In the 1960’s “large” telescopes that could be purchased ...



Optics - The Series

Using LRO Lunar Images to Quantify Achieved Instrument Resolution

Shane Santi – Founder & President

Dream Telescopes & Accessories, Inc. (Est. 2003)

Copyright 2017 – v4

Dream recently finished a custom 400mm Clear Aperture (CA) f2 IR telescope (see Fig. 1) used by the USAF. The 2-mirror portion of the system was a Classical Cassegrain with a focal ratio of f6.5. NASA Lunar Reconnaissance Orbiter (LRO) images were used to compare and quantify the 2-mirror instrument’s raw, single-frame image resolution. LRO images show that the instrument achieved sub arc-second raw performance from Nazareth, PA. This underscores the value and purpose of this author’s long-term (25 years) passion for understanding optics and structures to deeper levels, across three broad realms; optics, mechanics & thermodynamics.

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Fig. 1: Dream 400mm instrument - SolidWorks CAD model (left) & actual telescope during testing.

Like all Dream instruments it used carbon fiber extensively; 95% of the structural mass (no optics) was carbon fiber, using mostly Dream’s CFSC™, with smaller components being solid laminate carbon fiber (CF). Even the long (~400mm) 6-lens IR relay barrel and two internal lens-grouping spacers were carbon fiber. All CF work is done in-house and has been since Dream was established for this purpose in 2003. Most of the remaining structural mass is stainless steel, including numerous sizes of threaded inserts used. Dream designs and produces all inserts in-house, to achieve higher stiffness than Off-The-Shelf (OTS) inserts offer.

Both M1 and M2 were Dream’s engineered zeroDELTA™ lightweight mirrors. The two mirrors were independently tested in finishing but a test of the assembled instrument is useful because it accounts for a host of other factors that naked optic testing does not; impact of mirror mounts, optical alignment of the two mirrors, validation of spacing of M1 to M2, flexure of all components, focus shifts (from both mechanical and thermal sources), etc. This underscores Dream’s fundamental purpose; final, installed performance, not naked optic test documents.

The inherent focal plane of the 2-mirror Cassegrain was roughly 1/3rd of the way toward M2, not behind M1 like a conventional Cassegrainian focus. This arrangement and space limitation in this area of the instrument forced the use of a smaller test camera for under-the-sky testing. The camera’s detector was 3.6mm x 4.8mm with 3.75µm pixels. The pixel size in combination with the 400mm f6.5 2-mirror system yielded 0.3 arc-seconds/pixel. The detector’s peak sensitivity is centered on 550nm wavelength. No filter was used during testing.

Because the camera was small and very light, only 45 grams, a simple mount inside the carbon fiber M1 conical baffle was used to hold the camera. In testing, optimal focus was achieved with only a 0.46mm inward movement of the motorized M2, from the nominal M1 to M2 position. It’s important to keep the spacing of M1 to M2 within tolerance because going outside a given system’s tolerance will initially lead to increased off-axis aberrations (performance loss at the edges of the field) and eventually leads to on-axis performance loss as well.

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Fig. 2: Simulated field on Moon (left) and raw, single-frame test image from the instrument (right).

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Fig. 3: LRO image of FOV (left) and raw test image from Dream 400mm instrument (right).

The left image in Fig. 3 was taken by NASA’s LRO, which has mapped the Moon. The image in Fig. 3 was taken off the LRO public web site: .

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Fig. 4: Closer and closer LRO views of the target area for determining actual resolution.

Fig. 4 shows closer LRO views of the target crater used to determine resolution of the instrument. The left image in Fig. 5 is an LRO view with a lunar scale, showing 5km on top and 2mi on the bottom of the scale. The right image in Fig. 5 is a single-frame, raw (unprocessed) test image taken with the Dream 400mm f6.5 instrument.

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Fig. 5: LRO image (left) and a single-frame, raw test image (right) from the Dream 400mm instrument: shown in different scales.

Fig. 6 shows the closest view possible with LRO images. This shows the crater used to help quantify the resolution of the instrument is roughly 2km in diameter. But the single-frame, raw test image (Fig. 5 right) shows both light and dark within this diameter/crater. Conservatively the single-frame, raw test image has achieved lunar surface resolution of roughly 1.5km.

This lunar surface resolution can be converted into arc-second resolution from Earth. 0.3 arc-sec/pixel for the camera equates to 0.51-0.59km on the Moon, depending on the distance from Earth to Moon. 1.5km lunar surface resolution achieved in the single-frame, raw test image is 2.727 times greater than what 0.3 arc-sec/pixel equates to on the Moon (1.5km / 0.55km = 2.727). To determine the achieved arc-second resolution of the instrument; 2.727 x 0.3 arc-sec = 0.818 arc-second. The resolution achieved by the instrument, in a single-frame, raw test image, is roughly 0.818 arc-second.

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Fig. 6: LRO image with lunar scale showing target crater’s diameter.

To see a video of 254 consecutive 0.012 second single-frame, raw images highlighting the movement and distortions that atmosphere and the tracking mount caused, click here. The 254 separate, single-frame, raw images can be found here.

The seeing in the video is atmospheric, not mirror-seeing. Both mirrors were Dream's zeroDELTA™ engineered lightweight mirrors and Dream’s low-mass, high-stiffness and athermal carbon fiber instrument used Dream’s FAST™; Filtered Air System Technology. FAST™ fills the instrument chamber with filtered/clean air, as well as helping to maintain an extremely low temperature delta between all components inside the telescope and the outside ambient air. FAST™ also greatly reduces the number of mirror cleanings required, which helps to maintain higher reflectivity and increases time between re-coatings; lowest level of maintenance possible.

Achieving Higher Resolution –

The spacing of M1 to M2 was mentioned earlier because it is one of many factors that influences final performance of a system. Test documents of individual optical surfaces don’t tell the whole story. Another aspect to final performance is related to the radius of each mirror.

The radius tolerance for each mirror of this IR telescope was +/-0.1%. Using this tolerance M1’s radius could be 2413.0mm, +/-2.4mm, while M2’s radius could be 1143mm, +/-1.1mm. M1’s finished radius was only 0.218mm or 0.009% long; 2413.218mm final. M2’s finished radius was only 0.008mm (8µm) or 0.0007% long; 1143.008mm final.

Going outside a given system’s tolerance, for M1 to M2 spacing, radius tolerance for each optic, WFE, optical alignment, back focus, focus, etc., will all increase aberrations. It’s critical for an instrument designer to evaluate and control these and many other factors. Remember, there are very, very few errors that make a system better. Most can only do one thing; lower performance.

Without moving the instrument to a pristine mountaintop two simple alternatives can potentially provide even higher resolution in the single-frame, raw images:

• The use of a UV filter – shorter wavelengths provide higher resolution.

• Switching to a camera with smaller pixels.

Stacking 500 to thousands of the best 5% to XX% of the total number of raw images taken will greatly improve the processed image(s) as well. This was not done above because the goal was to show the raw performance of the Dream instrument, which is the result of over 25 years of studies, experiences and knowledge by this author. Dream’s specialty is producing optics and instruments that provide better raw data. Processing is left to our customers.

Processing is a subjective procedure that is never 100% clean. It can also show unrealistic performance. It also strips away the ability to make an apples to apples comparison between two instruments. Processing adds another layer that can taint the comparison(s). Systems that offer better raw images will always outperform those that need processing to achieve “quality.”

Achieving higher resolution also means better faint object detection; stronger signal. This can be a critical factor for military object and/or target tracking, satellite tracking, astronomical work like; Near Earth Objects (NEO), Kuiper Belt Objects (KBO), comets and much more.

Further Details About The zeroDELTA™ Mirrors –

The Dream zeroDELTA™ primary mirror, shown in Fig. 7, is 419mm physical OD, 63.5mm edge height and weighs just 4.3 kgs. There are no features within either the lightweight M1 or M2 that are greater than 3.2mm thick. This allows the mirrors to equalize exceptionally fast; a short Thermal Time Constant. This allows optimal performance of the mirrors due to a lack of figure distortion from internal temperature gradients and a lack of boundary layer thermals.

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Fig. 7: Dream’s zeroDELTA™ lightweight mirrors and carbon fiber instrument.

This author has been studying optics since 1993, as well as designing and using them for the past 15 years. Dream uses modern engineering tools to analyze the lightweight mirror designs for both polishing displacement (print through) and different gravity cases (final mirror mount & use).

Fig. 8 shows optical test data of the 400mm f2 instrument’s M1, which was finished in-house. The data was taken with one of the world’s highest resolution interferometers. This is needed in order to capture and show how small the Mid-Spatial Frequency (MSF, historically called primary ripple) errors are in this Dream zeroDELTA™ engineered lightweight mirror. The band numbers along the top of the graphic in Fig. 8 lists the physical size range of the specific MSF error spatial realms evaluated for each of the five columns.

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Fig. 8: Dream’s 400mm CA zeroDELTA™ concave M1 asphere.

This test data illustrates the value of Dream’s unusually high level of engineering, as well as the quality of Dream’s production of the substrate and processing of the mirror. The performance achieved in Fig. 8 was not done with expensive deterministic finishing but with conventional polishing. This validates the myriad of benefits within Dream’s highly engineered lightweight mirrors, showing their world-class benefits.

From low mass to almost no thermal footprint (no mirror seeing or figure distortion) to MSF errors that are no different than solid mirror processing. This allows customers to have all the benefits of a very lightweight, yet stiff mirror, without the cost of extreme processing or the performance loss associated with gross print through. It also opens the door for solid mirror users who are suffering from boundary layers issues to switch to a high-performance, lightweight mirror that doesn’t force them to have a government-sized budget.

The zeroDELTA™ M1 had a final face thickness of 2.5mm. This is a superb example of why Dream’s extensive engineering matters; to optimize the zeroDELTA™ mirrors to historically unprecedented levels of performance. The mirrors can have all of the mechanical and thermal benefits that come with extreme lightweight mirrors, delivered with a finish normally associated only with solid mirrors but at a price that makes them open to far more applications.

The thin faces in Dream’s zeroDELTA™ lightweight mirrors are deliberate because they offer;

• A shorter Thermal Time Constant: equalize to ambient temperature faster.

o Little to no internal temperature gradients to warp the mirrors’ figures.

o Little to no boundary layer degradation.

• Lower self-weight deflection: mechanical figure distortion.

• The use of sub ribs behind the face;

o Stiffens that face, controlling print through.

o Increases surface area, which promotes faster equilization.

• Moves the CG of the mirror farther away from the optical face;

o Shorter length lateral supports; stiffer.

o Lower mass due to the shorter length supports.

o Decrease in bending of the optical face as CG is moved farther from that face.

• A lighter mirror means it is easier to hold tight opto-mechanical tolerances; maintains optical alignment & focus better.

• It makes the instrument lighter, which allows faster slews, more accurate pointing and tracking, etc.

Dream’s in-house polishing averages 6-9Å RMS surface roughness. To date zeroDELTA™ mirrors have been finished to as low as 2Å RMS surface roughness. However, when required, the material can take a super-polish; 1Å or less. The material itself is not the limitation. The limits are the specific process and the metrology used.

One zeroDELTA™ mirror was finished to L/125 RMS surface (figure), then tested with the second highest resolution interferometer available ($400,000+) over five days, to determine the stability of Dream’s substrate. Temperature changed by at least +/-2°C over the five days, but the figure never dropped below L/125 RMS surface. The metrology expert who finished & tested this mirror has 45 years in optical finishing & metrology, working with space optics their entire career. They said the Dream zeroDELTA™ mirror could be polished to as smooth a surface and hold that surface just as well as hundreds of Zerodur, ULE, etc., zero-expansion mirrors they finished over their nearly half century career.

Evolution & Perspective –

In 1928 G.W. Ritchey wrote, “We shall look back and see how inefficient, how primitive it was to work with thick, solid mirrors, obsolete mirror-curves, ...”1

Ritchey began experimenting with lightweight mirrors on or before 1917. Ritchey was much more than an optical designer and optician. He designed and made his own polishing machines, cooled his telescope(s) during the day, as well as taking long exposures at night using photographic glass plates. He ground and polished those glass plates to match the best fit radius of the specific 2-mirror Ritchey-Chrétien telescope he was using at the time. He consistently used 0.5m and 0.6m telescopes to out-resolve telescopes of his time that were 1-1.5m in aperture.

A century ago Ritchey discussed numerous “refinements” in optics and mechanics that each improved the performance of telescopes. His critics often claimed that his “refinements were too technical, and especially too laborious” 2 to be practical. This is because most are unfamiliar with the high number of factors that are degrading the performance of their systems and most are living with errors that are fully within their control. Limited knowledge (ignorance) leads to the Dunning-Kruger Effect. No one can know everything. But we can all know more.

Most who use opto-mechanical systems requiring front-surface mirrors are unfamiliar with lightweight mirrors and their myriad of advantages. Those who are familiar often describe their relationship with lightweight mirrors as tumultuous, because traditional extreme lightweight mirrors force the use of expensive deterministic finishing or show difficulty in finishing them, as well as excessive print through.

Too many believe the performance they achieve is limited not by the instrument but by the atmosphere. The truth is that it is exceptionally rare to find an instrument that is not being degraded by numerous errors, which are in fact very much within our control. Regardless of the level of optical metrology test data for each “naked” optic, such test data is only a small fraction of the whole story. In fact how each optic is supported and tested is, by itself, a broad and deep topic, which touches at least three main areas: optics, mechanics and thermodynamics. Things as innocuous as the placement of a computer monitor relative to the interferometer, can have an affect on the “data.” In actual high-performance optics, not empty rhetoric, optical metrology of an individual surface is the burden of minimizing uncertainty in the data; a search for the truth. There is no one piece of test equipment that will provide 100% certainty because it is far more complex than that.

A focus on 1-dimensional aspects of optical systems, like the optical design and/or test reports for the figure quality of the mirrors, is a woefully shortsighted view of an instrument. It is ignoring a host of other factors that are affecting the performance of the installed instrument.

Although the best installed performance of an instrument is never driven by a small number of factors, everything starts with the mirrors. Mirrors are typically the largest optics of a system, which is why so much performance can be lost to mirror-seeing and mechanical errors in traditional opto-mechanical systems. When those historic thermal and mechanical issues are more appropriately addressed, it helps to expose even more areas of performance loss; additional areas that can also be refined, yielding further performance improvements.

Conversely, ignoring basic thermal and mechanical factors leads one to believe the system is at the limit of the atmosphere and is therefore performing at a high level. This leads a person to believe that no improvements can be or should be made. This is what led critics to believe 100 years ago that thermal and/or mechanical refinements are a fool’s errand. A century later this author hears the exact same rhetoric today.

If a colleague operating a 400mm ground or airborne instrument that uses solid mirrors tries to stiffen some aspect of the instrument and could independently prove that the specific aspect was indeed stiffer after the modification, their full instrument test might not show any improvement. If the person knows nothing about thermodynamics and material science, then their conclusion is that the “refinement” was pointless and the performance limiter must be the atmosphere, or something else outside their control. Ignorance is never bliss. It is an opportunity for the person standing behind you.

For the vast majority of applications requiring moderate to larger opto-mechanical systems, temperature is dynamic, not static. As such mirror seeing is also degrading the performance of the system in a dynamic way. Once that dynamic mirror seeing is addressed in a large manner, the thermal performance of the mirror is not only better, but it is stable as well. Dealing with thermal mass in a large, thorough way, will more than likely make the “refinements” to the structure’s stiffness detectable. This will typically expose more errors, including the fact that the back focus distance behind the corrective optics needs to be optimized to a smaller tolerance.

Regardless of the level of expertise and skill, no birdwatcher in the world can hear the call of a bird over the roar of a jet engine. Once the engine is turned off, that’s when the real work and fun begins. This is similar to putting on your first pair of glasses and realizing just how bad your sight was before glasses.

The level of items to scrutinize is similar to the children’s book, “If you give a moose a muffin.” Refinements are a journey not a destination, because each refinement can expose yet another, smaller error. Often leading one to revisit areas that were already scrutinized. Again, each time things are further optimized and improved, it will lower the size of errors you can detect; mandating, at some point, a revisit to a previous area that was improved upon.

Greatly improving the thermal performance of the mirrors can go a long way in bringing an optical system far closer to the level of performance it is capable of achieving. It is the first of many steps an instrument designer should take. When the right first few steps are taken, it exposes a wealth of other factors that can then be detected and scrutinized.

This 400mm Dream instrument was tested at just below 500’ elevation, in the center of a ~6000 person town that sits 75 miles west of NYC and 75 miles north of Philadelphia. Understanding a broad range of topics to deep levels has allowed Dream to design, produce and test opto-mechanical systems that can achieve sub-arc-second resolution in raw images, without being on a world-class mountaintop and without multi-million dollar budgets.

Savvy instrument designers and users looking for the best installed performance know that opto-mechanical systems are anything but simple. They also know that stiffness in substrates and stiffness in structures often comes at the cost of mass. Previously instruments could be up to three of the following four, but never all four;

1. Lightweight

2. High Stiffness

3. High Performance

4. Affordable; a fraction of the cost of government-level budgets

You could purchase an instrument that met your mass, stiffness and performance requirements, but it required a huge budget. You could lower the costs, but that would reduce one, to all three of the other parameters. Since 2003 Dream has been showing the next evolutionary step. Dream’s optimized technologies combine to create a paradigm shift where you can get all of the performance you want, without blowing the budget.

High Performance Doesn't Fear Change. It Defies The Status Quo.™

Processed Images From The 400mm Dream –

LRO images used in Fig. 4 and Fig. 5 have been copied below and arranged to provide a comparison to a processed image stack from the 400mm Dream. Fig. 10, 12 & 15 show the result of stacking & processing 150 of the 254 raw images taken through the 400mm Dream. We thank Jean-Pierre Chiappini for the processing of the test images.

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Fig. 9: LRO image. Fig. 10: Processed from 400mm Dream.

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Fig. 11: LRO image. Fig. 12: Processed from 400mm Dream.

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Fig. 13: LRO image. Fig. 14: Single-frame raw image – 0.4m Dream.

Click here to see a full frame image of the stacked & processed image.

Fig. 15: Processed from 400mm Dream.

"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: 40 years in professional optics,

using everything from $1m & $1.5m test

rooms to 144" Continuous Polishers.

1 The Journal Of The Royal Astronomical Society Of Canada, Vol. XXII, No. 9, November 1928.



2 The Journal Of The Royal Astronomical Society Of Canada, Vol. XXII, No. 6, July-August 1928.



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