Using LiMovie to extract event times from a video record
Let's open a sample video file and get our hands onto LiMovie. For our first demonstration we are going to use [5] 051010B_ZC2743.AVI from Data/Double Stars Total Lunar Occultations. When you open the file it will first show a white frame but when you press "Play" you should see the nice disappearance of ZC 2743 behind the lunar limb.
To measure the star you will need to click on the video image and select the object to be measured. When you do this the blue and red circles will move to the place you have clicked.
The measurement is done for the area inside the red circle. You will want the star to be in the center of the red circle. Then the area between the blue circles will be used for background measurements. You can adjust the size of the circles by changing their radius from the panel (highlighted in red on the above image). The measurement aperture will measure light from the star and the nearby background. To minimise measurement noise from the background, you should aim at having the measurement aperture quite small – so that the contribution from the background (and hance any associated noise) is small. However if you make the aperture too small, LiMovie may experience problems in correctly positioning the aperture – with a result that not all the light from the star is measured – and this would create noise. Hence the practical challenge is to set the aperture as small as possible, but large enough so that the star always falls well within the aperture.
The background aperture should generally be of much larger area – so that the derived background signal is more correctly derived. But you should not make it too large – in case the background being measured does not properly represent the background at the location of the star.
Desirably there should be a small gap between the inside of the background aperture and the outside of the measurement aperture – to make sure that no light from the star being measured can be recorded in the background aperture.
Once we have adjusted the cursor we could start the measurement. It is controlled by the set of buttons START, STOP and DataRemove highlighted in red on the above image. Once you press START LiMovie will start doing the measurements and will display the values in the upper right area of the window. You can press STOP at any time to stop the measurements. DataRemove will clear the data from the last time and will allow you to do new measurements. Once you have finished, click the Graph button for the light curve plot to be displayed.
We can see the step event clearly showing that this is a close double star. The step feature is caused by the light only coming from the secondary component when the main component is already behind the lunar limb. Okay lets now measure the times ....
If you keep both forms side by side you will notice that when you click on any of the points from the light curve plot, this point will turn red and the frame corresponding to this measurement will be shown in the main form of LiMovie.
Also the current frame number will be displayed in both the "Graph" and the main windows. In this case we have selected frame 249. If you select a different point from the light curve a different frame will be displayed. So now that we have positioned to the frame we want to measure the only thing left is to read the time from the on screen display.
But hang on what exactly is shown on the display? Is this 9:42:00 or is it 9:42:01? Why are the numbers blurred and overlapping?
Well firstly different timing devices show the time in a different way. In our case we have KIWI OSD. Before we continue please click on the Field Show button highlighted in red on the above image. Once you've done this you'll see two images instead of one and all numbers will appear very clear on them. These are the two fields composing the frame.
Fields and frames are the way the interlaced video technology works. Basically there are two separate fields that contain exactly half of the information on the frame and in our case we have the timestamps on each of the fields. The numbers circled in blue are the field numbers since the last reset of KIWI OSD which it uses as an internal counter. Then the first bit represents the hour, minute and seconds in UT and the following two numbers represent the milliseconds. In our case the exposure of the first field starts at 9:42:00.983 and finishes at 9:42:00.999. Then the exposure of the second field starts at 9:42:00.999 and finishes at 9:42:01.016. Note how the start and end time milliseconds of the two fields are swapped. This is done on purpose so when the frame is displayed the end of the first field will overlap with the start of the second frame and because they are exactly the same they will not appear blurred and will be clearly readable.
So the above frame has been recorded at 9:42:00.999 +/- 0.016 s. This value could be also retrieved directly from LiMovie for most of the video records that are using KIWI OSD.
Finally let us give you an idea of the information carried in this step event. Well firstly this proves that this star is a close double and the step event continues for 0.87s. Taking into consideration the positional angle these 0.87 sec correspond to 0.38" separation between the components. If you want to resolve this double star visually you will need at least 14 or 16 inch telescope.
Another video included in the WorkshopDVD of 24 Psc features a very easily detectable step event that continues for 0.3 sec. The double star has a separation angle of 0.09" and would require a telescope with aperture of almost 2 meters if you want to separate it visually! But in the same time we could "measure" this star using the Moon as a "tool" and using only a small telescope.
We learned how to ...
- Set the measurement cursor on the target star and adjust its signal and background area size
- Perform a measurement and plot the light curve
- Position the video to a selected point from the light curve
- Know the difference between a frame and a field
- Read the time inserted by KIWI OSD on a selected frame
 | Continue to the next part ... |
