Among the matters a double star observer must consider is that of a suitable focal length, or more specifically a suitable focal ratio. When observing double stars it is important to have the image scale large enough to enable accurate measurements to be made. In other words, the two stars must be far enough apart at the focal plane to be seen readily and measured accurately. Unfortunately, when using a filar micrometer a small image scale cannot be compensated by an eyepiece giving a high magnification. The reason is that in magnifying the image the wires of the micrometer are also magnified. The point of having a large image scale is to keep the wires of the micrometer small with respect to the distance between the two stars. Imagine having an image scale so small that the separation of the two stars at the focal plane was less then the diameter of the micrometer wire - the wire could hide both stars at once. So what is required is an image scale large enough to make the separation of the two stars greater than the diameter of the micrometer wires.
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