Planetary Dobsonian Review
The Planetary DOB
The image above shows my 6 inch f/10 planetary DOB. It was designed as a
kit by Stargazer Steve, and was used to make the Mars drawings on the Mars Drawings
I purchased my 6 inch f/10 DOB as a special design from Stargazer Steve
Dodson. He has his own site at Stargazer
Steve. If you don't know about Steve, he's a very accomplished telescope
maker and educator making his home in Canada. He sells a range of reflecting
telescopes of his own design packaged as kits.
With Steve's kits, optics are delivered already finished, and many parts,
including the mount elements, are pre-fabricated. So assembly is pretty easy,
usually taking only a day or two.
I'd been corresponding for a time with Steve about whether his 6 inch f/8
DOB would make a good planetary telescope. After some back and forth email,
Steve sent a response saying he knew what I wanted. A long focus Newtonian on
a DOB mount, with each component designed to give the best planetary
So he offered to make a 6 inch f/10 kit for me and quoted a price. The price
was a bit more than his regular 6 inch, but seemed fair given the special effort
he'd have to expend on the project. In retrospect, it was probably a
I agreed, and he delivered. The telescope arrived a few months later, well
packed and already partially assembled. All I had to do was put together the
Steve likes to make his DOBs with most of the telescope weight on the
primary end. This makes for a very low profile and small footprint DOB base, as
shown in the picture. On his regular DOB models, Steve does this with a
specially designed mirror mount that has room behind the mirror for some
My scope is quite a bit bigger than the normal Steve 6 inch because of the
performance I was seeking.
The tube is 6 feet long to accommodate the mirror focal length, and 9 inches
in diameter to keep tube currents out of the optical path. The longer tube
required a mirror design that would allow more counterweight to achieve Steve's
signature low profile mount.
Steve accomplished this with a wooden ring at the bottom end of the
telescope with 1 1/2 inch holes spaced around the ring. Counterweights are 1
1/2 inch bolts which can be mounted in the holes. I can use the number of bolts
necessary to accommodate the amount of counterweight needed to balance the
In Search of Perfect Images
DOB Curved Secondary Holder
The basic design elements that give great planetary performance are
numerous. An oversized tube to keep air currents out of the optical path. A
Pyrex primary to minimize cooling problems. A long focal length to give a
flatter field, minimize alignment errors, give better power per eyepiece, and
allow a smaller secondary.
The reduced sized secondary is only about 16% the diameter of the primary,
and gives negligible diffraction. The telescope also uses a curved secondary
holder to eliminate spikes.
The mount is made of heavy-duty birch plywood, with the sides of the DOB box
being double layered.
On the base are 3 Teflon pads. Mounted to the under of the DOB box is a
metal sheet that makes the bearing surface for the Teflon pads.
Heavy-Duty and Cleverly Designed
The elevation bearings, shown above, are two short lengths of 1/4 inch thick,
6 inch diameter aluminum pipe. Mounted to the sides of the telescope cradle,
the aluminum pipes ride on Teflon bearings.
The elevation bearing on the left side of the mount has a cleverly designed
adjustable brake to allow for variation in eyepiece weight.
The Telescope in Use
The result? A smooth operating mount that is very steady, and a telescope
that gives superb planetary images. There has been little for me to do but
enjoy the ride.
After using the telescope a lot, I did find that moving the telescope around
did present a problem, or at least an inconvenience. I had to lift the bulky 6
foot tube from the base, then move the base to a desired spot, then re-affix the
tube to the mount.
I kept bumping one of the elevation Teflon pads and working it loose when I
removed and replaced the telescope tube. I needed a way to move the telescope
without disassembling it.
First Refinement, Wheels
This prompted my first refinement. I mounted a pair of metal brackets, shown
above, to the base to allow me to slip an axle with wheels into position for
moving the assembled apparatus. I can easily remove the axle and wheels when
the telescope is where I want it. I just slightly tip the base and the wheels
drop out of slots that hold them in place.
Second Refinement -- Mirror Fan
I eventually added a second refinement: a computer muffin fan in the bottom
end of the telescope. The cooling fan is mounted to a circle of 1/8 inch
hardboard with a hole cut the size of the fan. Three elastic straps (ponytail
bands) suspend the fan in place, and absorb any vibrations generated by the fan
(the 3 red arrows).
With the elastic bands, the muffin fan doesn't cause any discernible
vibration, aids in cooling down the mirror and optical path, and breaks up the
turbulent convection layer that tends to sit over the primary's reflective
surface. It really lets the telescope deliver refractor-like images.
Third Refinement -- Setting Circles
For a long time I wanted to add setting circles to my long focus DOB. It
provides the best images of all my telescopes, both for planetary work and deep
sky work. But I had rarely used it for deep space work because it was just too
difficult for me to locate objects with it. I'm lazy now, and I don't do the
star-hopping technique anymore. Not since I added setting circles to my Pipe-Fitting Tripod. Since
then I use a calculator program that has star almanacs pre-loaded, and let it
tell me where to point. One could use a laptop planetarium program for the same
Or, even better, use the Star Pointer web page that
tells you where to point any properly aligned telescope at any object in
any of three popular star catalogs. It puts star pointing tables handily
available on any most and network web browser, whether on a laptop, Chromebook,
tablet, or smart phone.
But to use that technique, I needed setting circles on my DOB. I'd scratched
my head a number of times, never quite conceiving a design that seemed
feasible. Then I ran across Uncle
Rod's Astro Blog, and saw a technique far simpler than all the visions I'd
had. So I jumped on the technique, and what follows is my effort to emulate
Uncle Rod's clever design.
First, I created the azimuth setting circle by printing out the Setting Circle PDF
file to a sheet of 8.5 x 10 inch paper. I went to a copy store and had the
image reproduced on an 11 x 17 inch paper, giving me a 10 inch diameter setting
Then my wife used Mod Podge to affix the circle to a poster board, coating
the surface of the circle with Mod Podge as well. This gave a stiff setting
circle that was also a bit weatherized by the glue coating.
Az Setting Circle On Base
I then lifted the rocker box off my DOB base, and glued the setting circle
to the base as shown above, careful to have the setting circle centered and
with the zero reading at a convenient orientation.
Az Setting Circle View Slot
Finally, I cut a slot in the base at a position that would give me the best
view once the telescope was back in place, and added a pointer cut from thin
metal. The above shows the rocker box with viewing slot back in place on the
Az Circle W/Telescope In Place
The image above shows the slot with the DOB mount fully assembled and
with telescope in place.
DOB With Elevation Level
DOB W/Level Blowup
Then, following Uncle Rod's advice, I purchased an inexpensive
angle-reading level for the elevation setting circle. The version I purchased
has a magnet in the base, so I made use of this. My particular Stargazer Steve DOB design uses a
hexagonal box to hold the telescope, so I needed only to mount a small flat
plate to the box, which lets the magnet of the level hold it in place.
This simple design is superior to other designs I've made. In the past, I
made the elevation setting circle part of the mount, which necessitated precise
leveling of the mount in order to get good elevation readings. This design is
not as sensitive to how level the base is, as long as fairly level.
Because the level reads the elevation of the telescope irregardless of the
levelness of the base, and azimuth readings are not much affected by a base
that's close to level. The main error from poor leveling occurs when seeking
targets with high elevations.
All I need to do to use this setting circle configuration is to nudge the
base so that it reads an azimuth of zero when the telescope is pointing at
Polaris. In the future, I think I'll just add an azimuth bias option to my
calculator program. Then I'll be able to just get the mount oriented to read
within a few degrees of zero when aligned on Polaris, and I can enter the
reading as a bias which the calculator will compensate for when I select
Would I recommend a Stargazer Steve kit to a prospective buyer? Yes, with
no reservations. Steve's telescopes are cleverly designed, and built heavy duty
with quality parts. He is also very helpful if you need any assistance after
I've used my DOB for quite awhile now, and find that the design makes the
telescope easy to maneuver. The telescope is also very steady, a great accomplishment with such a large design.
If you want a DOB, but aren't really into putting it together yourself, for a comperable price to what I paid you can get an Orion SkyQuest XT8 Dobsonian Telescope. That's an 8 inch, versus my specially built 6 inch. The 8 inch has a shorter focal ratio, so it will be more sensitive to misalignment, and not have quite as flat of field of view. But it will actually have even more resolution on those perfect nights, and will be a better DSO telescope.