630406Eclipse4_p3

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Eclipse Bulletin No, 4 cont'd.

2. TOTALITY	

   (b) Corona.  colour. Here again it has been pointed out that the anateur can make
   significant contributions. "Observations have been made at previous eclipses which
   indicate that the colour of the inner corona is not the same as the outer corona
   but the results are discordant. The usual method is to take well calibrated photo-
   graphs of the corona using fflters of two colours (red and blue) and note intensity
   differences. The experiment, although simple in principle, is however a difficult
   one in practice since it is diffic4t to achieve the required accuracy of calibrat-
   ion and freedom from scattered light which may invalidate results."

   (c) Prominences. The prominences red, fiery tongues of light at the inner edge
   of the corona are sometimes very fine, sometimes missing entirely. This detail
   can be obtained only by photographing through telescope or binoculars.

3. SHADOW BANDS

       The phenomenon of the shadow bands is described in Bulletin No.3. They are
   very difficult to photograph on account of the low illumination and the speed of
   the movement.  Use a short exposure time (about 1/100 second), a large aperture
   (f/2.0) and a high speed film (Tri-X). The camera should face downward toward a
   white sheet on which is placed a compass and some scale for measuring widths of
   the bands. As explained in Bulletin No. 3, they should be observed both before and
   after totality. Because of their short duration, the photographer has to be on Us
   alert if he hopes to take more than one exposure.

4. LANDSCAPE ILLUMINATION

       It is interesting to record changes in landscape tllumination by selecting a
   subject presenting a fair range of contrast, aiming the camera and taking photo-
   graphs at regular intervals throughout the eclipse, say, every five minutes with
   one exposure at mid-totality, using a constant exposure.  Exposures made at the
   beginning and end will probably have to be somewhat over-exposed if anything is to
   register during totality.

5. SPECTROSCOPIC PHOTOGRAPHY	

   (a) Flash Spectrum.  The phenomenon of the flash spectrum is described in Bulletin
   No. 3. It is of very short duration (about 2 seconds), and so the photographer
   has little margin for error in taking his shots.  The flash spectrum can be photo-
   graphed by placing a prism or diffraction grating over the lens of an ordinary
   camera and timing the camera so that the spectrum enters the lens. If a prism is
   used, the lens should have a long Local length (about 100 mm.).  Black-and-white
   film should be used if measurements are intended; otberwise, colour film gives
   interesting results.  The shutter of the camera should be set to B (bulb), the
   lens wide open and focus at infinity, and a cable release used, The observer,
   watching through another prism or grating, should press the release as soon as
   the bright-line spectrum appears, and release it as soon as possible after the
   onset of totality. Another method, whereby the entire change in the spectrum can
   be obtained by winding the film continuously, will be described in a later bulletin
   covering special projects.


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