RONCHI
Simple Starter Kit.
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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.
OK.
You have a concave
Sphere or Paraboloid you want to test.
Let’s
stop prevaricating, researching, and hesitating and build a
simple
Ronchi tester and get it working.
Afterwards, you can
criticise this set-up, improve it, tidy it up, add polished knobs if you want
or even call me names if you like. But
if you have never done any Ronchi testing, you are simply in no position yet to
criticise or suggest improvements.
Philosophy
Ever wanted to burn off a Porsche while driving a VW ?
The tester is deliberately
kept very crude and can be made from common materials, which includes the
grating if necessary.
Yet it is as good as any
Ronchi tester and is perfectly capable of bringing a mirror of moderate F:NO to
a reasonable standard. Of course you
may wish to use other backup tests to guarantee a superior mirror, but most
users will be stunned at how well a carefully tested mirror using this rig will
perform.
Step 1
Will the Ronchi Test be adequate for my mirror ?
Chances are this is your first telescope mirror
– if not you will most likely not to be reading this material.
No matter the size or F:NO of your mirror, this
tester will be invaluable in reaching
the preliminary sphere required before finally
fine figuring to a paraboloidal shape.
It may or may not be capable of assuring a
superb mirror because the Ronchi test is really for spheres.
When testing paraboloids, the test becomes less
capable with deeper mirrors.
Considering
the ubiquitous 6 inch telescope mirror usually chosen by most beginners, the
chart below gives some indication of how close a Ronchi test will likely
approach perfection. Of course, nothing
here is fixed and a very careful first time user may approach the performance
of an experienced tester. But, for a
beginner, it is a useful guide.
Beware of the extremists. Some will claim the Ronchi test is useless.
Others will claim it can achieve anything.
As expected, the truth is somewhat in between.
THIS ASSUMES YOU WORK WITH COMPUTER
GENERATED SIMULATIONS
OF IDEAL RONCHIGRAMS FOR YOUR SPECIFIC MIRROR
TEST CAPABILITY
|
EXPERIENCED TESTER
|
FIRST
TIME TESTER
|
|
6 inch F:5 |
Possibly will produce a reasonable mirror but
definitely requires additional testing |
Good starting point but definitely requires
additional testing. |
|
6 inch F:8 |
Good mirror |
Probably good but not guaranteed |
|
6 inch F:10 |
Superb mirror |
Good mirror with care |
Remember that even a mediocre mirror will impress
most people and it is likely
to
equal or even surpass many shop bought items.
The examples above should not be taken to imply
that you need an F:10 telescope. This
should be determined by portability and your means of transport. I would
normally suggest F:8 for a beginner but insist you make that choice.
Step
2
What
Grating and Rig will I use ?
Do not bother making your own grating on a printer. It will be substandard.
Much better substitutes are
readily available.
If you do not believe this, further investigate information under
gratings.
You must now make a
choice depending on circumstances.
Possibility 1
If you live next door to
Edmonds or some professional supplier, and you have the money, buy an etched
and filled glass grating. It will not
degrade over time like most photographically made ones. Its performance will marginally surpass
even the best photographic grating and easily outperform mediocre ones, and it
may be used without a slit.[1] If you do not have this option, look at
possibility 2 below.
If you have made this
choice, you now need to build the simple Starter Rig. You can leave off the slit attachment if you like.
Possibility 2
Chances are that you are
isolated or in a hurry to start. In
this case, use a grating substitute with the confidence that it will give
excellent performance.
It must be used in the Slit/Grating mode.
This is
slightly more complex to set up but is, in my opinion, the preferred
option. If you subsequently get a
professional grating, the tester can still be used without the slit.
Do not listen to well meaning doom merchants. Simply go ahead following the described set-up.
If you have doubts about the capability of substitute gratings, see woven gratings.
If you have made this
choice, you now need to build the simple Starter
Rig, including the slit
attachment.
Step 3
How
will I hold my mirror during testing ?
It is very important
that you build some device to hold and align
the mirror during testing. This is especially true for two reasons.
·
Many
mirrors are damaged at this stage because of impatience. Balancing the mirror on a bench is not only
awkward; it can lead to chips and even a broken mirror.
·
The
test becomes difficult and frustrating because there is no easy way to raise or
lower the image. The easiest way to
build adjustment into the test situation is to make a mirror stand with
adjustable tilt. The lateral adjustment
is taken care of by moving the Ronchi rig sideways.
Luckily, the mirror stand can be cheap and crude, as long as it contains
the essential features.
For one example of a very simple solution, see Mirror Stand.
Step 3
How
do I set up the test ?
If you have never set up
the test before, it will be worth knowing either the Radius of curvature
or the Focal length of the mirror before starting the test.
Probably the easiest to
measure is the focal length. All you
need do is sharply focus an image of the sun on a wall or card and measure the
distance.
Now, measure out
double this focal length distance between the mirror on its stand and
the grating. It does not need to be
exact to begin with but should be accurate enough to form a small spot image the
same size as the LED source.
For some reason, many
incorrectly call the distance from mirror to grating the ‘focus’ distance. This
is confusing, and should be discouraged, because it is actually the Radius of
Curvature or twice the focal length of the mirror.
The following diagram shows
a slit source and grating occulter.
There is really no difference in setting up if the grating/grating mode
is used. In this case, the grating extends
over the lamp.
A diffuser MUST be used between lamp and
grating or between lamp and slit.
Test distance is Twice the Focal Length which
is the Radius of curvature.
The tilt of the stand and
the position of the test rig must now be adjusted so the returning image falls where
the grating and eye will be positioned during testing.
Aligning can be frustrating
the first few times. There are two main
ways [2]
to facilitate this adjustment. It is
best done in quite dull light. Once the
image is located and the mirror is aligned, the actual test may be done in
brighter light.
A very simple rig with grating and slit removed to aid mirror alignment
in dull light.
Temporarily mount a large
white sheet of cardboard or something similar just behind the testing rig. The photograph shows a sheet of Coolite
(polystyrene foam sheet) in use. This
is convenient because it is stiff and light.
Remove the slit to expose the light source – this passes a maximum of
light, which helps alignment. Now
adjust the mirror stand so an image of the lamp is seen on the white screen –
nudging it towards the position of grating and eye. The actual adjustments are
all best done by manipulating the position and tilt of the mirror stand. You will be able to see the image of the LED
and find it is quickly brought to the correct position.
Mount the grating (and the
slit attachment if appropriate) and place your eye just behind the grating.
From here on, the test rig
is carefully moved backwards and forwards and from side to side while watching
the Ronchi image. It is important to
keep track of whether you are inside or outside the Centre of Curvature. If you are not familiar with this,
deliberately move the rig backwards and forwards quite a distance and watch the
Ronchi bands expand and then contract again as you pass through the COC. Very close to the COC the pattern will break
up and become unreadable. This is
normal. Aim to have no more than about
5 bands (or even less) visible on the
mirror. This will occur a few mm inside
and outside COC.
Step 4
What
Ronchigram Guidelines will I need ?
For producing Spheres, the
Ronchi image will always give straight lines for any sphere and any grating
position. No computer simulation is
needed.
Parabolas, on the other
hand, give distinctive patterns. Depending
on the F:NO of the mirror, the spacing of the grating, and the position (which
determines number of bands seen), the
Ronchi image will change. Fortunately,
this is easily predicted using computer simulations. My program RonchiZ is
recommended but others are readily available.
See Software. Whatever you use, take the time to simulate
patterns that specifically match your mirror.
Note that it is unnecessary
to measure the position of the grating.
Adjusting the position of the grating so the correct number of bands is
visible does this automatically[3]. Some idea of band shapes is given under aspherizing
Interpretation and
Figuring
No information is included here about how to actually work the surface,
but
interpretation of imperfect Ronchigrams is covered under Basic Interpretation and Aspherizing.
Later you might like to visit Advanced interpretation.
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