Disadvantages

Diffraction micrometers have one drawback. As the grating consists of bars and slits with the same width, only 50% of the incident light from the double star will reach the telescope optics. Of this, about 50% of the residual light will end up in the zero-order images resulting in a total loss of 1.5 magnitudes compared with the unobstructed telescope. Another 20% goes into each of the first order images, the rest being lost in the additional satellites. Because of these losses the combination of a 20-cm telescope and a diffraction micrometer will allow observations of double stars as faint as about magnitude 7.0-7.5 with components which do not differ too much in brightness.

The diffraction micrometer formula includes the factor X, the wavelength of light. As the observation is made visually the satellite's exact distance from the primary star depends on the observer's own wavelength sensitivity but also on the stars' colours. The observer's most sensitive wavelength which should be used in the formula has to be established by comparisons with pairs with accurately known separations. A normal figure for X to start with might be 5650 A, or 0.000565 mm if p in the formula is in millimetres. This corresponds approximately with the effective wavelength of a white, class A spectral type star.

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