1. GAIA binary star simulations: 1) Further tests of sampling windows and observation strategies 2) Resolution-limits in sep/dm Staffan Söderhjelm Lund Observatory

2. Previous conclusions (GAIA I) across-scanWindow-effects (dsep, sigpi...) start at separation depending on the across-scan window-size. ‘Absolutely’ constant errors, relatively more conspicous for bright doubles Standard strategy ‘1B(ig)2S(mall)’ ‘Big’-window should not be 12x8, but rather 12x10 or 12x12

3. 1B2S For along-scan sep < 8 (pxl) and across-scan sep < 8, single 12x12 window with geometrical centre pointing For larger separations, individual 6x8 windows

4. New simulations for GAIA II Larger PSF (smaller optics) Fewer CCD:s, larger pixels Modified scanning (60”/sec scan-rate) Main parts of simulation/solution programs unchanged

5. Conclusions (as expected..) ‘Big’ window-heights need to be 12 or preferrably 14 pixels Typical 1B2S: 16x14 for along-scan sep<14 pixels, else two individual 6x10 windows Unsolved question: Observing strategy in dense fields

6. Ideal (post-mission) double star resolution limits Distant (no relative motion) binaries with assumed sep/dm and realistic colors Simple color-dependent PSF (width proportional to effective λ) Simulated data solved with single or DS model ‘Resolved’ if χ 2 significantly lower for DS

7. PSF-calibration in the DS solution For a barely resolved binary, the BBP gives an observed mean color Iteratively, and from spectral information, we assume the primary color can be estimated with some σ(V-I) The secondary color is obtained from the mean and the primary value, and an appropriate PSF is used for each component

8. Preliminary results For bright stars, the color calibration is a dominating effect (shape of central PSF), for faint stars much less so The derived resolution-limits are ‘hard-to-reach’ ideals, because finding the approximate DS parameters to improve will be difficult The limiting curves in the (lg_sep)/dm-plane are very close to earlier rough estimates, although based now on literally millions of DS solutions...