Double-image micrometers have some distinct advantages:
- less danger of systematic errors compared with filar micrometers;
- work well with imperfect clock drives;
- for close pairs the distance measurements are easier than with a filar micrometer;
- for close pairs - distance about 2'.'0 and less - with a magnitude difference of not more than 1.5 magnitudes the distance measurements (double-distances, stars on a straight line) are more accurate than with a filar micrometer.
On the other hand the disadvantages are:
- less brightness of the images (loss of about 0.75 magnitudes);
- limitation to relatively close pairs;
- the important parameter e/F (Lyot micrometer) is not easy to achieve.
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