354 research outputs found
Curvature sensors: noise and its propagation
The signal measured with a curvature sensor is here analyzed. In the outset,
we derive the required minimum number of sensing elements at the pupil edges,
in dependence on the total number of sensing elements. The distribution of the
sensor signal is further characterized in terms of its mean, variance, kurtosis
and skewness. It is established that while the approximation in terms of a
gaussian distribution is correct down to fairly low photon numbers, much higher
numbers are required to obtain meaningful sensor measurements for small
wavefront distortions. Finally, we indicate a closed expression for the error
propagation factor and for the photon-noise induced Strehl loss.Comment: Accepted for publication in the Adaptive Optics Feature of JOSA
Detection of phase singularities with a Shack-Hartmann wavefront sensor
While adaptive optical systems are able to remove moderate wavefront
distortions in scintillated optical beams, phase singularities that appear in
strongly scintillated beams can severely degrade the performance of such an
adaptive optical system. Therefore, the detection of these phase singularities
is an important aspect of strong scintillation adaptive optics. We investigate
the detection of phase singularities with the aid of a Shack-Hartmann wavefront
sensor and show that, in spite of some systematical deficiencies inherent to
the Shack-Hartmann wavefront sensor, it can be used for the reliable detection
of phase singularities, irrespective of their morphologies. We provide full
analytical results, together with numerical simulations of the detection
process.Comment: 23 pages, 9 figure
Stroke saturation on a MEMS deformable mirror for woofer-tweeter adaptive optics
High-contrast imaging of extrasolar planet candidates around a main-sequence
star has recently been realized from the ground using current adaptive optics
(AO) systems. Advancing such observations will be a task for the Gemini Planet
Imager, an upcoming "extreme" AO instrument. High-order "tweeter" and low-order
"woofer" deformable mirrors (DMs) will supply a >90%-Strehl correction, a
specialized coronagraph will suppress the stellar flux, and any planets can
then be imaged in the "dark hole" region. Residual wavefront error scatters
light into the DM-controlled dark hole, making planets difficult to image above
the noise. It is crucial in this regard that the high-density tweeter, a
micro-electrical mechanical systems (MEMS) DM, have sufficient stroke to deform
to the shapes required by atmospheric turbulence. Laboratory experiments were
conducted to determine the rate and circumstance of saturation, i.e. stroke
insufficiency. A 1024-actuator 1.5-um-stroke MEMS device was empirically tested
with software Kolmogorov-turbulence screens of r_0=10-15cm. The MEMS when
solitary suffered saturation ~4% of the time. Simulating a woofer DM with ~5-10
actuators across a 5-m primary mitigated MEMS saturation occurrence to a
fraction of a percent. While no adjacent actuators were saturated at opposing
positions, mid-to-high-spatial-frequency stroke did saturate more frequently
than expected, implying that correlations through the influence functions are
important. Analytical models underpredict the stroke requirements, so empirical
studies are important.Comment: 16 pages, 10 figure
Elevations in serum glycoprotein:N-acetylneuraminic acid transferases in rats bearing mammary tumors
Nonradioactive, ultrasensitive site-specific proteinâprotein photocrosslinking: interactions of α-helix 2 of TATA-binding protein with general transcription factor TFIIA and transcriptional repressor NC2
We have developed an approach that enables nonradioactive, ultrasensitive (attamole sensitivity) site-specific proteinâprotein photocrosslinking, and we have applied the approach to the analysis of interactions of α-helix 2 (H2) of human TATA-element binding protein (TBP) with general transcription factor TFIIA and transcriptional repressor NC2. We have found that TBP H2 can be crosslinked to TFIIA in the TFIIAâTBPâDNA complex and in higher order transcriptionâinitiation complexes, and we have mapped the crosslink to the âconnectorâ region of the TFIIA α/ÎČ subunit (TFIIAα/ÎČ). We further have found that TBP H2 can be crosslinked to NC2 in the NC2âTBPâDNA complex, and we have mapped the crosslink to the C-terminal âtailâ of the NC2 α-subunit (NC2α). Interactions of TBP H2 with the TFIIAα/ÎČ connector and the NC2α C-terminal tail were not observed in crystal structures of TFIIAâTBPâDNA and NC2âTBPâDNA complexes, since relevant segments of TFIIA and NC2 were not present in truncated TFIIA and NC2 derivatives used for crystallization. We propose that interactions of TBP H2 with the TFIIAα/ÎČ connector and the NC2α C-terminal tail provide an explanation for genetic results suggesting importance of TBP H2 in TBPâTFIIA interactions and TBPâNC2 interactions, and provide an explanationâsteric exclusionâfor competition between TFIIA and NC2
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