True Performance Never Fears Change

It Defies The Status Quo

20" f3.5 zeroDELTA lightweight mirror.

3.0" edge height, 20.4" phys OD, <20 pounds.
 
 
 
 

Dream's highly controlled in-house finishing averages 6-9Å RMS surface roughness, which is 2.2-3.3x better than the 20Å industry standard for visual spectrum.
 
 
 
 

Dream's in-house processing
 
 
 

Click above to gain access to videos showing the boundary layer. The use of zero-expansion mirror materials does not make the boundary layer performance losses go away.


Click here to learn about the centuries-old problem of the boundary layer.


This independent paper has excellent information about boundary layer performance losses. Learn more from these additional independent papers; 2, 3, 4, 5.
 
 
 
 
 
 
 
 
 
 
 
 

The record-breaking cold temperatures of December and early January (2017 into 2018) gave Dream a unique opportunity to test both the stability of its dedicated polishing & testing room, and the stability of a larger zeroDELTA lightweight mirror. On January 5th, 2018 numerous videos were taken of a 20" f3.5 paraboloidal mirror that was finished in-house just a day before. It was an ideal time to take these videos because the extreme cold had already lasted nearly a month, January 5th and 6th were the lowest temperatures seen, with the cold to break three to four days later.
Traditionally the larger the mirror, the harder it is to take stable test images:
******* there is so much more surface area (more area for room thermals to influence),
******* because the radius is typically longer (more volume for room thermals to influence) and
******* because the mirror itself typically has much higher thermal mass, leading to a slower thermal
******00time constant; far longer to equalize to +0.1°C to -0.2°C of ambient (room) temperature.
 
Anyone who has tried to polish and finish a large diameter mirror knows that it takes a long time to equalize. The thermals caused by the mirror create a substantial headache when trying to capture clear images during optical testing. Blurred test images hide small, and at times, larger details/errors, causing the optician to call the mirror finished too soon. Most who care about the mirror's final quality will let a mirror equalize overnight and test it in the morning.
Dream's zeroDELTA™ mirrors don't require this traditional overnight time because they can literally be tested minutes after coming off the polishing machine. This allows far more iterations during the critical stage of finishing the mirror. This greatly reduces optician fatigue, not quiting too early, and having the desire to do more iterations.
If a conventional mirror can't equalize in a temperature-controlled room, it will cause long-term performance losses in the astronomical telescope. G.W. Ritchey first recognized this problem with solid mirrors more than 100 years ago.

"We shall look back and see how inefficient, how primitive it was to work with thick, solid mirrors, obsolete mirror-curves, ..."

- George Willis Ritchey 1928

JRASC, Vol. XXII, No. 9, November 1928.

 
1_20180105_124413 shows a stable mechanical and thermal video at the Radius Of Curvature (ROC), 3.6
******************meters from the mirror.
2_20180105_124108 shows a stable video outside ROC.

After watching the two videos above you may say the videos don’t show much. They look like still images. That's exactly what every optician is trying to achieve. Dream's polishing & testing room is 68°F, +0/-2°F year-round. The above videos show the extreme stability of both the room and the mirror under test. We achieved this result every time we tested this and other zeroDELTA lightweight mirrors.
Look what happens when a hand is placed in the light path:
3_20180105_124906

The above video illustrates how difficult it is to achieve such stable test images. The mirror was sitting less than 18" from an exterior wall during all of the previous optical testing and these videos. During these videos the outside air temperature was 12°F, which was roughly the high for that day. We do not use a test tunnel or stirring fans to achieve these results. Learning about thermals, which lead Dream's owner to lightweight mirrors more than 20 years ago, has given Dream a broad and deep understanding of thermal issues, illustrated in these videos.
There are no features on this 20" zeroDELTAmirror greater than 0.16" (4mm) in thickness, which is what gives it such amazing thermal performance; lack of boundary layer issues and lack of figure distortion. This mirror is used inside Dream's 20" f3.5 Dream Astrograph telescope. A faster, perforated version of this mirror is used in Dream's line of R-C Cassegrain telescopes. All of Dream's athermal telescopes feature not just cutting edge engineered lightweight mirrors but also the highest percentage of engineered carbon fiber skinned sandwich core structures in the world. This is an ideal combination of extremely light, smooth and high-performance mirrors married with extremely light and exceedingly stiff carbon fiber structures.
We have yet to see thermals during optical testing that are being caused by the zeroDELTA lightweight mirrors themselves, which is further evidence to their world-class thermal performance and the reason for their brand name. Dream's highly engineered lightweight mirrors achieve the same smooth finishes professionals have come to admire in solid zero-expansion mirrors, without the thermal mass and mechanical baggage that comes with that traditional technology.

The top row above shows simulated views with zero errors. The bottom row shows the final and actual test images from this 20" f3.5 paraboloidal mirror (used in the videos on this page). If you would like to see a high-resolution version of the above and/or raw images, please contact Dream.

Contact Dream to discuss your project's needs.


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