Re: [APML] New All Sky Camera Images

[Aplanatic@aol.com]

Hi Gene,

To keep the argument simple, consider the on-axis case.  To find the 
effective f/ratio of the system, we need to find the diameter of the parallel 
bundle of rays -- from the star that's directly overhead -- that makes it 
into the aperture of the camera lens.  There is a virtual focus for these 
rays behind the sphere.  In other words, it's as if the star is located a 
distance R/2 behind the apex of the reflecting sphere.  So far, so good?  
Draw a cone from this virtual focus to the aperture of the camera lens.  This 
cone intersects the sphere at the effective aperture circle of the system.  
Parallel light rays from the star that lie outside of this circle do not make 
it into the lens.  

If you make a quick drawing of this situation you will see that the effective 
aperture is much smaller than the camera lens aperture.

Does this explanation help to form a picture of what's happening?

Dave Rowe

----------- Original Message -----------

> Aplanatic@aol.com wrote:
>  
>  > In general, the f/ratio of the system is increased from that of the 
camera 
> lens by
>  > the following factor:
>  > 
>  > (1 + 2d/R)
>  > 
>  > where R is the radius of the sphere and d is the distance from the 
camera 
> lens to the
>  > nearest point on the sphere.
>  
>  Maybe my brain is drained from work this week, but something doesn't seem 
>  right about this.  Shouldn't the f/ratio be decreasing as you are 
> effectively
>  shortening the focal length (assuming that the sphere covers the full 
> effective
>  aperture of the lens in use, no adjustment for reflectivity, etc.)?
>  
>  Gene Horr
>  genehorr@texas.net

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