(meteorobs) Glass domes (was "Whizzer" - magnified anddissected)

[Chris Peterson]

An optical glass like BK7 has an index of refraction of ~1.5, as does 
acrylic. Glass is somewhat less dispersive (Abbe 64) then acrylic (Abbe 
55). So on the whole, the two materials are optically similar, and will 
show the same degree of internal reflection with respect to off-axis 
light hitting a dome.

Again, the key to minimizing these kinds of effects is to make sure the 
entrance pupil is at the center of curvature of the dome- and that is 
true whatever the dome is made of.

Glass has two advantages- it is mechanically more robust, and it can be 
ground to a very precise surface. In the case of a dome for an allsky 
camera, the first is possibly important, depending on environment. As a 
dome weathers, its surface (or bulk) starts scattering light, which 
isn't good, and plastic usually weathers faster than glass. The second 
advantage probably isn't so important, since a molded acrylic dome can 
be optically accurate to the point that it isn't the limiting optical 
element.

Unless you use a ground glass dome (expensive!), acrylic domes are 
likely to be more accurate optically, because they are precision molded, 
whereas glass domes are usually made by a slumping process. The 
resulting glass dome will have a less uniform wall thickness, and 
therefore greater optical aberrations.

Chris

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

On 1/7/2012 6:14 AM, Jim Wooddell wrote:
> Hi Mark and all!
>
> The other issue is light refraction...if that is the right word.  Shine a
> light on the plastic dome at various angles and see what happens.  A
> question might be would a glass dome reduce or eliminate this???  I think it
> would.
> Not all sites are free from external light pollution.
>
>
> Jim
>