focus jpg

Diffraction vs. Hartmann - The Battle Rages

The next test compared diffraction to the Hartmann method. The procedure for this comparison was as follows:

1) Vega used as test star
2) One exposure with each method at each position
3) Starting on one side of focus and passing thru and out the other side
4) Hartmann mask 5 sec. exposure/diffraction focus 1 sec. exposure
5) C8 at F10

Set One - Exposures 1-4

As you can see, both methods make it clear that focus improves from left to right.

Hartmann Set One

Set Two - Exposures 5-8

Still improving and both methods are still holding up well.

Hartmann Set Two

Set Three - Exposures 9-12

Which one would you say is focussed? It actually lies between #1 and #2, but a bit closer to #1. A close examination of either method will show that. Personally, I think with diffraction it is a bit easier to tell, but the Hartmann is not too bad. Two other points are worth mentioning. One is that this is another demonstation of too-large step size. This series was done with the C8 at F10 using manual movement of the primary with DRO. That makes each step almost as big as the depth of focus. This explains why I wound up BETWEEN steps. The second point is that a three hole Hartmann would be a bit better than the two hole used here. The third hole would "flip sides" as focus is passed, making it easier to read.

Hartmann Set Three

Set Four - Exposures 12-16

And out the other side just to be complete!

Hartmann Set Four

Next: Automation!


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