Telescopes & recent observational techniques. ASTR 3010 Lecture 4 Chapters 3 & 6. Telescope mounts. Different Designs. Newtonian. Gregorian. Cassegrain. Focal Planes. Prime focus = large field of view, least number of optical elements (best imaging quality). Most radio telescopes.
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Chapters 3 & 6
useful to use a large instrument with the telescope
Fried parameter (r0): size of a typical lump of uniform air in the turbulent atmosphere (meter)
Coherent timescale (second) :
t0 = timescale of the change of turbulence
Typically: r0=10cm, t0=10msec FWHM=1” in the visible (0.5m)
Shorterexposuresallow to freezesomeatmosphericeffects
and reveal the spatial structure of the wavefront corrugation
Sequential 5sec exposure images in the K band on the ESO 3.6m telescope
A Speckle structure appearswhen the exposureisshorterthan the atmospherecoherencetime t0
1ms exposure at the focus of a 4m diameter telescope
Recombine 100s of short exposures to achieve the diffraction limitedimaging
400 100ms exposures
40sec single exposure
When a mirror is warmer that the air in an undisturbed enclosure, a convective equilibrium (full cascade) is reached after 10-15mn. The limit on the convective cell size is set by the mirror diameter
IPSF is peak intensity of an actual image, IAiry is the peak intensity of the Airy pattern
Perfect AO will have a Strehl ratio of 1.0.
GLAO : improve image quality over large FOV
Atmospheric coherence time