Understanding thermal behavior in Newtonian reflectors SkyArticle_09-2000

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1、telescopetechniquesUnderstandingThermalBehaviorinNewtonianReflectorsEvenareflectorwithfirst-rateopticswilldeliversubstandardimagesifintothecoolernightair.Astartestwillre-vealadancing,distorteddiffractionpat-thermalproblemsarenotremedied.ByBryanGreertern—thetelltalesignsofthe

2、rmalgrem-linsatwork.AstelescopeuserswesimplyHERMALPROBLEMSINNEW-acceptthis,butabetterunderstandingof toniantelescopesarealittlelikewhatisgoingoninsideyourtelescopegrowingold—they'reanun-tubeisthefirststeptowardafix.pleasantfactoflifeandonewewouldrathernotdwellontoomuch.MostD

3、eathofa"RefractorKiller"ofusliveinaclimatewheretherearesig-AfewyearsagoIhadaneducational,butnificanttemperaturedifferencesbetweenhumbling,experiencebuildingalong-indoorsandoutdoors,orlargetempera-focusNewtoniantelescope.Thegoalwasturechangesfromdusktodawn.Wehavetomakeaninstr

4、umentsofineitwouldlearnednottoexpectcrispimagesafterstandupinch-for-inchtoahigh-qualitytakingatelescopeoutofawarmhouseapochromaticrefractor.Myhandmade6-Above:Theauthorisseenhereadjustingthedigitalvideocamerausedinhisschlieren-imagingsetup.Thevideocam-eraisequippedwithaspecia

5、lfilterandpinholeadapterandmountedonathree-axispositioningstage.Withthisassemblyhewasabletostudythewavefronterrorspresentwhileatelescopemirrorcoolstothermalequilibrium.AllimagescourtesyBryanGreer.Right:Theextremetemperaturegradientscausedbyahotsolderingironshowupinvividcolor

6、intherainbow-schlierentest.Violethuesrepresentthestrongesttemper-aturegradient,andredonestheweakest.Becauserainbow-schlierenimagingallowsusto"see"heat,itprovidesaneffectivetoolforunderstandingthermalbehavior.Left:Aschlieren-shadowgramimageofawarm6-inchPyrexmirror(edge-onview

7、indottedoutline).Itclearlyshowstheturbulentthermalcurrentsthatrobimagequalityfromeventhefinestoptics.Inthiscase,thereisa25°Celsiusdifferentialbetweenthemirrorandthesurroundingair.Theboundarylayeristheregionwherealmostalltheheattransferandwave-frontdistortionoccur.Whilethisma

8、ybeanextremeexample,takingyourtelescopefromawarmcarorhouseoutintoacoldwinte