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Guiding Options


Introduction

GeMS requires Natural Guide Stars (NGSs) to compensate for tip-tilt and plate modes variation. The NGS are also used to compensate for differential flexure between CANOPUS and GSAOI and to compensate for the slow variation of the sodium layer altitude. The NGSs can be sensed using either the CANOPUS Wave Front Sensors (CWFS) or the GSAOI ODGW, or any possible combination of the above. Ideally, three of these guide stars should be available to compensate for the plate-dynamical errors, however CANOPUS may work with 2 or even 1 CWFS with reduced performance. One of the GSAOI ODGW star is used to monitor and compensate possible flexure drift between CANOPUS and GSAOI and can be significantly fainter than the CWFS.

Canopus WFS and GSAOI ODGW limiting magnitudes

The limiting magnitude for the three CANOPUS based WFS (CWFS) has been measured during the GeMS on-sky commissioning. The current limits are given below:

CWFS limiting magnitudes
Bright limit Faint limit
R=8.0 mag (Vega) R=15.5 mag (Vega)

The faint limit works well for IQ=20%, IQ=70%-ile and IQ=85%-ile conditions. For more detailed information on the performance of the GeMS, including the guide star magnitude limits consult the GeMS Performance web page.

Accurate determination of the ODGW limiting magnitude is pending. For the purpose of planning observations, it is required that ODGW stars be fainter fainter than H=8 mag (Vega) and brighter than H=14 mag (Vega).

Available guiding configurations and guide star selection

Only the Canopus NGSs (CWFS) can be used for fast tip/tilt correction. The GSAOI On-Detector Guide Windows (ODGW) are not yet implemented, and therefore not allowed for fast tip/tilt correction.

The following configurations can be accepted:

  • 3 CWFS stars + 1 ODGW star
  • 2 CWFS stars + 1 ODGW star
  • 1 CWFS star + 1 ODGW star

To compensate for tip-tilt and plate scale modes, the NGSs should be positioned as close as possible to an equilateral triangle about the science target. Best constellations (or best asterism) are the ones that cover most of the field, or the more distant the stars are, the lower the plate scale error will be. Care must be exercised in selecting these stars so that they remain accessible during dithered observations. Detailed information about how the constellation geometry and the guide star magnitudes can affect performance, and a description of the asterism search algorithm (MASCOT), can be found here.

When the asterism is not the optimal or the number of NGS is less than 3 NGS (2 or 1 NGS), the users might expect larger variation in the delivered FHWM across the GSAOI FoV when compared to an optimal asterism selection, affecting the PSF uniformity across the field. Note that the performance of the system not only depend on the selected NGS constellation, the number of NGSs and their brightnesses, but also depend on the laser guide star (LGS) photons return (this parameter varies seasonally), turbulence profiling (Cn2(h)), non-common path aberrations and other AO optimization and calibration parameters. The system performance can be evaluated and visualized using the Observing Tool. Instructions of how to visualize and evaluate the performance of a selected asterism are described in the GSAOI OT Details web page.

Examples

Example of fields using different guide star configurations are shown below.

Example1: 3 CWFS + 1 ODGW

An example of an optimal natural guide + flexure stars configuration is given below. The three CWFS stars are positioned as close as possible to an equilateral triangle. The magnitudes of the CWFS range from 9.7 to 12.3 in R and the magnitude of the ODGW star is 12.8 in H. The average Strehl ratio (in H-band) and the rms, calculated using the MASCOT algorithm, are shown in position editor at the bottom. The Strehl map is visualized in the position editor as narrow curved green/yellow lines (see the GSAOI OT Details page for details). In this example, we can expect a good performance, based on the analysis of the Strehl map and Strehl rms, i.e. an uniform PSF and good correction across the entire GSAOI field of view.

Screenshot of the OT PE (3 CWFS + 1 ODGW)

Example 2: 2 CWFS + 1 ODGW

An example of 2 NGS + ODGW flexure star configuration. Note that the use of galaxies was WFS is not an option. The magnitudes of the two available CWFS are 10.8 and 11.9 in R and the magnitude of the ODGW is 10 in H. The average Strehl (H-band) is around 18%, with a Strehl rms of 18%. The correction will be better close to the two CWFS stars and degraded toward the edge of the GASAOI field view. Based on the analysis of the Strehl map and the Strehl rms, you can expect a lower performance across the GSAOI field of view compared to an asterism with 3 CWFS + 1 ODGW.

Screenshot of the OT PE (2 CWFS + 1 ODGW)

Example 3: 1 CWFS + 1 ODGW:

An example of one NGS + ODGW flexure star configuration. The magnitude of the CWFS is R=11.8. The ODGW selected is the same as the CWFS (H=10.5 mag). The average Strehl (in H-band) is around 13% and the rms is quite large rms (~20%). The correction will be better close to the WFS star and degraded toward the edge of the GASAOI field view. Based on the analysis of the Strehl map and the Strehl rms, you can expect a more variable PSF and poorer correction across the GSAOI field of view. Note that the other targets in the GSAOI field of view are not point like sources and cannot be used as NGS.

Screenshot of the OT PE (1 CWFS + 1 ODGW)