True Performance Never Fears Change

It Defies The Status Quo


Modern optical metrology allows the quantification of three different spatial realms of the optical surface.

* Low Spatial Frequency (LSF) errors:   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.

* Mid-Spatial Frequency (MSF) errors:   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.

* High Spatial Frequency (HSF) errors:   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.

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.

The zeroDELTA mirrors are tested in their final athermal carbon fiber mirror mounts, in Dream's 5m vertical test tower.

**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
**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.

635mm zeroDELTAengineered lightweight mirror & 4D Technology 2019 PhaseCam 6000 (Gen V) dynamic interferometer inside Dream.

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.

**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.

Low & Mid-Spatial Frequency errors -
**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.

Low Spatial Frequency errors quantified

Mid-Spatial Frequency (MSF) errors quantified
 **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.

High Spatial Frequency errors -
**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.


Concave 247mm CA, f2.27 tested near center.

Concave 247mm CA, f2.27 near 50% zone.

Dream's in-house polishing

Dream's in-house polishing
**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.


**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.

"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
**- Ted Kamprath
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.
**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.


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.
**The ceiling of Dream's polishing & test room is equiped with a vertical test chamber for GenV inteferometetric testing of mirrors using a 4D Technology 2019 PhaseCam 6000 dynamic interferometer. This can allow comparative data of the mirror performance in the final mirror mount at different angles. It can also allow finishing of a primary mirror in its final mirror mount, at the most appropriate angle; one that mimics final use. This is one of many ways in which Dream can offer our customers the ultimate in delivered performance. For astronomical use, atmospheric research or any other upward-looking application, vertical testing should be a part of your specifications, not an after thought.

Click the image to the right to see the quality and tight specifications that can be achieved on a convex secondary mirror. Dream can provide finished & mounted precision optics, to optical assemblies, to full athermal instruments.

Contact Dream to discuss your project.

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space domain awareness (SDA), space Situational Awareness (SSA), Ball Aerospace, Lockheed martin, Boeing space, planewave instruments, NASA JPL, NASA goddard, lightweight telescope, lightweight precision optics, lightweight precision mirrors