Confocal techniques allow the user to achieve optically sectioned images with significantly enhanced axial and improved lateral resolution compared to widefield methods. Unfortunately, as one images more deeply within a sample, sample induced aberrations lead to a significant reduction in image resolution and contrast. Using adaptive optic techniques, we report on the effectiveness of a number of algorithms for removing sample induced aberrations. The viability and efficiency at a number of fitness parameters used in the optimisation routines is also considered.
We present a method for aberration correction in a confocal microscope that successfully combines both a spatial light modulator and a deformable membrane mirror. An active locking technique is used that benefits from the fast update rate of the deformable membrane mirror and the large effective stroke of the spatial light modulator. Concentrating on defocus, we were able to track 'best focus' over a distance of 80 μm with a lock RMS precision of 57 nm. In principle, this technique can be applied to any Zernike mode or aberration that can be accurately reproduced on the deformable membrane mirror.
VISTA is the Visible and Infrared Survey Telescope for Astronomy that has been designed by the UK ATC. The design incorporates two cameras covering the visible and near-IR wavelengths. The 4-m primary mirror has an active support system with 84 axial actuators that is used, in conjunction with the 5-axis support system for the 1.2-m secondary mirror, to maintain the image quality at seeing-limited conditions (~0.6 arcsec at Cerro Paranal, Chile). A system of curvature sensing is proposed to measure the low-order aberrations of the telescope and camera, which uses a pair of CCD arrays located at the edge of the camera focal-plane array. The analysis presented here uses simulated extra-focal images of point sources generated using the optical design program ZEMAX and demonstrates that this off-axis curvature-sensing technique will provide adequate measurements of low-order aberrations under the expected SNR, seeing conditions and field crowding. Even with the VISTA f-ratio of 3.26, an extra-focal distance of only 1 mm is shown to be sufficient to obtain rms wavefront errors accurate to a few tens of nanometres (under ideal and fully time-averaged seeing conditions). The demonstrated insensitivity to crowding means a field of 35 arcmins2, such as obtained with VISTA on a standard CCD, is sufficient to guarantee finding a suitably bright guide star.
VISTA is the Visible and Infrared Survey Telescope for Astronomy that has been designed by the UK ATC. The design incorporates two cameras covering the visible and near-IR wavelengths. An important concern in designing the telescope and cameras is the level of stray, scattered and background light. In the IR camera, K-band (approx. 2.0-2.3 microns) thermal emission from the telescope structure contributes ~50% of the sky background. Therefore a cold baffle is necessary. Due to the large field-of-view, and size of the required optics, a cold stop is precluded. Therefore baffling is provided by a long cryostat and a system of cold baffles with a coating that must absorb in-band light but reflect thermal radiation from the cryostat window, thus reducing the thermal load on the cryostat. In addition, the temporally and spatially variable OH airglow from the atmosphere is obscured by a warm (and hence non-absorbing) annular baffle around the secondary mirror. We report here on the modelling of the scattered and background light for VISTA. The model includes accurate models for the geometry of baffles and optical surface properties of all surfaces in the system. The optical specification is taken from a ZEMAX model and imported into TracePro to generate a fully three-dimensional telescope model, with a simple dome. For both cameras the analysis has been done for the case of scattered light from a full moon at various incident angles from 0 degrees (on axis) to 65 degrees. It is shown that a reflective baffle around the secondary mirror does not significantly impair the performance of the visible camera. Ghosting from bright stars in the field has also been calculated for both cameras. Results indicate that the level of scattered and ghosts is below the limits specified for VISTA.
The design of VISTA (Visible and Infrared Survey Telescope for Astronomy) requires close interaction between the science requirements, the optical and active mechanical design of the telescope and its instrumentation with the wavefront sensing. The optical design is based on an integrated approach of the telescope with tow separate cameras, one working in the IR waveband and the other working in the Visible waveband. The large field of view (2 degrees in the visible and 1.65 degrees in the IR), the seeing-limited resolution required (FWHM of 0.4 arcsec for the visible and 0.5 arcsec for the IR), the technological advance in active telescopes and large IR arrays and the f/1 quasi Ritchey-Chretien telescope design, makes this telescope a very powerful tool in performing high resolution and large astronomical surveys. A system analysis, modeling the various sources of errors such as optical aberrations, surface errors, control errors, environmental effects and detector effects is presented in this paper.
New legislation requiring the monitoring and reduction of polluting gases in both Europe and the US has increased the demand for reliable and affordable Open Path Monitoring Systems, suitable for hazardous area operation. We report on the research and development of a UV Fourier-transform spectrometer based system for open path monitoring of both hazardous and environmental gases. The device has no moving parts and is designed for unattended operation. Laboratory tests have successfully detected the present of and differentiated between SO2 and H2S present in a 1 meter test cell down to 1.0 ppm.m levels. the real time response of the system allows for the use of maximum entropy modeling to predict the size and location of a gas leak. Initial field tests with open air gas releases of SO2 and H2S have verified this. The multiple gas feature of the instrument allows for additional possible applications with regards to environmental monitoring.