Narrow-band filters are often used to constrain the chemical composition of astronomical objects through photometry. A challenge to derive accurate photometry is that narrow-band filters are based on ...interference of multiple reflections and refractions between thin layers of transparent dielectric material. When the light rays reach the surface of a filter not perpendicular to it, they cross the layers obliquely travelling a path longer than the thickness of the layers and different for each inclination. This results in a blue-shift of the central wavelength and a distortion of the transmission curve. Hence, particular care should be taken when narrow band filters are used in presence of small f-numbers and large non-telecentric angles, as frequent in the large field of view (FoV) instruments. Sometimes, the broadening and central wavelength shift of the transmission curve are considered and compensated in the design of filters for instruments with a small f-number. Here we consider the combined effect of small f-number, non-telecentricity and large FoV. Where single spectral lines are considered, a shift in central wavelength or a change in the shape of the transmission curve may introduce an instrumental dispersion in luminosity and in the linked color indices. We found that transmission curves of narrow band filters can be significantly different in shape than the nominal ones. The bottom limits for filters' effective FWHM for each f-number; the monotonic behavior of the blue-shift with distance from the center of FoV; the monotonic quality decrease of the transmission curves and the photometric dispersion introduced by the filters are computationally estimated. This work could represent a useful tool to evaluate the fitness of a particular filter at a particular facility.
Full sky coverage Adaptive Optics on Extremely Large Telescopes requires the
adoption of several Laser Guide Stars as references.With such large apertures,
the apparent elongation of the beacons is ...absolutely significant.With few
exceptions,WaveFront Sensors designed for Natural Guide Stars are adapted and
used in suboptimal mode in this context. We analyse and describe the
geometrical properties of a class of WaveFront Sensors that are specifically
designed to deal with Laser Guide Stars propagated from a location in the
immediate vicinity of the telescope aperture. We describe in three dimensions
the loci where the light of the Laser Guide Stars would focus in the focal
volume located behind the focal plane (where astronomical objects are
reimaged). We also describe the properties of several types of optomechanical
devices that, through refraction and reflections, act as perturbers for this
new class of pupil plane sensors, which we call ingot WaveFront Sensor. We give
the recipes both for the most reasonable complex version of these WaveFront
Sensors, with 6 pupils, and for the simplest one, with only 3 pupils. Both of
them are referred to the ELT case. Elements to have a qualitative idea of how
the sensitivity of such a new class of sensors compared to conventional ones
are outlined. We present a new class of WaveFront Sensors, by carrying out the
extension to the case of elongated sources at finite distance of the pyramid
WaveFront Sensor and pointing out which advantages of the pyramid are retained
and how it can be adopted to optimize the sensing.
MORFEO (Multi-conjugate adaptive Optics Relay For ELT Observations, formerly MAORY), the MCAO system for the ELT, will provide diffraction-limited optical quality to the large field camera MICADO. ...MORFEO has officially passed the Preliminary Design Review and it is entering the final design phase. We present the current status of the project, with a focus on the adaptive optics system aspects and expected milestones during the next project phase.
