Last

rasc.ca

insidethisissue - The Royal Astronomical Society of Canada

The acclimation time from a cool room temperature of 15 °Cdown to an outdoor temperature of -15 °C took far longer thanexpected. Both refractors were not totally acclimated even after1.5 hours, although the AP was giving acceptable views by thattime. The TMB did not show any detail at this point, while theAP and the Mak/Newt were struggling to give some Martiandetails. The details in this early stage in the Mak/Newt were asgood as the smaller AP, but the AP did not throw up as muchflare around the bright planet. It would have been the betterchoice for hunting Deimos and Phobos.After two hours, the TMB had settled down and wasFigure 3 — Three frosty tubes lined up in the snow at -28 °C. The150-mm Maksutov Newtonian (left) is my main planetary telescope,and was used as the control to evaluate seeing conditions andMartian detail. The TMB on a Losmandy G-11 mount (centre) is anice sturdy combination, while the AP (on the right) on a relativelysturdy alt/az mount proved to be a bit frustrating for high-poweredplanetary observing. Mars is high overhead.yielding detailed views, though it showed the planet as muchmore orange than either the AP or the brighter image in theMak/Newt. Perhaps this was due to better colour transmission,but more likely the saturated colour was due to the TMB givinga slightly dimmer view of the Red Planet. In the warmer falltimetesting, the AP had shown itself able to reach a slightlydeeper magnitude than the TMB.By 2.5 hours, the out-of-focus images showed no internalturbulence, indicating that both refractors had reached someform of thermal equilibrium (but see below) and both gavegood in-focus detail of a rather small (~13 arc seconds) imageof Mars. The defocusing test was done with some effort, as thetemperature had fallen to -28 °C and the two rack-and-pinionfocusers in the apos were getting very stiff — the opposite ofwhat was happening to the Crayford focuser on the Mak/Newt,which required only a slight twist on its tension knob, and thenremained delightfully smooth and effortless. The TMB focuserdid remain slightly easier to turn, but that could be due to therelative tension that was preset on each and something that Ileft untouched so as not to incur the annoyance of the scopes’owners. The two screws for securing the diagonal of the TMBwere much more assuring than the single screw of the AP, butboth worked without incident. The placement of the screws onthe TMB required that gloves be removed for access betweenthe large focuser tube and the diagonal, which was a painfulexperience at these temperatures.To compensate for the differences in focal length of thetwo apos, a very good Speers-WALER 5-8 mm zoom eyepiecewas used to allow for quick compensation of the focallengthdifferences between the scopes. It, along with someorthoscopics and Naglers, was used to keep magnifications asclose as possible, as magnification differences are telling in thesetelescopes. These eyepieces, along with a TeleVue Everbrightdiagonal, were transferred back and forth between the twoscopes for many repeated comparisons. When the eyepiece (inthe case of the Mak/Newt) or the eyepiece and diagonal (in thecase of the TMB) were put into place, there was always sometendency to fiddle with focus. Not so with the AP: installing theeyepiece and diagonal usually led straight to observing. Thismight suggest that the views in the first two perhaps were not assharp as that experienced in the AP but close inspection showedthat everything that was visible in one was seen in the others— just with more effort or time required. The views throughthe AP throughout the tests from September to January, whilesometimes matched, were seldom bettered by either the TMBor the Mak/Newt, although the differences, once conditionsstabilized, were very small.A binoviewer with a 2 triplet Barlow was also employed,as using both eyes is one of the biggest improvements that canbe made in planetary viewing. This allowed a great view of Marsand, with more power, made the differences between the scopesmore apparent. The AP handled the extra magnification withease; the other two were also capable with higher powers, butperhaps not with as much comfort.An interesting sideline test provided itself in the form ofthe rising Moon. When it was about 30 or so degrees above thehorizon, all three scopes were directed toward it, and used athigh power were directed towards it. Curiously, the 150-mmMak/Newt, which had acquitted itself so well with the overheadMars images, showed much more atmospheric turbulence thanthe two smaller 130-mm refractors. The amount of turbulenceseemed much more than one would have expected from themere 20-mm difference in objective diameters, yet there it was.The views of the Moon through the TMB were preferable to theviews through the Mak/Newt, even though the reverse was truefor views of Mars when it was high overhead. Such a turn ofevents speaks loudly for those who feel that objective size is arelevant factor in “cutting through the seeing conditions.” Itshould be added that the best views of the Moon that night wererendered by the AP.April / Avril 2008JRASCBuilding for the International Year of Astronomy (IYA2009)79

More magazines by this user
Similar magazines