As regards the control of the optical mirror, simplest and most widely used tests, especially at the amateur level, It is definitely the Ronchi Test.
This test uses a beam of light is transmitted and then reflected from the primary mirror, which is passed through a reticle with a series of low-frequency (4-8 linee/mm) located in the proximity of the focus point. In this way, on the mirror are projected a series of lines of shadow detectable visually or by means of a digital camera.
The shape assumed by the fringe it is possible to evaluate with precision the curvature that the mirror took during processing, and then it will be possible to determine a strategy for action to correct any mistakes.
It should be remembered that this test however is more qualitative and not quantitative (unless strictly control various parameters), so it will not tell the extent of any error, but it will tell us that this error type will. This does not mean that the test is of little value, rather it is a great help, especially in the phase of achievement of the sphere, due to its speed and ease of reading. I said especially in the attainment of the sphere stage, because in this case the lines you should see (for a perfect sphere) should also be perfectly straight and the evaluation to the eye of a straight line or of a possible distortion with respect to it is not so difficult, contrary to what happens in the evaluation of a perfect parabolic surface, whose lines are not straight, but curved trend with more or less accentuated depending on the number of lines per millimeter of the grating, the optical focal ratio, of the lattice position relative to the center of curvature etc ...
Then you will carry out this test if you want to test a spherical surface, While a parabolic can help, but generally these, we turn to other tests such as the Foucault Test.
Regarding the execution of the real test we need some tools that will be listed below.
NECESSARY EQUIPMENT:
- Ronchi pattern (4-8 linee/mm)
- Point source light or slit
- Support for the mirror
- Support for light-lattice source
METHOD’ OF EXECUTION:
First you need to place the mirror in a vertical manner, possibly secured in a holder who prevent the fall. Then you must place the support on which is mounted the grating and the light source at a distance about equal to the radius of curvature of the mirror (ie twice the focal length).
After that it must turn the bulb or the LED source and adjusts the position of the support so that the return light beam from the mirror goes to affect the Ronchi grating (the point where to put the grating will be at the point where you see the mirror fully lighted).
Now approaching the eye to the reticle we should observe the lines of shadow projected onto the surface of the mirror, and moving forward or backward you should see more or less line depending on how much you move away from the center of curvature.
Will not be displayed below the physical and geometrical principles that describe the diffraction and the path of the light required to describe these lines, and the complexity of some mathematical equations, and the fact that the most common defects that may be encountered, You can be explained by showing and commenting on some figures of shadow lines you should see.
MORE FAULTS’ COMMON
The shadow lines for a spherical mirror with the light source placed in the center of curvature, They must present themselves perfectly straight, such as those shown in Figure. Move longitudinally with grating will only change the number of visible lines. .
The images above all refer to the same sphere, But he is taken from different positions of the grid as you can read the comments on the photo itself.
From now on you will see only two images in the leftmost column will refer to an image taken with pattern in intrafocal (ie in a position closer to the mirror with respect to the center of curvature), while the right one will refer to an image taken by the grating in position extrafocal (or in a more distant position from the mirror with respect to the center of curvature).
Unlikely, the first attempt will be able to obtain the image of this type, with the lines perfectly straight, but more likely the lines tend to follow and highlight the defects on the surface almost inevitable during processing.
The most common of which are given below:
HOLE IN THE MIDDLE :
Description: It is a defect that involves the central part of the mirror, in which the surface is lower than the surface of a sphere. That is an area in which the reflected rays should be in focus at a point closer to the mirror than those in the rest of the surface. So indicates the presence of a “hole” central.
HILL CENTER :
Description: It is a defect that involves the central part of the mirror, in which the surface is higher than the surface of a sphere, therefore it indicates the presence of a central mound. That is an area in which the reflected rays should be in focus at a point farther from the mirror than those in the rest of the surface.
retorted BOARD :
Description: It is a defect that generally involves a limited area in the amplitude of the mirror edge, in which the surface is lowered with respect to a spherical surface. That is an area in which the reflected rays should be in focus at a point farther from the mirror than those in the rest of the surface. This is a common defect in the optical workings, and also difficult to remove because it is not possible to add material, but still toglierne, necessary to remove glass from the entire surface of the mirror to the exception of the edge zone.
FLOOR BOARD :
Description: It is a defect that generally involves a limited area in the amplitude of the mirror edge, in which the surface is raised with respect to a spherical surface. That is an area in which the reflected rays should be in focus at a point closer to the mirror than those in the rest of the surface. Unlike countered board, this defect presents fewer difficulties in its elimination.
FLOOR RING :
Description: It is a rather rare defect that indicates the presence of a generally annular amplitude limited area in which the surface is higher than the remaining spherical surface. That is an area in which the reflected rays should be in focus at a point farther from the mirror than those in the rest of the surface.
LOW RING :
Description: It is a fairly rare defect, although less relative to the ring floor and which indicates the presence of a generally annular zone limited amplitude in which the surface is lower than the remaining spherical surface. That is an area in which the reflected rays should be in focus at a point closer to the mirror than those in the rest of the surface.
ASTIGMATISM :
Description: l’ Astigmatism is a defect of the optical system that occurs when the surface is asymmetrical, ellipsoidal, oval, in which different sections of the surface have different radii of curvature. The only remedy for this defect lies in the return to the processing with the grits of carborundum to standardize the surface.