ECR: Update CALALIGN coefficients to correct ACIS-S fid positions based on SIM-Z offset

Overview

This ECR is to update the CALALIGN coefficients which specify corrections to the ACIS-S fiducial light positions based on SIM-Z position. As documented in the ECR to create the CALALIGN coefficients, the relative positions of some fiducial lights shift when the SIM is translated due to imperfections in the FTS retroreflector corner cube. The first ECR addressed very large shifts (up to 4 arcsec) in ACIS-I and HRC-I. This ECR is to insert coefficients for ACIS-S that produce corrections of up to 1.0 arcsec.

Motivation

Detailed analysis of aspect offsets showed an anomalous excess for a particular program of observations to observe quasar jets (PI: Marshall). The unifying unusual characteristic of these observations was an offset of -11.5 mm from the nominal ACIS-S SIM-Z position. The following plot of DY offset versus SIM-Z was then made:

The solid line is the best fit 2nd order polynomial to the data. This systematic trend can be removed by applying a correction to the effective fid light positions as a function of SIM-Z position.

New values

The CALALIGN1 block of the PCAD CALALIGN files contains the columns

FID_Y_CORR[5]    Polynomial coefficents for LSI correction
FID_Z_CORR[5]    Polynomial coefficents for LSI correction
FID_Y_LIM[2]     Offset limits for LSI correction
FID_Z_LIM[2]     Offset limits for LSI correction

Using the program plot_simz.pl, the following coefficient values were derived:

 DY coeffs: [0.0028072378 -0.0033344999 -3.2135358e-05 0.0  0.0] (mm)
 DZ coeffs: [-0.0044972804 0.0030785254 -0.000128211   0.0  0.0] (mm)

These were applied to each existing CALALIGN file using the code fix_calalign_files.pl.

Verification

Four ACIS-S obsids were reprocessed through aspect L1, and the new aspect solution was applied to the event file with reproject_events. The obsids are

                       OLD Source offset   NEW Source offset
Obsid  SIM_Z  Det          DY     DZ           DY      DZ
----   -----  ------     -----   -----       -----   -----
3108   -179   ACIS-S     -0.82   0.12         0.14   -0.17  
3111   -179   ACIS-S     -0.75   0.15         0.16   -0.05  
3574   -182   ACIS-S     -0.43   0.30         0.23    0.04  
3104   -190   ACIS-S     -0.00   0.11        -0.01    0.09  
3708   -194   ACIS-S      0.25  -0.34         0.07    0.07

The new CALALIGN values have the desired effect of greatly reducing the source aspect offset.

Two obsids (3111, 3708) were also processed through Aspect L1 and ACIS L2 by CXCDS ops (custom processing request 854). The results of this processing are consistent with the above values and demonstrate correct behavior of the new files within CXCDS pipeline processing.

For the other three detectors, there is no change in the coefficients. This was explicitly verified by doing an ASCII dump the entire contents of one N0004 align file and one N0005 (new) file and doing a graphical diff via the filemerge utility.

Analysis directory

The analysis to support and verify this update is in the directory /proj/sot/ska/analysis/fid_sim_z/2003_Sep_UPDATE.

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Last modified:12/27/13