The Speckle camera of the TNG

The adaptive optics system of TNG AdOpt@TNG is equipped with a real-time speckle camera having diffraction limited imaging capabilities at visible wavelegths.

The astronomical target is reimaged, through the AdOpt@TNG optical train and a dedicated objective, on an intensified (one stage MCP) TV camera. The frames (25 per second) are equidistributed between three PCs which provide to perform the mathematical transformations required. At the end of the integration time the collected and transformed data are available to the user. During this process no frames are lost. The paper cointaining the detailed description of the TNG speckle camera is downloadable here and for quick view of its capabilities please refer to the AdOpt@TNG first light (21 Dec 1998).

Two kind of data are available:

  • Power spectrum of the luminosity distribution of the target, suitable for the calculation of the autocorrelation function (Labeyrie method).

  • Shift and add of the single frames, based on the recentering of the brightest pixel.

The speckle camera doesn't provide direct diffraction limit imaging (i.e. Knox & Thompson method, Speckle Masking method).

The data are in the form of a 128 x128 pixel matrix, with a pixel size of 0.027", giving a field of view of 3.5" x 3.5". A set of filters is available:

Filters Available
Lambda [nm] Bandwidth [nm] Notes
475 30 b Strömgren
547 30 y Strömgren
550 10 C2
570 10 C2 continuum
650 10 ZrO
660 10 H alpha
670 10 TiO
580 100 wide band

The camera integration time (from 2 to 20ms) and gain are adjustable.

Suggestions for the observations

This speckle camera is suitable for the following purposes:

  • To resolve double o multiple stars of very small separation.
  • To determine shape and angular dimensions of objects with very small angular size.

In order to deconvolve the collected data from the atmospheric speckle transfer function (STF), a nearby single reference star must be observed.

Due to the short time scale, temporal and spatial, variations of the seeing behaviour, the STF obtained observing the reference star should not differ from the STF affecting the observed target: as for near infrared observations, every few minutes it is necessary to switch between reference and target as many times as requested by the total integration time.

The reference star must follow some constraints:

  • It must be a point like source (mandatory).
  • Its distance from the astronomical target must not exceed 5 degrees (strict).
  • Its magnitude must be comparable with the target's one (not so strict).

Despite this constraints it is quite easy to find a suitable reference star.

A mandatory constraint is concerning the astronomical target angular size: is must be smaller than one half of the field of view, that is 1.7".

The smaller are the single frame integration time, the filter bandwith and the seeing angular size and the larger is the signal to noise ratio achievable. In general speckle interferometry observations can be performed successfully when seeing is under 2".

In the case of binary or multiple systems the magnitude difference between the components strongly affects the sensitivity of the analysis: it is recommended that this difference is not larger than 3 magnitudes.

The limiting magnitude is forseen between 11 and 13 depending on the seeing conditions.

For any comments please contact Adriano Ghedina.