RONCHI

Simple  Starter  Kit.

GOTO   RONCHI   INDEX

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.

 


GOTO   RONCHI   INDEX




[1] I still believe the test is better and more versatile when used with a slit, but, with a quality grating, the slit-less option is convenient and does work quite 

   well.

[2] One way involves placing your head about 3 to 6 feet back behind the grating and moving your head until the image is seen.  Adjust so it is in line with the grating.  Then, slowly move your head forward, keeping the image in view until your eye is just behind the grating.  It also works if you move quite close to the mirror instead of behind it but this partially blocks the light.  I believe the method described in the main body of the text mush easier for a newcomer.

 

[3] Some do measure the grating position as an aid for final analysis but it is quite unnecessary during initial testing.  Ronchi testing is incredibly useful during the prefiguring stage, when the Ronchi test is best left in its simplest form.  There is more on this under ‘aspherizing’.