Plossl Shootout: the 7.5mm Celestron Ultima vs the 8mm TeleVue Plossl
For Plossl users this is one of the new debates ever since the Sky and Telescope article proclaimed the Ultima as best. So being the ever skeptic, I decided to see for myself.
The Celestron Ultima is a modified Plossl design. Their marketing literature reports a 5-element hybrid optical design (the standard Plossl design is 4 elements), long eye relief, wide field, each air-to-glass surface fully multicoated to enhance contrast levels, computer designed to keep visual aberrations to an absolute minimum, inside surfaces and edges of all lenses blackened to keep out reflected light, and exterior housing surfaces are flat black with rubber grips for secure handling.
The TeleVue following has a long and rich history, this design by Al Nagler was after all what helped build his current fame and is even hold patents in Japan. TeleVue’s marketing literature states distinctions such as: diffraction limited "on-axis" performance, expensive high-index glasses to reduce astigmatism and lateral color, sharper images than any other brand of 4 or 5 element "Plossl" or Orthoscopic design, correction for fast focal ratio telescopes, full multicoatings for greatest light transmission and contrast, blackened lens edges and internal anti-reflection threads for maximum contrast and image detail.
Certainly this Ultima newcomer to the field can’t unseat the long time TeleVue favorite! Or can it?
SETTING THE STAGE…
The equipment I used is the “commoners” telescope, the Newtonian. I have as my primary instrument the Orion SkyQuest XT10 Intelliscope. This is a 10” production scope with a fast f4.7 primary mirror. The Intelliscope means it has a “GOTO”-like computer attached for easy location of celestial objects. A great platform with lots of light gathering, easy to move around, and really fast point-and-shoot observing because of the easy to use Intelliscope computer and the always ready-for-action dobsonian mount. Yes, there will always be better mirrors out there for those seeking the last few percent of performance they can extract out of a system. But you know what…if the seeing is very good I easily get a very good image at 56x per inch that is downright snappy in clarity when the air holds its stillest. Plus, with the money I saved on the platform, I get to have fun expending it on accessories – so more fun.
Before we start, everyone should be aware that fast Newtonians have their issues, as they require an eyepiece (EP) that is well corrected unless you have an expensive Paracorr or the like to correct the coma fast optics present to EPs. If they are not well corrected then the off-axis area of the field of view is often labored with stars becoming streaks or “seagulls.” So how these EPs perform really should not be extended to how they will perform on a longer focal ratio Newtonian (they will probably perform better), nor how they will perform in all the ways tested on a refractor, whether it be achromatic, apochromatic, or other design. I’m not sure if anyone has really detailed the exact scientific reasons for this different behavior between the reflector and refractor, it certainly is not due to the added diffraction issues from the secondary and spider, but it is real phenomenon as it is observed that some EPs which perform exceptionally on a Newtonian can be horrible on a similar aperture refractor and visa versa. I guess when you think about it though, with all the elements in the observational chain, air currents, humidity, sky glow, clarity of the atmosphere, temperature, the entire optical chain from telescope through to eyepiece through to human eye and human physiology and psychology, you can see it is really quite a large multivariate equation affecting the final image. Therefore, the affect that the primary objective (i.e., a mirror or a glass lens) may have to the entire equation could easily become a sensitive factor depending on the condition of the other elements in the chain. But that discussion is for another time, let’s get on to preparing for the Shootout.
For this evaluation, I will do what I call a Shootout. Not a series of long and drawn out observing sessions trying to eek out the minutest differences, but a rapid comparison of the major characteristics that are most visually impacting for general observing that can be done in approximately an hour. Yes, we could scrutinize over several hours for many observing evenings to determine, for instance, which EP was really sharper or had more contrast. But if it takes you that long to determine a difference, aren’t you really saying that they are for all practical purposes the same or extremely close in performance? So that type of comparison is good if you have a critical need for every little bit of performance. What I want here is to just know if one EP really outright better, or are they in the same class and close in performance. That is the essence of a Shootout comparison, to discover if two EPs are in different classes of optical performance or are suited for different observing needs.
The primary comparisons will be:
1. Physical Characteristics
a. Fit, Finish, Cosmetic Appeal
2. Optical Characteristics
a. Eye Relief
b. Apparent Field of View (AFOV)
3. Optical Performance
b. Light Throughput
e. Deficiencies: Astigmatism, Coma, Scatter, Ghosting, Axis Zones, etc.
Criteria 1 and 2 don’t make or break how an EP performs, but are often important because they influence the intrinsic “feeling” you come away with. So I don’t want to play these down since they often leave that last taste in our mouth so to speak. They are important, but purely on a personal, and very individualistic level.
I believe characteristic number 3 is the heart of the matter related to an EP. So when I draw my final conclusions, that category will be most important. Finally, there is money to be considered. But it’s really impossible to tell what any reader’s circumstance may be, so I don’t factor that into the equation and instead leave that assessment up to each reader. However, in the case of this Shootout, it won’t be a consideration since the two EPs are comparably priced at a 2006 street price mostly between $75-$85.
LET THE COMPETITION BEGIN…
1. Physical Characteristics.
Fit and finish for both EPs is great. Both appear to be made well, sturdy, have substantial weight due to their use of chromed brass barrels (which I prefer because it makes confidently handling them in cold weather a little easier), and cosmetically they look good. So both I’d rate equal here. On ergonomics, I rate the Ultima squarely better due to the easily removable eye guard, the diamonded rubber strip around the lens housing, and the larger lens housing which allows a more sure grip as well as identifies the Ultima easily in the dark. The TV also has an integrated eye guard which makes it easy to grip as well. However, the TV is smaller than the Ultima, plus I found I kept hitting the eye guard with my head in use because it extended up further than the eye relief really allowed. This closeness of course ensured a good light shield, but the down side is that it meant my head contact jittered the scope. It also felt weird having to put my eye inside the rubber tunnel of the eye guard to observe the full field of view making me always cognizant of the EP instead of it becoming transparent so the image could be the sole focus. So I don’t judge this a good design aspect for a short focal length, and therefore higher power EP. The TV eye guard does come off, but the lens housing is custom made for the guard so when it is off, cosmetically it looks like something is missing due to the large empty groove around the housing. Not a good cosmetic look without the eye guard. Overall, not a real big deal, but I’d have to give the ergonomics category to the Ultima as a square win due primarily to the problems that the ridgid integrated eye guard caused in observing with the TV, and its smaller size made it less sure to handle.
2. Optical Characteristics.
This category I term “characteristics” instead of “performance” for the optics because they are not about how accurately the optics present the image. Instead they are really positional characteristics of how the image is framed and where it is formed above the top lens. In my book, the items I list as optical characteristics are largely a subjective preference unique to each observer. For some of us, these are nothing more than personal preferences and may have no optical impact at all. For instance, since I am not an eyeglass wearer, eye relief is not a big deal for me. So readers need to take that into note with my evaluations and judge for themselves if the eye relief numbers are sufficient for them.
My criteria for when eye relief becomes a problem is if it is so short that my eye lashes are constantly depositing oil on the top lens, or if my head must contact the EP housing to get close enough. The former can be an issue because many optical designs hate anything on the top lens. In the case of the Plossl design, I notice that even a minute oil streak on the lens will make the image go out of focus as your head movements change the position of your eye over the EP. In fact, it can be easily mistaken for bad seeing! I recall one evening thinking how bad the seeing was with the image of Saturn periodically going clear then slightly blurred. But this was confusing since it just looked so clear and still when I just looked up into the night sky. When I did a closer examination of the lens element, there was a hairline oil streak causing the problem. Wiping it off returned Saturn to exquisite resolution with no fluctuations in the image. A word of caution here is that if the eye relief causes the need for constant cleaning, then the chances for accidents naturally increases. If you rub too hard or wipe a dust particle across the surface putting a minute scratch in the coating during a cleaning, over time an abundance of these microscopic scratches will result in light scatter and a reduction of resolution.
In the end, between both our EPs with no eye guards attached, the eye relief was about the same for each EP. It was close, but not overly so – i.e., although my eyebrow had to touch the housing, my eyelashes were not coming in contact with the top lens. So both get an equal rating. The Ultima is listed at 5mm eye relief and the TV is rated at 6mm.
Next I examined the AFOV. Simple? Yes. But also highly important since the trends today are for more and more AFOV in the popular eyepieces. The importance of this factor for observing is that the wider the AFOV, generally the more enjoyable the experience is perceived; at least it sometimes provides more impactful lasting memories. For this evaluation I wanted a nice clear star field to examine. Not a field too dense with too many dim elements, but something bright that made an easy picture as I popped between the two EPs. So I turned my scope to the Beehive. At the focal length of these EPs, 160x was too much power. So I slewed over to M36. Perfect! First up, the TV. Excellent view and very rich looking star field. Out it went and in with the Ultima. It was close, perhaps the same, maybe a degree or two more for the Ultima, but not enough to easily tell, especially with the eye guard in place on the TV and having to dip into it to see the entire FOV. So I’d call it even for all practical purposes. In their marketing literature, the Ultima is rated at a 51 degree AFOV and the TeleVue is rated at 50 degrees.
OK. The easy stuff is done. Now for the multitude of optical characteristics. There’s a lot to go through, so rather than choosing a each characteristic and observing a specific target to evaluate that, instead I will choose five classes of subjects and evaluate all the criteria via those five observation classes. This methodology is quicker. I chose a single bright star (Capella), two easy doubles (Polaris and Mizar), two star clusters (M36 and M37), a nebula (M42), and an extended bright object, for this a planet (Saturn) and the Moon.
Easy always first, so up with Capella. I centered the bright star in the field of view of the Ultima and it provided a nice white image in a nicely sharp point. Just the normal amount of scatter I’m used to, so well controlled. Also, no ghosting of the image. Color looked good with somewhat of a minor red fringe on the left. Hardly noticeable, but there. I then moved it through the field to observe how large the typically better on-axis area was and to see when the star may exhibit bloating or other issues. Starting about 40% of the distance from the edge to center, bloating began and got progressively worse in the Ultima. At 10% from the edge the star bloated about 50% and began to become a pancake in the last 5%. While not the “best” of performance for a Plossl design, it was not too awful since 60% of the AFOV was clean and the initial bloating was extremely mild.
Next I placed the TV in and brought the focuser up. Capella presented what appeared to be a slightly larger and slightly warmer star image. It also exhibited a slight color fringe, perhaps a little less than the Ultima and instead of red was more yellow-brown and less distinct. As I moved the star across the field, a very minor bloat did not start until about 35% from the edge and did not get any worse until the extreme edge where it bloated a little more and pancaked slightly. So overall, a much better performance on star bloat. Color correction maintained itself throughout the field. So a clear win to the TV relative to distortions and a much larger on-axis performance area.
Since tone is sometimes difficult to determine, I then went to Mizar. Mizar is great because it usually fills the field of view with no other distracting stars and both elements are bright and white. Sure enough, the Ultima put up a whiter image compared to the TV. The Ultima also showed a finer pinpoint star image than the TV. So the Ultima sqyuarely won (for me) in terms of tone and resolution or spot size was a square win.
Before I move on, let me tell you a little more about tone. Tone is the characteristic “color” an EP lens and its coating imparts to the final image. EPs impart some sort of color, or are they neutral - actually neutral is really hard to determine. But it’s usually easy to tell if an image looks “warm” with more browns and yellows or “cool” with more blues or whites. These are the two major terms. In my experience, the University Optics HD Orthoscopics are cool or white. All their images just sparkle bright and give the impression when you walk away of white clean images. Siebert Star Splitters are even better. The Edmund RKEs many call neutral, but to me they remind me more of a vintage glass tone, not warm but more of a clear yellowish cast. Who knows, perhaps this is what neutral really is and more stars are really this color as opposed to the nice white or blue we like them to be? So each of us has a preference for coloring, mine is either cool or white most of the time. However, there are times when I prefer warm since for me warm accentuates planets and for some reason makes red and orange stars appear more vivid. Ultimately, I notice that I come away feeling like what I saw was “prettier” if the EP casts a blue to whitish coloration -- not neutral.
Enough on tone so let’s move to Polaris for another double observation. My goal here is the dimmer double. Polaris is magnitude 2 and its companion is magnitude 9. So I’ll be able to judge some of the light throughput differences between the two and also re-verify that I get a better pinpoint with the Ultima on the dim companion since dimmer stars resolve to finer points. The more pinpoint image held true in the Ultima compared to the TV. Light throughput was difficult to judge however, they both put up a good image of the companion, maybe a tad better for the Ultima, so will hold judgment on that. And the minor light scatter seemed about the same, not quite reaching the companion for both.
Let’s get some nebula action now, so over to the Great Orion Nebula, M42. With the Ultima in the focuser, the Trapezium came into full view. Always a spectacular sight and component E was also clearly visible as a dim ruddy colored star. Contrast of the nebula did not look as good as I expected but I could see lots of good eddies or molts in the brighter parts of the nebula. That was great as I usually only get that in my Orthoscopic designs.
Now time to see what the TV can do. Bringing the focus up via the rack and pinion with the TV in the slot, then moving to fine focus with the helical fine focuser the Trapezium snapped. Unfortunately the first impression was again, not as fine of a pinpoint star image which I was beginning to enjoy through the Ultima. However, the surrounding black seemed blacker, so a little better contrast as that is popularly termed (field blackness is a more accurate term, not more contrast, but we will stick with the popular usage for the moment). The greater contrast also allowed seeing more extensive nebulosity than what the Ultima presented. However, not as fine detail in the eddies or moltiness was visible as was in the Ultima. Throughput was also not as good as component E visibility was not constant and at times I had to go to averted viewing to catch it. The warmer tone of the TV also showed itself here with the Trapezium not putting up so much of a blue-white image as the Ultima did. So looks like the Ultima was stacking up as a square winner in many optical performance characteristics.
Although the Trapezium E component was clearly more easily seen in the Ultima, I wanted to verify this light throughput issue. So I swung the scope over to the M37 Open Star Cluster with it’s many fine and dim stars. The TV showed a nice image with a good black background. At times, it seems like M37 has meandering paths of dust through it, a nice affect. So a good image, and the prominent orange-red star was nicely colored. Moving to the Ultima, it was immediately apparent that there was less of a black background. I looked with more concentration to examine this phenomenon and determine why. There were more stars…many more fine dim pinpoints than the TV showed! And the orange-red star was less orange-red -- but I expected this since when an EP produces a finer pinpoint, you see the color less. An old trick of double star splitters is to defocus slightly to show the colors better. So I did this. Sure enough the orange-red star became nicely more colored. However, the defocus lost the many more dim stars, and as a consequence it made it appear that the contrast or field blackness increased! So now with a slight defocus, the image of M37 appeared essentially the same as it did in the TV with the meandering dark lanes throughout. Very interesting! Wait, I just can’t let Open Clusters go, I love them. I want to peek at M35 and M36 since they are nearby. Wow! Nice. I liked the Ultima’s look for Open Clusters -- bright stars seem whiter and more prominent, it’s a “good” look for me.
Well, at this point I seemed to have enough data for all the comparisons. However, bright extended objects behave differently than bright point sources and diffuse dim sources. Therefore, visiting a planet is required. In this case I turned to Saturn and here’s where a big surprise reared its head. The TV was in place for Saturn. Rack-and-pinion focus, then fine helical, and a snap into beautiful clarity. Cloud belts clearly visible and the Cassini clear around all the visible ring, including the front most part where it is thinnest. Also quite sharp and defined was the shadow of the planet orb on the rings and the shadow of the forward ring section on the planet orb. Both of these shadows I enjoy observing because they make the planet orb appear more three dimensional. So I watched Saturn and it stayed nice and crisp until is started to soften when about 25% from the edge. As it moved off though, most of the Cassini stayed visible as a black line, just not as sharp. A very nice planetary performance.
The TV then came out and Ultima in. Big in-focus movement for the Ultima, actually almost all the way in for my focuser. Now the helical fine focus. Hmmm. We have a problem here. The image is not snapping into a fine focus. As a matter of fact, at its best focus, the image was no where near as good as with the TV. Had the seeing changed that rapidly or was the lens element dirtied? I checked the lens element and it was perfect. Briefly, I put the TV back in and everything nice and sharp as before. So it looks like what has shaped up is that the TV provides better resolution on bright extended objects, and the Ultima better resolution on bright point objects and dim extended. I reconfirmed that the Ultima was not throwing up as good of an image with Saturn, and it remained so. Plus, as Saturn drifted to the 35% from the edge area, where Capella began bloating, Saturn became very much softer and then I lost the Cassini division all together, especially at the 20% point from the edge onward. So even on-axis I could not get the entire Cassini consistently in the Ultima, nor where any of the planet or ring shadows anywhere near as pronounced as in the TV. So the TV is a square win in resolution on planets.
But who will win on the Moon? I would expect the Ultima given it’s more pinpoint stars, but the TV showed best on Saturn presumably because of better color correction, however this should not be a factor on the Moon. Only one way to find out. Turning to the thin cresent Moon of early April, the Ultima went up first. A seemingly excellent image. A lot of good definition in Crater Petevius which was just off the terminator. The Fissure Rimae Petavius was clearly defined as was the central mountains in the crater. As I looked towards the edge of the field, everything stayed nicely in focus but became soft around 25% from the edge of the field. This was a little less than I expected since I noticed star bloat at around 35%. But that bloating was minor, so evidently not enough to affect the crater definitions. Overall, quite an excellent view all around the field with actual blurring not occurring until the last 5% or so from the edge.
Next the TV. I was expecting less performance given the Moon is relatively colorless and the Ultima put up more resolute stars. To my surprise I got more detail on the Moon from the TV! With Crater Petevius as the standard, inside the main crater just on the other side of the central mountain, opposite the Fissure, there was a row of three individual small mountains clearly defined. I did not remember seeing these through the Ultima. I quickly put the Ultima back in to verify, and sure enough, only a blurred hint of them was there. With the TV back in, they were nice and sharply defined. So the TV gets a square win on this extended object also. Looking to the edge of field, craters stayed sharp on par with the Ultima, getting slightly soft around 25% from the edge and blurring at around 5%.
AND THE WINNER IS…
So let’s sum up the results of the Shootout between the 7.5mm Celestron Ultima modified 5 element Plossl and the 8mm TeleVue 4 element Plossl:
From the table above, I think we can clearly say that we don’t really have an optical winner overall. Instead we have one of these EPs excelling on extended objects like the planets and the Moon, and the other excelling on star fields and diffuse extended objects like nebula. So I guess who the winner is depends on what you observe most!
A real strength of the Ultima was its rendition of stars. I can only imagine how much better delicate Open and Globular Clusters may look at a very dark site if the M37 experience played itself out like it did in my moderately light polluted Northern Virginia suburb. And even though the Ultima had a smaller on-axis area of clarity, with a stationary star field you didn’t really notice any issues with the stars near the edge since they retained their round shape in the bloat test until the extreme edge. So the Ultima really is an excellent star field performer and much better than the TV.
When things like this happen, it is really a tough call. Overall, if I had no current line of Plossls in my inventory and I had to choose between the two brands, what would I do? “If” the rest of the Ultima line at longer focal lengths similarly excelled on stars over the TV line (I would want to confirm this first with their 10mm and 12.5mm), I believe the best solution would be to go with the Ultima line and in addition to their 5mm and 7.5mm top end, supplement that focal length range with some good Orthoscopics (Siebert Optics or University Optics) or Naglers for higher power planetary and lunar work. I really must give significant weight to the Ultima’s better ergonomics, whiter color tone, throughput of more and finer stars in star clusters, being able to catch Trapezium E better, and more easily detecting molts and eddies in nebulosity as clear advantages over the TV. But remember that the Plossl design is really for mid-power work. So the 7 to 8mm focal lengths are really a venture into high power realms where other designs have been shown to excel.
Glad I could share my experiences evaluating these two fine EPs, and hope this Shootout has provided you with some useful insights.
Click here for more about the 8mm TeleVue Plossl. -Ed.
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