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Twinkle Twinkle Little Star

From the author of   Sizing Up The Universe

22:00 EDT July 5, 2009.
ToUcam on 4" Takahashi FSQ (focal length = 530mm).
5 second videos, 1/25 second exposures, 10 frames/second, binned 2x2.

Stars twinkle because of differential refraction of the starlight as it passes through Earth's atmosphere. In a telescope, we don't normally see them twinkle because the aperture of the telescope is much larger than the the pupil of a human eye. Hence, the intensity fluctuations caused by the atmosphere "average-out" more thoroughly over a telescope's pupil than over an eye's pupil. In a telescope, on nights of average to bad "seeing", we see distortions but the overall intensity remains fairly constant. But, the small aperture of the human eye allows us to witness intensity variations too.

Below are shown two 5-second animated gifs (looping forever). The one on the left shows Arcturus significantly defocused in my 4 inch refractor. In this animation, we can clearly see the intensity variations across the pupil of the telescope.

The animation on the right shows Arcturus in focus. But, I made one more important change. I put a cardboard mask over the front of my refractor. The cardboard mask has a small section cut out. Covering that small section I put a piece of aluminum foil from which I cut out a small circular opening about the size of a human's pupil. As shown in the pictures below the animations, I placed this mask over the refractor's dew shield. The right-hand picture is a close-up of the mask showing the small circular pupil cut from the aluminum foil.

I also did a frame-by-frame analysis of the focused image. The difference is Arcturus' intensity between the brightest and the dimmest frames was only about a factor of 2 (a little bit less than this actually). So, the star doesn't completely wink out as it twinkles---it just dims.

PS. Arcturus is not a "little" star.

Arcturus defocused.
Arcturus w/ "pupil-sized" pupil mask.

Telescope w/ "pupil-sized" pupil mask.
Close-up of "pupil-sized" pupil mask.


Spica was much closer to the horizon and therefore twinkling with more gusto. I was able to take a defocused full-aperture video but, unfortunately, it was too dim to take a masked focused image. Here's an animation of unmasked, defocused Spica...

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