The originator of this thread said:
I've watched Ed Ting rate the 8inch dob as his "if i could only have 1 scope", even more than the 10 inch due to size,awkwardness etc.
Since we have started debating eight-inch versus ten-inch Dobsons, I will pipe up again. I have built and made considerable use of one of each, as well as a twelve-and-a-half inch. The eight-inch was first -- it was f/5.1, and was in pretty much the classic style -- solid tube with the balance point nearer the middle than the bottom, plywood rocker and ground board. It did use a conventional mirror cell, since I happened to have one around. The 12.5 was second -- built for a local club using an existing f/6.3 mirror, with similar construction to the eight inch, but with carpet-and-strap mirror support in the traditional "box" with a big Sonotube sticking out of it. The ten was my third and last so far -- f/5.0, truss tube, quite lightweight wooden construction in a more modern style, designed for easy take-apart and airline travel, and by far the most compact when so disassembled. All three had Pyrex mirrors.
I am mostly a deep-sky observer, and all three telescopes worked well for me, the sole problem being that the 12.5 was too big for me to transport since I did not then own a van; I used it in my driveway for a while before turning it over to the club.
The telescopes were generally stored and transported at ambient temperature, so cool-down and associated thermal effects were rarely a problem. I occasionally could detect a pool of cool air on the "down" side of the 8-inch's tube, but it did not affect low-magnification observing, and removing the vinyl cap at the bottom of the tube made it go away when it was a problem with higher magnification.
I never felt the need for eyepieces with particularly wide apparent fields of view with these instruments: A low-magnification eyepiece with an apparent field of view of 65 to 70 degrees gives at least a one-degree true field with any of them, and that is more than sufficient for locating things if you have a decent finder to start with. With the 8- and 10-inch telescopes, I would generally start with a 20 mm Meade Research-Grade Erfle, with a 68 or so degree apparent field of view, and magnifications of 51 and 62 respectively. With the 12.5, I would use a 32 mm Erfle. Those magnifications were as often as not perfectly adequate for deep-sky work, and when I wanted more I would switch to something like a Meade 7 mm wide-field (similar apparent field to an Erfle) or, later, an 8 mm Brandon (45 degree apparent field, more or less). I never used a coma corrector, because when you use an Erfle with a fast Newtonian, the dominant off-axis aberration you see is not the coma of the primary mirror but the much greater inherent astigmatism of the Erfle itself. (Try an old Erfle in a fast Newtonian with a ParaCorr and see what we were up against when Erfles were the best there was.) If I were doing another Dobson I might plan for a ParaCorr and maybe some whizzier wide-field eyepieces -- or not, since I have never been bothered by their lack. And in the 8- and 10-inch instruments I am describing, adding these items as an afterthought would have made the telescopes very difficult to balance: Can you say "Dippy Bird"? :-) :-) :-)
Collimation was never a big deal with these three telescopes: I had to tweak the eight-inch every few setups, mostly because the diagonal support tended to rotate about the optical axis when transported, but the ten survived five nights' worth of tear-down, setup, and transportation between my sea-level hotel in Hilo, Hawai'i, and the Summit Observatories Visitor Center, 9300 feet up on Mauna Kea, with no collimation adjustment required. I stored the 12.5 in my garage while I had it, and merely rolled it out to the driveway to observe; that process was too gentle to mess with the collimation.
Wind did not bother the 8-inch much -- its tube had nearly the same area on either side of the pivot points. The 10 tended to weathercock in the wind with the fabric light shroud installed, but it was stable -- though less well-baffled -- when I took it off. I don't recall using the 12.5 in more than a gentle breeze.
Since the 8- and 10-inch telescopes were both f/5, when I used the same eyepiece in both, they showed nearly identical aberrations at the edge of the field. Note that most commercial 8-inch Dobsons are f/6 whereas most commercial 10s are faster: Comparing edge aberrations for them might favor the 8-inch, both because the longer focal ratio is more forgiving of eyepieces and because the eyepieces required for the comparison would need to have slightly longer focal lengths at 8-inch f/6 than at 8-inch f/5, in order to obtain the same magnification.
The 10-inch was lighter in weight, less bulky, and much easier to operate than the 8-inch, but that was due to my taking advantage of the community's experience with truss-tube types to come up with a better design than I used for the eight-inch.
I hope my experience may be of some use to the topic originator.
Clear sky ...
