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By Richard Hedrick

Astronomy TECHNOLOGY TODAY 31

The PlaneWave Instruments CDK

At PlaneWave Instruments we engineerand manufacture the Corrected Dall-Kirkham (CDK) telescope. This is a won-derful optical design which providesbeautiful performance across a large focalplane from a system of remarkably simpleoptical elements.

The design utilizes an ellipsoidal primary mirror, a spherical secondary mir-ror, and a two-element lens group near thefocal plane of the telescope as shown inImage 1. All of these components are opti-mized to work in concert in order to createsuperbly pinpoint stars across the entirefocal plane.

This is not simply a Dall-Kirkham witha corrector. Rather the CDK is an opticalsystem of its own in which each componentis optimized to work in concert with theothers to best cancel spherical aberration,off-axis coma and astigmatism, and to createa flat field.

CDK HistoryThe path by which the CDK design

came to be is an interesting one. There maybe others who have independently come upwith this optical design, but I will tell thestory of how the CDK was discovered forPlaneWave Instruments. The first designwas made for visual use as a hobbiest project.

In 1991 a group of us started a tele-scope making class at El Camino College inTorrance, California, and quickly developedfull-blown telescope making fever. Manygreat telescopes were made in the class, rang-ing in apertures from 6 inches to 28 inches,but eventually the core group of telescopemakers in the class decided to make a large

Image 1: The layout of the CDK telescope.

An Insider's View of the Origins of PlaneWave Instruments and Its CDK Telescopes

telescope. This group consisted of JoeHaberman, Jason Fournier, Don Quok, andme, and was lead by the professor of the

class, Perry Hacking.Perry wanted to build a

really big telescope for vi-sual use and wanted to doit all by hand. Of course,we spent a great deal oftime talking about whatthe perfect size telescopewould be. The questioneventually came down tohow big of a telescope wecould fit into two stan-dard-size pickups, versuswhat was the largest ama-teur-made telescope thatwe knew of at the time: a41-inch Newtonian. Sonaturally we decided tomake ours “1” bigger, a 42-inch Newt. It didn’t hurtthat “42” was also the an-swer to “the life, the uni-verse, and everything” in

the popular series, The Hitchhikers Guide tothe Galaxy.

The problem with a big Newtonian isthat the ladder required can be frighteninglytall, especially when used in the dark. So wethought of making a very fast Newtonian –f/3.3. The problem in turn with such a fastNewtonian is off-axis coma, so we startedlooking at correctors for the Newtoniansand eventually settled on a Wynne correc-tor similar to that used by Palomar. Thatwas our starting point and we optimized thedesign for our system.

In April 1998 we placed an order for alight-weight 42-inch Pyrex mirror blank.Since we were associated with a college wegot a good deal on the blank, but that alsomeant getting knocked back in the line fordelivery – all told, it took 4.5 years to getthe blank. Meanwhile, we had concludedthat it would be too difficult for us to makethe corrector lenses as Wynne correctors fea-ture some very thin lenses with extremelyshort radii.

Then a local amateur telescope maker,Dave Rowe, happened by. Dave livednearby and had heard of the telescope mak-

32 Astronomy TECHNOLOGY TODAY

THE PLANEWAVE INSTRUMENTS CDK

Image 2: First light for the CDK 42-inch Dobsonian telescope in front of PlaneWave Instruments.

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ing class. One evening, while dining at theMexican restaurant across from El Camino,Perry explained our efforts to Dave and onthe way out, Dave asked whether we hadconsidered a Cassegrain. Perry explainedthat the secondary mirror of a ClassicalCassegrain was too difficult for us to make –Perry said he found it scary (and still does)– so Dave asked about a Dall-Kirkham andPerry concluded that the off-axis perform-ance of that design was too poor.

The next weekend Dave was backagain, asking suspiciously leading questions,as if an idea was brewing. One questionPerry particularly recalls was: “If you wereto place a corrector in front of the focalplane of a Dall-Kirkham, where would youguess would be the best place?” Perry ven-tured that something around 200 mmseemed a good trade between surface curva-tures and the diameters of the elements, atwhich point Dave revealed that he had anidea to make a fairly simple corrector for aD-K. Perry recalls being very excited at theidea.

At the next weekly meet-ing, Dave delivered the opti-cal prescription. ApparentlyDave already had a basic de-sign he had originally comeup with back in 1996 and hadoptimized it for our project.The result was an incredibledesign that gave wonderfuloff-axis performance fromlenses that were neverthelessrelatively easy to make, andwith the aid of a tertiary mir-ror the eyepiece would onlybe about six feet off theground. Eureka! The CDKwas born and Dave became amember of the team.

The CDK 42 is onlynow being completed. By thetime we'd finished the optics,everyone’s lives had gottenbusy (kids, work, etc) and sothe telescope sat for manyyears. Only recently, in April



Image 3: The prototype CDK18 made by Celestron. Picturedare Bob Peasley, mechanical engineer, Joseph Lupica,President of Celestron, and Richard Hedrick, then VP of Engineering at Celestron. The photo was taken in 2003.

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of 2009, did we finish putting it togetherand perform the first star test (Image 2).

During the grinding and polishing ofthe 42-inch primary, we had a lot of time totalk about “the perfect imaging telescope,”and Dave Rowe and I discussed possible de-signs for a flagship telescope for Celestronwhere I was then VP of Engineering.

After much brainstorming, I broughtthe idea to Celestron and it was decided thatit would make a prototype CDK. The firstone we constructed was the 18-inch seen inImage 3. After more planning and strategiz-ing with the sales and marketing departments,as well as with Celestron’s distributors, we de-cided on production of a 20-inch CDK. Atthat point, Celestron hired Joe Haberman, thebest mirror maker in the telescope makingclass, who had also by then started a small op-tics company called Haberman Optics.

After Celestron's announcement of theC20 and shipment of two units, severalevents culminated in the conclusion that itno longer made sense to produce the tele-scope. Joe and I were, of course, extremelydisappointed – it was such a wonderful de-sign and we knew it was only a matter oftime before others produced the telescope.

During this time the company was soldand the C20 was officially dead. Celestron'spresident and I discussed the possibility ofmy independent production of the C20since Celestron no longer wanted too. Fur-thermore, a university had placed an orderthat Celestron would not be able to fill, soCelestron gave Joe and me permission toproduce the telescope needed to fill thatorder. That was the beginning of PlaneWaveInstruments. Within six months, Joe and Ileft Celestron to pursue PlaneWave fulltime. Celestron was gracious enough to con-vey the drawings of the C20, now called theCDK20, so we didn’t have to start fromscratch. Thank you to Joe Lupica, President,and to David Shen, owner of Celestron –both have been very supportive of our newventure.

CDK PerformancePlaneWave Instruments currently manu-

factures the CDK in three apertures: the

CDK20, the CDK17 and the CDK12.5,with apertures of 20, 17 and 12.5 inches re-spectively, selling for $32,500, $22,000 and$9990.

There has been a lot of talk about namingtelescope designs and I want to be accurate withours. We call it a CDK – short for “CorrectedDall-Kirkham.” But in truth, this is not sim-ply a Dall-Kirkham with a corrector added.While, the design does start as a Dall-Kirkham,to which a lens group is added near the focus,it is then optimized – the system is optimizedas a whole. It might be more accurate to call ita “Modified Corrected Dall-Kirkham,” but“MCDK” seems like a bit much.

Spot DiagramsWith any high performance telescope, I

consider it important to demonstrate opticalperformance with meaningful spot diagramsand Figures 4 through 6 provide those forthe CDK12.5, the CDK17, and theCDK20. These spot diagrams are shown inthree wavelengths and are for a flat field.The spot diagrams are presented on theright side of each figure and diffraction sim-ulations on the left. The small squares are 9by 9 micron – the same size as a typicalCCD pixel. The spot diagrams show howlight travels through the optical design if be-having as a ray and the diffraction simula-

Image 4: Shown are a spot diagram and diffraction simulation for the CDK20. The performance21 mm off-axis is 6 micron rms spot sizes. 21 mm off-axis is the corner for a full-frame CCD.



Image 5: Spot diagram and diffraction simulation for the CDK17.The performance 21 mm off-axis is 6.5 micron rms spot sizes. 21mm off-axis is the corner for a full-frame CCD.

Image 6: The spot diagram and diffraction simulation for theCDK12.5. The performance 21 mm off-axis is 12 micron rms spotsizes. 21 mm off-axis is the corner for a full-frame CCD.





Image 7: Image of NGC7331 taken with a CDK12.5 and an SBIGSTL11000 camera. This is a single 600-second image that has beendark subtracted and flat fielded. The image is 42 mm on the diagonaland is meant to show the pinpoint stars across the entire field.

Image 8: Blow up of the upper left-hand corner of Image 7 showingthe pinpoint stars 21 mm off-axis of the CDK telescope.

tions demonstrate how the optical designperforms if you treat light as a wave. Bothare useful tools in understanding how a tel-escope will perform.

But spot diagrams can be misleading.Many companies demonstrate spots of onlyone wavelength making it impossible to tell

how it will perform under different wave-lengths. Some diagrams also hedge bets byshowing performance for a curved field. Ifthe telescope is to be used only visually, thatmay not be a problem because the eye cancompensate for some amount of field cur-vature. But, if the telescope is used for im-

aging, then such reports can be misleading.If a telescope has a curved field, stars in thecenter will be in focus and stars at the fieldedges will be out of focus. If the spot dia-gram is displayed for a curved field, off-axisstars will look much better in the diagramthan the system will deliver when imaging.

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Another way to hedge spot diagrams is toconceal the scale or spot size. Such a diagrammay show a favorable comparison against arelatively poorly performing design, whilenot allowing meaningful comparison tohigh-performing designs for lack of mean-ingful spot scale.

This is why we try to provide the fullestmeaningful disclosure of our optical design.Our spot diagrams reflect performance on aflat focal plane and are shown in three wave-lengths so you are getting the completestory. We also provide a scale to compareagainst as well as the RMS spot size of off-axis stars (typically the point of interest ason-axis spots are going to be limited by dif-fraction).

Image PerformanceSo, in our efforts to explain the merits

of the CDK design, we provide what weknow to be meaningful spot diagrams. But,the more we show people these spot dia-grams, the more we realize that they fail toconvey the full story. We eventually con-



Image 9: An example of a 52-mm field of view taken with an Apogee U16 camera and aCDK12.5. On the left is a star near the center of the field and on the right a star near thelower left corner of the field 25 mm off-axis.

cluded that showing actual results of stars onand off axis was a much better way to go(see the examples in Images 7, 8, and 9).Image 7 presents a full frame of NGC7331– the field is 42 mm on the diagonal. Image8 shows a blow up of the upper left handcorner of that image and demonstrates thepin-point stars produced at the edge of thefield. Image 9 was taken with a CDK12.5with an Apogee U16 camera that has a 52-mm field on the diagonal. The image pres-ents a star near the center of the field and astar near the corner of the field, again best il-lustrating the incredible performance of theoptical design.

PlaneWave CDK FeaturesThe PlaneWave CDK is not just a high

performance optical design, it is also a well-engineered instrument. As explained earlier,the features start with the optical perform-ance. The design has a flat, coma-free field,with no off-axis astigmatism over a 52-mmfocal plane. The system is f/6.8 for theCDK17 and the CDK20, and f/8 for theCDK12.5. The CDK17 and CDK20 comewith a 3.5-inch ID focuser that can be manually rotated 360 degrees, and theCDK12.5 features a 2.75-inch ID focuserwhich can also be rotated. Both focusers runon a lead screw so there is no slipping ormovement as the angle relative to gravitychanges.

The primary mirrors are conical shapedand thus have lower thermal mass, so equi-libration is fairly fast. The primary mirrorsare glued to the mirror cell at the center ofmass so no external torques act on the mir-ror as the gravity vector changes. The mir-rors are also laser aligned to the mirror cellso their optical axes are true to the mechan-ical axis. This ensures that you won’t end upwith a tilted field at the focal plane andmakes collimation surprisingly easy for sucha high-performance imaging platform. Sincethe secondary mirror is spherical, collima-tion is accomplished by tip-tilt of the sec-ondary only. There is no need to go to greatthe lengths necessary to center the secondaryover the optical axis of the primary as withan R-C design.



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Image 10: M33 taken with an Apogee U16 with a 52-mm field corner to corner taken on aCDK20. Notice the perfect stars across the entire field. Image courtesy of Andre Paquette.

The CDK17 and 20 optical tubes usedual carbon-fiber trusses to minimize thermal expansion and contraction and toprovide a light-weight, stiff structure. TheCDK12.5 uses a closed carbon-fiber tube.And the large dovetail used to mount thetelescopes features an expansion joint toallow for the different thermal expansion of aluminum versus the carbon fiber struc-ture.

These features, along with the benefitsinherent to the CDK optical design, ensure that each PlaneWave telescope is ahigh-performing, well-engineered instru-ment.

What is New at PlaneWavePlaneWave Instruments is currently

working on several exciting new projects, in-cluding a 0.66 reducer we are soon intro-ducing that will work with the CDK12.5,CDK17 and CDK20. We also have aCDK24 under design that is probably aboutsix months away from production. It willhave a focal ratio of around f/6.8 and willfeature more back focus than the currentmodels. It will also cover an even larger 70-mm focal plane.

We are also working on an observatory-class complete system designated theCDK700, a 0.7 meter alt-az CDK with

Nasmyth focus and with direct-drive motorsand high resolution encoders on each axis.The CDK700 will be around f/6.5 and willalso cover a 70-mm field of view. The direct-drive motors have zero backlash and zero periodic and non-periodic error. This telescope is intended to bring the featuresand advantages of larger professional tele-scopes to a much wider market and we an-ticipate delivery of the first unit by the endof this year.

I invite you to visit www.planewavein-struments.com for regular updates on theseand other new products.



CAD drawing of the CDK700. Thisis a Nasmyth focus 0.7 meter alt-az telescope.

Image 11: A portion of the Veil Nebula taken with the CDK17 and the SLT11000 camera. Itcombines LRGBs of 20 minutes each and is courtesy of Johannes Schedler. With a 42-mm diagonal, this image is another example of the full-field performance of the CDK design.

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