Screen Ruler        Great Shefford Observatory


Overview        2

Screen ruler windows        2

Assumptions        3

Simple measurement        4

Setting start and end positions of the star trail        5

Measure when asteroid meets background star        6

More rigorous measurement        7

Refining Screen Ruler start and end positions        7

Plotting the asteroid path across background stars        8

Appendix        10

Keep on Top        10

Displaying coordinates via the mouse        10

Information displayed via mouse position        10


Screen Ruler is designed to quickly identify when an asteroid moves in front of background stars and galaxies during image stacking. Appropriate images can then be excluded from the stacking process so that subsequent measurement of the asteroid position (and magnitude) are unaffected by background objects.

Example: 2018 PT23 on 2018 Aug. 16, fast moving NEO, moving at 93”/minute in p.a. 300° (lower left to upper right):

Stack of 60 consecutive images shows stars interfering with target measurement

After discarding the exposures where the asteroid was passing close to stars, the result is a much cleaner target for measurement

Screen ruler windows

When Screen ruler starts it displays two windows


  1. The images being stacked are spaced regularly in time
  2. The images being stacked will have consecutive sequential numbering somewhere in the file name that can be entered as the start and end numbers in the Scale window
  3. If a) or b) are not attainable then the time of exposure is expected to be available for each image (exposure start time, midtime or end, it doesn’t matter which, just use one consistently throughout)
  4. In the examples presented here Astrometrica is used to stack images and its “Blink current Images” function (Ctrl+B) is used in the “More rigorous measurement” instructions

Simple measurement

This uses Screen Ruler to measure along a star trail to find which exposures recorded the asteroid passing the star. It works well if a single bright star is involved with the asteroid image

Example: In an Astrometrica stack the target asteroid can be seen between two star trails:

Enter the start and end exposure numbers (1 and 60) into the Scale window, Optionally the start and end exposure start times (20:57:15 and 21:00:45) can also be entered.

Key in times with or without colon separators, colons will be added automatically when the cursor leaves the field. An end time less than start time is assumed to refer to the next day:

Setting start and end positions of the star trail

Drag the Screen Ruler window over the stacked image and resize it to cover the whole length of the brighter of the two star trails. Use Shift+left click to set the start position of the star trail and Shift+right click to set the end position of the same trail (note, the asteroid was moving in p.a. 300°, from bottom left to top right, so the star trail starts at top right and ends at bottom left):

The Screen Ruler has drawn a line from the start to the end coordinates set by the mouse, the thickness and colour of the line can be adjusted and the transparency of the Screen Ruler window can be changed. See help on the Scale window for details.

Now, as the mouse is moved over the Screen Ruler the coordinates of where the yellow perpendicular line intersects the drawn line are read out in units of the start and end exposure numbers and also the start and end exposure times (if times have been entered on the Scale window).

Measure when asteroid meets background star

Here the mouse is positioned on the star trail, just before the asteroid, towards the end of the trail, the indicated exposure number is 48 and the equivalent exposure start time is 21:00:01 UT

The mouse is then moved so that the yellow perpendicular line is just after the asteroid, the equivalent readout is exposure 51, start exposure time 21:00:12 UT (Both readings are shown in the screen shot here to demonstrate the mouse movement, only one reading is actually displayed at any one time)

So a re-stacking of images 1-47 and 52-60 can be done, which will stop the bright star from interfering with the asteroid measurement.

More rigorous measurement

In the last example there was actually at least one other, fainter star trail interfering with the asteroid image. To remove all background objects from the stacked image the following stacks will be needed:

Stack 1        A stack with zero motion of many or all of the available images, to reveal faint background objects

Stack 2        A stack of all available images using the asteroid’s motion, to reveal the asteroid, even if there is interference from stars. A Median stack might be appropriate if there is a large amount of interference masking the asteroid completely

Stack 3        Optionally a stack of just the first and last available images, to define the start and end of star trails accurately, sometimes necessary in a rich starfield

Refining Screen Ruler start and end positions

Use stack 2 to set the Screen Ruler from start point to end point along a star trail as shown in Simple measurement. If this is difficult to do because of overlapping star trails then set the start and end points approximately with stack 2 but then refine using stack 3. In Astrometrica use the “Blink current Images” function to cycle betweeen stacks 2 and 3. Stack 3 should show a round star image close to the start and end of the Screen Ruler line set up using stack 2. Fine adjustment of the start and end positions can now be made using the arrow keys (start position) and Ctrl+arrow keys (end position)

Plotting the asteroid path across background stars

Having accurately set the star trail line in Screen Ruler, use the Astrometrica “Blink current Images” function to cycle through stacks 1 and 2 (zero motion stack and asteroid motion stack). Leave it displaying stack 2, showing the asteroid as a point and stars as trails

Rather than seeing where a star trailed over the asteroid in Simple measurement, this procedure will see where the asteroid trailed over background stars using stack 1 (zero motion). Therefore the asteroid starts at the opposite end of the Screen Ruler line, in the previous example images, the asteroid starts from lower left and moves to upper right. Use the “Swap start and end coordinates” button on the Scale window to force the Screen Ruler to dsplay the image sequence numbers in reverse order, rather than having to redraw the line using the mouse.

The settings in the Scale window remain as originally entered, just the displayed coordinates in the Screen Ruler window are swapped.

Now, without resizing the window, drag the Screen Ruler window title bar so that the asteroid start position of the line is exactly positioned over the asteroid in stack 2 (to the lower left).

Click back to Astrometrica and cycle the image displayed in the blink window to stack 1 (zero motion). The Screen Ruler start position marks exactly where the asteroid was when image 1 was taken and the line shows its path in subsequent exposures.

It is now obvious that there are three stars that the asteroid passed close by. Moving the mouse along the ruler allows identifying exposures 24-28, 48-52 and 55-58 as ones to be excluded from the stack (note the screen shot shows the start and end readings for each star to demonstrate the procedure but only one reading is actually displayed at the mouse location at any one time)


Keep on Top

The Screen Ruler is designed to operate as the topmost window, so that it floats above Astrometrica. When needing to work exclusively in Astrometrica, e.g. when re-stacking images then either uncheck the “Keep on top?” setting in the Scale window, or more conveniently, minimise the Screen Ruler (the Minimise control is found only on the Screen Ruler window but minimises both the Screen Ruler and the Screen Ruler Scale windows at the same time).

Displaying coordinates via the mouse

The Screen Ruler only displays coordinates via the mouse position when it has focus. If Astrometrica is clicked on, with the Ruler in front, then Astrometrica has focus and even though the mouse may travel over the Screen Ruler window, no readings will be shown. Click on the Screen Ruler window to give it focus and it will start displaying coordinates again.

Information displayed via mouse position

The format of the displayed information is as follows, using this example:

58 = Current mouse position (interpolated from start to end position)

(2) = Current mouse position (interpolating from end to start position)

21:00:37 = Current mouse position exposure time (interpolated from start to end time)

 20:57:23 = Current mouse position exposure time (interpolated from end to start time)

Clicking the “Swap Start and end coordinates” button just causes the two sets of displayed numbers to be swapped, i.e. in the example above, the following would be displayed:

The status area at the bottom of the ruler displays the current position of the mouse within the Screen Ruler window, together with the equivalent coordinates of the start and end of the line, measured in pixels, the origin (0,0) is top left, the x coordinate increases to the right and the y coordinates increases downwards.

Screen Ruler © Peter Birtwhistle 2010-2018, Great Shefford Observatory