European Southern Observatory
New Technology Telescope
There are three different procedures to set the NTT Active Optics System (AOS). The first is to use a default setting which has been calibrated for the zenith position. The default setting is obtained through an initialization procedure which is run every afternoon. The second method is to correct the default setting for gravitationally induced deformations, using predefined look-up tables. These tables include corrections for decentring coma and defocus, but not for higher order effects. The third method is to do a full wave front analysis, the so-called image analysis, and to calculate the mirror settings from this. This method can be used either in open or in closed loop.
A low order Zernike polynomial is fitted to the map of the displacement vectors. The accuracy or validity of the solution is estimated from the rms. residual deviations with respect to this polynomial. If the rms. is poor, the corrections are normally not applied to the mirror.
The results of the image analysis are displayed in the Graphic User Interface of the Main Active Optics Panel and in the Active Optics Engineering Interface.
The optical quality of the telescope is given in the Transverse aberration field. The important parameters are:
A poor rms. can be an indication that the air inside the dome is not well mixed. The standard integration time for the image analysis is 30 sec. On this time scale the wave front aberrations generated by the free atmosphere are effectively averaged out unless the wind is very low. This may not be true for wave front aberrations generated by the local air in the dome, depending on parameters, such as, temperature inhomogeneities, the wind speed and the air flow pattern in the enclosure. Active optics will only correct the low temporal frequency contents of these local air effects. Under unfavourable conditions, for example when the primary mirror is more than 1 degree warmer than the ambient air, these effects can lead to a severe degradation of the image quality.
As it was mentioned above, the image analysis system can be used in parallel mode, during the science exposures. In this mode, a dichroic is inserted in front of the guide probe which deflects most of the light of the guide star to the Shack-Hartmann grid. The corrections are calculated during the exposure, and after the exposure is finished a decision can be taken whether or not the corrections should be applied. The force setting will take approximately three minutes. This mode requires a sufficiently bright guide star which is not always available. Also, the parallel mode presently only works with zero rotator offset. In imaging mode this will in most cases be acceptable, but in spectroscopy a rotator offset may have been applied, either to align the slit with the object(s) or to orient the slit along the parallactic angle.
When the seeing conditions are good, it is advisable to make a new image analysis every time the telescope is moved of more than 15-20deg in altitude when close to the zenith (altitude > 60deg) or ~10deg when at lower altitudes. Note that a change in azimuth has no effect on the optics, and that these values can be relaxed when the seeing is bad. When staying for a long time on a same field, keep a careful eye on the image quality of the EMMI/SUSI frames. Images elongated by more than about 10% (to check use the MIDAS command CENTER/IQE or the Real Time Display functions) indicate residual astigmatism (in the presence of some defocus) which is generally the first indicator of imperfect settings. In any case, the judgement of the night assistants can be fully trusted: do not hesitate to ask their opinion, and follow their recommendations.
For the majority of NTT observations, whether imaging or spectroscopy, better seeing will give improved signal to noise ratio. Under good conditions, the time spent on an extra image analysis will generally be a good investment. It is recommended to use the parallel mode as much as possible, and to apply the corrections when appropriate, using the guidelines above. However, there are several conditions where little or no improvement can be expected. The first is when the seeing is poor (significantly above 1 arcsec). If the seeing is poor, the default setting is normally sufficient. Second, if wind shake of the telescope is important (especially when observing into the wind, at wind speeds of 10m/s or more). Finally, if there is little wind the solutions will not be good: this typically happens when the wind drops below 2-3 m/s. In the last case, either try doing an image analysis with the telescope pointing into the wind, or wait for the wind to pick up again. Problems may also occur when M1 is significantly warmer than the ambient temperature.
If the automatic mode is used, it should be used for every preset because the corrections are differential relative to the previous values. Thus, if the seeing conditions deteriorate strongly and image analysis becomes unnecessary, the telescope should be pointed close to the zenith before disabling the automatic correction mode in order to return to the default set-up.
There used to be a small offset between the telescope focus at the image analysis camera, and the focus at the instrument. This is not the case anymore, and a telescope focus is generally performed only just after the begining of night.
comments to: NTT Webmasters <firstname.lastname@example.org>
Last Update: April 12,2000 (GM)