Ronchi pattern

One of the tools needed to run the Ronchi test is the namesake grating. This is a series of opaque parallel lines spaced equally so you have a spatial frequency that can go from 4 all 8 per millimeter. A pattern with a lower frequency can be used if it is necessary to test a parabolic mirror with very thorough focal ratio, While a greater number of lines per millimeter would cause more diffraction of light that would disturb the vision of lines of shadow. When speaking of lines per millimeter is always meant the number of opaque lines (the black) per millimeter, so in a grid of 4 lines/mm there are 4 black and four white lines every millimeter (ie each line has a thickness 0.125 mm).

The grating is easily available and can be purchased in specialty stores, or with a very minimal expense it can be built at home, due to the simplicity of realization of this instrument.

The methods most commonly used for its construction, I'm:

  • Printing from PC
  • photographic Method
  • mechanical Method

 

PC PRINTING

This method is the simplest and quickest of the three suggested as it is to print on glossy paper an image of a pattern that can be drawn by means of suitable CAD programs, or from the following link You can download the pdf file ready for printing:

Download: Pattern 4Linee-mm

The frequency limit of lines / mm can be obtained it depends on the maximum number of DPI (dots per inch) the printer is able to generate (Note that sometimes when you open an image with a program that change depending on how many DPI can endure) .

After having printed the image must be made to dispose the press in a special support that it gives rigidity.
A pattern made in this way is able to be useful for this purpose, even if it is not of high quality because the printed black lines are not perfectly opaque, but it is more or less opaque depending on how it is deposited ink and the transparent spaces, due to the presence of bright sheet, will always be a little dull. In addition the edges of the lines are not perfectly defined, but they tend to always generate ripples due to ink spread on paper.

And here is an example of a lattice so that:

IMG_5256

And this is what you see if you go to zoom with a small microscope on the lines to see them in detail. Printing was done with ink-jet on a special tracing paper:

CAM00947

This however was always made on the same type of paper, and always with inkjet printer, However, by positioning the lines so that it is parallel to the direction of the printer cursor scrolling. The improvements are evident and indicate that this solution is to be preferred:

ink long.

this instead refers to a press always on the same type of paper, But with the use of a professional laser printer:

laser

As you can see, get a great lattice with this method is not as easy as it seems, You need to look for the right card and the right printer (laser printers are preferred). This obviously does not mean that they can not obtain good results, but be careful and check every time if what is quality or not printed.

PHOTOGRAPHIC METHOD

The photographic method consists in photographing a sheet on which are printed a series of parallel black lines, with a photographic stain film so as to obtain (once developed) an 'image imprinted in the film that will be used as a pattern of Ronchi.
The film therefore has a crucial role in this process, and it is recommended to use one and grain black and white fine in order to achieve maximum clarity and resolution possible.
First you need to print on a sheet of white paper a series of at least 50-60 equidistant parallel black lines of a known thickness (P.S: black and white lines must be of equal thickness), and place it in a position such that it is well lit and easy to photograph.
At this point one has to position the camera,, possibly fixed to a suitable support,, at a distance such as to obtain the image of the pattern on the film having the number of lines per millimeter adequate.
To determine the distance it must be considered that the linear dimension of the image on the photographic plate is given by this equation:

Dim. linear on the film = (focal lens) x (dim. linear design) x (dist. design target)

And then inserting some parameters and developing the accounts will come to get the distance at which position the machine as a function of the number of lines / mm to be obtained and the thickness of the lines that have been printed on the sheet:

dist. design-target = 2 x(focal lens) x (spess. line) x (linee/mm)

Taking an example, if it is printed with a pattern of lines 4 mm thickness on the sheet of paper and using a lens from 50 mm and you want to get a lattice from 6 lines / mm there must be positioned with the camera:

dist. design-target = 2 x 50 x 4x 6 = 2400 mm = 2.4 m

Or you can make a series of pictures from different distances and then use those that are more suitable.

With such a fact pattern, you will always have that space between the opaque lines are not perfectly transparent in so far as there is the presence of the film and this causes always and in any case a small scattering of light during the test, that goes to decrease the contrast of the image and the sharpness of the contour lines (This also happens with the lattice obtained from the press), which however it is not able to jeopardize the goodness of the results obtained by the test.

And here is an example of a pattern made with this method:

IMG_5250

and the detail of the lines:

IMG_5259

MECHANICAL METHOD

A pattern obtained by the mechanical method is much more complicated to implement than a lattice obtained with the other two methods, but it has the great advantage of getting some very opaque lines, alternating with perfectly transparent lines, and this is possible using copper wire or very thin fishing (circa 0.1-0.2 mm diametro), a support and glue.
As a support can be used, for example, a piece of aluminum, copper, wood or any material as long as a certain stiffness of approximately 40x30x2 mm size (the thickness depends on the used laminate) on which to drill a circular hole of at least 15-20 mm diameter or open a square or rectangular window of similar size. Sara from this window (forum) that we will pass the return light of the mirror and that will affect the reticle.

The photos that accompany the procedure are based on a pattern made with 2 cotton threads, to make this otherwise highly visible images made with very thin wires (as it would be necessary) would be not clear. P.S: absolutely do not use wire NEVER cotton to make the reticle. At the bottom of the page in the pictures of the details of the lines you will understand why.

First you need to cut from the spool 2 wires of adequate length (2-2.5 m) and take one end of each of these wires and approach them as we would then wrap a perfectly simultaneously next to 'another.
One proceeds by gluing these ends on the left side of the support and will continue wrapping the wires next to each 'other, around the longer side of the support so that they are parallel as much as possible to the minor sides.
Once you are maintaining a sufficient number of windings (the grating must have a width of at least 10-15 mm) pasting both wires to the support.

IMG_5226

Now very carefully, We must disconnect the end of only one of 2 wires (attention that often tend to fall off both, What if that happens irreparable effects on the work done so far) and it must unroll up to remove it completely from the support (attention to when it comes to also disconnect the other end).

IMG_5230
So we must proceed to glue well the only thread left over the entire length of both longer sides of the support because in the next operation you will have to cut the wire so as to remove it completely from one of the two faces of the lamination.

examples together

In this way we get a series of perfectly opaque lines due to the presence of air by separate wire (therefore perfectly transparent lines).

IMG_5237

finished lattice

E 'of paramount importance to ensure that the glue applied is completely dry before proceeding with the next operation, so do not try to force the issue or is in serious danger of ruin in a moment all the work forcing you to start all over again.

As I said before the wires that allow to obtain good results are those made of copper or nylon of which the details of the lines will be shown. It will also be shown on the detail lines made with cotton threads to show how these wires are not suitable for the purpose:

all lattices

Comments (2)

  1. Avatar

    Giovanni

    The mechanical method , exposed here did not know him. However, I suggest a small variation: with metotodo photo, instead draw dark and light lines, It is hasty and perhaps better to photograph the teeth of a good black comb, place at a suitable distance and backlight.

  2. Avatar

    Bartolomei Mirco

    hello John, interesting alternative, although in my opinion definitely the most expeditious solution but less powerful, It is given by the fact that all the combs designed for plastic injection have the teeth slightly tapered to facilitate removal from the mold, which generates lines that are not perfectly parallel on photographic film.
    The white and the black lines you can easily design and print from the PC and the simple copy-paste the process is also quick. So I would opt for this solution, but nobody forbids to do otherwise, especially if it works anyway… :good:
    Ciao… :bye:

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