDO during this contact have been transferred to USINT -- but DO NOT ASSUME USINT IS COMPLETELY AWARE OF THEM UNTIL YOU HAVE CONFIRMATION FROM YOUR CONTACT. Due to the thermal budget of the orbit worked into the long term schedule, new preferences and constraints are very difficult to accommodate at this time. If the situation is dire or has a very strong science driver we can contact the director's office on your behalf. SPLIT OBSIDS Due to the thermal budget of each orbit, some obsids are split into multiple obsids during the 32 days leading up to the observation. The policy is that when an approved observation is split, both ends are approved so long as they are being done in the same week or are pool targets. Observers will only be notified if both portions of the split do not end up in the same week and the target was not a pool target. -------------------------------------------------------------------- Important issues to consider: HAVE YOU SELECTED OPTIONAL CCDS? In Cycle 8, the policy of selecting optional CCDs was implemented. Because of changes in the Chandra thermal environment, the ACIS Power Supply and Mechanism Controller (PSMC) has been steadily warming over the course of the mission. Under current thermal conditions and assuming an initial PSMC temperature of less than +30~C, observations at pitch angles less than 60 degrees which are longer than ~50 ks and which consume maximum power within ACIS (6 CCDs clocking) are likely to approach or exceed the Yellow High thermal limit for the PSMC. To counter this, all observers are asked to review their CCD selections and determine which, if any, of their CCDs can be turned off to prevent the PSMC from overheating. If no optional CCDs are selected, six CCDs are being clocked and the observation MUST be scheduled at a pitch angle less than 60 degrees, then the observation is likely to be split into two (or more) observations. If no optional CCDs are selected, six CCDs are being clocked and the observation is not constrained in such a way as to prohibit it, the observation is likely to be scheduled at a time for which the pitch angle is greater than 60 degrees. If the observation is assigned a time in the Long Term Schedule for which the pitch angle is smaller than 60 degrees, the observation may be rescheduled for a later date when the pitch angle is larger based on the detailed scheduling of the time slot for that observation. While these restrictions apply to all observations, those most likely to be affected are those for which the approved observing time is larger than 25 ks and at least five required CCDs. Note that even if your observation is very short, it may occur in a sequence of observations at bad pitch angles, and it is therefore important that you consider whether any chips are optional unless your target is at an ecliptic latitude greater than 60 degrees. IS SENSITIVITY BELOW 1.0 keV IMPORTANT TO YOUR SCIENCE? It was discovered during Cycle 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/detailed_info.html Then click: "ACIS QE Degradation" 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. 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. The CTI correction is applied by default in the standard processing of the FI CCD data. For a general discussion of the ACIS energy resolution the GO is advised to review the information on the CXC Calibration page: http://cxc.harvard.edu/cal/Acis/detailed_info.html Then click: "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 the S-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. In the past, the BI background rate (after the standard event screening) was greater than twice the quiescent rate for up to 30% of the time. This flare fraction has declined with time, and is practically zero at present. We do not know if/when the flares will return. The front illuminated (FI) devices have always been little affected by flares. Note that a re-observation 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 been increasing since 2004. It has now reached (for FI CCDs) or surpassed (for the BI CCDs) the rates observed at launch. For more details, check the relevant section of the calibration web page. http://cxc.harvard.edu/cal/Acis/detailed_info.html Then click: "ACIS Background" Start with the "General discussion". 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 substantially fills that chip, consider using S1 for background 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/ (clean in background) Please note that the RMF generation is the same for Very Faint mode as it is for Faint mode. It is important to realize 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 calibration web page for a discussion of background flares and the telemetry limit. http://cxc.harvard.edu/cal/Acis/detailed_info.html Then click: "ACIS Background" Review section 1.3 of "General discussion". In order to reduce the total background rate and the likelihood of telemetry saturation, VF observations MUST use no more than 5 CCDs or an energy filter with a 13 keV upper cutoff. If the target is brighter than 5-10 cts/s, one has to take more drastic steps, such as turning off more chips or using Faint mode. In addition, the use of S1 & S3 require the energy filter as specified above regardless of the number of CCDs. 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 6.14, at http://cxc.harvard.edu/udocs/docs/POG/MPOG/node11.html#SECTION046163000000000000000 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 Tables 6.7 and 6.10 in the December 2005 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/cal/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.