Mike Jones article.qxd:Layout 1 - Astronomy Technology Today

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BAFFLE OPTIMIZATION FOR CASSEGRAIN TELESCOPESFigure 2. Ray 1 is traced first, giving the secondary mirror and primary hole diameter.Figure 3. Ray 2 is traced next, starting at the upper rim of the secondary baffle.acting as a central obstruction. Either extremecreates a larger central obstruction;the design goal is to find the minimumblockage that still shields the full focalplane. As will be seen, and contrary tosome prior published work, this does notnecessarily mean that the secondary andprimary baffle obscurations have the samevalues at the optimum.Several papers have appeared over thedecades describing various graphical andcomputer methods for achieving orapproximating optimal baffle design.Andrew Young’s 1967 paper 2 was the firstto describe an iterative algorithm thataccounts for mirror curvature, and thefirst to discuss the option of inclusion ofbaffle design in optical design codes usingmacro languages. Rochelle Prescott’spaper 3 simplifies baffle optimization toa graphical method, but approximatesthe mirrors as flat surfaces. The paperby Hales 4 presents a lengthy but closedform,calculator-friendly algorithm foroptimal baffle design, with the onlyThe Largest Telescope & Binocular Dealerin the New York Metropolitan AreaFast DeliverySame Dayon ManyProductsCCTSCAMERA CONCEPTS & TELESCOPE SOLUTIONSwww.cameraconcepts.comAt CCTS, the owners, Jeff and Greta, take a hands-on approach tocustomer service. Call them direct at 631-335-1279YOU NAME IT…WE HAVE IT!• Televue 13mm and 8mm Ethos in Stock and Ready forFree Shipping• Full Line Meade Telescopes and Accessories Dealer• Full Line Coronado Dealer• Full Line Daystar Dealer• Celestron Premier Showcase Dealer• Full Line Orion Dealer - Buy Direct from Us at ReducedShipping Charges• Largest Library of Astronomy Books in the Northeast• Dew-not Heaters• Baader Solar Filters• Arcturus Exclusive Photo/Visual Accessories• Feathertouch Focusers in stock• Digital Camera Adapters• Denkmeier Binoviewer Cosmetic defect units available -great price• Desert Storm Covers are Back Starting at Only $25• Universal Astronomics Unimount Light DeluxeOnly $299.99• Celestron BIG Binos at a Small Price Tag• Fingerless Astro Gloves with Built in Thinsulate MittenOnly $18.00• CCTS Twist Pack Eyepiece Cases• And So Much More We’re OverflowingWe travel coast to coast to over 20 star parties and events a year - check out our website for the schedule.We’ll see you there! Or Visit us at 10 South Ocean Ave Patchogue, NY 11772 • 631-475-1118Arcturus OpticsA CCTS Exclusive!The only 5 element APOBarlows on the markettoday! As low as $129Ultra Wide eyepiece with awhopping 20mm of eye relief!As low as $89.95Super Wide eyepieces allwith 70 degree apparentfields! As low as $4952 Astronomy TECHNOLOGY TODAY
BAFFLE OPTIMIZATION FOR CASSEGRAIN TELESCOPESapproximations being that both mirrorsare spherical rather than aspheric. Thisapproximation has minimal impact on themechanical dimensions of the baffles.Hales’ method does not require iteration,but does require solution of a fourth orderquartic and selection of the proper root.The Young and Hales papers also bothbegin with the assumption that thesecondary and primary obscurations mustbe solved to be equal, which is not actuallythe case.Baffle OptimizationI developed the method shown in thisATT article independent of publishedwork, at first in Visual Basic and implementedin the CassDesign program, andthen in the ZEMAX 5 macro language.This method makes no approximations,and can handle higher-order aspheric mirrorsas well as conics. This approximationfree,3-ray baffle numerical optimizationmethod presented here is different fromprevious published methods in that it iterativelysolves for the baffle design thatprovides the best shielding, rather thantrying to force the secondary and primaryobscurations to be the same.Optimization of Cassegrain bafflingcan be accomplished by tracing three rays,and iterating the baffle design to achievesimultaneous intersections of the threerays in a common plane, described separatelyin Figures 2, 3, and 4, and plottedtogether in Figure 5.The basic telescope radii, spacing andconics are designed first, with a first guessat the secondary mirror diameter beingthe primary mirror diameter dividedby the secondary amplification. Thetelescope field of view (FOV) and imagedimensions are specified to accommodatethe desired range of viewing and imagingdevices. For square or rectangularimaging arrays, the maximum FOV isthe radial distance out to the corner of theformat.To start the iteration process, a beginningvalue for the secondary baffleFigure 4. Ray 3 is the cross-ray from the top of the secondary baffle to the bottom of the image.Figure 5. All three rays used for baffle optimization, and their intersection points.Astronomy TECHNOLOGY TODAY 53
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