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; 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.
* 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 at or near the center, closer to the edge and potentially other samples between these two, depending on the size of the optic.
**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.
**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. Without defining these simple parameters, conversations about optics will remain firmly traditional, as opposed to modern.

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.

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.

 

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 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!"
- Ted Kamprath
39 years in professional optics, using everything from million dollar test rooms to 144" Continuous Polishers. 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, +0/-2°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 20" zeroDELTA mirror finished January 4th, 2018, during record cold temperatures.
**The ceiling of Dream's polishing & test room is equiped with a 1.12m x 1.12m trapdoor that allows vertical & near-vertical testing of mirrors. 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, an ideal way to ensure the highest quality and performance in final use, assuming astronomical use, atmospheric research or any other upward-looking application.

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 full athermal instruments, to finished & mounted precision optical assemblies.

Contact Dream to discuss your project's needs.


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