Natural Guide Star (NGS)
ALTAIR with NGS uses a natural guide star (of sufficient brightness and sufficiently close to the science target, as described below) to monitor distortions in the incident wavefront and correct for them using the deformable mirror as well as provide tip/tilt corrections to the telescope's secondary mirror.
NGS guide star requirements
ALTAIR requires a guide star within 25 arcsec of the center of the field. Best performance is obtained when the guide star is much closer to the science target than this. The guide star can be the science target itself, if the target is sufficiently bright and point-like (see below).
The guide star should be as bright as possible. Two correction regimes can be distinguished:
(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 guide star magnitude increases.
Due to processing limitations of the Altair CPU, the faintest limiting magnitudes can only be reached under optimal conditions (IQ=20% or ~0.5 arcsec native seeing and CC=50%, photometric conditions). In most cases for guide stars fainter than V=13, better performance will be achieved using LGS. Limiting magnitudes for full and partial correction as a function of the guide star spectral type are given in the table below:
|V-band limiting magnitude and color for full and partial correction regimes|
|Limiting mag for
The guide star may be an extended object provided that it is bright enough with a sharp core with a FWHM < 0.6 arcsec.
Assumes NIRI f/32, V=12 guide star on axis, CC50 IQ70 (clear, good seeing) conditions, and airmass = 1.2. Note that Strehl ratio decreases rapidly with decreasing wavelength and that the core FWHM is roughly constant with wavelength.
|Filter||Core Strehl ratio||Core fwhm||Halo fwhm|
|K'||0.17||0.08 arc sec||0.61 arc sec|
|H||0.10||0.07 arc sec||0.66 arc sec|
|J||0.04||0.08 arc sec||0.72 arc sec|
|Y||0.01||0.08 arc sec||0.77 arc sec|
Note that the performance will degrade with:
- Fainter guide stars
- Increased angular separation from guide star to observed object
- Extended guide "stars"
Although it is possible to use fairly faint guide stars, the combination of a faint guide star with an off-axis angle (e.g. faint guide star 25 arcsec off-axis) will lead to very poor performance. Use the NIRI ITC (with Telescope configuration set to "Altair") for individual cases; note that core FWHMs produced by the ITC are unrealistic. See the image library for examples.
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. 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.
NGS observing condition requirements
ALTAIR's performance is more sensitive to observing conditions than the other Gemini facility wavefront sensors. We recommend requesting image quality conditions equal or better than IQ=70% (i.e. FWHM in V of less than 0.8 arcsec at zenith). 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 at zenith), but low Strehl ratios (around 10% in the K band) and FWHMs of about 0.1 arcsec for bright sources will result. 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 around 0.3 mag for CC=70%, and an extinction of about 1 magnitude for CC=80%.
We do not recommend using ALTAIR in CC=80% or worse because, in these conditions, the opacity tends to vary quickly by extreme amounts. Any guide star lock in CC=80% 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=80% may be possible. However, even for the brightest guide stars, IQ=85% CC=80% is not generally possible or useful.
Note that ALTAIR LGS requires IQ=70%, CC=50%, as detailed on the Laser Guide Star page.