Long-exposure Images Research Articles

SEEING MEASUREMENTS ON MOUNT GRAHAM

ABSTRACT A transportable 30-cm aperture telescope was constructed and used with a CCD camera to measure stellar image motion as seen from the summit of Mount Graham in southeastern Arizona. Based on observations during eight nights in October and November 1984, the average value of Fried's length r0 (Fried, 1967) at 0.5 micron wavelength is 13 cm at the zenith, implying a long-exposure visual image full width at half maximum of 0.8 arc second. Since only a small number of nights have been tested, the statistical uncertainty in the average seeing conditions is large and these results should be regarded as preliminary. No large differences were found among three different sites or between different heights of the telescope above the ground. Direct measurements of thermal turbulence in the first few tens of meters above ground indicate that telescopes may be located within a few meters of the ground and well below treetop height without significant seeing degradation.

Achieving high resolution in Earth-based optical astronomy

Progress in astronomy is now a function of unexpected discoveries that have resulted from improvement of its instruments, and specifically of methods for observation of fainter objects and observation with higher angular resolution. The resolution of ordinary astrophotographs obtained with long exposure times is the same for all objects brighter than the limit and is determined only by the atmospheric turbulence generated at existing observatories by orographic atmospheric circulation disturbances. A serious effort to find localities where these disturbances are minor and where the atmosphere is closer to free would make it possible to increase the amount of clear-sky time with images that are very good by present standards (0.5-1'' in a long exposure) to several times those that prevail at existing observatories. Nonclassical, interference methods deliver much better (by factors up to 103) resolution at the cost of very insignificant (factors up to 103–104) deterioration of the ability to register faint objects. The Michelson interferometer at its modern technical level and speckle interferometry are most promising among these methods. Their sensitivity limits are determined by the quantum nature of light and depend strongly, ~β−2, on long-exposure image quality. The intensity interferometer gives even better (now down to 10−3'' ) resolution, but its low sensitivity permits the use of this method only for bright stars. Adaptive optics does not appear to be widely useful in the astronomy of faint objects. A generalized relation is given for sensitivity as a function of resolution in the various techniques for resolution improvement, and prospects for their future development are discussed.

Optical Resolution Through a Randomly Inhomogeneous Medium for Very Long and Very Short Exposures

A theoretical foundation is developed for relating the statistics of wave distortion to optical resolution. The average resolution of very-long- and very-short-exposure images is studied in terms of the phase- and log-amplitude-structure functions, whose sum we call the “wave-structure function.” Those results which are comparable are in agreement with the findings of Hufnagel and Stanley who studied the average modulation transfer function of long-exposure images. It is found that the average short-exposure resolution can be significantly better than the average long-exposure resolution, but only if the wave distortion does not include substantial intensity variation.