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Mirage

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#1 toucansam

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Posted 23 April 2020 - 03:20 AM

Sorry, this isn't exactly astronomy related, but it is optics related.

Scintillation.  AKA mirage.


Rifle shooters adjust their magnified optics slightly out of focus with the target in order to observe mirage.  This allows them to read the wind, both speed and direction.

Tonight I was observing the city lights about six miles away with an APO refractor.  Very clear and cool night, very still air.  At least at my location.  Mirage was unreal tonight; half the city looked like it was sitting on a lake.  Lots of reflected light and mirage.  Beautiful. . 

 

I observed a strange effect that I cannot explain.  When I moved out of focus one direction, the scintillation effect was very clearly moving left-to-right.  Moving out of focus the other direction, it was moving right-to-left.

I have used spotting scopes and rifle scopes alike to focus on mirage and read the wind. I have never seen it change direction based on which direction I was moving the focuser.

Can someone explain this effect to a noob?



#2 TOMDEY

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Posted 23 April 2020 - 04:06 AM

Cool! OK... I just figured it out. For your night time view of out of focus distant lights... the air mass-cons will move in the same "correct" direction as the air when you rack the eyepiece inward. The direction will be opposite when you focus outward. I'll just leave it at that, but can derive the actual air's transverse velocity etc. etc. if anyone is interested. That's a function of the entrance pupil diameter of your scope, the range of the (out of focus) source (star or street light), and the range of the refractive turbulence.

 

This topic is a sub-category of Schlieren Test used in wind tunnels, etc. Also Shadow Bands sometimes seen during the "Diamond Ring" brief phases of Total Solar Eclipses. I designed and built detectors for those things.    Tom

 

~click on~   >>>

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  • 40 SOLAR ECLIPSE 20170821 Tom's Shadow Band Recorder.jpg


#3 TOMDEY

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Posted 23 April 2020 - 04:07 AM

nother pic >>>    Tom

 

~click on~   >>>

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  • 41 all-horizon sunrise Tom Solar Eclipse  Tom's Shadowband Recorder.jpg


#4 toucansam

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Posted 23 April 2020 - 04:12 AM

Thanks!!!!  Trouble is, its a radial focus scope and the parts that are changing position are inside the tube, hahahaha.  So I have no idea if CW or CCW is equiv to moving my EP "inward."




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