He's the grid Mike.
To use:
Quote from "The Giant Planet Jupiter" John H. Rogers.
"As the planet's tilt if very small , the distance east or west of the CM can be converted directly into relative longitude. If a spot is X deg from the CM, the distance from CM to spot divided by the distance from CM to limb is Sin X. For speed and simplicity, the author measures photographs using a transparent grid, ruled with lines corresponding to 10 deg intervals of longitude. This is placed over the image with the outmost lines touching the limb at the latitude of the spot to be measured."
I use it by capturing series of avies, a few either side of "Preceding Edge Transit", then either side of what looks like central transit the a few either side of "Following Edge Transit". I process all the avies and check with the grid to find the closest to central GRS transit. I then check by taking the Preceding and Following images and subtracting the times to get the difference. I then divide the difference, add that to the Preceding time to get central GRS transit time. I then check this off on the first guestimate I made. The guestimates are usually pretty accurate using the grid.
Now that the date and time UT of transit is known the longitude can then be worked out using the ephemeris tables. After collecting a series of transits you can then work out any longitude drift of the GRS (or any object you chose as your datum)
You will notice in the second image I've used only the red channel. I found it made it much easier to measure that way, especially on those nights where the seeing is mush.
The third image is the results of a series of measurements showing how the GRS has changed longitude from the 10/4/06 to the 16/5/06
Hope this helps. You know how I like to ramble
