by Jeff Barbour
The following is a scope comparison between a vintage Vixen Optical-sourced Celestron C102HD F9.8 achromatic refractor ("Vicki") and an Intes-sourced Orion Argonaut ("Argo"). The comparo was done from Backyard Boulder Creek last night (Friday May 17) under passable conditions and featured the Moon and a lowering Jupiter. It is part of a more comprehensive comparo that began at last weeks SCAC Star Party at Bonny Doon Airport. (There's another star party tonite, anyone interested can drop bye http://astro.santa-cruz.ca.us/ for directions.)
PS: Phil and James this is the scope we played with a bit last Monday nite at MB.
As it turns out, the early evening of Friday May 17, proved to be a bit anomalous for this time of year. Wispy cirrus clouds moved in late in the afternoon. The typical windage abated. Temperatures stabilized earlier than usual.
Seeing overhead reached 7/10 stability. Good, but not crystal-clear focus could be achieved on the Moon's limb. Meanwhile, a much descended Jupiter peaked out at about 6/10 before beginning to degrade.
At first my sense was that 150mm Argo revealed significantly more detail on the lunar surface than 102mm Vicki. But in seeking out "on the limit" detail in craters Posidonius and Theophilus, I soon realized that at 120x both scopes displayed about the same resolution.
Within Posidonius lie a series of mounds north and west of Posidonius-A, (the near-central craterlet). Although the kind of detail Argo can reveal in the crater was not present early on, both scopes revealed the mounds - but without resolving the close set western pair.
Later (around 9:45 in the evening), revisited Posidonius. Despite being considerably less advantaged in terms of sky position, stability had improved. It was here that Argo cleanly resolved the mound pair direct (at 180x) while Vicki (at 192x) could only hint at resolution during eye movement. Thus the advantage of sheer aperture revealed itself. As expected, a large central obstruction is less an issue where decent contrast exists between neighboring features.
Earlier I also visited Crater Theophilus. Theophilus is dominated by a group of four central prominences. These four are trailed to the east by a diminutive and more distant fifth mound. The central prominences resolved in both scopes. The hope was that the fifth, shadow strewn mound, would not be detectable through Vicki. This proved incorrect. Vicki revealed the faint fifth mound as well.
Also checked the region of Crater Messier early on. Messiers long "Bird of Paradise" tail has always held a fascination for me and its quite beautiful at just about any time during the lunar cycle. On this occasion several nearby craterlets were inspected. None proved unresolvable in the achromat.
Thus through 7/10 stability overhead skies and at a common magnification of 120x, Argo was unable to best the 4 incher in terms of resolution - something that is easily accomplished when comparisions of this type are made with the 80mm Pup. However, Argo's "color" on the Moon is more of a "bone-white" with a touch of yellow. While Vicki's is more pure yellow. In terms of "Seleno-aesthetic" - Argo came off the winner...
Jupiter was als an interesting study. Sky conditions some 40 degrees above the horizon were less favorable than overhead (6/10 at best). At all magnifications used Argo appeared to get finer edge-focus. Meanwhile, Vicki had less difficulty revealing the NTB and rift in the SEB. Argo did a better job on the darker polar regions - especially the SPR where faint bands and mottling were more easily detected than through the achromat.
At 120x both scopes could reveal the two main equitorial belts. Vicki gave occasional snatches of the NTB, while Argo revealed more of any delicate shadings. Vicki made the belts appear of greater contrast to the zones, but without the same hints of edge irregularities and fine detail seen through Argo.
Bumping magnification up (180x Argo, 192x Vicki) really helped both scopes in terms of revealing the NTB. It also showed the four Galileans to be tiny disks within an aura of atmospheric flare. At this point Argo picked out a faint transiting barge on the NTB. Vicki had more trouble so doing. Vicki seemed to give a better view of the SEB rift however - so things are not cut and dried, no not at all.
As the sky darkened Argo, had more and more trouble with image glare. Meanwhile Vicki continued to give decent high contrast views of the two main belts. For a lark, I bumped Vicki to 300x and was surprised to be able to still detect the NTB. (This was possible through Argo at 320x as well.)
Throughout this period, I had both scopes setup side by side. Lacking clock drives on the SkyView Deluxe mounts in use, had to re-center each study after a changeover. This complicated things but not by much. Argo always impressed me with the "naturalness" of the planet's color, while Vicki made the two main belts leap out with greater presence. Jupiter's edge was always sharper through Argo and was totally free of chromaticism. The magenta that did not end up in Jupiter's disk ended up in a faint halo around the planets edge through Vicki.
This whole picture of comparisons is now strangely garbled. The four inch showed more contrast between belts and bands, while the six was able to reveal more discrete detail and fine mottling. It will take quite a number of nights viewing the sky through both scopes under varying conditions before some sense can be made out of it all. With Jupiter now going the way of the other four classical planets, it is unlikely that enough quality observations will be possible before the planet combusts in the setting Sun. Even so, more viewings are planned.