(meteorobs) Fisheye lens misunderstanding and photography update

[Chris Peterson]

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