Zhumell 10 inch Dobsonian Reflector

Zhumell 10 inch DobsonianSince 2007 my primary telescope has been the Z-10: a Zhumell 10 inch (250 mm) Newtonian reflector on a Dobsonian mount. Many might argue that so much aperture is wasted in washed-out skies, but I found the Z-10 suits my needs quite well. It is sometimes claimed that in light-polluted skies larger apertures only further brighten the background and diminish contrast; that in washed-out skies you can actually see more with smaller apertures. I’ve found that to be simply another urban legend: the Z-10 does quite well in pulling in DSOs from my front yard despite the heavy light-pollution. It consistently shows objects that I can’t see with smaller apertures. The Z-10 also just fits in my car (a VW Passat), allowing me to easily take it to nearby parks or to far-away dark-sites.

I’m reasonably satisfied with the Z-10, although it isn’t a perfect telescope by any means. The optics are okay (but not great) and the mount is reasonably steady. As with all Dobs, I’ve made some modifications to make it better suited to my needs....


As I mentioned, I’m quite pleased with the Z-10’s performance in my washed-out skies. From my light-polluted front yard or a near-by park, the Z-10 has shown me all of the Messier objects (although some low-surface brightness Messiers like M33, M74, and M101 are quite challenging) and it provides enough resolution on globular clusters to make most quite spectacular looking (I find unresolved globulars rather unsatisfying). Under these same washed-out skies, my 4 inch Newtonian can’t reveal even a third of the Messiers. The Z-10 also proves itself against stellar objects, with enough light gathering power to catch 14th magnitude quasars, despite the light-pollution (some magnification and a good transparent night are required). Despite the urban legend, the 10 inch aperture also does better than my friends’ 6 and 8 inch telescopes. The difference is quite noticeable compared to a 6 inch scope, but more subtle compared to an 8 inch scope. As an illustration, one night observing in park near my home, M97 was an averted-vision object in my Z-10 (M97 sat in a big dome of light-pollution), but simply couldn’t be seen in my friend's Celestron C8. So in my experience, bigger apertures rule even in heavily light-polluted skies.

Z-10 overview

Z-10 with light shieldThe optics on the Z-10 are okay, but not great. I get reasonably sharp images at lower to medium powers, but at around 200x the images degrade. They loose their sharpness and it is hard to get a crisp focus. The generally poor seeing around here makes it hard to blame this on the optics with any certainty. The in-focus diffraction pattern consistently shows nice concentric rings, while the out-focus pattern is always pretty crazy looking; I'm not a star-testing expert, but I believe this suggests an optical defect. On the other hand, I have been able to split Porrima (γ Vir) with the Z-10 and I’ve been able to glimpse spectacular views of both Jupiter and Saturn during those rare instances when the image steadies. Someday I’ll have the mirrors tested.

The Z-10 holds alignment reasonably well: even after travel by car it usually needs only a slight tweak to collimate so long as I’ve been careful not to jar it sharply. The dual-speed focuser works very well, and the 8x50 RACI finder has a field of view that is sharp from edge to edge and provides nice low-power views. The mount is stable and moves fairly smoothy with an appropriate amount of stiction. I can track objects at 300x without difficulty (although the narrow field of view means that I’m tracking more than observing at that magnification). There is vibration when touching the scope but it damps out quickly in a second or two.

Out of the box, my Z-10 was front-heavy and the way the tension springs connected from the mount to the telescope azimuth hub was rather awkward and inconvenient. But both of these were easy to fix.


The balance problem was easy to fix in a very flexible way: for under $10 I got a fanny pack and a 2 lb (1 kg) hand barbell. I put the barbell in the fanny pack, and I strap the fanny pack around the bottom of the telescope tube to even out the balance. This also makes it easy to add additional weight when I have any heavy eyepieces or other front-end gear. I replaced the tension springs easily and inexpensively with shorter springs and double-ended bolt snaps that make mounting and unmounting the telescope a breeze. I got the idea to do this from the Visual Astronomy web site, where you can find detailed instructions. I also added better alignment knobs to both the primary and secondary mirror holders.

Zhumell with bolt snaps and replacement tension spring

A well known problem with the Z-10 is that the nuts holding the azimuth hubs to the tube come loose. Mine started coming loose, so I added lock-washers to these and haven’t had a problem since.

I really like the RACI finder, but I found it difficult to sight along the finder or the Z-10 itself when pointing the telescope, so I installed a second mounting base which I use for either a red-dot finder or a laser-pointer (I much prefer the laser-pointer, as explained in my earlier article on finders). The Z-10's tube doesn't extend very far beyond the focuser, so to help keep stray light out of the optical path I made a cheap light-shield. I also flocked the key parts of the inside of the tube, as I described in a recent article.  

Z-10 showing the focuser, finder, and added laserpointer


All-in-all, I’m pretty pleased with the Z-10. It has sufficient light-gather power to show many interesting astronomical sights, and the aperture is put to good use even in washed-out skies. The Z-10 is one-man portable and fits across the back seat of an ordinary car. Most of the Z-10’s short-falls were easy to fix. I do wish it had better optics, and I may eventually have the mirror re-figured. But the views are good enough that I’m in no rush to do this.



z10,how about the z12

 any reviews on the 12 inch ???

The Z12

I'll gladly review the Z12 if someone gives me one :)
Seriously, the Z10 and Z12 are nearly identical in all aspects, so I would expect the Z12 overall to be like the Z10: a solid telescope, reasonably put together, with okay but not outstanding optics.  I've noticed that the newer models of both the Z10 and Z12 have fancier altitude hubs.  The older hubs like the ones on my Z10 work fine, especially with the customization I described.  So the improved hubs should be fine, too. 


Thanks,im deciding between zhumell or the apatura,10,12

ordered the apertura 12"

 after a few months of reading many forums on the internet  i pulled the trigger on this one from optismart.. i have been going over all your info and archives and keep learning and learning. you put out a lot of great info that seems to fit my observation location here on long island,ny about 25 miles outside of new york city!!..the scope came yesterday after great communication with the people at optismart. they actually called and emailed me 4 times over the last 4 days since i put the order in.. so far very pleased with them and your site still has soooo much to offer..thanks phil.

New Apertura 12"

Congratulations on your new telescope!  It will give you many years of enjoyment and show you countless sights. 
Thanks for your kind comments regarding this web site.  I have been a bit remiss in posting new material (life has been busy lately), but promise to get some new articles up soon.

Zhumell z12 user comments

I've been using the z12 for a year under urban skies....Bortle 6-7 with plenty of local lighting issues

It works great....overall good quality, acceptable entry level optics...The included 30mm SuperView ep is actually good enough for most folks to keep.The 9mm Plossl, not a chance....The 8x50 RACI finder is decent quality and easily aligned with two delicate plastic screws.....The new style altitude bearings are smooth and easily adjustable, along with a center of gravity adjustment too..The lazy susan turntable is smooth and adjustable, although the small brass bearing that goes under the adjustment knob isn't built to last......The 2" Crayford style focuser is plenty good enough for the visual observer....

The supplied laser collimator works ok once it's own collimation is checked and adjusted if needed....The collimation and mirror lock knobs at the primary are nice enough to not need any upgrade. I heeded the suggestion of many, and removed the primary cell and check the primary mirror clips for proper adjustment...This is recommended for any GSO manufactured newtonian telescope that has primary mirror clips...Many users have found them to be too tight as shipped.

The primary springs and proper fitting bushings makes an upgrade required, though an easy task to DIY...The OEM springs are just not up to the task of moving the weight of the 12" mirror. I added nylon bushings INSIDE the OEM brass bushings to eliminate the slop that was present. Don't forget to lube the collimation adjustment screws while you have them out...The reward is easy, smooth movement for collimation and it will hold collimation much better...

Replacing the phillips head adjustment screws on the secondary with thumb knobs is a good idea, along with adding a couple of plastic washers between the collimation screws and the secondary mirror, again, an easy DIY.....Smoother, tool-free adjustment is the reward....

Overall, the scope is built like a tank and well appointed as shipped. The GSO optics have a reputation for overall decent quality and identical to other GSO manufactured brands out there.... I added a HALO to mine, since so few guide stars are available at my observing location. I had a TELRAD, but as a newcomer, and with light polluted sky, I found it more useless than not... Using the HALO along with Stellarium on my netbook makes finding DSO's in real time almost too easy. Using an Explore Scientific 9mm 100 degree series ep, the chance of being "on target" is about 75%....Using an ES 18mm 82 degree series EP finds the target in the FOV 99+% of the time.....

Overall, if the Zhumell z12 were still available, I would highly recommend it as a solid value for an entry level telescope. IF there were anything to be remotely be considered a negative, it would be the size and weight. It fits in my Honda Element with no problem...Heavy is relative...I've moved music gear all my life and worked construction most of it too...Your milage and experience may vary....
Size does matter....same as location, location, location.....

My experience is that larger apertures require higher power for observing the dim fuzzies in less than ideal urban environments....With the correct FL ,there is a sweet spot where the background is dimmed just right, and the contrast is just right too....With the 12" z12, the 9mm 100* worked extremely well even with less than great seeing. The 18mm 82* so far is unsurpassed as a finder EP...Wide and sharp as a tack, virtually no coma, and contrast that would make Baader blush....

great night, it all came together

 Yep it all came together last Saturday night when I hosted my 3rd annual Fantasy Baseball(Yahoo) BBQ Bash on my deck in the backyard.It's been two months since I got my AD12 from Opticsmart and of course I was praying for clear skies.Well it all came together like a perfect storm!!. The grill got going with hot dogs and hamburgers, guys started arriving at 4pm and the beers were flowing.All the time we were talking baseball I kept an eye on the few small clouds going by overhead hoping they weren't going to grow into thunder heads.
  Well 9pm came around and the sky was getting  dark so it was time to release the hounds.I started with a few double stars like Alberio,Epsilon Lyra and the Mizar–Alcor thing- a- ma- jig. Now it was time to pull out all the stops and I went right to m57(ring nebula). First with the 2", Superview 30mm, 68 degree FOV which got everybody going " wow". Then I put in the 1.25", Super-Plossl 9mm, 52 degree FOV which got a bigger "wow". While I had everyone's attention I went right to m13(Hercules Globular Cluster). Well needless to say they were all quite impressed(and all quite drunk).
 We ended the evening by playing a few rounds of Texas hold 'em and everyone went home to their wives and girlfriends very content.
 For me it's time to start thinking about getting some more eyepieces and a barlow lens.