We report our finding that the South Pole is the darkest known Earth-based site for near-infrared astronomical observations. For this reason it has great potential for the most sensitive surveys of distant or faint objects. We find that the south polar sky background is substantially darker in the standard near-infrared 7, H, and K filters, and in an optimized X DARK filter centered at 2.36 /¿m. In particular, the ä'dark background at the South Pole is only 162± 67 /xJy arc sec -2 at the zenith. This is consistent with the results described in an accompanying paper by Ashley et al. (1996, PASP, 108, 721) and is comparable to the sky brightness measured by high-altitude balloon at 2.4 /im (Matsumoto et al. 1994, PASP, 106, 1217.
Ground based high-contrast imaging (e.g. extrasolar giant planet detection) has demanding wavefront control requirements two orders of magnitude more precise than standard adaptive optics systems. We demonstrate that these requirements can be achieved with a 1024-Micro-Electrical-Mechanical-Systems (MEMS) deformable mirror having an actuator spacing of 340 microm and a stroke of approximately 1 microm, over an active aperture 27 actuators across. We have flattened the mirror to a residual wavefront error of 0.54 nm rms within the range of controllable spatial frequencies. Individual contributors to final wavefront quality, such as voltage response and uniformity, have been identified and characterized.
We present results from a series of optical (g and r-band) and near-infrared (K ′ -band) observations of the region of the sky including the entire XTE and ROSAT error circles for the "Bursting Pulsar" GRO J1744-28. These data were taken with the Astrophysical Research Consortium's 3.5-m telescope at Apache Point Observatory and with the 2.2-m telescope at the European Southern Observatory. We see no new object, nor any significant brightening of any known object, in these error circles, with the exception of an object detected in our 8 February 1996 image. This object has already been proposed as a near-infrared counterpart to GRO J1744-28. While it is seen in only two of our ten 8 February frames, there is no evidence that this is an instrumental artifact, suggesting the possibility of near-infrared flares from GRO J1744-28, similar to those that have been reported from the Rapid Burster. The distance to the "Bursting Pulsar" must be more than 2 kpc, and we suggest that it is more than 7 kpc.
Current high-contrast "extreme" adaptive optics (ExAO) systems are partially limited by deformable mirror technology. Mirror requirements specify thousands of actuators, all of which must be functional within the clear aperture, and which give nanometer flatness yet micron stroke when operated in closed loop. 1 Micro-electricalmechanical-systems (MEMS) deformable mirrors have been shown to meet ExAO actuator yield, wavefront error, and cost considerations. This study presents the performance of Boston Micromachines' 1024-actuator continuous-facesheet MEMS deformable mirrors under tests for actuator stability, position repeatability, and practical operating stroke. To explore whether MEMS actuators are susceptible to temporal variation, a series of long-term stability experiments were conducted. Each actuator was held fixed and the motion over 40 minutes was measured. The median displacement of all the actuators tested was 0.08 nm surface, inclusive of system error. MEMS devices are also appealing for adaptive optics architectures based on open-loop correction. In experiments of actuator position repeatability, 100% of the tested actuators returned repeatedly to their starting point with a precision of < 1 nm surface. Finally, MEMS devices were tested for maximum stroke achieved under application of spatially varying one-dimensional sinusoids. Given a specified amplitude in voltage, the measured stroke was 1 µm surface at the low spatial frequencies, decreasing to 0.2 µm surface for the highest spatial frequency. Stroke varied somewhat linearly as inverse spatial frequency, with a flattening in the relation at the high spatial frequency end.
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