Summary
Capella was observed in April 2002 and the data were processed with the standard pipelines to Level 1a. Spectra of the level 1a tg_lam values (in energy) were created with HAK s/w and bright lines fit. The centriods of bright HEG and MEG lines on the plus and minus sides of the ACIS-S array were used to estimate any changes required in the spacing of the ACIS-S CCDs, that is changes to the sizes of the "chip gaps".
The result for this one observation/measurement is that the chips should be shifted in TDETX direction by the following amounts in pixels:
S0 : 0.53 S1 : 0.31 S2 : 0.20 S3 : 0 (0 by definition) S4 : -0.03 S5 : -0.23
That these values are all less than or about 1/2 a pixel is confirmation of a stable (and well adjusted) chip geometry (the current chip gaps were determined back in the Fall of 1999 soon after launch!)
Software
The measured centroid values in energy were manually entered in the file chip_gap_solve.2583.pro. This file also has the IDL calculations to solve for the chip gaps and chip offsets from the given measured line locations. The output of running the file (by doing IDL> @chip_gap_solve.2583.pro) has been captured in the file chip_gap_solve.2583.txt.
Note that errors are not included in the s/w at this time. Single
measurements have uncertainties around 0.2 pixel... Note that 11
measurement differences are used to determine 6 unknowns: the 5
gap changes and one HEG-MEG offset value.
Please send any comments to
Dan Dewey at dd@space.mit.edu.