Re: [ATM] Re: [atm_free] The zonal Foucault test is free ofinherentcorrection bias - some supporting graphs

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

The book by Strong does recommend use of a pellicle
made of lacquer on glass, removed by water. I didn't
know what a pellicle was, so I just looked it up and
found this at 

http://optical-components.globalspec.com/LearnMore/Optics_Optical_Components/Optical_Components/Pellicle_Beamsplitters

"About Pellicle Beamsplitters

"Pellicle beamsplitters are constructed from thin,
nitrocellulose films that are stretched and cemented
to rigid supporting rings. Pellicles are very delicate
and liable to resonate to certain vibration
frequencies.  Unlike thicker beamsplitters, pellicle
beamsplitters have the major advantage of eliminating
second surface reflections by making them coincident
with the original beam. 

"Pellicle beamsplitters are available either coated or
uncoated.  Uncoated pellicles yield an average
reflectivity of 8% from 375–2400 nm. Uncoated
pellicles reflect 10% of the incident light when used
at 45 degrees, thus providing a useful, low-intensity
side beam, thus they are often used in beam sampling
applications. Coated pellicle beamsplitters provide
equal reflectance and transmission over the same
range.  Coated pellicles are generally manufactured
with two different coatings, dielectric and Inconel®. 
Dielectric coatings are designed for a specific
wavelength.  Inconel coating contains nickel,
chromium, cobalt, and iron. A thin Inconel film
balances the transmission of each element, resulting
in a wavelength neutral coating.

"Pellicle beamsplitters have several advantages over
plate beamsplitters.  Unlike plates, the thinness of
pellicle beamsplitters causes very little ghosting. 
Additionally, they foster a very low incidence of
chromatic and spherical aberrations.  Finally, they
cause only negligible alterations in the optical path.


"However, pellicle beamsplitters are very fragile
devices and are sensitive to environmental factors. 
They are easily damaged, and should never be touched. 
When cleaned, pellicles should never be wiped or
bushed.  They should simply be allowed to dry in
clean, moving air.  Their drum-like design (film
stretched over a frame) makes them susceptible to
acoustic interference.  Care should be taken to
minimize this type of disturbance while they are in
use.
...
"Related keywords: pellicle beamsplitter, pellicle
beam splitter, beamsplitter pellicle"

--- "Richard F.L.R. Snashall" <rflrs@rcn.com> wrote:
> Guy Brandenburg wrote:
> 
> > I am currently browsing a newly-acquired copy of
> John
> > Strong's book on laboratory physical techniques
> > (formerly out of print but available from Lindsay
> > Books as a reprint). He points out that Foucault
> tests
> > are sensitive to parallax, and has a method of
> working
> > around that by using a half-silvered mirror so
> that
> > the examiner's eye and the ray of light coming out
> > towards the mirror are in exactly the same spot.
> > I know that I always have about an inch or two of
> > parallax when I use a Couder mask.
> >
> > Guy
> 
> I had always heard that one of the difficulties here
> was the aberration
> introduced in the transmission through.  That was
> reduced by using
> a pellicle (sp?).
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 

Guy  Brandenburg
Washington, DC
My home page:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
_______________________________________________
ATM mailing list http://www.atmlist.net/