Oct 072015
 

I was asked to help out some users align their image sequences of the recent eclipse of the Moon and this is the result using DVS.

[vimeo 141665784]

The following is my idea of the best way to proceed.

1. Use DSSR to Guide (optional)

DSSR can guide on full disk Moons and this has the advantage of minimising the frame to frame drift which speeds up DVS alignment. To do this, zoom out the video screen until the Moon is small enough that you can select the whole disk as a guide target. Reducing the size of the guide target reduces DSSR’s cpu hit and makes guiding easier.

2. Set Similarity to 10

Similarity is used to eliminate false matches in cases like sunspot groups where multiple possible matches occur. For eclipses, there will only be one Moon per frame so the Similarity filter is redundant. Setting it at 10 turns it off.

3. Set Size Parameter to Exactly Match Moon Diameter

You need to make the anchor box the same size as the lunar disk. First set the Drift parameter to 1 to minimise the time taken to display the anchor box after you click. Then click on the centre of the disk and adjust the Size parameter until the anchor box is the same size as the disk.

eclipse

You can then adjust the Drift setting to suit your image set wobbliness or turn on the search full screen option.

4. Optimise Anchor Swap Setting

This is the KEY parameter for eclipses. It determines how often the current alignment target is swapped out for the current best match. The quickest way in the long term is to make a second set of images at a reduced scale. DVS processing time goes up roughly as the 4th power of image size. Using half size images will align 16 times faster and quarter size images will align 256 times faster.

This speed increase allows you to quickly run a series of alignments with different swap settings and see which best aligns the moon. I tried settings of 1, 5, 10, 20 and then with the parameter unchecked. Setting 5 gave the best results.

5. Produce Intermediate Image Set

The great thing about step 4 is that you now have a good rough alignment of your animation which you can apply to your original image set. Load the original set and then click the Data button on the alignment module. Select the best data file from step 4 and enter 2 for half sized images, 4 for quarter sized images, etc.

Now export your frames as an image sequence.

6. Final Alignment

Load the image set from step 5. You can use a smaller drift value because the Moon should be aligned to within a few pixels. This greatly speeds up processing time and you can then do any manual alignment tweaks, crop, add overlays and export as normal.

This may seem tedious but I found that a set of 328 images from Jim Fakatselis of Peppermill Skies Observatory took 2h30m to align from scratch. Producing the 5 runs in step 4 took a total of 25 minutes and the final alignment took only 36 minutes because it was already roughly aligned. The method above was 2.5 times faster and the results were optimised because of step 4.

7. Tricks and Tips

  1. Avoid overexposing the bright parts of the Moon. Burning out the surface to white means that DVS has no features to align on. You can see this at the start of the animation above where the burnt out limb causes alignment drift.
  2. Avoid abrupt changes to exposure settings. These cause DVS to jump which you can see about 2/3rds way thru the video above.
  3. As noted above, use DSSR to guide your scope during the eclipse. Even a rough guidance will dramatically improve DVS processing time.
  4. Use DTA to export a copy of your original images to DVS format. This will allow you to add clocks, date and time overlays to your animation in DVS.
  5. Turn off your camera’s auto orientation feature.

Finally, here is another of Jim’s eclipse sequences captured on a second scope. Note how the Moon moves about 2/3rds thru due to the overexposed limb.

[vimeo 141995085]

 

 Posted by at 8:52 am

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