Ronchi Patterns
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.
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