Shows various test rigs which are easy and cheap to build.
GOTO RONCHI INDEX
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.
DELUXE
WITH FINE TILT
ADJUSTMENT
Many seem
worried about aligning the grating and slit so they are exactly parallel. While important, I believe the difficulty
has been much overstated.
If you can
adjust the tilt of either while watching the Ronchi image, it is easy to see
which is the best position.
One good
indicator of exact alignment is the appearance of faint interference bands
between the main Ronchi Bands.
A mirror
with gross errors does not show these fine bands so this is an indication that
the surface under test is improving.
They are only
visible with a reasonably fine slit. A
Ronchi arrangement not showing these bands, even with a scribed slit, indicates
some deficiency that you should investigate.
If a
Grating/Grating setup will not show these fine lines, you should change over to
a Slit/Grating arrangement.
This unit is
the same as the Basic rig, but with interchangeable slit adjustable for tilt
with a long lever, which gives excellent control.
It allows
quick slit changes as they are glued onto a 5/16 UNF nut. Simply screw on until
the slit is roughly aligned. The nut is
not tightened.
Fine
adjustment is controlled with the lever while watching the test image.
If you are
doing a lot of testing using a slit, this is a nice way to go. There are many other possible approaches.
Although
called the "Deluxe" rig, it in keeping with the spirit of "simple and available to anyone"
because only simple materials and equipment are needed.
Performance
is excellent.
A RED LED
source is located in the pivoting bolt.
On the front of this is a nut which has stuck to it a scribed glass
slit. The rear of the glass
"slit" is ground with 240 - 500 grit.
It would be
possible to replace the glass scribed slit with a blade slit if this was
mounted on a
small metal
disk.
The grating
is mounted in the 35 mm slide mount.
This protects, yet allows easy manipulation.
Since the
Phase Grating material is so cheap, it may be regarded as disposable.
Small pieces
may be clipped in place allowing testing closer to the axis.
The tilt of
the 35 mm slide can be adjusted easily using your fingers, but final adjustment
via
the long
lever is easier.
In this
example, the long lever is a piece of 1/8 inch thick plastic threaded for the
bolt which
is tightened
into the plastic and even glued in place if necessary so both turn as one unit.
The
mainframe is tapped for the bolt. This
is left untightened so the bolt pivots on the thread.
Either a
blade or scribed glass slit may be glued to the front nut. If a blade slit is used, some ground glass
must be inserted behind it.
If a scribed
glass slit is used, the rear may simply be ground as a diffuser. See diffuser and scribed
slit.
You can make
a nut with a pinhole source if you want, but pinholes really require a brighter
light source. It is very difficult to
make a high quality pinhole and there are very few occasions when one is
needed. See pinholes.
The
diffuser is important for even illumination, especially if you wish to take
photographs.
GOTO RONCHI INDEX