|
|
 |
|
Abstract Review
| Corresponding Author |
|
|
|
| Authors |
| Name | | Affiliation |
|
Andrei Tokovinin |
|
CTIO, NOAO |
|
|
| Abstract |
| Session | | 1 (Instrumentation and observations to quantify the magnitude and distribution of atmospheric optical turbulence.) |
| Title | | 'An instrument to measure the atmospheric time constant' |
| Abstract | | In the context of site selection for future generations of telescopes, the coherence time is a particularly important parameter that determines the sensitivity of interferometers and the performance of adaptive-optics devices. But there is currently no suitable, simple technique to measure the coherence time: either the instrument is not well suited for site monitoring, or the method is burdened by intrinsic uncertainties and biases. Site testing and monitoring campaigns rely, therefore, predominantly on the assessment of the seeing.
To close the current gap, a method has been recently proposed to measure the coherence time with a small telescope (Kellerer&Tokovinin, A&A, in press). It suggests to shift the image of a star somewhat out of focus, which converts it - due to a suitably enlarged central blind area of the telescope - to a ring. Insertion of a lens with proper spherical aberration sharpens the ring into a narrow circle, and atmospheric turbulence causes then distortions which can be conveniently assessed, because, to a first approximation, they appear as ring-radius changes. A relation between the temporal properties of the radius variations and the coherence time has been developed in the framework of the Kolmogorov theory of turbulence.
First measurements with this Fast Defocus Monitor, FADE, were obtained at Cerro Tololo, Chile, in November 2006. Ring images were recorded during five nights using a broad range of instrument settings. The measurements and their uncertainties are currently being analyzed. The resulting seeing and coherence-time values are to be compared with simultaneous measurements from the MASS and DIMM site-monitoring instruments.
|
|
Return to Program and Events
|
|
|
|
