The Window Sill Observatory

May 15, 2012

New Astrometry Module for DVS

Filed under: Software, Tutorials — Dave @ 12:09 pm

DVS now has a new Astrometry module that allows you to measure the movement of an object between a pair of images. You can use this to calculate velocities of asteroids, comets, minor planets, satellites, etc. It can also be used to measure object parallax angle during transits such as the 2012 Venus transit.

Here is the relevant section from the latest DVS manual.


The astrometry module measures the movement of an object between a pair of images. It does this by creating a base line between 2 fixed points A and B. The movement of a moving point C is then calculated by DVS. It is intended to measure movement of comets, minor planets, asteroids, satellites etc. and to help with parallax measurements during transits.

It also works in sub-pixel mode for more accuracy when you apply a divisor greater than 1 in the Align dialog.


  1. Run DVS and load your image pair.

  2. Set the divisor setting in the Align dialog as required for sub-pixel accuracy.

  3. Click the Astrometry button in the top toolbar.

  4. Left-click on reference point A on frame 1 and click button A.

  5. Left-click on reference point B on frame 1 and click button B.

  6. Left-click on moving point C on frame 1 and click button C.

DVS will then display the movement of the target C in pixels. Repeat steps 4 to 6 using different reference points to repeat the calculation. DVS will display the number of results as well as the average movement.

Click Copy to copy the results screen to the clipboard. Click the Clear button to clear the results screen and the stored results.


  1. Your image pair should have the same image scale – i.e. each pixel should subtend the same angle. DVS will compensate for small differences in image scale.

  2. Your image pair should have the same orientation – e.g. North up. DVS will compensate for small differences in orientation between images.

  3. DVS must find points A, B and C in both images. You can check this is happening by moving to frame 2 after you select each point and checking whether DVS finds the target. Make sure you return to frame 1 after this check.

  4. Images taken with different equipment may be very different and DVS may not find the points on image 2 using your usual settings. Try lowering the Similarity setting to say 500 to help DVS to find the target.

  5. You can use different target box sizes and maximum drifts for each point.

  6. The target box for the moving object should be just big enough to include it.

Below are some test images to try out the new module.

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