D. Dravins, L.Lindegren, E.Mezey & A.T.Young
ATMOSPHERIC INTENSITY SCINTILLATION OF STARS.
III. Effects for Different Telescope Apertures
PASP 110, 610-633 (1998); erratum: PASP 110, 1118 (1998)
Fig. 1. Power spectral density of
scintillation in telescopes of different size. The symbols are values
measured on La Palma for a sequence of small apertures. Their fit
to a sequence of synthetic spectra predicts
the scintillation also in very large telescopes up to 8 m diameter.
Bold curves are for fully open apertures. A central obscuration (secondary
mirror) increases the scintillation power, while apodization decreases
it for high temporal frequencies.
Fig. 2. Power spectral content of
scintillation in different apertures, i.e. the amount of integrated power,
as function of frequency. Observations and simulations are as in
Figure 1. This illustrates where in the spectrum the power is located.
For smaller apertures, the power distinctly shifts towards higher frequencies.
This trend continues until aperture diameters around 5 cm, where the structures
in the "flying shadows" begin to get resolved.
Fig. 3. Scintillation autocovariances,
showing dependence on wind direction (position on the sky) and on central
obscuration (secondary mirror). For a circular and open 20 cm aperture,
the function is shown for the zenith, and for two wind-azimuth angles at
zenith distance Z = 35 deg. The scintillation in a 2.5 m telescope
is much less but shows somewhat complex time structure, caused by its 90
cm secondary mirror. The plot shows autocovariance + epsilon;
= 0.0002 for the smaller and = 0.00003 for
the larger aperture. The true zero levels (= epsilon) are marked.
Although this figure contains synthetic data only, both amplitudes and
timescales were fitted to empirically determined values for summer
conditions on La Palma.
Fig. 4. Transmission profiles of
apodization masks used on La Palma. These "unsharp" telescope apertures
were made from airbrush-painted MylarTM
films. The radial dependence of optical transmission
in two of these is shown, as measured on a microphotometer. The amplitude
scale (left) is relevant for computing diffractive effects of light, while
the scale at right is the ordinary light intensity.
Fig. 5. Measured and synthetic autocovariances
for sharp and apodized apertures. The "sharp" one is a clear
window of similar material as the "strongly apodized" one (Fig. 4).
The increased autocovariance (power) for the apodized aperture is caused
by its smaller effective diameter (due to its apodized edges), mimicking
a smaller sharp aperture. The differences at the highest frequencies
are seen in Fig. 6.
Fig. 6. Measured and synthetic scintillation
power spectra for sharp and apodized apertures. As in Fig. 5, the
increased power at most frequencies for the apodized aperture originates
from it being effectively smaller. However, for the highest frequencies,
there is a tendency for the spectrum to fall off steeply enough, that the
power seen with the apodized aperture becomes less than with the
Fig. 7. Synthetic power spectra for
apertures with and without cental obscuration; with and without apodization.
The scintillation power at 10-100 Hz may differ by an order of magnitude
between telescopes that otherwise would appear to be nearly equivalent.
For investigations that are limited by atmospheric
effects, this shows the potential for improving sensitivity by optimizing
the geometry of the telescope's entrance pupil. These synthetic data
were normalized to observed summer conditions on La Palma, both in power
Fig.8. Scintillation measured through
masks with two apertures, at different separations and position angles.
If the same flying-shadow pattern crosses both apertures, a secondary peak
appears in the autocorrelation, revealing the flying-shadow speed and direction.
The autocorrelation changes significantly with position angle: the secondary
peak is reproducible only within a rather narrow range (about 30 degrees).
For apertures separated by 30 cm, typical delays of 20 ms indicate a flying-shadow
speed of about 15 m/s.
Fig. 9. Double and single apertures,
and different colors. Autocorrelations were measured through a mask
with two 10 cm apertures, separated by 45 cm. The position angle
was adjusted to show a secondary peak due to flying shadows crossing at
a speed of about 15 m/s. This secondary peak remains essentially
unchanged between 400 and 700 nm, but the function is strongly different
from that seen in a single 10 cm aperture.
Fig. 10. Telescope concepts for reducing
scintillation "noise" in stellar observations. The passive system
(left) incorporates a photometer that rapidly (< 10 ms) and with good
spatial resolution (< 10 cm) measures the two-dimensional brightness
distribution over the entrance pupil, thus resolving the spatial, temporal
and chromatic signatures of scintillation. The active system (right)
incorporates second-order adaptive optics which measures the pupil illumination
in real time, and corrects the two-dimensional intensity
excursions across it (e.g., by imaging it through an adaptive two-dimensional
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