RONCHI  RIGS

Deluxe

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.


 


 

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.

 

 

 


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