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Modern optical metrology allows the
quantification of three different spatial realms of the optical
surface. |
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* Low Spatial Frequency (LSF) errors: |
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Full optical surface. These are the largest
errors in the mirror; coma, astigmatism, trefoil, spherical aberration,
larger zones, etc. Generally referred to as the figure of the
mirror or irregularity of the lens. |
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* Mid-Spatial Frequency (MSF) errors: |
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Full optical surface. These are smaller
than Low Spatial but not all the way down to the Ångstrom-level
RMS surface roughness that a coherence scanning interferometer
measures. MSF historically was called primary ripple but was
undefined spatially and quantitatively. Only specific commercial
interferometers produced since ~2014 have been capable of quantifying
MSF properly. Older interferometers cannot quantify this spatial
realm. |
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* High Spatial Frequency (HSF) errors: |
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Often evaluating less than 1 square mm
of area at a time, since it is designed to quantify the smallest
scale errors. Samples are normally taken near the center, closer
to the edge and potentially other samples between these two,
depending on the size of the optic. Called out as surface roughness.
Examples briefly discussed here and here. |
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Click either photo (left or
right) to watch a video about the interferometry technology developed
for JWST and funded by NASA. Dream uses this technology to finish
its world-class
lightweight mirrors in house. |
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The zeroDELTA™ mirrors are tested in their final
athermal carbon fiber mirror mounts, in Dream's 5m vertical test
tower. |
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**With a modern
phase-shifting or similar Gen V interferometer, equipped with
a 1k x 1k (1 million data points across the full detector) or
larger detector, the full optical surface can be tested for LSF
and MSF errors, but not the smallest scale errors (HSF). The
smallest scale errors are quantified using a separate coherence
scanning interferometer. MSF
errors, optics with low MSF errors, Low mid spatial frequency
errors, space optics, precision optics, newspace optics |
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**Properly
finishing an optical surface is keenly important because roughness
creates scatter. Light that doesn't reflect or refract where
it should will decrease contrast, resolution and sensitivity
of the optical system. |
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635mm zeroDELTA™ engineered lightweight
mirror & 4D Technology 2019 PhaseCam
6000
(Gen V)
dynamic
interferometer inside Dream. |
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8.2 lb independent test head (right)
allows testing of optics at different angles and in the presence
of vibrations. A modern, enabling technology by itself. |
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**One of the
most common statements heard in optics is to describe a surface
as smooth. But this is ambiguous in two ways. 1)What spatial
realm is smooth; low, MSF or high spatial frequency errors? There
can be any number of combinations, like a smooth LSF but fairly
rough MSF and HSF. 2)What is "smooth" or "rough?"
What level the surface is deemed as smooth depends on
the spatial frequency being discussed and the wavelength or waveband
of interest, as well as the project requirements. Without defining
these simple parameters, such conversations are meaningless. |
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Low & Mid-Spatial Frequency errors - |
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**Below is
extremely high resolution interferometry data of low and MSF
errors of a 0.4m primary mirror while zenith-pointing (optical
axis) and on its carbon fiber athermal mirror mount. For any
application that uses the mirror at more of an upward angle,
as opposed to a horizontal optical axis, this test configuration
and mechanics is substantially more accurate than the industry
standard two rigid pegs supporting the mirror while the optical
axis is horizontal. Any first year mechanical engineering student
knows why. |
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Low Spatial Frequency errors
quantified |
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Mid-Spatial Frequency (MSF)
errors quantified |
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**This
paper shows an example 396mm CA, f1.376
mirror finished with low Mid-Spatial Frequency (MSF) errors,
as well as low RMS surface roughness, even though the mirror
is 420mm physical OD, 62.5mm edge height and weighs only 4.32
kgs. |
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High Spatial Frequency errors - |
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**Click the
below thumbnails to see RMS surface roughness data for two different
areas of a 0.25m Dream zeroDELTA™ engineered, lightweight mirror. Dream's in-house polishing (of both spheres
& aspheres) averages 6-9Å (0.6-0.9nm) RMS surface roughness but the zeroDELTA™
lightweight mirrors have been finished
to 2Å. They can be superpolished
as well; 1Å or less. Dream's zeroDELTA™
lightweight mirrors offer the optical surface performance and
substrate stability of a solid Zerodur (glass-ceramic, zero-expansion)
mirror but at 1/5th to 1/10th the cost and a thermal
time constant that is often hundreds
of times shorter. |
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Concave 247mm
CA, f2.27 tested near center. |
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Concave 247mm
CA, f2.27 near 50% zone. |
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Dream's in-house polishing |
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Dream's in-house polishing |
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**4D
Technologies NanoCam Sq shown below
left, measuring a fast 0.4m Dream zeroDELTA™
mirror. The Nanocam was designed for testing RMS surface roughness
of large diameter mirrors, like meter-class mirrors. The Zygo
NewView is shown on the right measuring
a 0.3m Dream zeroDELTA™ mirror near the outer edge. This Zygo unit
is designed for measuring HSF on medium to small diameter surfaces. |
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**This
white paper discusses the performance
of the 2-mirror portion of a 0.4m
f2 IR telescope. The 2-mirror portion
of the telescope achieved 0.83 arc-second image resolution from
a less than ideal location. Dream finished this aspheric primary
mirror to within 0.009% of the nominal radius. Our typical radius
tolerance is +/-0.1%, but as this example shows, Dream can achieve
far tighter. |
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"Your company does phenomenal work. There is a lot
of thought and heart that goes into your products. Dream's engineering
sets their lightweight mirrors apart from competitors. Your engineering
goes beyond the lightweight aspect. You focus on actual performance!" low mid-spatial frequency errors |
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**-
Ted Kamprath |
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Over 40 years in professional
optics, using everything from $1.5m test rooms to 144" Continuous
Polishers (CP). He's spent his career using the latest in technologies,
methods, materials & science to finish precision optics. |
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**Dream's
dedicated polishing and testing room is 68°F, +/-1°F
year-round. Although Dream's zeroDELTA™ lightweight mirrors equalize exceedingly fast,
the tight temperature control is ideal for test equipment repeatability,
as well as consistency & control of the engineered pitch
& polishing compound that Dream uses. |
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Click on the image to the left
to gain access to both extraordinary videos and test images from
a 0.5m zeroDELTA™ mirror finished January 4th, 2018, during record
cold temperatures.
20.5" physical OD, 3.075"
edge height, no features thicker than 4mm, yet weighs only 20
pounds. |
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**Dream's polish & test room is 20 degrees C, +/-0.6 degrees C year-round and is equipped with a 5m vertical
test tower for gen V (modern) inteferometetric testing of mirrors using
a 4D Technology
2019 PhaseCam
6000
dynamic interferometer. In 2023 the interferometer was upgraded to the higher resolution 6010 model. The engineered lightweight mirrors are tested in their final, athermal carbon fiber mirror mounts (also engineered and produced inside Dream), at the most appropriate
angle; one that mimics final use. This is one of many ways in which Dream offers its customers the ultimate in installed performance.
For astronomical use, atmospheric research, SDA/SSA or any
other upward-looking application, vertical testing in the final mirror mount should be
a part of your specifications, not an after thought. |
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The image to the right shows a finished & coated aspheric convex secondary mirror that was finished to L/125 RMS surface.
Dream can provide: finished precision optics, mounted optical assemblies and/or full athermal
instruments. |
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Contact
Dream to discuss your project. |
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