Telescopic Definition in a Hazy Sky

Abstract

The diminution of star disks during temporary haze or even by a thin passing cloud has been particularly noticed by Sir John Herschel. The effect of this haze is apparently to diminish the intensity of diffraction phenomena. The markings on the full moon have never been seen so blackened and distinct as through the haze of a London fog. On one occasion a very gentle east wind brought down a London fog to Reading. That evening the air was extremely still, but embrowned with the haze. Castor bore a magnifying power of 600 with a Wray 51/4 acbromatic. The object-glass was being adjusted to the axis of the tube. Finally an intense jet black ring was seen to surround each star of this celebrated double, supplemented with one bright perfectly defined diffraction-ring, then a fainter ring at a further distance could be descried. The perfection of this definition has never since been attained. The haze had settled down into still air. The eastern breeze had died away. The brown fog remained in the sky; diffraction assumed its most perfect form. The moon bore any power I could muster with absolutely steady definition. These states of the atmosphere are extremely rare. I can now record a repetition of this wonderful steadiness. On November 9 Col. Abadie, Mr. Maunsel, and myself, were observing Saturn with an 81/2 Calver mirror. The crape veil and belts were well shown with the pale blue Polar cap. Ball's division was intensely black. The outer ring was narrow, and its dark grey tint contrasted strongly with the brilliant whiteness of the inner ring. I was greatly astonished, however, to descry Encke's division on the outer ring usually called A. The shadows of the ball were well marked. The eastward shadow much stronger than the western. A very brilliant narrow band edged the northern belt near the plane of the rings. The inner edge of the ring B projected a thin shadow on the ball. A lady who had no previous knowledge of the belts, entering the observatory last night, said they appeared to consist of several fine lines of parallel bands separated by very fine brighter bands. At 11 p.m. there was a very wide narrow halo round the moon. Saturn appeared much bedimmed, and Jupiter shorn of his brilliance. Yet Col. Abadie writes: “Jupiter was a sight to be remembered. The distinctness of all the belts was enhanced by bright zones; one to the north of the uppermost belt was particularly to be noticed. The clouds between the equatorial belts changed in appearance from being very fleecy to a long wavy appearance” (we were observing about three hours). The moon was too dim for observation. Encke's division is so seldom seen in my experience, its appearance in the field of so small a telescope was a matter of great surprise; and but for the concurrent testimony of two others for about a space of three hours, I certainly should have greatly doubted the accuracy of the observation. At all events, it is a valuable demonstration of the value of the chiaro-oscuro in astronomical research.