Simulated geometric Ronchi patterns for an 8 inch F:6 paraboloidal mirror with a ¼ wave turned down edge (TDE)
extending over the outer 5 % of the diameter on each side of the mirror.
Copyright – P. J. Smith
But permission is given to distribute this material in unaltered form as long as it is not sold for profit.
This set of simulated Ronchigrams may help some detect and interpret turned down edges.
The simulations are only Geometric which of course means that diffraction effects will modify these images.
Most Ronchi tests have areas of virtually unusable fringes at the extreme left and right because of the diffraction effects. There will also be weaker diffraction bands between the Ronchi bands more or less running parallel to the bands which are not shown here.
The mirror profile is drawn under each image.
Note that the TDE on the surface is ¼ wave (0.55 um) but the effect on the wavefront is ½ wave.
Although the TDE covers only 5% of the diameter on each side, since this represents a much greater area (being at max diameter) the effect on good performance will be quite noticeable and action must be taken to reduce it.
The set of Ronchi images begin further from the mirror, progress through Polar Centre of Curvature, and end up forward of this point ie. Closer to the mirror.
These images will be nothing new to experienced Ronchi users but they may be useful to help beginners.
¼ wave of TDE on an 8 inch F:6 mirror.
Note that, depending on the span of the TDE, the edge pattern may look different. If the TDE is confined to a smaller span, the bands hook more suddenly, but if the TDE covers a wider span, the bands show a more gentle bend.
If the depth of the TDE is the same, the case where it extends further in from the edge will have the worst impact on performance, even though it is usually less obvious on the Ronchi bands..
Note that the best portion of the pattern to detect and interpret the Turned Down Edge is neither at the extreme left or right position or on the centreline.
It is most useful to examine the change of shape of the bands as they depart the edge of the mirror at, for example, the 7 or 8 OClock positions.
In all cases, the Geometric bands compress at the extreme left and right positions, which is exactly the effect of the Diffraction effects at the extreme left and right positions. This makes interpretation in this region virtually impossible.
Some may find the second image useful to aid interpretation.
Find the position where the surface begins to change slope downward (R = 90%) and trace vertically upward on the grid. Then run around this zone on the mirror noticing how the edges of the Ronchi bands behave at this zone radius.
The greatest slope in this case occurs at the edge so this is where the band has been displaced the most (inwards in this case).
If the TDE slope is constant at the edge the bands kink over and then extend more or less in line with the original line shapes. All this is much easier to see with spheres.