Last

rasc.ca

insidethisissue - The Royal Astronomical Society of Canada

The TMB seems to suffer from a long cool-down periodthat keeps it from performing as well as it can. With the twoscopes being so similar otherwise, this was puzzling. A possibleanswer lies in details of the optical design. While most opticaldesigners are rather reticent to talk about such subtleties, aHungarian optical company is willing to venture an opinion onthe issue.The problem is that air is a very bad heat conductor (about1000x worse than oil), and especially if the air space is wide,the internal member of the triplet lens is separated from theenvironment thermally.... When we take such a lens out to thecooler environment, the center member can lose only verylimited amount of energy through the air spaces, but its edgeis in contact with the metal cell of the lens, which cools downquickly. This way, the center of the middle element remainswarm for a long time, while the edge of this middle elementstart to cools down suddenly. The result is under-correction,and this remains there for a long time, because the centermember loses a significant amount of energy at the edge overthe entire cooling process. On the contrary, oil spaced lensesare good heat conductors internally, so their center elementscan lose a lot of heat over the front and back surfaces of thelens. So, they cool down much faster, and also, during thecooling process, the internal heat distribution of the lens ismuch more homogenous compared to an air spaced lens, asmost of the heat is lost through the front surface and less at thelens edge. So, these optics can better and quicker adapt to thechanging temperature, thus they can deliver high resolutionimages sooner than their air spaced brothers.— GPU OpticalPerhaps the superior thermal equalization of the AP canbe attributed to its oil-spaced objective, versus the TMB’s airspacedone. It would be interesting to test this theory out withthe newer air-spaced triplet objective that is now found in thenewer AP telescopes.Star tests of the two apos in the deep-freeze situation alsoshowed a distinct difference. The AP tested as near to perfect asI am familiar with. There was the slightest of colours to be seeneither side of focus, and I have seen more distinct diffractionrings, but the even intensity of the rings on both sides of focusshowed excellent figure. The TMB star test showed moredistinct diffraction rings than the AP. This is suggestive of a verysmooth surface, but the inside rings also showed less intensitywhen inside focus. This is described by H.R. Suiter as indicativeof under-correction (see page 182 of Star Testing AstronomicalTelescopes), in agreement with the explanation given from GPUOptical. Perhaps the TMB never reached total equilibrium andits cold-weather performance was thus compromised. To controlfor purported temperature effects on the TMB, an artificial startest was conducted indoors and it continued to show someunder-correction. Indoor test conditions were insufficient toconclusively decide the issue.ConclusionsMake no mistake, these two apos are very good telescopes.They provide views that are as good as any other telescope ofthis size and some that are larger. At only 2/3 the cost, the TMBrepresents an encouraging step forward in the annals of qualityrefractor viewing. At its price, it still might not be “an apofor the masses,” but it has brought that much-desired designcloser to affordability. But the AstroPhysics apo still delivers aperformance that makes it the more competent of the two. Ifcost is no consideration, then it would seem the desired choice.With cost a factor, the choice is not so clear and options openup that are wholly dependent on the particular person.Time with these wonderful telescopes has done nothing tocure me of scope-aholism. In fact, it has worsened the conditionsomewhat. However the close performance of the Mak/Newt, at1/3 the cost of the less expensive of the two refractors, will beenough to keep me out of apo-heaven. The TMB and the Mak/Newt were quite close in performance for much of the testingand I have learned to live with some of the larger closed-tube’sshortcomings — especially now that I have some appreciationfor just how long these heavy refractors take to achieve thermalequilibrium themselves. Yes, I will still have to go through thesacred collimation ritual on rare occasions, but not so often asto have it take away observing time.And for those who are incensed by my apo-atheism, I offereven more fuel for the inquisition. Pareidolia is a psychologicalphenomenon involving a vague and random stimulus such as animage or sound, being perceived as recognizable and meaningful.Common examples include images or figures in clouds. A famousexample these days is the “Face of Mars” picture.Figure 4 — The Face on Mars. Courtesy NASA.80 Building for the International Year of Astronomy (IYA2009)JRASC April / avril 2008

More magazines by this user
Similar magazines