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Abstract: After proposing a new method of deriving the atmospheric time constant fromthe speed of focus variations Kellerer and Tokovinin 2007, we now implement itwith the new instrument, FADE. FADE uses a 36-cm Celestron telescope that ismodified to transform stellar point images into a ring by increasing thecentral obstruction and combining defocus with spherical aberration. Sequencesof images recorded with a fast CCD detector are processed to determine thedefocus and its variations in time from the ring radii. The temporal structurefunction of the defocus is fitted with a model to derive the atmospheric seeingand time constant. We investigated by numerical simulation the data reductionalgorithm and instrumental biases. Bias caused by instrumental effects, such asoptical aberrations, detector noise, acquisition frequency, etc., isquantified. The ring image must be well-focused, i.e. must have a sufficientlysharp radial profile, otherwise, scintillation seriously affects the results.An acquisition frequency of 700 Hz appears adequate. FADE was operated for 5nights at the Cerro Tololo observatory in parallel with the regular sitemonitor. Reasonable agreement between the results from the two instruments hasbeen obtained.

Autor: A. Tokovinin, A. Kellerer, V. Coude Du Foresto

Fuente: https://arxiv.org/

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