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 at 8 lines per mm. A pattern less frequently can be used if you must test a parabolic mirror with focal ratio very thorough, 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 ones) for mm, so in a grid of 4 lines/mm there are 4 black lines and white every four mm (that is, each line thickness 0.125 mm).
The grating is easily available and can be purchased in specialty stores, or with very little expenditure can be built at home, due to the simplicity of realization of this instrument.
The most commonly used methods for its construction, are:
- PC printing
- Photographic method
- Mechanical method
PRINTING FROM PC
This method is the easiest and fastest of the three suggested as it is to print on glossy paper the image of a mesh that can be drawn through appropriate CAD programs, or from the following link can be downloaded your pdf file ready for printing:
Download: 4lines pattern-mm
Frequency capping of lines/mm obtainable depends on the maximum number of DPI (dots per inch) that the printer is capable of generating (Beware that sometimes when you open an image with a program that the change depending on how many DPI it can bear) .
After printing the image it is necessary to have the press into a cradle that gives rigidity.
A pattern made in this way can be useful for the purpose, Although it is not of high quality because the black lines printed are not perfectly opaque, but they are more or less opaque depending on how settled the ink and transparent spaces, due to the presence of the shiny sheet, will always be a bit dull. In addition the edges of lines are not perfectly defined, but they tend to generate ripples caused by the spreading of ink on paper.
And here's an example of a lattice as well done:
And this is what you see if you go to zoom with a small microscope on the lines to open in detail. The press was made with special paper for inkjet printer transparencies:
This was achieved on the same type of paper, and always with inkjet printer, However, placing the lines so that they are parallel to the flow direction of the cursor the printer. The improvements are evident and indicate that this solution is preferable:
This refers to a print on the same type of paper, However, with the use of a professional laser printer:
As you can see, get a great pattern with this method isn't as easy as it sounds, need find the right paper and printer (those lasers are preferred). This of course doesn't mean you can't get good results, but you have to be careful and check every time if the Board is of quality or not.
The photographic method is to photograph a sheet on which were printed a series of parallel black lines, with a film camera in order to obtain (when grown) an image etched in the film that we will use as Ronchi grating.
The film then plays a key role in this process, and it is recommended to use a fine grain and black and white so you get maximum sharpness and resolution possible.
First you need to print on a sheet of white paper a series of at least 50-60 parallel black lines regularly spaced bars of a known thickness (P.S: black and white lines must be of equal thickness), and place it in a location that is well lit and easy to photograph.
At this point you have to place the document camera, possibly fixed to a suitable support, at a distance such as to obtain the image pattern on the film with the appropriate number of lines per millimeter.
To determine the distance it must be held that the linear dimension of the image on the photographic plate is given by the equation:
Dim. linear on the film = (lens focal) x (Dim. linear drawing) x (Dist. target design)
And then inserting some parameters and developing accounts you get to get the distance in which to place the coffee maker on the basis of the number of lines/mm to be obtained and the thickness of the lines that were printed on the sheet:
Dist. drawing-target = 2 x (lens focal) x (spess. line) x (line/mm)
Doing an example if you have printed a pattern with lines 4 mm thick on the sheet of paper and use a 50 mm and you want to get a pattern from 6 line/mm you should position with the camera:
Dist. drawing-target = 2 x 50 x 4 x 6 = 2400 mm = 2.4 m
Or you can take a series of photos from different distances and then use the ones that are most appropriate.
With a pattern so done, you will always have that space between the opaque lines are not perfectly transparent as there is the presence of film and this always causes a small scattering of light during the test, going to decrease image contrast and sharpness of contour lines (This also happens with the lattice obtained by press), that however is not capable of affecting the quality of results obtained from test.
And here's an example of pattern made with this method:
and the detail lines:
A lattice obtained by mechanical method is much more complicated to implement than a lattice obtained by the other two methods, but has the great advantage of getting really opaque lines, alternate lines perfectly transparent, and this can be done by using very thin copper wire or fishing (about 0.1-0.2 mm in diameter), a backing and glue.
As support you can use eg a piece of aluminum, copper, wood or any material provided by a certain rigidity of size of about 40x30x2 mm (the thickness depends on the laminate used) on which to drill a circular hole of at least 15-20 mm in diameter, or open a window similar sized square or rectangular. Sara right from this window (hole) We'll return mirror and light which will affect the pattern.
The photos that accompany the procedure are based on a pattern made with 2 cotton threads, this to make them visible otherwise images made with very thin wires (as would be required) would not be clear. P.S: absolutely never use cotton threads to do the lattice. At the bottom of the page in the photos of the details of the lines you will understand why.
First you need to cut from the spool 2 proper length wires (2-2.5 m) and take one end of each of these threads and bring them closer as we will then wrap them simultaneously one perfectly next to each other.
It proceeds pasting these ends on the left side of the media and will be continued by wrapping the wires next to each other, around the larger side of the media so that they are parallel to the sides as much as possible minors.
Once maintaining a sufficient number of windings (the pattern must have a width of at least 10-15 mm) you paste both wires to the support.
Now very carefully, disconnect the end of one of the 2 wires (attention that often tend to come off both, What happens if work irredeemably compromises so far) and should roll out up to remove it completely from the media (careful when you get to pull the other end).
Then proceed to paste well single strand remained along the entire length of both sides are longer than support because in the next step you will have to cut the thread in order to remove it completely from one of the two sides of the sheet.
In this way we will get a series of lines perfectly opaque due to the presence of the wire separated by air (then by lines perfectly transparent).
It is vital to make sure the glue applied is completely dry before proceeding to the next step, so don't try to force the times or risk seriously damaging in a moment all their hard work forcing you to do it all over again.
As mentioned before the wires which can obtain good results are those copper or nylon that will show details of lines. Details will be displayed on the lines made with cotton threads to show how these threads are not suitable for the purpose: