RONCHI
Grating/Grating mode
Setting up the Grating/Grating arrangement.
Copyright – P. J.
Smith
But permission is given
to distribute this material in unaltered form as long as it is not sold for
profit.
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General
Some test situations, where
the source is close to the grating, allow the same grating to be extended in
front of a light and diffuser. Thus
part of the grating acts as the source and part of the same grating as the
occulter.
In the majority of test
situations this is impossible because source and image are widely separated.
But in the case of testing
a concave mirror at centre of curvature, or of testing a lens by
autocollimation, the arrangement becomes possible. Since testing concave mirrors at Centre of Curvature is all most
ATM’s ever do, some seem to think this is the only way a Ronchi test can ever
be done. For a variety of other test
situations see Advanced set-ups
and Null testing.
Porter and Anderson seem to
be the first to use this mode. The
following is an extract from their original article[1]
Before deciding on this
mode of operation you should study the photographs comparing performance
between Grating/Grating and Slit/Grating set-ups using different types of
gratings. See Grating/Grating V Slit/Grating comparison. Then decide if a Grating/Grating mode is
best for you.
Simple Implementation
Area source
An incandescent source with
a filament arranged in a line is an obvious choice and it does work well behind
a diffuser. This is probably best if
you want really intense illumination.
For sheer brute force a 12 V Quartz Iodide headlight bulb is hard to
beat but is seldom needed unless more than one unsilvered surface are in the
test beam. Excess heat may be a problem[2].
A simple, perfectly
adequate light source which will evenly illuminate an area of grating is a
bright LED with its collimating front lens left intact. A small flashlight bulb also works but LED’s
are small and cool.
This produces an area
rather than a line source of light but the extremely high efficiency of a
grating source as opposed to a pinhole or even a slit makes this entirely
adequate for visual work. A diffuser
(see Diffuser) is placed just in front of
the LED with the grating just in front of this. The reason it is best to
leave the collimating lens surface in place on the LED is because this produces
more even illumination over the area of the grating.
LEDs of 5 and 10 mm
diameter are readily available. 5 mm is
usually sufficient and is easier to mount close to the axis but for special
cases a 10 mm one would have advantages.
A larger grating source is easier to locate and makes for more tolerant
set-up. Be careful when using a very large illuminated grating as source since
it is easy to use an active portion of source and occulter that is separated by
more than the minimum distance. Sometimes you will stray off axis more than you
would like. Extremely simple set-ups
using this arrangement are the Basic and Emergency rigs. The Basic rig also is
suited to a slit although the Deluxe rig is better in this role.
Since the Viewing Grating
is the same as the Source Grating, there is no alignment to be done and the
grating is always in position. The main
problem is allowing for unobstructed head and eye access .
With some ingenuity and
blue-tac it is easy to conceive other simple systems.
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