Effects of differential extinction across the Earth's atmosphere on astronomical photometry are reviewed. Under the same circumstances when a rotating extended object is observed spectrometrically, a residual radial velocity is obtained which varies during the day. In the case of integral sunlight observations, this effect was calculated along the

Aims:The role of longitudinal structuring of the surrounding corona on the modes of oscillation of a coronal magnetic flux tube was studied in Donnelly et al. (2006) for a piecewise uniform profile. Here we investigate whether a more realistic continuous exponential profile changes the conclusions drawn from that paper. Methods: A partial

ELMER is a multi-purpose Camera&Spectrograph in the optical range for the GTC. ELMERs electro-mechanical design was based in well known, straightforward and reliable solutions, hence each mechanism is based in a wheel driven by a pre-loaded worm gear transmission, except focusing of the focal plane on the CCD which is based in a translation stage

ELMER is a visible imager-spectrograph currently scheduled to be in operation at the GTC on Day One. The design phase of the ELMER Structure and Mechanisms ended on November 2002. Since then, after manufacturing and mechanical integration, an exhaustive test campaign at factory was carried out. We present the fabrication and assembly highlights and

The magnetic structures of the Sun are very inhomogeneous, with irregularities smaller than the smallest sizes that we resolve from Earth. Such irregularities are not properly accounted for by standard magnetic field diagnostic techniques. We have identified a quantitatively important bias that has remained unnoticed hitherto. Intense magnetic