Directionality of individual cone photoreceptors in the parafoveal region.

The pointing direction of cone photoreceptors can be inferred from the Stiles-Crawford Effect of the First Kind (SCE-I) measurement. Healthy retinas have tightly packed cones with a SCE-I function peak either centered in the pupil or with a slight nasal bias. Various retinal pathologies can change the profile of the SCE-I function implying that the arrangement or the light capturing properties of the cone photoreceptors are affected. Measuring the SCE-I may reveal early signs of photoreceptor change before actual cell apoptosis occurs. In vivo retinal imaging with adaptive optics (AO) was used to measure the pointing direction of individual cones at eight retinal locations in four control human subjects. Retinal images were acquired by translating an aperture in the light delivery arm through 19 different locations across a subject's entrance pupil. Angular tuning properties of individual cones were calculated by fitting a Gaussian to the reflected intensity profile of each cone projected onto the pupil. Results were compared to those from an accepted psychophysical SCE-I measurement technique. The maximal difference in cone directionality of an ensemble of cones, ρ¯, between the major and minor axes of the Gaussian fit was 0.05 versus 0.29mm(-2) in one subject. All four subjects were found to have a mean nasal bias of 0.81mm with a standard deviation of ±0.30mm in the peak position at all retinal locations with mean ρ¯ value decreasing by 23% with increasing retinal eccentricity. Results show that cones in the parafoveal region converge towards the center of the pupillary aperture, confirming the anterior pointing alignment hypothesis.

Rapid measurement of individual cone photoreceptor pointing using focus diversity.

A novel method is presented to rapidly measure the pointing direction of individual human cone photoreceptors using adaptive-optics (AO) retinal imaging. For a fixed entrance pupil position, the focal plane is rapidly modulated to image the guided light in various axial planes. For cones with different pointing directions, this focus diversity will cause a shift in their apparent position, allowing for their relative pointing to be determined. For four normal human subjects, retinal images were acquired, registered, and the positions of individual cones tracked throughout the dataset. Variation in cone tilt was 0.02 radians, agreeing with other objective measurements on the same subjects at the same retinal locations.

James Webb Space Telescope segment phasing using differential optical transfer functions.

Differential optical transfer function (dOTF) is an image-based, noniterative wavefront sensing method that uses two star images with a single small change in the pupil. We describe two possible methods for introducing the required pupil modification to the James Webb Space Telescope, one using a small (<λ/4) displacement of a single segment's actuator and another that uses small misalignments of the NIRCam's filter wheel. While both methods should work with NIRCam, the actuator method will allow both MIRI and NIRISS to be used for segment phasing, which is a new functionality. Since the actuator method requires only small displacements, it should provide a fast and safe phasing alternative that reduces the mission risk and can be performed frequently for alignment monitoring and maintenance. Since a single actuator modification can be seen by all three cameras, it should be possible to calibrate the non-common-path aberrations between them. Large segment discontinuities can be measured using dOTFs in two filter bands. Using two images of a star field, aberrations along multiple lines of sight through the telescope can be measured simultaneously. Also, since dOTF gives the pupil field amplitude as well as the phase, it could provide a first approximation or constraint to the planned iterative phase retrieval algorithms.

Calibrating a high-resolution wavefront corrector with a static focal-plane camera.

We present a method to calibrate a high-resolution wavefront (WF)-correcting device with a single, static camera, located in the focal-plane; no moving of any component is needed. The method is based on a localized diversity and differential optical transfer functions to compute both the phase and amplitude in the pupil plane located upstream of the last imaging optics. An experiment with a spatial light modulator shows that the calibration is sufficient to robustly operate a focal-plane WF sensing algorithm controlling a WF corrector with 40,000 degrees of freedom. We estimate that the locations of identical WF corrector elements are determined with a spatial resolution of 0.3% compared to the pupil diameter.

In vivo imaging of the human rod photoreceptor mosaic.

Although single cone receptors have been imaged in vivo, to our knowledge there has been no observation of rods in the living normal eye. Using an adaptive optics ophthalmoscope and post processing, evidence of a rod mosaic was observed at 5° and 10° eccentricities in the horizontal temporal retina. For four normal human subjects, small structures were observed between the larger cones and were observed repeatedly at the same locations on different days, and with varying wavelengths. Image analysis gave spacings that agree well with rod measurements from histological data.