This page is still under construction - we are filling it as commissioning and results goes - more details on performance will be added when SV is finished.

This page addresses the performance of GeMS in terms of image quality and sky coverage. Details about sensitivity, zero points and throughput can be found in the GSAOI page.

GEMS Performance

A. Strehl ratio and FWHM

Based on commissioning results, we have started to define the performance ranges that could be achieved with GeMS. Table below summarizes SR and FWHM measured for different seeing conditions. These are for a constellation of 3 bright Natural Guide Stars (NGS). Performance for less than 3 NGS can be evaluated and visualized from the OT. Instructions are described here.

Important Note: The average Strehl, rms Strehl and the minimum and maximum Strehl delivered by the Mascot algorithm, and visualized in the OT Position Editor, corresponds to the best Image Quality Conditions (IQ=20%-ile). The Observing Tools does not provide, yet, Strehl values and maps for worse IQ conditions (IQ=70%-ile and 85%-ile). The Strehl ratios and FWHM values listed in the table below can be used to asses the performance of the proposed observations.

Table 1: GeMS performance for different seeing conditions (seeing @ 550nm)

	J	H	K	Natural seeing	Gemini IQ
Strehl Ratio (%)	10	20	30	<0.45"	IQ20
FWHM (mas)	70mas	75mas	80mas	<0.45"	IQ20
Strehl Ratio (%)	5	10	15	0.45" - 0.8"	IQ70
FWHM (mas)	120mas	110mas	100mas	0.45" - 0.8"	IQ70
FWHM (mas)	150mas	130mas	120mas	0.8" - 1.0"	(good) IQ85
FWHM (mas)	300mas	250mas	200mas	1.0" - 1.1"	(bad) IQ85

Below is an example of performance obtained in good seeing conditions:

This image is a composite of thirteen 15 seconds images, obtained during GeMS tuning tests during the night of December 18. The average angular resolution on this composite image is about 0.055 arc second, with a Strehl ratio of 30%.

B. NGS limiting magnitude

The magnitude limit of the three Canopus probes has been measured during commissioning. Current limits are given in the table below:

Table 2: Natural Guide Star limiting magnitude

	Bright Limit	Faint Limit
Canopus WFS	8.0	15.5

Magnitudes are given in R-band, in the Vega system.

Note that these are not the final expected limits. Due to design errors and alignment issues, the original setup suffers from more than a 2 magnitude sensitivity loss. A new NGSWFS module is currently under design and construction, and final magnitudes should reach R~17.5 . We expect this new module to be installed and tested during 2013. More details on this work can be found here

C. Sky coverage

We have estimated the sky coverage achievable by GeMS by running random pointings on the portion of the sky reachable at Gemini South. Assuming a limiting magnitude of R=17.5 (which should the case after the NGSWFS upgrade), we find that about 56% of the pointing have 3 GS or more. Only 13% have no guide star at all. The map below shows how the fields are distributed in the sky. Black dots are fields with 3NGS, Green=no star, blue=1 NGS, red=2 NGS.

 

The sky distribution plot demonstrates what could readily be expected: The highest probability to find 3TTGS is in the galactic plane, and actually extend quite far out of it. Out, closer to the galactic poles, we have an about equal mix, with no obvious spatial signature, of pointings with 0 (green), 1 (blue) and 2 (red) TTGS. Below, we have looked at the SR distribution histogram for all these fields:

In general, when 3 GS are available, the Strehl is good. The peak in the average Strehl probability distribution around 85% is solely due to 3GS groups quite uniform. The peak around 1% in the relative rms Strehl probability distribution plot is solely due to 3 GS groups.

Note that the SR reported here only accounts for the contribution of Tip-Tilt and Plate scale mode, and does not include any high-order effect. That's why values are higher than actually measured on the images. These numbers have to be treated as scaling factor that will apply to the laser corrected images.

D. PSF uniformity over the field

This feature is, as such, unique to MCAO. Uniform PSF vastly improve the accuracy of the image analysis. More generally, it is the experience AO users that data reduction is a critical problem, because of (1) the lack of proper and simultaneous PSF calibration and (2) PSF spatial variability in the field. For some programs (e.g. stellar population, sparse to moderately crowded field) a PSF can be found in the field itself, by definition, however small the field is. For the majority of the wide field programs (high Z clusters, galaxy morphology/evolution, YSOs, solar system, ISM), this is not the case. Having a large, uniform field goes a long way toward solving this problem: if a star is present in the field of view, it can be used for the whole 85''x85'' uniform field. Since, by definition there are three m < 19 stars to serve as tip-tilt guide stars in a 2 arcmin diameter field, the probability of having at least one in the central 1 square arcmin field is high (60%). The PSFs are made by a nearly diffraction peak and a halo. The halo is due to the turbulence that is not conjugated to the deformable mirror, and the ratio peak / halo then depends on the atmospheric conditions. We also observe a residual static shape on the PSFs that is probably due to mis-calibration of the centroid gains. Below we show some examples of PSFs that have been extracted at different location over the 85x85 GSAOI field of view.

E. Elevation, weather and seeing limitations

Limitation in elevation. While the telescope elevation hard-limit is 17 degrees, the limitation for propagating the laser is 40 degrees in elevation. This limitation is imposed by the LGSWFS zoom mechanism, which cannot mechanically keep the LGS in focus at lower elevations. In addition, the LGS return flux decreases signicantly at high airmass which also affects the performance.

Safety concerns with clouds. Because of safety concerns, the laser cannot be propagated in clouds that could hide any nearby aircraft from our laser spotters. The laser can be propagated in thin cirrus, however as expected the cirrus will reduce the laser brightness and affect the AO performance.

High winds. High winds entering into the dome will create wind shake and could make the Tip-Tilt loop unstable. We found that the TT loop could survive with wind on the secondary of up to 2.5 - 3.0m/s.

Transiting objects. Objects that transit near to zenith also impose a limitation. Since the telescope is on an alt-azimuth mount, the instruments, mounted at Cassegrain focus, must rotate to maintain the image orientation. This effect is especially pronounced as an object transits through zenith. As Canopus is also mounted at the Cassegrain focus, it is rotating too. However because the LGSs are launched from a fixed mirror on the back of the secondary mirror, they will be seen as rotating on sky (while the natural image remains fixed). Thus the laser constellation must also follow and de-rotate to keep the LGS spots fixed on the LGSWFS. The speed at which the laser can rotate is limited by the safe rotational speed of a mirror inside the BTO. We found that for an object transiting at an elevation of 80 degrees the laser loop would hold up to +/-2 minutes of the transit.

Poor seeing. Finally, for seeing worse than 1.5 arcseconds, the MCAO loops begin to lose their stability.

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Summary:

• GeMS performance is very uniform over a 85x85arcsec square field, both in terms of Strehl ratios and more general PSF characteristics;


• Images are close to be diffraction-limited, in terms of FWHM, over the full field of view;


• Strehl ratio under median seeing conditions varies from 15% to 50% in the 1-2.5 micron range and 0-30 degrees zenith angle, with relative uniformity (relative Strehl ratio standard deviation) from 1 to 5% ;


• Three natural guide stars are needed to get the best compensation from GeMS. Based on current limitation in magnitude limits (R=16) we estimate a sky coverage of ~55% for all the portion of the sky reachable from Gemini South, i.e. dec < +20, even when sky background noise and windshake jitter are taken into account;


• The overall performance is a weak function of the exact match between the deformable mirror conjugation altitude and the location of the turbulent layers;


• Under median seeing conditions, GeMS brings a 1.5 to 1.7 magnitude sensitivity gain over the 1-2.5 micron range on point sources with respect to seeing limited imaging. Gains with respect to HST/NICMOS, in the same conditions, are 0.3 (J) and 1.2 (K) magnitudes;


• Multiplex gains with respect to Classical AO systems (like Altair) are factors of 10 to 20 in area;

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