SOLAR ECLIPSE

Hokitika Guardian, 30 August 1930, Page 6

SOLAR ECLIPSE

PHOTOGRAPHING THE SON. GOOD INSTRUMENTS. RUSH PERIOD : 94 SECONDS. Some of the most important of the ■ scientific instruments to be used by the New Zealand Eclipse Expedition next October, have been. assembled and tested at the Dominion Observatory, ■ Kelburn, by Dr C. E. Adams, Dominion Astronomer, and Mr P. W. Glover, B.Sc., F.R.A.S., a well-known New Zealand astronomical worker. The ! most interseting part of i tion’s equipment—the coronograph, 1 which will be used for photographing ‘ the elusive corona which springs into view during the short period of totality. This corona is a flaming envelope surrounding the sun, and is normally lost to sight through the brightness of the mass of the sun itself. When the whole disc of the sun is obscured the corona becomes visible. The apparatus consists off a long, almost horizontal shaft, together with x circular inclined mirror fixed fan

close to it. The shaft is the 19ft camera .for photographing the eclipsed sun \and its’ surrounding corona... . At the end of the shaft near the circular morror is • fixed the four-inch object lens..’of the..camera. It has a long fecal length, and the plate is fixed at the other end,' 19ft .away;. This elongated camera,. although an important part /of the apparatus, is much simpler than the remainder.

THE PROBLEM AND ITS SOLUTION. Betfore describing the separate mirror and its mechanism it is worth while sketching the methods by which solar eclipse photograhpy is undertaen. In the first place the sun, while the eclipse is in progress, is moving continuously through the sky. If the corona is to be photographed a comparatively long exposure is necessary, seeing that it is only .visible when the suns disc is obscuredand the sky considerably darkened.,.'lt' becomes, therefore, a problem of keeping the sun’s image in exactry the same position on the photographic plat.e throughout the exposure. In that case there are only a few possible riiethod. The camera could be arranged to follow the motion of the sun- accurately, or finally, both the camera and plate might be kept steady and some even more artificial device used for keeping the image of the sun in its place. Mr Glover prepared a parper last year in which data was worked out for the coming eclipse for a station situated at Niuafou, using the moving plate method. Copies of this paper have been requested by‘ the large American party who are understood to be following this method in their coronal photography at Niuafou. The New Zealand party, however, are using the last of the: three methods -mentioned above. The long camera and plate are kept stationary,, and the sun’s rays are deflected into the lens by. means of the circular morror.

The mirror, which is a 12in. one, perfectly flat, is arranged on a mecnanism that moves it so that the rays from tlie sun which are reflected into the camera are kept in the same position. For tins reason the mirror is mounted on an axis parallel to the axis of the earth. Telescopes in which the axis of rotation is parallel to the axis of the earth are known, as equatorials, and their main advantage is that they can be kept pointing to a fixed star by mean's of the “clock.” This “clock” is geared to the axis of the instrument and turns it slowly through the same angle in an hour as the earth turns in an hour, or—what is the . same tiling—as the sky seems to move in an hour. The mechanism which turns the coeiostat mirror «o tjiat the sun’s rays are thrown constantly to the same portion of the photographic plate is a very similar one to that employed in equatorials. It has this difference, however—that it turns at only half the speed of the sun. This is to allow for reflection, which doubles corresponding angles.

THE MOST VITAL PART. In the photograph the coeiostat mirror is seen mounted on its equatorial axis. At the top of this axis is the theodolite which will be used for adjust ing the axis of the coeiostat to its appropriate direction. Behind and below these, on the heavy iron .frame which supports the instrument, is the geared mechanism operated, from the “clock”. The latter is connected to the coeiostat and its back mechanism by a rotating shaft equipped with a universal joint. Hanging below, a tripod are the heavy weights which keep the clock and. hence the mirror itself in action. The adjustment of the axis off the mirror, to meet the latitude of Niuafou and the conditions of the eclipse will be a difficult and most responsible part of the job. The object glass of the long camera will be about three feet away from the mirrow, and the camera itself will tilt silghtly upward.

At the island the eclipse begins at five minutes to eight on the morning of October 22. The sun will become more and more obscured until finally the whole sun will he blocked out at eight minutes past nine. The period of totality last 94 seconds, and during that tipie the coeiostat and camera will be carrying through an important job and will hqye to be functioning perfectly.