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Guiding Options

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Introduction

GeMS requires Natural Guide Stars (NGSs) to compensate for tip-tilt and plate-scale modes variation. The NGSs can be sensed using 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.

In July 2019, the Natural Guide Star Wave Front Sensor (NGS WFS) unit on Canopus was replaced by a new guiding unit called 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.The on-sky characterization of the new guiding unit have been taken in place in October 2019. The NGS2 on-sky commissioning results show an improvement in sensitivity of about ~2.5 magnitudes, compared to the previous NGS guiding unit. The commissioning results show also a vastly improvement in acquisition and offsetting. The NGS2 Canopus WFS (CWFS) limiting magnitudes were partially verified during the on-sky commissioning in October 2019. The new limiting magnitudes for the Canopus WFS are listed in the Canopus WFS limiting magnitudes section below. More information about the NGS2 can be found in the CANOPUS Wave Front Sensors (CWFS) web page.

The focus sensing to compensate for sodium layer altitude variations with the former NGS system was obtained using CWFS3. With NGS2 this is not possible. The slow focus information is obtained using an off-axis Peripheral WFS 1 (PWFS1) star. Since the PWFS1 is not used for guiding and only to compensate the sodium layer variations, stars with fainter magnitudes can be used. The off-axis PWFS1 limiting magnitudes are given in the PWFS1 limiting magnitudes section.


Canopus WFS limiting magnitudes

The limiting magnitudes (bright and faint) of the Canopus WFSs (CWFSs) for different image quality constraints are presented in the table below. The bright limits are based on previous measurements and have not been characterized yet. The faint limits values are based on a measurements obtained during the second night of the NGS2 commissioning. The characterization of the NGS2 limiting magnitudes (faint and bright) for different sky brightness and image quality constraints will continue during the December 2019 GeMS run. Therefore, the values may be subject to change.

CWFS Limiting magnitudes (R Vega mag)
IQ CC SB Bright Faint
20%-ile 50%-ile Any 10.0 17.7
70%-ile 50%-ile Any 9.5 17.3
85%-ile 50%-ile Any 9.0 16.9

Important Note: The difference in brightness between the three CWFSs should not be larger than 3 mag. This limitation is pending confirmation and may be subject to change.


PWFS1 limiting magnitudes

The limiting magnitudes of the PWFS1 for different image quality constraints are presented in the table below. Only the faint limits are listed. The values listed in the table are an estimation and have to be verified during the next on-sky commissioning run planned for December 2019. Therefore, the values may be subject to change.

PWFS1 Limiting Magnitudes (R Vega mag)
IQ CC SB Faint Limits
20%-ile 50%-ile Any 16.70
70%-ile 50%-ile Any 16.45
85%-ile 50%-ile Any 16.20


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.


Examples

Example of fields using NGS2 and 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 12.96 to 15.71 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%/SB=Any 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 errors, 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.

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 10.6 and 11.6 in R band. The expected average Strehl ratio and FWHM in Ks-band for IQ=70%/CC=50%/SB=Any are 11.3% 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.

Example 3: 1 CWFS:

An example of one NGS star configuration. Left: DSS 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%/SB=Any 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 galaxy cluster MACSJ 0416.1-2403 observed with GeMS/GSAOI (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.

 Fig.8 from Schirmer et al. (2015)


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