(meteorobs) Fisheye lens misunderstanding and photography update

[Chris Peterson]

Somewhat counter-intuitively, the aliasing is dependent only on the pixel 
size of the camera and the focal ratio of the lens. The field-of-view 
(essentially, the focal length) doesn't matter at all. The physical image 
size of a point source is the same for all [perfect] lenses of a given focal 
ratio. For instance, any f/4 lens will produce an Airy disc size of 5.4 um, 
which is about the same size as a typical DSLR pixel. So you'll usually see 
aliasing, at least until the meteor becomes so bright that the tails of the 
PSF are above the noise floor of neighboring pixels, and then you see the 
characteristic wide flare of imaged fireballs.

I wouldn't defocus the camera to reduce filtering. You can achieve the same 
thing by the careful use of blurring filters during processing, and that 
gives you the chance to experiment with different methods- an opportunity 
that is lost if the raw data is already filtered by the optics.

Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com


----- Original Message ----- 
From: "Leo S" <l.stachowicz at btinternet.com>
To: "Global Meteor Observing Forum" <meteorobs at meteorobs.org>
Sent: Monday, December 07, 2009 12:54 PM
Subject: Re: (meteorobs) Fisheye lens misunderstanding and photography 
update


> Thanks Thomas,
>
> The "spiral" part threw me. I see what you mean now. I get this on all
> my bodies as well.
>
> The best way to avoid it is to not crop so much. Not going too wide with
> the lens is going to help with this obviously.
>
> I suppose that slightly defocussing the lens might help, acting just
> like a "poor quality" lens or AA filter as Chris mentioned. I'm not
> sure, but it might be worth trying? Chris?
>
> Leo