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Using the K3CCDTools Drift Explorer for Drift Alignment
Submitted: Friday, 3rd July 2009 by Al Sheehan

K3CCDTools is a popular piece of astro software for image capture and processing. One of the really useful tools that make K3CCDTools a “must have” is the Drift Explorer. The Drift Explorer provides real time analysis and plotting of deviations in RA and Dec over time. So it can be used to view and measure the accuracy of your RA drive including periodic error, as well as measure and fine tune your polar alignment.


Click to Enlarge
Figure 1 - K3CCDTools Drift Screen

Figure 1 shows the Drift Explorer window in K3CCDTools. On the left is, of course, the normal video capture screen showing with the K3CCDTools reticle tool overlaid. The button that activates the reticle is immediately below “Help” in the menu. The button to the right of the reticle button with the blue and red graphs on it is the Drift Explorer button.

To simplify using the Drift Explorer, align the camera so that the “horizontal” (wide) axis of the frame is aligned in RA. This is not mandatory, and the alternative will be discussed later.

Once the target star is in the FOV, select the Drift Explorer. The reticle will appear over the capture window automatically when the Drift Explorer window opens. The target select button is the first button at the top left of the Drift Explorer window. This allows one star to be selected for drift analysis.

Once the star is selected, the Drift Explorer immediately starts plotting 2 graphs – one each for deviations in RA and Dec over time. The graphs will show some noise due to seeing, but the centerline of the graph will reflect drift. So it’s very easy to see the effect of any adjustments by the changing slope of the Dec graph, or the rate of drift. The aim when drift aligning is to get the Dec graph to follow the time axis (zero arc seconds) as closely as possible. From Figure 1 you can see that after tracking for 223 seconds, the Dec graph is essentially still on the zero arc seconds deviation line, and above the graphs you can see that the alignment accuracy is about 0.1 arc seconds of drift per minute.

The second button on the Drift Explorer window is the “Reset Target” button. Use this after each adjustment to reset the graphs to zero and restart the calculations for rate of drift. Note that the time axis is not reset by the Reset Target button, so sometimes I prefer to use the “Select Target” button to deselect the star and reselect it again, to restart the time axis.

The third button on the Drift Explorer window is the “Measure Angle” button. This is used to determine the angle of the camera relative to Right Ascension, which is useful especially if you can’t align the camera so that RA appears horizontal across the captured image. The procedure to use the measure angle button is as follows:

  • Select the target star
  • Stop the RA drive on your scope so the star starts to drift in RA
  • Press the Measure Angle Button
  • Press the Measure Angle Button again before the star drifts out of the FOV so K3CCDTools can calculate the angle of drift in the camera
  • Restart the RA drive on your scope (hopefully before the star disappears from the FOV)

Obviously, this can get a bit hectic when using a small CCD webcam, and especially if the options to start and stop RA are hidden in menus for your scope controller (like mine). I have found having a helper to start and stop the Measure Angle button to allow me to start and stop RA works well when the RA start and stop are buried in menus.

Once the angle of the camera is measured with respect to RA, then k3CCDTools can correctly and accurately calculate and present the RA and Dec drift graphs regardless what angle the camera is to RA.

The Drift Explorer has many other buttons and functions which are not critical to basic usage of the Drift Explorer – in fact, you’ll see from Figure 1 that the Drift Explorer can also be used for guiding, but that’s for another tutorial.


Article by Al Sheehan (sheeny). Discuss this article on the IceInSpace Forum.

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