RONCHI RIGS
The Close in Brute
Shows various test rigs which are easy and cheap to build.
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Copyright – P. J.
Smith – pjifl@bigpond.com.au
But permission is
given to distribute this material in unaltered form as long as it is not sold
for profit.
THE
CLOSE IN BRUTE
This was made
for a specific purpose, accepting its obvious disadvantages.
The purpose
was to allow :-
1.
Slit and
Grating to be as close to the optical axis as possible without using a beam
splitter. Despite this, the slit is
quite long. It is worth remembering that a beam splitter introduces aberrations
because of its thickness if the beam under test traverses the glass between
mirror and grating or mirror and slit.
2.
Eye to be as
close as possible to the Grating or Knife Edge. In this respect, this unit is a pleasure to use if one wears
spectacles.
It is
possible to use a 50 W Quartz Iodide lamp pumping light into this monster.
Of course
the efficiency is lousy. Despite this,
it does provide a cheap, readily available tester that handles reflections from
more than one surface as in many null tests.
The most
obvious potential objection is overheating of the apparatus and air around the
test area but this is less objectionable than one would suppose because the
main source of heat is displaced sideways away from the optical axis.
With a
variable voltage supply, the tester may be used at full power for limited
periods, which reduces the problem to tolerable proportions. If used at full power for long periods, the
Duct Tape is not a good solution. It
is, however, possible to upgrade the device by using other heat resisting
materials, yet retaining the same principle.
Since the
heat source is remote from the vicinity of the slit, there are no problems with
heat effects on the optical path.
The glass
light guide works well. It is best made
from glass about 2 - 3 mm thick. The 45
deg. edge may be finished in 20 minutes by freehand hand grinding and
polishing. There is no need to use grit
coarser than 250 and the polishing may be done on cloth with Cerium Oxide. A tough cloth such as denim is ideal.
Slight
rounding of the hypotenuse during polishing of this surface may not be a bad
thing.
The front
and rear surfaces must be left polished and not painted for best
internal reflection. This is why the metal
shims are placed front and back.
Another reason is the need to shield the eye from any spilled light
emitted from the end of the glass.
A simple
solution to the variable voltage supply is to use 6 or 12 V as needed. Just
plug into whichever is wanted. . I use
a very old TV transformer (yes, they contained power ‘trannies’ once). For normal use the 6 V winding is used. Full power for shorter intervals is obtained
by using the 12 V winding.
Painting and
Scribing a slit on the narrow surface takes care but it is possible. It should last at least a year with careful
use. Some slight chipping around the "slit" does not matter much as
it is possible to choose another portion of the slit. Do not worry if the paint is not perfectly opaque. [1] A photograph of the slit end of one of these
rigs can be found under the scribed slit
Some time
must be spent aligning the Grating or Knife Edge with the slit before fixing it
in place.
Again, with
care, it is not a problem.
I find
myself using this monster far more than expected. It performs well, is a compact unit, and there is never any
problem finding the return image.
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