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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