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This page copyright 2012 Steve Ford http://geeky-boy.com and is licensed under a Creative Commons Attribution 3.0 Unported License.

Equatorial Mounts


One of the religious wars that perpetually breaks out in the sci.astro.amateur newsgroup is an argument over which is better: Dobsonian (Dob) or Equatorial (EQ) mounts. Having never used a Dob, I can't really compare. (But Keith Wichman can!) However, I am frequently struck by the arguments that pro-Dob people give.

I should mention that I have a Celestar-8 scope on the standard wedge mount. I've never used a German-style mount - it might suffer more from the claimed disadvantages of EQ. I don't know. I'll only speak to Celestron's wedge. Also, I'll ignore the more expensive Dobs that include motor drives. The debate generally rages around less expensive scopes.

Setup Time

One big argument used against EQs is setup time. Dobs take very little time and effort to set up, which supposedly removes a barrier to using the scope. I laugh when I see this argument.

When I'm ready to use my scope, I open the door, pick up the scope, walk outside, and set it down. Done. With a Dob, I assume I would have to make two trips. EQ wins.

Closely related is the time it takes to polar align an EQ. I polar align as I set the scope down. North is right over there (see me pointing?), so I set the scope down with the polar axis pointing roughly North. Done. Zero extra time. EQ still wins.

What about the setting circles? I almost never use them (see below for rare exceptions).

What about when I'm not going to my back yard? OK, now it is more time-consuming. I have to undo three bolts. That takes about four minutes, including threading the bolts back in the mount so they won't get lost. Then when I'm on-site, I must re-assemble - that's another 4 minutes. Will that 8-minute overhead cause me to use the scope less often? Good grief NO! The overhead involved in just loading the car with everything, driving to a site, and unloading the car FAR outweighs the extra 8 minutes. That is the overhead that causes 98% of my viewing to be from my back yard; it would be no different with a Dob.

So, for setup time the EQ either wins or ties. (See next section for child-related issues.)

Size

My Celestar-8 is about at my limit for carrying it outside easily. I'm not very strong; even a 10-inch SCT would be more than I could handle. In that case, the take-apart and put-back-together times would become significant for back-yard viewing, and I probably would use a Dob more often.

For children, the situation is more significant. Depending on the age and strength of the child, even a 4.5-inch reflector on a tripod might be more than he could handle by himself. And if at all possible, the child should be able to accomplish setup without help.

Again, my lack of direct experience hinders me; there may well be a range of ages and scope sizes for which a child could handle a Dob but not a tripod-mounted scope. In this case, the answer is obvious. Go with the Dob. If the child has to wait for somebody else to help set up the scope, it will be used a small fraction of the times that a self-serve scope would be used. And, as everybody agrees, the best scope is the one that gets used the most.

Closely related is the hight of the eyepiece. Small Dobs are usually more kid-friendly. With an EQ, the real shrimps need a ladder to see things close to the horizon! As before, if a tripod-mounted scope is of a size that makes it difficult for a child to reach the finder, then a smaller mount is in order. It's hard to beat a 4-inch Dob in that respect.

So, at least for adults, the size only matters for 10-inch or bigger scopes. For my 8-inch, there is no advantage or disadvantage either way.

Difficulty of Use

It is true that an EQ is significantly more difficult to use, if you use the setting circles to find objects. I don't, and other beginners probably shouldn't either. Again, I can't really compare since I've never used a Dob, but it only took me one evening to get used to the initial strangeness of sweeping it around the sky. Maybe it's harder for German-style EQs, I don't know. But my wedge is perfectly easy to point.

I can imagine that the EQ mounts might look more complicated than Dob mounts. I love gadgets, so that's actually a minor advantage for me. But I think that one evening's worth of using an EQ is enough to get people over that aspect of it.

The only complexity left is the fact that after you've swept an EQ to point approximately at the intended target, you have to lock it down. I believe that this is something you don't have to do with a DOB. Once you've locked an EQ, you have to fine-tune it with knobs instead of nudges. I guess that is a little more complicated. However, most people I know like to turn knobs! They want to think they're doing something a bit more high-tech than just wrestling with a big tube.

I will admit that twisting a knob is slightly more complicated than giving a tube a push.

But there is one area where the EQ is easier to use - tracking objects across the sky. With a Dob, you have to nudge it. For beginners, they will invariably nudge it the wrong way and will have to re-nudge it a few times. One slip and it's time to star-hop again. With an EQ, the whole point is to make tracking easy. Turn a knob. If you turned it the wrong way, turn it back. Almost zero chance of "losing" the object.

Once again, with children it might be more of an issue. Perhaps they would have trouble keeping the scope pointed roughly at the object while they lock it down. Perhaps they will have trouble reaching the knobs while looking through the eyepiece.

But, at least for adults, I believe that the advantages and disadvantages cancel each other out, leaving them equally easy to use, even for beginners.

Learning the Sky

Here is a pro-Dob argument that genuinely annoys me. A Dob "forces" you to learn the sky. Maybe I'm just a cowboy at heart, but I don't like people trying to force me to do something for my own good. Convince me that it's a good idea to learn the sky, and I'll do it no matter what mount I have. As it happens, I already know the value of learning the sky, and I have been doing exactly that. But the notion that the unwashed masses are too stupid to understand the value of learning the sky ... well that puts me off no end. (It almost makes me want to recommend EQs just out of spite.)

When I first got my Celestar 8, I used the setting circles instead of learning to star-hop. It let me see some pretty cool things right, from the start! However, it didn't take long for me to get tired of aligning, so I started to star-hop. So for me, the setting circles were actually a stepping-stone to learning the sky and star-hopping.

Maybe the issue is different with a computerized model. Perhaps there are people who will never try to learn star hopping, will use the scope for a year, and then say, "Is that all there is?" These people may well benefit from a few sessions of star-hopping; they might discover an unexpected treasure chest of joy. On the other hand, there may be just as many people who have enough trouble learning to star-hop that they give it up in frustration. A little bit of success with setting circles might do them wonders.

But I claim that it's not an argument against EQ mounts. It's an argument in favor of better education of beginners.

Using the Setting Circles

I mentioned above that I don't use the setting circles very much. However, there are two circumstances where I do use them, and it is a great help to me.

I live in a heavily light-polluted area. Sometimes, when hunting for an object in a star-poor field, I simply can't get there. I'm slowly learning some techniques on how to do it, but it still happens. In that case, it's nice to be able to take a few minutes to get my polar alignment a little more accurate, set the circles, and point right at the object. Then, much like solving a maze starting from the destination, I work my way out, finding landmarks as I go.

Secondly, here's a technique that I'm somewhat proud of, as I discovered it without any hints. My back yard is such that the "horizons" are fairly high. During the first 3 months of having my scope, I could not find Mercury. Any time it is high enough off the horizon to be visible above the surrounding trees and houses, the sky is way too bright to find it, even with binoculars (at least my crummy 7x35 pair).

Then, I figured it out. I woke up shortly before dawn. I used SkyMap to get the RA and DEC of Mercury. Then, I polar aligned and set the circles using bright stars. Finally, I pointed the scope at the right coordinates (below the horizon), switched on the motor, and went back inside for breakfast. A half-hour later, the scope was pointing above the tree line, and Mercury was right there, surrounded by a pink sky. No hint of it in the finder scope (a crummy 6x30), but clear as a bell in the eyepiece.

No way in the world you could do that with a Dob.

Expense

As the scopes get larger, it rapidly becomes very difficult to mount them on a tripod. For a 4-inch, a Dob is only slightly less expensive than an EQ. An 8-inch SCT is significantly more expensive than an 8-inch Dob.

I'm very fortunate that I had the money to get what I really wanted, an 8-inch SCT (it's the largest aperture for the degree of portability that I want). But for people on a tighter budget, it would be hard to resist the extra size that a given dollar will buy you with a Dob.

Conclusion

I can imagine several circumstances where a Dob is definitely the way to go. Large aperture, a child who can't carry a tripod-mount, a cost-constrained budget, or a person who, for whatever reason, just has more fun with a Dob.

But even for beginning adults, I see cost as being the only reason to favor a Dob over an EQ mount.