(meteorobs) Fisheye lens misunderstanding and photography update

[brahi at op.pl]

http://adsabs.harvard.edu/abs/1995A%26AS..112..173B

greetings

Przemyslaw Zoladek
 
"Global Meteor Observing Forum" <meteorobs at meteorobs.org> napisał(a): 
 > Is aliasing really that important? There's two types and both have been 
 > covered in this discussion very well. One is optical distortion (curved 
 > image) caused by the ultra wide FOV (field of view) lens, and the other 
 > is a pixelated portion of an image which also distorts the meteors image 
 > and could make it appear curved or blurry. What is the desired outcome 
 > of this discussion?
 > 
 > What I would like to learn is how to determine altitude and azimuth of 
 > the retardation point of a bolide event as recorded from a fish-eye lens 
 > from an all-sky cam. It's easier to figure from a camera with a 90 
 > degree FOV pointing horizontally in any given direction, but an all sky 
 > camera pointing straight up with a 180 degree FOV is much harder to 
 > calculate. The horizon to horizon view coupled with the distortion of 
 > the image and aliasing can cause mis calculations.
 > 
 > Can someone please explain how to determine altitude and azimuth please 
 > from an an all sky image? Links? Papers? Discussions?
 > 
 > Thanks...
 > 
 > Regards,
 > Eric
 > 
 > 
 > 
 > Chris Peterson wrote:
 > > It is true aliasing, resulting from having higher spatial frequency 
 > > components in the image than the spatial sampling frequency (the pixel 
 > > spacing). It will show up in any optical system producing a small spot size 
 > > compared with the pixel size, regardless of how linear the meteor path is on 
 > > the image. The "spiral" effect may be a combination of the aliasing and the 
 > > curved path- that's related to the Moiré effect referenced by Leo. But it's 
 > > still fundamentally caused by aliasing. A meteor image crossing multiple 
 > > pixels also makes it easier to see the limiting spatial resolution of the 
 > > image- that is, the fact that the distance between pixels in the final image 
 > > is larger than the resolution of our eye. We essentially have a pixelated 
 > > image because of that.
 > >
 > > The only condition under which the effect would be hidden (although the 
 > > aliasing still exists) is when the meteor so exactly follows a single row or 
 > > column of pixels that no other pixels are exposed. Not a likely event.
 > >
 > > Chris
 > >
 > > *****************************************
 > > Chris L Peterson
 > > Cloudbait Observatory
 > > http://www.cloudbait.com
 > >
 > >
 > > ----- Original Message ----- 
 > > From: "Peter Hirons" <peter at galley.ie>
 > > To: "'Global Meteor Observing Forum'" <meteorobs at meteorobs.org>
 > > Sent: Monday, December 07, 2009 1:20 PM
 > > Subject: Re: (meteorobs) Fisheye lens misunderstanding and photography 
 > > update
 > >
 > >
 > >   
 > >> I wouldn't call this aliasing - it's just the effect of the fish-eye lens
 > >> in turning straight lines into curves.  Try taking a photo of a building
 > >> in daylight.
 > >>
 > >> You can get software to correct this, but I'd stick to a linear lens as
 > >> wide as you can get.  You probably only want to be looking towards the
 > >> radiant most of the time anyway.
 > >>
 > >> Peter
 > >>     
 > >
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 > >   
 > 
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