630610Eclipse7_p2

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Eclipse Bulletin No. 7 cont'd.                                                -2-


3. Photography of the Corona: It is seen from the table in the last paragraph that
the outer corona requires about five times the exposure sufficient for the inner
corona. To record the fainter details without over-exposing the bright inner
parts, some device must be used to reduce the light reaching the film in increasing
amotuits toward the centre of the image to, say, 1/2 that entering at the edge. For
this purpose, three methods may be recommended:
(1) a rotating sector shutter in the image plane, centred on the image;
(2) a circular filter dense at the centre and thinning out toward the edge;
(3) a small disc (comparable in size with the diameter of the image) placed
      inside focus . The positions of afl three are shown in the diagram below:

           (Diagram here.)


4.  Spectroscopic Projects: Photography of the flash spectrum was described in
    Bulletin No.4 and need not be repeated. However, for photographing the spectrum
    of the corona or of the light in the sky, an entire]y different approach must be
    used. A telescope must be used to gather light. Instead of the eyepiece, a
    slit is mounted in the focal plane and the usual train of collimating lens,
    grating or prism, and camera lens must be employed as shown in the diagram.
    While a diffraction grating has the advantage of giving a constant dispersion,
    the brighter spectrum from a prism permits shorter exposures.

    The writer has been unable to find any information concerning exposure times;
    however, the corona is usual]y rather less bright than the full moon, and on this
    basis it is suggested that those undertaking s‡ectroscopic work should practise
    on the moon, and then, knowing the necessary exposure for the moon, give twice or
    four times this exposure at the eclipse itself (preferably both, if there is time).


    (Diagram here.)


5.  Measurement of the Colour of the Corona: Past experiments suggest that the colour
    of the inner corona is not the same as that of the outer corona, but results vary.
    This sort of information can be obtained by measuring the darkening at different
    points On photographs of the corona taken through red and blue filters, and can-
    paring red and blue for different parts of the corona. This method, although
    simple in principle, is easily made worthless by insufficient]y accurate calibra-
    tion and by spurious effects caused by scattered light.

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