(meteorobs) "Whizzer" - magnified and dissected

[Chris Peterson]

My experience has been that acrylic hemispherical domes tend to be quite 
good optically- better than glass by far, unless you get ground and 
polished glass, which is hard to find and VERY expensive.

Acrylic is a very dimensionally stable material, and the molds appear to 
be quite good. As a result, the domes introduce very little distortion. 
The key to using them is to make sure that the entrance pupil of the 
camera lens is at the center of curvature. If you do this, all the rays 
making it to the sensor are essentially perpendicular to the surface of 
the dome. If the lens pokes up into the dome, however, there's quite a 
bit more distortion.

A bigger problem with the domes- and this applies to glass as well as 
plastic- is internal reflections. That's why we often see distorted 
fireballs (but not dimmer meteors). The fix for this is to use a dome 
with AR coatings on both sides, but those are hard to come by. I did 
have one coated for an experiment, and it produced much nicer images of 
bright meteors. If I ever make a large batch of cameras at once, I'll 
probably use coated domes, since most of the cost is setup. If you coat 
a bunch at once, the price gets quite reasonable.

FWIW, I don't think we're seeing any structure at all in this meteor, 
and we're not resolving any of the plasma envelope. At the pixel scale 
of Tom's camera, that fireball is a point source. We're simply seeing 
the optical broadening of a very bright object caused by diffraction, 
scatter, and internal reflections in the optics. Think about how large 
the Moon (0.5°) appears on these cameras, covering many pixels, even 
though the angle subtended by the Moon only amounts to a couple of 
pixels on the zenith, and less at lower altitudes. A meteor plasma 
trail, with a typical luminous diameter of a meter or less, subtends 
millidegrees- a fraction of a pixel.

Chris

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

On 1/5/2012 10:55 AM, George Gliba wrote:
>
> How about surplus glass optical domes, like the one below?
>
> http://www.surplusshed.com/pages/item/m4065.html
>
> Starry Skies,
> GWG
>