Since 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....

Myth-busting

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 14^th 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

The 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.

Modifications

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.

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.  

Summary

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.