Altair Performance and Use

See also the commissioning performance summary of the Strehl ratio, FWHM and 50% encircled energy.

No ITC is available for L' AO imaging. Experiments show that with ALTAIR the background per pixel increases by 3.2X. Thus, if AO reduces a 0.3" FWHM image of a point source at L' to 0.1" FWHM, the improvement in S/N is roughly 2.5 over NIRI standalone imaging. Note that on extended sources performance at L' is severely degraded.

No ITC is available for spectroscopy mode. See the NIRI Performance web page for information on estimating the sensitivity.

Splitting of the light is done by a dichroic, therefore on a wavelength basis, so there is no vignetting.

See the Altair Observing Strategies page for aspects of guide star acquisition.

Altair's performance is more sensitive to observing conditions than the other Gemini facility wavefront sensors. We recommend requesting image quality conditions better than IQ=70% (i.e. FWHM in V of less than 0.8 arcsec). Altair performs well in these conditions. It is possible to get some correction in IQ=85% (i.e. FWHM in V of less than 1.2), but you will typically be getting low Strehl ratios (around 10%) and FWHM of about 0.1 arcsec for bright sources. When conditions are much worse than IQ=85%, the uncorrected size of the star exceeds the Altair pupil size causing Altair's control loop to diverge.

Cloud cover also adversely affects Altair's performance. Cloud cover will of course reduce the magnitude of any guide star due to extinction. Clumpy clouds further reduce Altair's performance as the centroid gain calculation, which relies on steady flux on the wavefront sensor will tend to produce spurious results. Under uniform cloud cover conditions such as thin cirrus, one can expect performance to degrade in line with the Gemini Cloud Cover constraints: no effect for CC=50% or better, an extinction of 0.3 mag for CC=70%, and an extinction of about 3 magnitudes for CC=90%. We do not recommend using Altair in CC=90% or worse because, in these conditions, the opacity tends to vary quickly by extreme amounts. Any guide star lock in CC=90% conditions is typically very tenuous except for the very brightest guide stars (V less than approximately 7 magnitudes).

In summary, we recommend using Altair NGS in IQ=70% and CC=70% conditions or better. In some cases, such as with very bright guide stars (V magnitudes less than 7), IQ=85% / CC=70% or IQ=70% / CC=90% may be possible. However, even for the brightest guide stars, IQ=85% / CC=90% is not generally possible or useful. Note that Altair LGS requires IQ=70%, CC=50%, as detailed on the Laser Guide Star page.

The GS should be as bright as possible. One distinguishes two correction regimes (a) full correction regime where the best image quality can be expected and (b) partial correction regime where the delivered image quality degrades as the GS magnitude increases. Limiting magnitude for full and partial correction as a function of the GS spectral type is given in the table below:

The guide star may be an extended object provided that it has a sharp core with a FWHM < 0.6 arcsec.

Note that the performance will degrade with:

• Fainter guide stars
• Angular separation from guide star to observed object
• Extended guide star

Here is a short list of important points to note:

• In its default configuration without the Field Lens, the Altair isoplanatic angle is relatively small. A Strehl attenuation of 50% occurs at about 7 arcsec from the guide star in K band, so do not be overly optimistic and choose guide star as close as possible from the object. Use of the Field Lens dramatically increases this 7 arcsec limit. The field lens is new for 2006A, so PIs wanting to use it should review the Field Lens summary.
• Choosing a guide star that is both faint and off-axis will lead to very poor compensation.

LGS performance can be divided into "high Strehl" and "low Strehl" modes. We define "high Strehl" as full correction, and expect this to be accomplished with the use of a natural tip/tilt star with R<15 and within 15 arcseconds of the science target. Full correction should be able to deliver Strehls of over 20% in the K-band and 10% in the H-band. We define "low Strehl" as partial correction in any conditions where a reasonable correction can take place. Strehls will be generally lower than the full correction values, lower for more distant tip/tilt stars or for more separation between the science target. Limits of the system are R<18.5 within 25 arcseconds of the guide star. Full performance notes are on the Laser Guide Star page.