![]() AstroImageJ with opened and zoomed M42 Trapezium image Also you can use sliders at the bottom to adjust levels, so the object to measure will be well defined. Then you can pan and zoom image to find your object. After you start AstroImageJ you open image file ( File -> Open). and this data you need to export to spread sheet and calculate separation and position angle.then you need to select stars, so software will determine its center and calculate position.after image is opened you need to run plate solve on it, so the image scale, coordinates and orientation is calculated.The whole process consists of following steps: ![]() There is also dedicated software REDUC for double stars measurements, but I do not have yet a chance to use it. With some care it is possible to achieve accuracy of 0.1″ or better, so for some cases you can observe changes in position after a year or two (like for example 70 Oph, Gamma Virginis, or Zeta Cnc). In the example below I used free AstroImageJ software to measuring double stars, but any other software that can handle plate solving and astrometry can be used for this (like MaxIm DL, MPO Canopus or Astrometrica). When position of stars is determined with good accuracy it is pretty straightforward then to calculate separation of any star pair. Astrometry with CCD (or CMOS) cameras can be very useful tool for measuring separation and position angle of double and multiple stars.
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