RONCHI 

Gratings

 

Summarises Grating types, and performance in a  Ronchi Test with different sources.   Range from expensive specialist equipment to cheap readily available common items. 

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Copyright – P. J. Smith

But permission is given to distribute this material in unaltered form as long as it is not sold for profit.


There are gratings and gratings and their performance cannot be separated from the source used.  This seems poorly understood by most.

Understanding how different types of gratings perform in different test configurations is the key to a rational choice of grating/source combinations. At the moment we have a huge range of test set-ups and people are reluctant to consider alternatives.  Some insist on the ‘best’ equipment, unaware that a readily available, cheap substitute may perform as well.  Others blindly follow the practices they have seen used without asking questions.

Even worse, the advice given by well meaning people when transferred to another country or society may be very counterproductive.  By advising someone in a developing country that they have to perform a Ronchi test in a particular way may cause abandonment of the project when perfectly useable, or even superior materials are already close at hand.

Rethinking the test can result in :-

1.      Better performance.

2.      Avoiding set-ups that are inferior.

3.      Simpler set-ups.

4.      Use of readily available materials that perform as well as expensive set-ups.  This results in

(a)    Being able to set-up an excellent Ronchi test without waiting for special components.

(b)    No need to import and pay high prices

 

It is impossible to separate the performance of gratings from the light source used because an excellent quality source may retrieve a mediocre grating.  In general, there are three modes of operation to consider.  Each has advantages and disadvantages but users should be aware that all give good results with the right equipment. If you want more details on sources see Ronchi sources.

 

The three basic modes are of operation :-

1.      Grating source used with a Grating occulter – the Grating/Grating mode .

2.      Slit source used with a Grating occulter – the Slit/Grating mode.

3.      Pinhole source used with a Grating occulter See Pinhole sources.

 


Types of Gratings

Few realize that many common materials work extremely well as Ronchi gratings.  If you have easy access to professional gratings and have always considered anything else inferior, take the time to look at typical results obtained with some of the materials below.

Some of these results explode common myths and indicate that readily available items materials could often better the performance of many of the traditionally recommended ones. 

Even the worst substitutes are quite useful if the Ronchi test is done as a preliminary to another test.

Carefully chosen substitutes can do much better and, if the surface is not too deep, may be used as the final test.  This is especially true of spherical surfaces.

Recommendations to newcomers on what to use as a grating, should be based more on availability of materials than any other factor.  Those making recommendations should be very aware that availability of items in one country might be very different in another.

 

Spend some time comparing the following gratings and their performance.

 

Inkjet and Laser printed gratings are often recommended as readily available substitutes where computers and office equipment is common.  In general there are much better choices available and these are not recommended. 

 

Wire and Solid Gratings.  Wire wound gratings were some of the first used. Others tried different threads, even hair. These are shunned by some but work well when set up correctly.  Some have experimented with objects such as very fine combs with passable results. Excellent if you are willing to spend the time and effort but not necessary.

 

Woven Gratings may be used to produce excellent Ronchigrams when using an appropriate set-up.  This may come as a shock to many who have never seen them used.  Many woven substitute materials are available anywhere in the world so this is recommended as being a good practical substitute which works well when used in the appropriate way.

 

Professional and photographically made Gratings are usually recommended if available.  Etched and filled gratings on a polished glass substrate are very good.  Well-made, photographically derived gratings are usually good but are not quite the epitome some think.  Recommended if readily available,  otherwise don’t bother.

 

Phase gratings have interesting properties.  Usually it is difficult to make or obtain one but sometimes you will find a common material that acts as a passable substitute.  Mainly for experimenters but can be useful.

 

Non Linear Gratings allow specialized versions of the Ronchi test to be performed.  Mainly for experimenters and special applications.  For normal mirror making, don’t bother.

 

Single Wire tests are usually done in a different way to normal Ronchigrams but there are also similarities. This is included for completeness, especially because my simulation program RonchiZ simulates wire test results.  Not normally necessary for simple mirrors but very useful for Aspherizing extremely deep mirrors.

 

An opaque Grating may be used as the source.  One system utilises highly reflecting paper or alfoil behind a clear grating.  It is then illuminated with a moderately bright light beam.  Reports indicate this may be used in daylight.


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