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A Review of Stargazer Steve's Planetary DOB Design

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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 page.

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 performance.

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 bargain.

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 mount.

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 counterweight material.

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 telescope.





In Search of Perfect Images

DOB Tube
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

DOB Base

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

DOB 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

Dob 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 purpose.

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 circle.

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.

Dobsonian Azimuth Setting Circle
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.

Dobsonian Azimuth Setting Circle
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 base.

Dobsonian Azimuth Circle Viewing Slot
Az Circle W/Telescope In Place

The image above shows the slot with the DOB mount fully assembled and with telescope in place.

Dobsonian Elevation Setting Circle
DOB With Elevation Level
Dobsonian Elevation Setting Circle
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 targets.

Recommendations

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 purchase.

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.