version 1.0.36 Rev 22Apr2022 

Screen Ruler is a resizeable, semi-transparent window that can be placed over other windows. It contains a moveable and resizeable line used as a ruler to read off measurements from underlying images. It is designed to speed up the identification of astronomical images in a sequence taken of a moving object such as a minor planet or comet, where the object passes close to field stars or galaxies. These images can then be left out from further processing, allowing more accurate measurement of the object's position and brightness. 

Screen Ruler is designed to be used in situations where the minor planet or comet is too faint to be visible in individual images, only being revealed after digitally stacking multiple images together using software such as Astrometrica. If the moving object is bright and easily seen on individual exposures there is no need to use Screen Ruler, it will be obvious which images need to be left out. Also in some instances a good result may be obtained by using median stacking functionality, but, especially with smaller focal length systems, this may not give optimal results.

The Ruler is drawn on the Screen Ruler window to represent the motion of an asteroid or comet. Information about the images being used is entered in the Screen Ruler Control window, allowing the location of the mouse to indicate the images where the minor planet or comet is close to background objects.

Screen Ruler can be used in several different scenarios, these are covered in tutorials:

  • Simple, using a linear interpolation between the first and last images used in a stacked image
  • Intermediate, using images stacked for the object's motion and another with zero motion to increase precision and to identify all field stars involved
  • Advanced, same as Intermediate, but extracting exposure information directly from the FITS images being stacked, giving extra precision and allowing the selected images to be copied to a separate folder for ease of stacking for the final measurements.

Please see the tutorials for further details.

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