by Roger Venable (International Occultation Timing Association) November 2009

1. Record your occultation video in your usual way.

2. In the field at the time of the occultation event, make a darkfield video
        recording, and also make flatfield video recordings. It is usually
        best to make several flatfield recordings at different brightnesses.
        See note.

3. Get software: Limovie version 0920 or higher, Registax version 4.0 or higher,
        VirtualDub version 1.6.14 or higher, and AviSynth version 2.5 or higher.
        See note.

4. When you digitize or convert the data recording to an avi file, also
        digitize or convert the darkfield and flatfield recordings to avi files.

5. In VirtualDub, use the "levels" video filter to convert the luma 601 videos,
        with a brightness range of 16 to 235, to the pc range of 0 to 255. You
        must do this with the data videos, the darkfield video, and
        the flatfield videos. Run the dub for each of them, and save each as a new
        avi. See note.

6. Open the flatfield recordings in Limovie and study their brightnesses in the
        areas of the field of view containing the occultation star and any
        comparison stars that you may use. To do this, enlarge the background
        circle to its maxiumum of 25 pixels radius, and look at the "BKG/Frame"
        value in the lower left part of the interface. Compare this value among
        your flatfield recordings. You should use the flatfield video that is
        brightest without having saturation of pixels in the critical areas. (It
        doesn't matter if other areas of the flatfield are saturated.)

7. Use Registax to make averages of the darkfield video. See note.

8. Open VirtualDub and open the jpg file you just created. (It will open.) Check
        the "levels" video filter to be sure that it is not being applied,
        because you just used it is step 5 and you don't want to use it again now.
        Then save the darkfield as an avi file. This creates a one-frame avi file
        of the darkfield.

9. Open the pc-range occultation event avi in Limovie, and measure it at multiple
        points,        but avoiding hot pixels, emphasizing measurements in the areas of
        the field near the occultation star. This will enable you to estimate the
        range of brightness of the critical areas of the event avi. Make estimates
        of the *range* of brightness between the darkest and lightest pixels of
        the background illumination of the occultation video. To do this, set the
        aperture radius to zero, which causes it to include only one pixel, and
        run a number of brief measurement runs at different points. In the csv
        files so created, look for the 'Aperture' values, not the 'Measurement'
        values.

10. Using VirtualDub, open the occultation event avi. Set the
        "brightness/contrast" video filter by adjusting the brightness upward by
        moving the slider to the right. Each gradation on the slider's scale is 16
        Limovie-value brightness points per pixel. Move it a distance
        corresponding approximately to 1/2 of the range of brightness that you
        ascertained in step 6, or perhaps slightly more than that amount. Then run
        the dub. Then save the resultant file as your new data avi file. See note.

11. Make an avs file that uses the overlay function of AviSynth to subtract the
        darkfield avi from the data avi. It should look like this:

    a = AVIsource("C:\path1\occultationevent.avi")
    b = AVIsource("C:\path2\oneframedarkfield.avi")
    overlay(a,b,mode="subtract",pc_range=true)

        See note.

12. Open this avs file with VirtualDub. Check the "brightness/contrast" filter to
        make sure it is not being applied, for you used it in step 10 and you
        don't want to use it again now. Then run the dub. Then save the resultant
        video as an avi file. This is your darkfield-adjusted event avi. Notice
        that the numbers of the inserted timing data are damaged by the darkfield
        subtraction. Do not delete the original event file, because you will need
        it for frame counts and timing data.

Dealing With The Flatfield

13. Make an avs file to subtract the darkfield from the flatfield avi, similarly
        to what you did with the event avi. It should look like this:

    a = AVIsource("C:\path3\flatfield.avi")
    b = AVIsource("C:\path4\oneframedarkfield.avi")
    overlay(a,b,mode="subtract",pc_range=true)   

        See note.

14. Open this avs file with VirtualDub, and run the dub. Then save it as your
        dark-adjusted flatfield avi file.

15. Use Registax to make averages of the new flatfield-with-dark-subtracted avi.
        This is done by loading it into Registax, clicking on the
        "Flat/Dark/Reference" menu and then clicking [pay attention now] "Create
        Darkframe". Do NOT click "Create Flatfield" because that is
        a mysterious and incompletely documented function. Registax will then
        stack and average the flatfield-with-dark-subtracted as though it were a
        darkfield. Save it as a jpg file that calls it a flatfield, not a
        darkfield.

16. In VirtualDub, open the flatfield one-frame jpg file, and save it as an avi.

17. Open Windows Paintbrush. There will be a blank image there, all white. Use the
        image attributes function to adjust the blank image's dimensions to match
        those of your avi's. This will probably be 720 pixels horizontally by 480
        vertically. When you set it up that way, you will note that the horizontal
        pixels are numbered from 0 through 719, and the vertical from 0 through
        479. Save this image as a jpg, with a name something like "allwhite.jpg".

18. Open the plain white jpg in VirtualDub, and then save it as an avi. This will
        create a one-frame avi that is plain white.

19. Open the one-frame flatfield avi file in Limovie, and measure it at multiple
        points, emphasizing measurements in the areas of the flatfield that
        correspond to the areas that contain the occultation star near the time of
        the event in the data video. First, make estimates of the *mean*
        brightness of the background. To do this, set the background outer radius
        at 25, and by setting the aperture radius to zero you will be able to set
        the background inner radius at 1. Then read the background value, called
        "BKG/Frame" in the lower left corner of the interface. Click around the
        critical areas of the field to get an idea of what the mean value is.
        Remember this mean brightness number. Let's call it 'm'. Then calculate
        (2m - 255)/16. Let's call that 'n.' It may be postive or negative. You
        will use this number in an operation in step 21. Second, leaving the
        aperture and background radius settings as they are, read the one-pixel
        aperture value at multiple points of this flatfield in the critical areas.
        This value is in the box labelled "Frame" at the very lower left corner of
        the interface. You need        to add the "Frame" number to the BKG/Frame value
        to get the total brightness of the pixel. Click around the critical areas
        to ascertain the highest value you can find of such a pixel. Call that
        value (after adding the BKG/Frame value) 'p.' Calculate (255/p)*100. Let's
        call that 'q.' You will use this number, too, in an operation in step 21.

20. Make an avs file that manipulates the one-frame flatfield avi. Let 'a' be the
        flatfield avi, and let 'b' be the pure white one-frame avi. Here is the
        avs file:

    a = AVIsource("C:\path5\oneframeflatfield.avi")
    b = AVIsource("C:\path6\allwhite.avi")
    overlay(a,b,mode="exclusion",pc_range=true)

        See note.
   
21. Open this avs file in VirtualDub. Before you run the dub, add a video filter
        called 'brightness/contrast.' Adjust this filter's brightness slider
        leftward or rightward by a number of gradations equal to n (from step 19.)
        Negative n is to the left, positive to the right. (Each notch on the
        slider adjusts the entire file by 16 in brightness on the 0 to 255 scale.
        The resolution of this adjustment is 1.6, because there are 10 pixels of
        slider movement per notch. So, if n is 0.65, you will want to move the
        slider 0.65 x 10 pixels, or 7 pixels, to the right.) Leave the contrast
        slider at 100%. Then click "OK." Then, click on the 'brightness/contrast'
        filter a second time, so as to bring up a second instance of this filter,
        to run after the first instance. This time, set the brightness slider to
        normal (the middle position,) and move the contrast slider to q. (The
        contrast slider has a resolution of 6.25%. You should move it to the
        highest level you can that is *less than* q, so as not to exceed q.) Then
        click "OK" twice to return to the main interface. Then run the dub. Save
        the result as an avi file, called something like, 'AdjustedFlatfield.avi.'
        The new flatfield avi is a negative of the original dark-subtracted
        flatfield, with the same proportions of contrast in its dark and light
        areas, but with the contrast stretched to the maximum allowed in the 255
        level grayscale.

22. Make another avs file, this one to apply the fully adjusted flatfield to the
        event avi by multiplication (not by division!) It should look like this:

    a = AVIsource("C:\path7\DarkAdjustedEvent.avi")
    b = AVIsource("C:\path8\FullyAdjustedOneFrameFlatfield.avi")
    overlay(a,b,mode="multiply",pc_range=true)

23. Open this avs file in VirtualDub. Be sure the "brightness/contrast" filter is
        not applied, because you applied it in step 21, and you don't want it
        here. Then run the dub. Then save the result as an avi. This is your fully
        calibrated event avi.

Final Adjustment & Processing

24. The event avi so created will likely appear dark. To make the event more
        readily visible without damaging any brightness data, you can open
        VirtualDub and apply the "brightness/contrast" filter. Apply brightness
        changes alone to avoid affecting any relative proportions of brightness
        or noise. Apply contrast settings to enhance the visibility of a faint
        event while increasing noise by the same proportion. When you have
        adjusted the avi to your satisfaction, save it as an avi file.

25. Open the final avi in Limovie and measure the brightnesses of stars in the
        usual way.