(meteorobs) Fisheye lens misunderstanding and photography update

[Meteorites USA]

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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