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Stellar Projection During Lunar Occultation

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#1 Brian Albin

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Posted 05 August 2021 - 08:43 PM

Hello,

I have been reading of observers seeing a star appear on the face of the moon during an occultation. Or sometimes instead of appearing on the disc, on other occurrences the star hesitated on the limbus before suddenly disappearing.
I will ask in the Science forum about possible causes of this appearance, but I wanted to first ask in this forum if any of you have seen this. It apparently is rare and not the usual thing to happen during an occultation.

 

I thank you for your reports,

Brian



#2 james7ca

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Posted 05 August 2021 - 09:42 PM

Could be caused by changes in the seeing conditions which would make both the star and the limb of the moon move with respect to one another. So, periodically it could appear that the star was actually over the face of the moon (not really, just that your eye and brain would integrate the changing positions and think that they overlap).

 

Furthermore, because of atmospheric refraction you can still see the sun after it has already gone below your horizon. The sun, the moon, and the stars that appear on the horizon are actually about 30 arc minutes below the horizon. Not really the same situation as the momentary changes in position that you'd see during atmospheric turbulence, but it does show the power of refraction caused by the air that surrounds us.


Edited by james7ca, 05 August 2021 - 09:42 PM.


#3 TOMDEY

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Posted 05 August 2021 - 10:17 PM

Digital video image capture can do exactly those things. The displacements are typically far more than optical effects can explain. I got that when recording lightning bolts at night. If you are post-examining sequential frames, often showing impossibly displaced bolts. Things like exceeding the antiblooming threshold, inadequate time between frames to drain/purge excess electrons, very bright object on very dark surround, compressed file format (especially spatial/temporal differential e.g. jpg), etc. The biggest cause is electronic shuttering of the frames by shift-registering columns/rows into protected interleaved readout.

 

So these things are artefactual to the camera architecture --- not actually present in the image falling on the array.    Tom


Edited by TOMDEY, 05 August 2021 - 10:18 PM.



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