Il DIMM per TNG - Dario Mancini, inventore, ingegnere ed astronomo

Generalities

The effects of atmospheric turbulence on astronomical observations is a great limit for the ground-based telescopes, producing random perturbations on the starlight crossing through the turbulence layers. The evaluation of  optical distortion of the wavefront coming from a point source is usually derived from the analysis of the stellar image apparent size, in particular  measuring the Full Width at Half Maximum (FHWM) of a long exposure image at the focus of a telescope. This goal is not easy because the stellar images at a telescope are affected also by diffusion of  the radiation or electrons in the detector, vibrations of the telescope, tracking and focus  problems. A seeing Differential Image Motion Monitor (DIMM) was conceived for the evaluating qualitative and quantitative seeing. It was developed by ESO firstly for Chile Paranal site testing (tower made by Dario and Guido Mancini);  later a DIMM was adopted by IAC on Roque de los Muchachos Observatory (ORM).

Following the positive  experiences of ESO and IAC, in 1997 a DIMM was been installed for seeing monitoring for Galileo National Telescope (TNG) at ORM.

The design and practical  realization of tower, foundation, concrete, telescope, control system and control software, has been completely done by Dario Mancini and some collaborators of his staff at the Astronomical Observatory of Capodimonte (OAC) and at ORM. Unlike of the previous systems, TNG DIMM is fully automated and no personnel was required during the measures. This means that high system reliability in terms of quality and  continuity of the service has been reached.

The differential Method

The DIMM basic principle is the differential method. The telescope aperture is forced by two circular holes with same diameter D and at a given distance d between them. In perfect atmospheric conditions, the light comes as a plane wave determining two stable identical images or spots. The atmospheric turbulence effect gives a delay to the light coming through the two holes, causing a relative movement of the two spots obtained on the CCD detector, one in respect to each other. Their mean movement is proportional to the atmospheric seeing. This principle avoids tracking errors inducted by monitoring one star only on the focal plane. In fact, the noise inducted by telescope tracking is subtracted and doing so it is not needed a special optical quality, making more cheap the whole system. Furthermore the measurement is insensible to the system mechanical vibrations.  
The advantages of the method are several:  

• no good optical quality is required  
  

• it is independent of the telescope size
  

• it is no sensitive to tracking or mechanical telescope spurious motions
  

• it is wavefront independent
  

But how we can obtain the seeing values? The answer is: statistically! It consists in calculating the wavefront incidence angle covariance on both parallel and perpendicular directions of sub-pupils axes.

The DIMM Tower

The instrument is placed in open air on a 5m high tower, about 100m far from TNG dome. The DIMM support tower is a steel based structure, in order to reduce external noise in the measurement due to its very low thermal capacity. A metallic structure presents some advantages in respect to a concrete one, for the.following considerations: is easy to be assembled or removed; it requires smaller and cheaper foundations. In case of further locations, fondation can be hidden below ground level and rebuilt in another place ; it is characterized by a smaller global thermal inertia (fast adaptation of the structure to temperature fluctuations) means no additional local seeing effects during the night; the structure resists the wind pressure and is characterized by high resonance frequencies; it is similar to an antenna support, thus permitting to be considered as not permanent. The structure consists of two main parts: the DIMM and the personnel towers. The first one has been designed in order to minimize the top rotation in case of wind turbolence and in case of any induced vibrations. Its design is similar to a telescope serrurier thus allowing the connection of different devices to the top ring. Tower performances are strongly dependent on foundation stiffness that are designed, in order as similar as possible to a fixed joint.
The personnel support tower is metallic too, has been designed to resist to the worst weather condition, 200Km/h wind and ice,carring easily and safely heavy weigth, project's priority is/was security of personnel and instrumentation. It has been studied also to minimize the vibrations trasmitted through the foundations to the DIMM tower.