Zhumell Z10 10" Dobsonian Review

By Cathy James

Figure 1.

The Situation

Back in February of 2009, I wrote, "A modest 8" Dob may still be in my future, opening up the DSO window a little wider and giving better views of globular clusters. However, since I am so often seeing-limited, I am not sure that more aperture will significantly improve planetary views here on 90% of observing nights. Hopefully we will be in a position to buy a house next year, which will help with storage and ease of carry-out and setup."

Flash forward a year and half, and my situation has changed dramatically. Following another move, I have left the cloudy skies of New England for the sunshine of Northern California. Buying a house at California prices is not going to happen any time soon, but my rental condo in Dublin, CA has a very nice deck, and the skies are darker (by suburban standards) than anything to which I had access in Massachusetts. Glare is a minor problem in a community that mainly uses full-cutoff fixtures and is screened from the surrounding towns by hills.

I used to own an Orion XT10 back in my North Carolina years, and I fell in love with the beauty of globular clusters as seen through a larger aperture. (I could never understand the point of looking at globs back when all I had was an 80mm. The first look at a Messier globular through the XT10 converted me to a new class of object!) The desire to look at these beautiful objects flared up again, and while I considered compromising on an 8" Dob, I knew that in the end I would be disappointed with anything short of the aperture I had used six years earlier.

I considered buying another Orion XT10, but research showed that the company had changed suppliers. The new XT10 is f/4.7 rather than the f/4.9 – f/5.0 I had owned previously. This may not sound like a large change, but collimation begins to become significantly more troublesome at focal ratios much faster than f/5, and eyepieces that work at slower ratios begin to lose their luster. (I am not a Nagler fan, preferring inexpensive oculars that I am not afraid to drop, fingerprint, or what have you.)

Fortunately, I learned that the basic design of the 10" f/5 was still available from Zhumell, the original manufacturer, and had received good ratings from most Internet reviewers.

The telescope was ordered from telescopes.com, the only U.S. supplier (as far as I am aware) of Zhumell telescopes. The order was placed Wednesday, and FedEx delivered the scope to my doorstep Saturday morning.

Telescope Assembly

The scope and mount were well-packaged, but the parts were not as well protected as the Celestron Omni XLT 127 which I have previously reviewed. The tube is only held in place in its cardboard box with the usual shaped pieces of styrofoam. The only damage I could see was a slight dent in each of the metal disks that form the azimuth bearing, but this turned out to have no effect on operation and is completely invisible once assembled.

Figure 2.

Figure 3.

Building the Base

In my previous review of the XLT 127, I wrote, "The instructions are clear, and include pictures showing the names of each component and drawing lines to the named component. This is very helpful when the instructions tell you to remove or tighten a particular knob or bolt; you know immediately which one is being discussed." If that defines one end of the instruction spectrum, the Z10 is at the other; the company does not ship any assembly instructions at all. None, nada.

At first I thought that my order had accidentally had the instructions omitted, but a quick look at comments of other owners on the Net showed that (1) no instructions are shipped with the telescope, (2) instructions can be downloaded online from www.retrevo.com, and (3) these instructions are quite out of date and misleading. A buyer who has no experience building a Dobsonian might be quite puzzled trying to figure out which screw is used in which hole, and how the pieces of the azimuth bearing fit together. If you have never owned a telescope before and decide to buy a Z10, I strongly encourage you to get the members of your local astronomy club to help put it together. Once it's assembled, you will have no trouble using it.

(As I was putting the finishing touches on this review, I discovered that up-to-date instructions are in fact available in a 5MB PDF file at http://www.telescopes.com/images/pdf/ZHUE024.pdf. Please read through this so that you can spare yourself the issues that I had.)

Figure 4.

In my case, I had built a nearly identical scope ten years earlier, so I had minimal problems. The only tricky step was inserting the bolt that serves as the azimuth rotation axis. This bolt is provided along with a small flat washer, a small bumpy washer, a large- diameter washer, and a short piece of aluminum tubing; lacking the instruction PDF, I had no indication of the order in which these should be put on the bolt.

The Optical Tube Assembly

Figure 5.

The OTA is heavy, as you should expect for a 10" Dob. It would be extremely difficult for me to move the base and tube at the same time. It takes a good bit of my strength to pick up either one by itself, especially when one is being very careful not to bump the tube against anything. As much as I would like a 12" or 16" scope, there is no chance that I will buy anything larger than a 10" unless I am able to have a permanent observatory in my backyard one day. Cooldown is also a significant issue for large Dobsonians. For all but the most dedicated amateurs, I would suggest letting 10" be your upper limit of aperture. (You can always look through other owners big scopes at star parties or club outings, but beware; it may spoil you for your own equipment and give you a bad case of aperture fever!)

The tube comes completely assembled, so little needs to be said about it. However, the altitude bearings are quite different from the spring-tension design that was used a decade ago. Instead, there is a knob on each bearing that allows tension to be adjusted. It can be a bit tricky to get the tension right.

The really unusual thing about the new bearing design is the ability to change the pivot point on the telescope tube. In other words, you can adjust the distance between the primary mirror and the altitude bearing axis. This offers flexibility to handle a range of different eyepiece and finder weights. Unfortunately, the idea sounds better than it works in practice. It is not possible to adjust it with the OTA in place, so you wind up putting the OTA on the base, finding that it doesn't balance, removing it, loosening two screws, adjusting it a small distance, tightening the screws, putting it back on the base, finding that you've overshot the mark, removing it again, etc, etc. You will wind up finding the best compromise balance point and then swearing never to adjust it again, and you certainly won't change it just because you've switched from a 1.25" to a 2" eyepiece.

Figure 6.

Figure 7.

Accessories

The Z10 comes with an 8x50 right-angle correct-image finder, a laser collimator, a 30mm 2" barrel 68 degree field of view eyepiece, and a 9mm Plossl.

The finder attaches using the standard dovetail and is thus interchangeable with most other finders. Installation is very simple.

It is not easy to sight down the primary optical tube accurately enough to put guide stars in the finder. The first night that I used the scope, the only finder attached to it was the 8x50 RACI, and it was a struggle to put any objects or guide stars into the view. I knew this was likely to be a problem, so at the same time I ordered the scope I also called Astronomics and ordered a Telrad. Installing the Telrad made a HUGE improvement in operation, and I would encourage anyone planning to purchase a Z10 to budget for one. (I plan to write a Telrad vs. Rigel QuikFinder shootout at some future point.)

As to the laser collimator...well, thereby hangs a tale.

Collimation

Readers who have some experience collimating Newtonians will realize that, unlike Cats, both ends need to be properly adjusted. And those fancy laser collimators are really only useful for tinkering with the primary mirror.

Fortunately, I have a Cheshire eyepiece. This inexpensive and extremely useful tool should be in the toolkit of every Dob owner. I plunked it into the focuser and nearly choked; the image looked like a horribly out-of-adjustment, worst-case scenario taken from a manual on collimation! The shadow of the secondary wasn't even close to being centered.

Adjusting secondaries on Newts is, quite frankly, a pain. I had to use a large screwdriver to break loose the VERY tight center screw holding the diagonal mirror, loosen it further to allow the secondary to move down the tube toward the primary, tighten all three radial screws to hold it in place, and finally adjust the orientation of the secondary until it was centered in the Cheshire. The very fact that the secondary screw was so tight tells me that the misalignment probably did not occur in shipping, but was already poorly aligned at the factory. Again, if you are a novice, please get someone from your local astro club to help. No telescope will perform well without proper collimation, and you *can* *not* assume that it will be delivered to you in usable alignment!

The two-speed Crayford focuser seems to be of very decent quality, but it does not have alignment adjustment screws. Fortunately, once the secondary was properly adjusted the focuser proved to be reasonably well-aligned. (More on the focuser later...)

Once that was done, it was not difficult to get the primary aligned with the Cheshire. As a test, I plugged in the laser collimator and found that it did not indicate the same center as the Cheshire. Someday I plan to do (and write up) more experiments to find out how far off the perfect laser alignment is from the perfect Cheshire alignment, and how high-magnification views look at each setting.

Jupiter

The first object to which the Z10 was pointed was mighty Zeus, the king of the Solar System. I was puzzled as to why a ghost image the same size as the planetary disk was floating just a bit offset from the main image. Tinkering with the focus and changing eyepieces didn't help. Popping in the Cheshire showed that once again the collimation was WAY off. But, but, I only moved the telescope 10 feet out onto the deck...how can that be? Worse, attempts to recollimate didn't seem to be getting anywhere.

To make a long story short, the problem turned out to be the adapter ring that lets 1.25" eyepieces be used in the 2" focuser. When switching to the supplied 2" 30mm eyepiece and back, I had not seated the adapter properly. Once this was firmly locked in place flush against the focuser, it was trivial to re-collimate and get good views. (Very probably the collimation had been fine at the start of the observing session, but in trying to "fix" a nonexistant problem I had created one.)

For those who have heard (or worse, experienced) horror stories related to the glue-like grease commonly found on inexpensive focusers, you'll be delighted to hear that nothing like that is used on the Zhumell.

Collimation worries behind me, the detail visible on Jupiter was a big step up from the C5 (Omni XLT 127, that is, reviewed elsewhere on this site) that has become my primary scope. Seeing has been poor ever since the Z10 arrived, so I have not been able to run the power up on the Moon and planets, but even at low power the resolution of fine details on the face of Jupiter is clearly better in the larger aperture.

Deep Sky

Using inexpensive oculars such as simple Orion Plossls, University Optics Abbe Orthoscopics, and even Owl superwides (definitely not designed for f/5) gave great views. In my first week of ownership, I viewed M15, M2, M30, M31, M32, M110, M57, Gamma Arietis, and Albireo. (The poor quality of seeing can be judged from my failure to fully split the double-double, Epsilon Lyrae, which should have been trivially easy.)

Small-aperture scopes just show globulars as faint fuzzies, much less impressive than many open clusters. But when I bought my first 10" back around 1999, I fell in love with globs. When I sold that 10" in 2004, it was a shock to find that my favorites (M15 in particular) were just faint blurs again in my Short Tube 80. Moving up to the C5 helped, but even with 5" of aperture the bright, loosest globs in sky still feel as though they are almost on the verge of being resolved...but not quite. (M22 may be an exception to this, but the viewing season for this far-south cluster is very short.)

The Z10 is clearly across the resolution threshold. There's no doubt that you are resolving M15, M2, and M30. I'll look forward to checking out M79 this winter and M3, M10, M12, and M13 next year. (On a later night, I set up the C5 next to the Z10, pointed both at M15, and went back and forth making comparisons at the same power. The differences between the two were subtle, but real; in the C5, the cluster appeared to be on the verge of resolving at its edges, while in the Z10 the edges were solidly resolved and a number of stars at the edges stood out distinctly.)

On a later observing night in late November, the transparency was good but seeing remained poor above 140X. The view of M42 was great, with significant detail visible even at 50X. At 139X, it was lovely. Much of M15 was resolved, although not all the way to the core. The galactic core of M77 was readily visible at 50X, and at 139X a hint of a glow was visible beyond the core, although the actual spiral structure was not visible. No trace of NGC1055 was seen. Even 10" of aperture is not a substitute for truly dark skies. (This November night had a limiting magnitude between 4 and 5.) The most impressive object was the open cluster M103. In the C5, the cluster was clearly visible but quite bland, with only a few stars seen and all appearing white. In the Z10, not only were substantially more stars visible, but I could readily see color in the brightest ones.

Although I pushed the power as high as 278X on some objects that first autumn night, the seeing was always a limiting factor. The problem was not cooldown, because the inside/outside temperature differential was only a few degrees (ah, Northern California autumn...), the scope was left out over 3 hours before observing began, and the fan was running.

Figure 8.

Yes, one feature of the new Z10 that wasn't offered back in 2000 is a small, built-in fan. You simply add 8 AA batteries (I suggest rechargeables) to the supplied battery case, then plug the cable into the side of the mirror assembly and it begins to run. I have not used it enough to know how effective it is, but with a mirror this large I expect it will make quite a difference on really cold nights.

The 68 degree AFOV, 30mm 2" works surprisingly well. While it is not sharp to the field edge (what did you expect?), the aberrations are easily ignored. As long as you aren't expecting TeleVue-class performance, I think you'll be quite pleased. Very rough comparisons with my Owl eyepieces seem to indicate that the AFOV is about what's promised. Assuming no vignetting (a key assumption), this eyepiece would give a true FOV of about 1.6 degrees, which can certainly be useful in looking at wide objects such as open clusters and low-surface-brightness galaxies. I don't have enough experience with this eyepiece yet (and none at all under dark skies) to say much more than that.

Moon

So far I have only pointed the Zhumell at the Moon on a single night. I'm normally a fairly serious lunar observer, but lately the weather, my personal schedule, and other commitments have kept me away from the eyepiece when Luna was well-placed.

However, at 80X on a night of poor seeing, the view of the wrinkle ridges in the maria was very impressive, although it was difficult to get a sharp focus. Part of this was bad seeing, but part was that the fine adjustment of the 2-speed focuser wasn't working. Spinning and spinning the fine adjustment produced no motion at all.

Focuser

Figure 9.

It wasn't until the next evening that I studied the focuser closely, inside the living room under good lighting. There are two screws sticking out of the bottom of the focuser (bottom as seen from a normal viewing position). They look nearly identical, and with no manual supplied it's not obvious how to adjust them.

A little experimentation quickly answered the question. The bottom screw is used to lock the focuser in place after achieving good focus. When it is snug, turning the focuser adjustment knobs has no effect. When the lower screw is loose, the upper screw controls the amount of friction in the focus tube movement. A loose upper screw will make it very easy to change focus, but any slight bump will knock it out of adjustment. Tightening the upper screw will help the tube stay in place (and hold focus) while still making it possible to change focus.

If (like me) you grew up on rack-and-pinion focusers, the idea of turning a focuser knob without the focus changing will throw you for a loop. But since Crayfords use friction rather than gears to move the focus tube, it makes sense once you understand it.

Conclusions

Readers of this review may feel that I've been unduly negative. In reality, I'm quite happy with the Z10, and taking the price into account I'm delighted. However, I want prospective owners to understand that while there is a gem hiding inside the box, it may take a bit of polishing before it shines brightly. Nothing that I did should be at all difficult for an experienced amateur—or a new observer who has the assistance of an old hand.

Come back in a year and I'll give an updated review once I have more experience with this little rough diamond.

Cathy James