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How to find the best NGS Asterism
GeMS needs natural guide stars to compensate for Tip-Tilt and Plate Scale modes. They are also use to compensate for the differential flexure between Canopus and GSAOI and to compensate for the slow variation of the sodium layer altitude.
At the beginning of the GeMS+GSAOI operation, *only one guiding mode* will be available: 3 CANOPUS TTGS (CWFS) + 1 GSAOI ODGW star for flexure compensation. Other combinations will be tested and exercised during the GeMS + GSAOI commissioning.
Complementary information can be found here.
Performance vs. Asterism:
Depending on the constellation geometry and guide star magnitude, the expected performance will be different. Best constellation are the ones that cover most of the field, or the more distant the stars are, the lower the plate scale error will be. More generally, at the first order, we want to maximize the area of the triangle delimited by the three stars. However, this has to be mitigated by the noise propagation, i.e. the magnitudes of the stars. As a rule of thumb: “best” asterisms have a reasonably sized asterism of bright stars.
To provide the users with a quantitative selections criteria, we have implemented an algorithm efficiently find the “best asterism” of 3 stars in the group of N stars located inside the NGS patrol field of a specific sciencefield. We define here “best asterism” as the asterism that will provide the highest AND most homogeneous correction level over the GSAOI field of view. More details on the algorithm can be found in the following documents: "Compensation of the null modes by Gemini MCAO" by F. Rigaut and "Methods for correcting tilt anisoplanatism in laser-guide-star-based multiconjugate adaptive optics" by B. Ellerbroek and F. Rigaut.
This algorithm has been implemented in the P1T and the OT and automatically finds the best asterism based on the catalog magnitude. The algorithm returns a list of valid tip/tilt guide star asterisms along with average, rms, min, and max Strehl values and Strehl maps. The best asterism based on the user criteria is selected by default.
What happens if only one NGS is available?
The PSF size and shape across the field will be depending of the distance with the natural star used as guide star like any other classic one laser guide star + one natural guide star AO system... BUT... as we are still using 5 laser guide stars across the field the drop in performances is much smaller. Past experience (see figure 9 of Schirmer & al.) have shown that the PSF and ellipticity is noticeable but still well under the seeing size and the full field of GSAOI can deliver useful science data.