FGS Alignment Results Presented for the STIS team by Linda Dressel TIPS - Nov 21, 2002

The Need for FGS Realignment FGS alignment updates were last made in Dec 1997 for FGS3 Oct 1998 for FGS1r Mar 2000 for FGS2r The FGSes undergo alignment shifts which are generally largest at installation, then decline with time. The alignment error in FGS2r was becoming large enough to cause numerous near-misses in STIS acquisitions.

Two-stage STIS acquisition: First attempt:Move and re-image:

Pointing errors for FGS1r dominant

Pointing errors for FGS2r dominant

Pointing errors for FGS3 dominant

FGS1r V2 error vs time ( – ) FGS1r V3 error vs time ( – )

FGS2r V2 error vs time ( – ) FGS2r V3 error vs time ( – )

FGS3 V2 error vs time ( – ) FGS3 V3 error vs time ( – )

The solution: realign FGS1r and FGS2r to FGS3 Caveat: The program used to compute the alignment corrections from FGS observations of a star cluster has instabilities.

Action: implement solutions that are in agreement with the STIS pointing analysis FGS1r and FGS2r were realigned relative to FGS3 on 20 October. Consistent solutions were obtained from data taken from 28 August to 7 October.

Predicted improvement in STIS pointing

Observed median V2, V3: -0.12”, +0.19” (+/-0.20”) Predicted median V2, V3: -0.15”, -0.04” (+/-0.17”) Difference: +0.03”, +0.23” (+/-0.27”) FGS1r

Observed median V2, V3: +0.10”, +0.01” (+/-0.20”) Predicted median V2, V3: -0.02”, -0.22” (+/-0.17”) Difference: +0.12”, +0.23” (+/-0.27) FGS2r

Observed median V2, V3: -0.36”, -0.07” (+/-0.20”) Predicted median V2, V3: -0.27”, -0.32” (+/-0.17”) Difference: -0.09”, +0.25” (+/-0.27”) FGS3

Final check: Tycho 2 coordinates

CONCLUSION: It worked!