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Guiding Options

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GeMS requires Natural Guide Stars (NGSs) to compensate for tip-tilt and plate-scale modes variation. The NGSs can be sensed using only the CANOPUS Wave Front Sensors (CWFS). Ideally, three of these guide stars should be available to compensate for the plate-scale dynamical errors, however GeMS may work with 2 or even 1 CWFS with reduced performance. The observation with one or two CWFSs implies a slow degradation of the full-width half maximum (FWHM) over the GSAOI field of view, proportional to the distance to the CWFS (see Schirmer et al. 2015 for details about the GeMS performance with one NGS). The degradation in the performance is smaller than the typical anisoplanatic effect observed with a single AO system.

The original design of GeMS includes a loop to compensate for flexure between the Canopus bench and the instrument based on the GSAOI ODGW measurements. However, it has been found that the flexure effects is negligible during a typical GSAOI exposure. Therefore the ODGW is not currently employed to correct for flexure. Only Canopus based WFSs (CWFS) are defined for GeMS/GSAOI observations during the PhaseI and PhaseII processes.

In July 2019, the Natural Guide Star Wave Front Sensor (NGSWFS) unit on Canopus has been replaced by a new guiding unit - the Natural Guide Star Next Generation Sensor (NGSNGS or NGS2). NGS2 is based on a single Electron-Multiplied CCD (EMCCD) focal plane array and uses configurable guiding windows (multi-region of interest) to read at rates up to 800 Hz. It is expected that NGS2 will provide a sensitivity gain of more than 2 magnitudes, and improve/solve operational issues related to acquisition and offsetting. The NGS2 CWFS limiting magnitudes will be verified during the on-sky commissioning planned for October 2019. Proposers of new observations with GeMS and GSAOI must continue to use the current Canopus WFS limiting magnitudes given below. The faint and bright limits will be update after the NGS2 commissioning run ends.

Canopus WFS limiting magnitudes

The current limiting magnitudes for the three Canopus WFSs (CWFSs) are given in the table 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%/70%-ile/85%-ile and SB=Any conditions. For more detailed information on the performance of the GeMS, including the guide star magnitude limits consult the GeMS Performance web page.

Available guiding configurations and guide star selection

The NGSs can be sensed using only the CANOPUS Wave Front Sensors (CWFS).

The following configurations can be accepted:

  • 3 CWFS stars
  • 2 CWFS stars
  • 1 CWFS 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 variations in the delivered FHWM and in the Strehl ratio across the GSAOI FoV when compared to an optimal asterism selection (see Schirmer et al. 2015 for details). 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.


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

Example 1: 3 CWFSs

An example of an optimal natural guide 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 9.9 to 12.2 in R band. The average Strehl ratio in Ks-band, the associated error, the minimum and maximum values, calculated using the MASCOT algorithm, and the average FWHM expected for IQ=70%/CC=50% conditions 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). Note that the Strehl map displayed in the position editor does not take into account the Laser Guide Stars, however the map provides a good representation of the expected performance. The Strehl ratio and associated error, the min, max Strehl values and the FWHM provided at the bottom of the OT Position Editor are scaled to match the value given in the GeMS performance web page. In this example, we can expect a good performance, i.e. an uniform PSF and good correction across the entire GSAOI field of view.

Screenshot of the OT PE (3 CWFS)

Example 2: 2 CWFSs

An example of 2 NGS stars configuration. Note that the use of a galaxy as a CWFS is NOT an option. The magnitudes of the two available CWFSs are 10.9 and 11.9 in R band. The expected average Strehl ratio and FWHM in Ks-band are around 11% and 0.09 arcsec respectively. Note that because the Strehl map does not take account the location and contribution of the Laser Guide Stars, the lines shown in the position editor do not reflect the true Strehl variation across the GSAOI field of view. However, the values displayed at the bottom of the Position Editor are correct. The correction will be better close to the two CWFS stars and degraded toward the edge of the GSAOI field view. Therefore you may expect a lower performance across the GSAOI field of view compared to an asterism with 3 CWFS: a fairly uniform PSF but with increasing FWHM values, proportional to the distance to the CWFSs.

Screenshot of the OT PE (2 CWFS)

Example 3: 1 CWFS:

An example of one NGS star configuration. Left: Image of the Gemini Frontier Field galaxy cluster MACSJ 0416.1-2403 displayed in the OT position editor. The magnitude of the CWFS is 13.4 mag in R-band. The expected average Strehl and FWHM in Ks-band for IQ=70%/CC=50% are 6.9% and 0.090 arcsec respectively. The correction will be better close to the WFS star and degraded toward the edge of the GSAOI field view. Therefore you can expect a fairly uniform PSF, but with variable and poorer FWHM across the GSAOI field of view toward the edges. Note that only point-like sources can be used as NGS. Right: Ks-band image of the the galaxy cluster MACSJ 0416.1-2403 observed with GeMS/GSAOI from Schirmer et al. (2015, Fig. 8). As expected, the PSF is fairly uniform, but the FWHM and Strehl ratio show variations toward the edges of the image. The FWHM increases from 72 to 122 mas while the Strehl decreases from 15% to 3% from the location of the NGS. The average Strehl ratio and FWHM shown in the image are consistent with the values provided by the MASCOT algorithm in the OT.

Screenshot of the OT PE (1 CWFS) Fig.8 from Schirmer et al. (2015)

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