====================================================== CAL ====================================================== (1) The CUC encourages the calibration group to update the new calibration web pages and summary document as they work toward the Chandra calibration goals. Our plan is to ensure that the calibration uncertainties listed in the calibration summary document are consistent with the publically released versions of the CALDB and CIAO. The summary document will therefore be updated any time a new version of CIAO or the CALDB is released that improves the accuracy of the calibration. (2) At our next meeting the CUC would like to hear a presentation that describes the prioritized plan for how to go from the current status to the goals in order that calibration group resources are directed toward efforts that produce new calibration results for the broadest range of Chandra users. We can present a summary of a prioritized plan at the next CUC meeting. The information below describes our current activities and expected completion dates and may be helpful background material for the committee. A prioritized list of present calibration activities and expected completion dates can be found at: http://asc.harvard.edu/cal/docs/cal_priorities.html. Separate lists are given for the HRC, ACIS, HRMA, LETG, and the HETG. The activities are sub-divided into projects scheduled for completion by June 2004, Sept. 2004, or longer term. Based on this plan, we should meet or exceed 19 out of the 24 pre-flight calibration requirements by Sept. 2004. Due to the radiation damage suffered by ACIS and the effects of contamination build-up, we presented revised goals for ACIS calibration to the CUC at the Jan. 2004 meeting. By Sept. 2004, we should meet or exceed 21 of the 24 goals listed in our calibration summary document. Most of the remaining issues are related to image reconstruction with the HRC, which is listed as a longer term project. Sept. 2004 is a critical date since the calibration team will begin a reduction in staffing due to budgetary constraints. Beyond Sept, 2004, the most resource intensive issue will be re-calibrating ACIS after the bake-out. (3) Diab Jerius spoke about efforts to model the on-axis Chandra point-spread-function. It is clear that getting to this goal will require a concerted effort over some time and that a well formulated plan would help guide the upcoming research. Diab Jerius has written a document summarizing the remaining HRMA calibration uncertainties, including the PSF. These issues have been assigned either a high, medium, or low priority. The expected near-future resources in the optics team are sufficient to address the high priority items (e.g., PSF issues) over the next year, and probably most of the medium priority issues over the next two to three years. Given the forecasted budgets for staffing, it is unlikely that the lower priority items will be addressed. An executive summary of Diab's document will be distributed to the CUC at the next meeting. (4) CUC would like to see a plan that assesses the value and direction of further detailed modeling of the Chandra PSF and that lays out some practical goals for the work. See response to item (3). (5) The CXC should look at the "big picture" questions of how priorities among various calibration activities are set and how to best use the available CXC resources for calibration and analysis activities. Based on expected reductions in staff during the next two years, we have reviewed the calibration priorities. However, if the impact on calibration of the ACIS bakeout requires more effort than anticipated, this will affect the calibration schedule for other items. See response to item (2) for additional information. (6) About improvements in the LETGS Dispersion relation... The CUC would like to see the CXC take a similar approach to that outlined in recommendation [8] of the complete CUC document before putting further effort into improving the LETG/HRC-S wavelength scale. RESPONSE FROM 16 April 2004: The current LETGS dispersion relation analysis described in preliminary fashion at the 2004 January CUC meeting involves a non-linear empirical adjustment to the wavelength scale in order to match the positions of spectral lines with precisely known rest wavelengths. This is aimed at addressing gross deviations in some spectral lines characterized by shifts relative to true positions by a full line width or more. Salient problems date back to the first LETG observations obtained post-launch where these wavelength errors were visible by eye when comparing positive and negative orders. These errors affect analysis in at least four ways: 1/ Unreliable absolute wavelengths compromise Doppler studies; 2/ Fitting and deblending of spectra can fail because lines do not appear at their correct wavelengths; 3/ Co-adding of ± orders leads to spurious broadening of lines owing to differences in observed wavelengths in the different orders; 4/ Non-linearities stretch and compress line widths such that significant deviations from the LRF are observed, especially with coadded ± orders. This again can adversely affect spectral fitting and line broadening and shift studies. Some specific practical problems with scientific analysis of Capella spectra owing to these dispersion relation errors were reported by the SRON team at the Calibration workshop of 2002. One particular LETGS GO observation (ObsID 3757) is greatly compromised by wavelength errors because the scientific analysis is based on searching for relatively small Doppler shifts (~50 km/s), and a report describing the severe impact of current errors was sent to the LETG team by the GO. The current gross empirical adjustment analysis is expected to be near completion at the end of the third quarter of this year. A timely assessment of the benefits of further improvements to the dispersion relation would be of value once the results of this analysis have been evaluated an its impact on reducing dispersion relation errors have been determined. It is anticipated that this could form part of the LETGS presentation to the 2005 January CUC. ADDITIONAL MATERIAL ADDED 25 June 2004: The comments above refer to improvements in the "LETGS Dispersion relation" and "LETG/HRC-S wavelength scale." A better and more fundamental description of the problem would be "event position errors in the HRC-S," which not only cause non-linear errors in the apparent wavelength of LETG spectral features but also in their shape. These errors are highly dependent on event position on the HRC-S (in detector coordinates as opposed to sky or grating coordinates) and can be roughly grouped into two types: 1) Degap errors--The current degap corrections assume that corrections are symmetric around the tap center, at which the correction is defined to be zero. This is usually incorrect, with asymmetric corrections and a non-zero offset from the tap center. Position errors are typically 1-2 pixels but exceed 5 pixels in portions of roughly a dozen taps. The current degap coefficients also do not properly extend events to the edges of the taps, usually leading to residual 1- or 2-pixel gaps between taps. Vinay Kashyap has written a SPIE paper describing recent efforts to improve the HRC-S degapping coefficients (attached/link). Improvement in event positions is significant but incomplete, and further study will lead to greater improvements. 2) Localized errors--These are typically of order 1-2 pixels and occur in detector regions of size~10-20 pixels (in the dispersion direction), and can not be corrected by the relatively smooth degap correction functions (currently 5th-order polynomials for the 256-pixel taps). Mapping out these errors is more difficult, but there has been substantial progress and we believe that empirical corrections can be made with great benefit. These position errors affect scientific analysis in the following ways, which are often interrelated: 1) Positive and negative order spectra can not be added together for analysis because of significant line shifts and differences in line profiles (usually worse in the negative order) between the two spectra. 2) For the same reason, spectra from different observations can not be added together unless one is lucky enough to obtain spectra that fall on the same spot on the HRC-S (to within ~200 microns). 3) The detectability of weaker lines is thus reduced. 4) Distorted line profiles: reduce the S/N of lines, change the apparent line-to-continuum ratio (important in abundance determinations) impair velocity analyses (based on Doppler widths and shifts) useful for studying binary stars, cataclysmic variables, strong flows in flares, etc. increase line blending (which affects line diagnostics for density, temperature, etc.) 5) Wavelength errors: may lead to misidentification of lines often require tedious "fixes" of line wavelengths in order to get spectral fitting programs to work properly reduce the utility of Chandra spectra in providing wavelength benchmarks via the Emission Line Project, etc. 6) Incomplete spectral models (that omit weak but cumulatively significant lines) and improper continuum-level determinations are the primary causes of incorrect line-strength determinations, which lead to incorrect inferences of density, temperature, abundances, etc. (7) The CUC would like to hear about the following topics at its next meeting: Chandra/XMM-Newton cross calibration We will present the latest results on Chandra XMM-Newton calibration.