Scope Comparison: Celestron C102-HD, FS-102 Takahashi & MK-67 MCT
Why Vicki? About Vicki (and Her Kin) Of APO's and ACHRO's To Obstruct or Not to Obstruct? Lunar-Planetary Under Lackluster Seeing Lunar-Planetary Under Good Seeing A Good Home for Vicki Vicki's New Home!
Why Vicki?
For several months now I've been angling for a way to spend quality time with more scopes and to find a way to fund Astro.Geekjoy. (Have you dear reader, ever noticed the dearth of banner ads etc. on this website?)
In mulling over the situation, came up with an idea that could accomplish both goals. Why not buy "used" scopes, spend some time observing through them, and sell them to appreciative others who frequent the website? Meanwhile, the amateur astronomical community could be benefited by a growing body of scope comparisons done by a single observer under reasonably well-documented conditions. Along the way I could also verify the various calculators found on the site and, once larger scopes come my way, expand the range of deepsky studies as seen from Backyard Boulder Creek. Make sense?
And thus, Vicki. Vicki, an older Vixen-Optical sourced, Celestron CH-102 102mm F9.8 achromat. Purchased for the reasonable price of 180 dollars from one John as advertised on AstroMart. Initially evaluated by "T" "theAstronomer" who lives within a couple hours of John in the Great State of Ohio. (Home of the fictional "James T. Kirk, Captain of Star Trek's USS Enterprise.) Vicki, transported by T during a flight out to "Hotel California" now recently returned to Ohio bearing a treasure-trove of astronomical adventures sans Vicki.
About Vicki (and Her Kin)
At F9.8 and 1000mm focal length, Vicki is a "moderately focal-ratiated" 4 inch achromat. An F5 version is also sourced from Celestron. But such scopes are incapable of the kind of chromatic correction possible with the classic F12 to F15 scopes of past eras. Speaking of the past, at one time the slow 4 inch was the standard of lunar-planetary, and double star amateur astronomy. But such slow-achros long tube lenghts, and inability to capture wide fields of view placed them on the evolutionary slow track once faster crown and flint doublet pairs could be mass-produced with "an acceptable" amount of color correction.
An F10 achromat represents a compromise between the unwieldy F15 units of the past and the fast achros of the present. It is probably the best that can be done with crown and flint glass in the present. A fine F10 instrument makes for an acceptable lunar-planetary-doubles scope while also supporting use as a 30x sweeper. (This even with use of 1.25 inch eyepieces.)
But not all F10 achromats are the same. To get the most out of one requires diffraction limited optics and careful spacing of the doublet pair to minimize chromatic aberrations.
It is common knowledge among optophiles that the current batch of Chinese-sourced 102mm refractors distributed by Celestron, Meade, and other resellers do not meet the low- chromaticism, diffraction-limited requirements of a good lunar-planetary-double star scope. Although the Chinese do a fair job with the fabrication of their doublets, they are unable to "fine-tune" spacing well-enough to minimize chromatic aberration. Nor do they take great care in collimating the optical-train. Thus the task of doublet-spacing and collimation - especially in faster models - falls squarely into the hands of the amateur who owns one. OR in the hands of those resellers (such as StellarVue) which tackle the task either by "rejecting" scopes or implementing the required adjustments - for a price. Thus you could probably purchase a "well-tempered" chinese-sourced F10 4 inch scope for around 500 dollars new and improved. OR you could take the time to do the work yourself!
But Celestron did not always purchase scopes manufactured in China for resale as the C102-HD. Before the "Chinese-invasion", the model was manufactured by the Vixen Optical Company of Japan. As such, it featured all-metal construction (dew shield and objective cell included). And was also quality tested and tuned - at the factory. A process which adds a great deal to production cost and soon sent the price well beyond Celestron's $500 resell cap. (A cap which includes CG4 mount and a number of low-cost accessories.) Thus Vixen Optical versions of the C102-HD are no longer sold and along with so passed the C102-HD model's reputation for "good optics, solid mechanical fit, and attractive finish".
So today you can purchase a used Chinese-sourced C102-HD for around $100, or a Vixen-sourced model for three times as much.
But even among Vixen Optical sourced units there is room for diversion of quality. Would "Vicki" prove an especially fine example of its breed? Or a dud? Well, on the night of Saturday, May 11, 2002, Vicki saw first light. And not only that, Vicki had a chance to run up against one of the finest commonly available 4 inch apochromats of the day - a Takahashi FS-102 possessing 95% plus strehl-rated optics completely free of chromatic aberration. How did Vicki fare?
What would you expect? Do you think a $400 F10 achromat would be as "chroma-free" as a 2.5K apochromat? Duh! Do you think that a "diffraction limited" doublet would give Jupiter views comparable to a 1/6th wave florite-elemented object glass? Well... Do you think double stars would resolve with equal clarity and precision? Or that deepsky performance and aesthetics would be well-matched? Hmmm. Read on!
Of APO's and ACHRO's
The following scope comparison occured from about 8:00 to 11:00 PDST during a Santa Cruz Astronomy Club (SCAC) Star Party at the club's very fine Bonny Doon observing site. Skies were quite deep (a little less than magnitude 6.0) and stability 8-/10 (both ratings based on conditions directly overhead). A number of club members (including Dan, Mark, Ralf, and myself) looked through both scopes. A fifth observer ("T" - theAstronomer of AstroTalk) was also present. The following notes were duped from a more complete star party report found here at Astro.Geekjoy. It also includes a few comparison views vis-a-vis my "tweaked" ST80 - the Pup whose review is also available on this website.
From that report:
All first lights pose difficulties. On this particular evening Vicki encountered two. For SCAC club member Ralf and I immediately identified problems with fastener threads on the scope's tube rings. The normal fasteners were worn and the alternatives were too shallow to properly accept the thumbwheel fasteners from the mount head. A little ingenuity soon had the OTA securely mounted - with a little assist from a young associate who found some "shimming" material - twigs.
The second problem followed from the type of inch and a quarter adapter found on the focuser. This particular adapter was a wee bit too long for the combination of diagonal and 35mm eyepiece to achieve focus. However, it was soon found that other eyepieces - 25mm Ultrascopics and smaller did fine.
Although Vicki has been here since T's arrival Wednesday, I had yet to look through the scope. Meanwhile fellow SCAC and AstroTalk member Dan would soon arrive with his superb FS102 Takahashi apochromat. And thus Vicki was about to be weighed in the balance. Would the 1000mm focal length, 102mm achromat prove to be the fine instrument needed to kick off Astro.Geekjoy's first fundraiser? Or would I end up spending the next several months carefully respacing doublets, and aligning objectives in order to achieve barely passable views of Moon, planets and double stars? Would Vicki's performance be more akin to that of the ST80 Pup or, like Dan's Takahashi, rise toward the level of 150mm MCT Argo?
These were some of the questions that would be answered this very evening - should sky prove supportive.
From that very first view of Venus, T and I knew that Vicki's focused images were delightfully low on chromatic aberration. This was one nicely set up collection of crown and flint glass! Sweeping the scope further east, Jupiter was quickly acquired. At some 200X, all the various belts and zones normally seen through 150mm Argo under decent seeing were possible. Would the image of the "Banded One" pass muster with that provided by Dan's nearby and now fully operational "deathscope"?
That first view through Dan's Tak caused me some consternation - until I realized that he had selected his "star party magnification" of 120X. There was no way to make a fruitful comparison between Vicki and Taki with such a disparity in magnifications. So, Dan dropped in a barlow lens and ran the magnification up to something that more closely approximated Vicki's 200X. In so doing the gap closed considerably, but it also remained clear that Taki's florite-elemented 1/6+ wave optics gave a crisper presentation of the belts along with enhanced contrast and more natural color. So in this one area - observing the amateur's planet - Vicki was good, but Taki was excellent.
During the course of the evening, there was only one other area of observation on which the superb optics of the apochromat displayed a clear advantage over the achromat. This was seen while observing globular cluster M13 later in the evening. There was something tighter and more contrasty about the way the Tak showed the many 12th magnitude stars just inside the upper limit of direct perception. While star-chaining was possible with the Tak, Vicki made the effect less obvious.
Between views of Jupiter and that of the "Great Cluster" in Hercules, Dan and I turned up a number of other studies... Mostly double stars. Neither of the scopes could successfully resolve 1.3 arc second Iota Leonis. Such a close and disparate double would have placed the faint companion right on the primary's frontier. Interestingly, we looked up a second double of similar seperation, 1.3 arc second Gamma Virginis (Porrima). It was here that Vicki showed a clear advantage over the Tak. This was seen as an hourglass split of the two third magnitude components, whereas the Tak was only able to show a slight pinch in the waist between them. Dan and I speculated on why Vicki would have given a visibly superior resolve. We concluded that the airy disks of the two bright stars were actually smaller because some luminosity had transfered to the first diffraction ring. While the APO displayed slightly larger airy disks (as evinced by the lesser luminosity of the diffraction rings).
Dan and I also dropped by 2.1 arc second Xi UMA and more northern Nu - a very difficult disparate. Xi showed nice wide separation at the roughly 300x magnification just used to attempt Porrima resolution. Meanwhile, Nu's 10th magnitude companion was more steadily held in the Tak but still remained visible in the Vixen. Nu by the way, is a pair that hangs right on the limits of Argo's resolution under the half magnitude less transparent skies seen from Backyard Boulder Creek. This truly argues the value of an unobstructed vision of the sky and all its' denizens.
Both scopes were turned on Epsilon Lyrae at different times throughout the evening. At 120X, Vicki's view was that of four tightly glowing balls of luminosity, well distinguished in gap of separation. Gorgeous. The real test was 6th magnitude Pi Aquilae, and here Vicki did not let us down. Again, two perfect little balls of light, well distinguished some 1.4 arc seconds in seperation.
However there were certain "failures" with a couple doubles during the course of the evening. Seeing conditions, though good, did not support resolution of the difficult Antares and Delta Cygnii disparates. Confidence remains high however, that a future night of better seeing will make both companions plain.
As the sky drew darker, double star observing gave way to that of galaxies. Dan and I turned the four inchers on the M81/82 pairing. And both scopes gave fine, high contrast views. The real test was M81's neighboring irregular dwarf - NGC3077. This particular 10th magnitude galaxy often proves difficult to the 80mm Pup achromat and probably offers the greatest insight into differences of even a few additional millimeters of aperture. For you see, both four inchers revealed this faint, face-on irregular obvious to the eye, while the Pup placed more serious demands on the skill of the observer just to detect it.
About this time, the Virgo galaxy field culminated to the south. I turned both the Pup and Vicki on what is arguably the most difficult of the Messier galaxies to detect with any sense of definition - M98 west of 6 Coma Berenices. While the Pup, under these fine 6.0- ZULM conditions could just detect the edge-on as such under moderate aversion, the four incher made presentation direct to the eye. There is truly a world of difference between a three inch and a four inch scope when it comes to small galaxies and faint nebulae.
Speaking of sky conditions, Dan and I turned the four inchers on M44 to determine how difficult the 12.7 test star would be. At around 110X in magnification in each scope, the eye could hold the star direct about 50% of the time through the Tak, while the achromat only achieved this fleetingly. Assuming seeing stability of 8/10 at that time, the calculated limiting magnitude on this occasion was 5.8 - a fine sky indeed!
In discussing various views through both scopes, Dan and I both agreed that the achromat gave views that were about "80%" of those provided by the far more expensive apochromat when it came to Jupiter and globular clusters. Since the various stars viewed showed a visibly brighter first diffraction ring, it was also clear to our thinking that the scope's overall correction (chroma and spherical aberration combined) was "strictly diffraction-limited". Dan mentioned that his version of a startest revealed no perceptible zonal errors. My own confirmed 1/4 wave optics undercorrected with the telltale "achromatic starpoint" that lingers several waves extrafocally. A behaviour also displayed by the Pup whose spherical aberration, after doublet respacing, is comparable to Vicki's but takes a large hit in chromaticism due to fast focal ratio.
So far so good. Vicki had stood the test well! And best of all, proved that a moderate focal length four inch achromat could provide for an evening of great fun and enjoyment beneath a fine night sky. But what about when things weren't so good? And how about compared to a six incher?
| Focused Star | Disparate Double | Extrafocal Startest | Planet Saturn | Planet Jupiter |
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Aberrator Simulation of Vixen C102HD Performance Under Perfect Seeing (1/4th wave SA undercorrected)
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Aberrator Simulation of FS-102 Performance Under Perfect Seeing (1/6 wave SA overcorrected)
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Aberrator Simulation of MK-67 Standard Performance Under Perfect Seeing (1/6 wave SA undercorrected)
To Obstruct or Not to Obstruct?
The "perfect scope" is inexpensive but composed of high quality components, small but robust of construction, provides superb image quality of great contrast and illumination, is easy to use, transport, and setup and does not exist - anywhere.
The closest we might come to such a scope would be a fast, seven inch apochromatic refractor of no more than 1000mm focal lenght. Such a scope would cost no more than seven thousand dollars to purchase and mount. It would be designed to be broken down quickly and re-assemble with minumum fuss and re-alignment. Dew shield, optics tube, visual back and focuser could be handled as "carry on" baggage. The mount needed to support it, could fit in a robust travel case of no more than 60 inches diagonal length.
Such a scope would possess 95% strehl ratio optics. (95% of a star's light would end up in its airy disk.) It would hold stars direct down to magnitude 13.5 at 2mm exit pupils and reveal details on the Gas Giants to the limit of atmospheric seeing conditions found most places on the earth - outside the grounds of the world's great observatories. Our hypothetical scope would show the cores of 1 arc-minute sized galaxies to near magnitude 14. Resolve globular clusters to magnitude 9. Elongate matched pair doubles to .5 arcseconds and 1.5 magnitude disparates to .7 arcsecs. It would also be able to achieve 2 degree "rich fields" at 30x.
There is no comparison between such a scope and a 4 inch F10 achromat of similar focal length. Nor is there any comparison between such a scope and a 150mm, F12 MCT such as Argo. Nor could Dan's formidable 4 inch Tak stand much of a chance given the limitations of some 12.5 square inches of light collecting area. Meanwhile, even the finest SCTs of twice 7 inches in aperture would fall down in such a comparison. Why? Because of limited lunar-planetary performance due to oft-questionable optics, challenging sky conditions, and that large "plug" in the middle of the optical train.
So we start with what we have. Vicki, for its mere 4 inches of achromatic aperture, is a pretty good scope. - No not as superb as an apochromat of similar aperture - but surprisingly fine. Now the question becomes, how does Vicki compare with Argo? Can someone who has followed the seasons of Moon, planets, double stars, clusters, nebulae, and galaxies through a six inch find pleasure in taking a "step back"?
Last night (Thursday, May 16, 2002) I learned a thing or two...
Sky conditions these parts have not been good early in the evenings. Spring is the time of year when the greatest thermal delta exists between "night and day" here in the Santa Cruz Mountains. Late afternoons often are announced by blustery winds. Evening dominated by thermals rising off the mountains to the west and from nearby structures and infrastructure. On a typical night, stability begins at 5/10 then improves to 7/10 around midnight. Transparency is a tad better than Winter (5.0 ZULM), but poorer than Summer (5.5 ZULM). (Too much water content round these parts in the Winter.)
Thursday, May 16, 2002 was a typical Spring. Clear and warm during the day. Bluster in the afternoon. Unstable in the evening. Stiller by midnight. I started out with a view of Jupiter through both scopes. - Two equatorial bands plus occasional glimpses of the NTB. Classic 5+/10 stability. Then, after a great deal of trouble tracking it down in the darkening sky followed Polaris-B's emergence at 120x both scopes.
The edge went to Vicki. Are you surprised? I'm not. Argo is temperamental about seeing. A good four inch gives better images up to a seeing stability of about 7/10. A superb 4 inch hangs in there to 8/10. But at 9/10, Argo blows 'em away. Why? Because of image scale illumination at high magnifications. Even Jupiter. Especially Jupiter. The reality is that Argo has 96 plus stehl ratio rated optics - sort of. Why sort of? Because of that 35% linear central obstruction that's why. So its optics are actually closer to 78% strehl - almost but not quite a "diffraction limited" 150mm. And it is that "almost" that kills Argo when the seeing is less than good...
So Vicki did a better job of holding Polaris 8th magnitude companion at 120x against the dusk than Argo. One good reason for owning a refractor under less than optimal seeing conditions.
But I also noticed something else. Vicki is NOT a fun scope to orient towards studies in the night sky. It is in fact under-mounted on a Skyview Deluxe/CG4 equatorial. It is very difficult to reach the manual slewing cables. And you will often find yourself sitting cross-legged on the ground hunched over while viewing anywhere near the zenith. Score one for Argo. Can be used comfortably on an inexpensive mount. Meanwhile, Vicki needs a Vixen Grand Polaris with electronic slewing. Nothing less. Thus total cost 400 dollars for the OTA plus 800 dollars for a fine mount/tripod combo. Argo 800 dollars OTA plus 300 dollar mount. Close to a draw in price wouldn't you say?
Through the course of the evening, I used the same eyepiece combinations on both scopes. Vicki used the 3x ultrascopic barlow with 25mm Ultrascopic eyepiece to achieve 120x. I dropped the 15mm Ultrascopic in Argo to get the same magnification. Vicki was operating at .85mms of exit pupil. While Argo was at 1.25. Using the Limiting Threshold Magnitude Calculator Argo should hold stars to magnitude 12.7 while Vicki's upper limit should have been 12.0. This particular evening included a 25% Moon in Gemini. I turned both scopes on M44 for a check of my favorite 12.7 magnitude Praesepe star. Vicki revealed the star on full avert (adds one magnitude to reach) while Argo could accomplish the same hold under moderate aversion (adds .7 magnitudes). Effectively, under 5/10 seeing the scopes were limited to 11.7 and 12.0 magnitudes respectively. Nothing deep about this sky, but what was there Vicki showed better...
Made two double star checks as well. The more or less matched-magnitude Castor pair was your basic "star-foo-yung". Pair of barely discernable central brightenings tied together by a single oval of ill-defined luminosity. Sure sign of 5-/10 seeing in that part of the sky. Dimmer, closer matched pair Xi Ursae Majoris gave precisely the same, but dimmer presentation overhead. At best 6/10 seeing on the zenith. Had Jupiter been overhead there was a chance that a meaningful "marginal seeing" comparison was possible. But the Amateur's Planet now resides outside the middle third of the sky, and "Spring Fever" makes such a comparo improbable.
Left both scopes setup outside and went inside. The plan was to return toward midnight and do some deepsky questing after the Moon exited the scene and stability improved.
Speaking of the Moon, early checks through both scopes gave a slight edge to Argo. Here the six incher is able to ignore its central plug a bit on the basis of high feature contrast. Even despite the marginal seeing. And oh yes, color through Vicki along the edge, though perceptible is nowhere near objectionable...
Round 11:00 returned to the scopes. Sky stability was now much improved. Both scopes showed tight little airy disks against an almost black background sky. (Vicki gets the edge in this department - due to smaller exit pupil at the same magnification.) Stability improved to 7/10. A 5.3 magnitude star could just be held direct (as mush) in the Hercules Keystone. (NOTE: My eyesight is 20-40 corrected, a dissolved star counts for me!)
Under these conditions Vicki should reveal stars direct to magnitude 11.9, while Argo should hold stars to magnitude 12.1. Now this difference, along with Argo's .8 arcsecond resolving power (versus Vicki's 1.2 arcsecs) is pretty crucial. For you see, Argo's view of M13 was very nice. Several dozen very pointillistic stars directly held along with a much compressed core region. Even a few stars standing out against the core itself. Star chaining also seen. A view equal to Dan's Tak under half-magnitude deeper conditions at the recent SCAC star party.
But, as at the star party, Vicki struggled to resolve more than two dozen components. No sign of star chaining. Larger more nebulous core. Decent but unexciting view. To get that requires a magnitude deeper skies. Skies that are very rare - even at the Bonny Doon observing site.
BTW: Comet IK lay on the same right ascension axis as M13. It was twice as large and had equal surface brightness to the globular. A fine sight in the 9x50mm finderscope mounted on Vicki for the occasion. In the main tube, it was far less exciting than The Great Cluster due to a lack of "resolvable" structure.
While near the Great Cluster slew about a half-degree northeast to pick out edge on galaxy NGC6207. Found that I could just catch the galaxies core region through the 4 inch as a blurry star. Moderate aversion gave a sense of N-S orientation. Meanwhile Argo required only the slightest aversion to accomplish the same sense. But still this is an 11.6 magnitude galaxy that often requires a 200mm SCT to view with any certainty!
Also turned up globular's M92 and NGC6229. M92 through Argo looked about the way M13 did through Vicki. A couple dozen stars "softeyes". No star chaining, No resolvable core components. Through Vicki, moderate aversion needed to resolve members. Meanwhile 9.4 magnitude 4.5 arc-minute sized, 12.4 magnitude average surface brightness NGC6229 showed about the same through both scopes. Argo hinted at roughness and held the bright nucleus direct while Vicki needed some aversion. So once you get past the "resolvable stage" globular clusters look pretty much the same in both scopes.
This was also verified while observing the "Hedgehog" globular M56. Maybe a dozen resolvable components on softeyes through Argo. While Vicki required eye movement to detect half that number. (This adds about 1.5 magnitudes to my own personal visual sensitivity.)
Turned both scopes on the Double Double. Nice clean resolve for Argo both pairs. Vicki had a bit more difficulty with the more disparate Epsilon-1 pair. Argo's smaller airy disks were a boon - once seeing stability improved to support it!
Also dropped in on the superb Albireo pairing. Argo showed the golden color of the primary with greater subtlety while adding a bit more blue to the aqua secondary. The scope after all is a "six inch apochromat"!
Finished up with the "King of Rings". Fine views through both scopes of the annularity. 12.3 magnitude star in the "Challice of the Ring" a wee bit tough for Vicki. 13.1 magnitude star near the Ring needed slight aversion through Argo and moderate aversion through Vicki. Thus, under 7/10 stability, 5.0 ULM conditions Vicki maxed out at about magnitude 12.4 and Argo, 12.7.
In sum, under marginal seeing conditions the 102mm achromat performs slightly better than the 150mm MCT. Once seeing achieves 7/10 stability the advantage falls to the MCT.
Now how do the two scopes compare to our F5.5 180mm apochromat? Argo comes distinctively closer under 7+/10 seeing stability and 5.3 plus unaided limiting magnitude transparency skies. The four inch does better under poorer conditions.
In terms of overall performance, the MCT is the superior scope. Larger aperture. Much shorter tube lenghts. Truer color. Vastly superior double star and lunar reslution. Higher image scale on planets (seeing permitting). All this says "Get a Mak".
Lunar-Planetary Under Lackluster Seeing
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 too was an interesting study. Sky conditions some 40 degrees above the horizon were less favorable than overhead (6/10 at best). At all magnifications, 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. Should any new insights be revealed I will post them here on this webpage.
Lunar-Planetary Under Good Seeing
The following is an extract from a an SCAC Star Party report dated May 18, 2002. The venue was the club's Bonny Doon observing site - a site renouned for numerous and consistent nights of fine transparency and good to excellent seeing stability...
And what was necessary was to setup Argo outside the locked gate and check out seeing conditions. Based on the stillness around me, and the tubular clouds overhead, things were very promising. But wouldn't know how good until I turned Argo on the Moon at 180x. There got a nice clean limb. But there was also a wee bit of tremble around high contrast studies. My best guess 8-/10. Perfect for comparing the 102mm achromat and the 150mm MCT on fine lunar features. And Jupiter? Very nice. Even outside the skies middle third, 7+/10 stability. Dry and still. Oh Boy...
Soon had Vicki setup alongside Argo. And just as I started the comparo SCAC club member Michelle arrived. This confirmed that there was, in fact, a star party scheduled for the evening. Ten minutes later, just as I was getting some sassy views of Selene and Jupe through both scopes, the "kids" arrived. So mind you, please understand that the conclusions I've arrived at of Vicki's performance vis a vis Argo are not well supported by a large number of datapoints. But anyway let me tell you what came to light...
Both Vicki and Argo displayed all Jupe's major and minor belts and zones - including a nicely delineated Equatorial Belt - the Holy Grail of Jupiter viewing. Add to this an obvious GRS rotating in across the disk - with well-defined "socket" and hints of internal detail (at least through Argo). Include a small barge and a faint oval in the NEB plus some belt edge irregularites and you get the sense that all was well with the sky. But not quite so. Argo detected "trembles" in Jupes image where Vicki showed none. Effectively such trembles were beyond the achromats resolution. And these same "trembles" made it difficult to give the edge to Argo on the view - either of Jupe or of Selene.
Speaking of Selene, spent a lot of time with Michelle observing the Ariadaeus and Hyginus cleft regions. These were positioned due west of the terminator along our satellites equator. Splendid presentation! Every time I'd pick out something in Argo though, Vicki would also reveal it. But not quite as sharp at 192x as Argo at 180x. And like Jupiter, no visible tremble, nor occasional "high contrast image twinning". Argo's skies were not Vicki's skies. This proved out later when resolving Castor. Vicki showed a pair of clean airy disks with single diffracts. While Argo's 50% smaller airy disk was tough to pick out within each bright stars spurious image. The pairs spurious images had expanded to include the first diffraction ring. And such a diffraction ring would normally lie just inside Vicki's airy disk had the two scope views been superimposed.
On this fine evening, also turned the scopes on Posidonius. Detail much finer than the previous evening. Both instruments cleanly resolved the "Double-Mound" west of Posidonius A. Quite a trick for a four inch scope. One that requires 7/10 seeing through Argo. However, Argo has gone much deeper during especially splendid nights of seeing. And neither of the two very small craterlets seen near the Posidonius eastern wall was resolvable this occasion.
I asked the kids which scope gave the better view of Selene. By acclamation, Argo. I asked them which gave a better view of Jupiter. Again by acclaim Vicki. Vicki was also voted most popular in viewing the Castor pair. Now it is a well established fact that a six inch MCT, despite obstruction, behaves like a six inch when viewing lunar features. But the fact that a 4 inch achromat - and not an apochromat - could win out on Jupiter is rather astonishing. Or is it? To my eye the achromat gave higher contrast views of Jupiterean "macro-features". - Things like belts, bands, etc that include extended fields and regular outlines. No question about it, such features appeared darker and more contrasty. BUT embedded micro-features were tougher to resolve. The interior of the GRS was more obviously mottled in Argo. While the "socket" circumscribing that same feature was easier to detect in Vicki. White ovals and barges were often picked out in Argo first, while the EB was first noticed in Vicki.
As in previous comparisons, matched magnifications - not exit pupils. This means that Jupiter was more luminous to the eye through Argo. This reduced the contrast of extended features. But the scopes greater resolving power did a better job of picking out small ideosyncracies within those features. Finally had I actually matched exit pupils - say the optimal .7mm in both scopes, Argo would have boasted 210x while Vicki 150x. At such magnifications, fine details would have been lost through Vicki while macro-features may have gone either way. (Thinner belts would be disadvantaged while thicker ones would be even more contrasty.) Meanwhile at 210x, Argo's Jupiter would have dimmed and even more micro-features revealed.
So the "kids" liked Vicki's Jupiter - and so did I. A four inch scope is just about optimal for this planet. Optimal because such scopes incorporate less turbulence in the image, while providing just the right balance of illumination and contrast. Could you time transit events with it? Sure but not as many. Would you get those once in a lifetime blow out views when the sky is incredibly steady? Probably not. Solid consistent performers them unobstructed diffraction-limited four inchers...
A Good Home for Vicki
Now I am not selling Argo. And to some degree I have biased my reader toward acquiring the more expensive MCT plus a less expensive mount rather than the less expensive achromat and more expensive mount. But somewhere out there in astro-land is a scopist who already has a fine Losmandy or Vixen mount that would like to acquire a good refractor at a fifth the price of an apochromat. Perhaps that amateur often views through marginal skies? Or often experiences great temperature variations that kill a Mak due to tube currents? Perhaps this amateur simply likes the long, elegant lines of an F10 4 inch? Perhaps there are memories of a scope long lost that need satisfaction? A finally there is only the desire to help out Astro.Geekjoy? For whatever reason, if you are interested in taking Vicki to home and heart, send us an email and tell use why you want Vicki and how much you can afford to contribute to support Astro.Geekjoy.
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Vicki's New Home!
Meet Vicki's new home, it's the same as the old home. Vicki, Argo and the Pup welcome you!