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What Size Solid tube Dob Should You Buy?

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What Aperture Solid Tube Dobsonian Should I Purchase?

Introduction: Perhaps one of the most common questions asked by new astronomers (right after: which first telescope should I get?) is: which of the "standard" solid tube Dobs should I get? Dobs are very popular among astronomers of all ages and experience levels, and the market is full of a wide variety of such instruments, ranging from basic (yet still quite nice) entry level models to incredibly beautiful and functional custom Dobs that are works of art.

The mass-produced, solid tube Dobs (Dobsonian-mounted Newtonian reflectors) that are considered "standard" come in four sizes based on common steps in aperture: 6" F/8, 8" F/5.9 to F/6, 10" F/4.7 to F/5, and 12" F/4.9 to F/5. The three smallest telescopes can easily fit across the back seat of a car, while the largest one needs about an extra foot of room for the longer and larger tube. Two companies (GSO and Synta) in Asian produce all the main components of these scopes, and a variety of reputable companies supply them to the public around the world. They are all quality instruments and though the accessories and details (finderscopes, focuser type, etc.) vary from one company or size to another, the biggest decision for an astronomer looking to purchase one of these Dobs is usually which size to purchase? That is the subject of this article since I feel that my own experiences may help others make a decision in this regard.

8" Dob vs. 10" Dob - Intro: About a year ago, I bought an XT10i Dob from Orion, and it is a great scope. Unfortunately, back problems and portability concerns started to rear their ugly head, and I debated for a while what to do. After experimenting with various ideas, I eventually sold the XT10i and bought the XT8i instead, which has also turned out to be a great scope. Portability was key, as dictated by the back problems and the drawbacks to living in a 3rd floor apartment (stairs... lots of stairs!)

Now, it is always frustrating to give up aperture, though thankfully my back problems have subsided and my telescope is no longer a source of disagreement between my astronomers desires (more aperture!) and my back (what are you doing carrying this water heater around?!) Someday, I'll probably look into a truss-tube scope that can provide more light-gathering power without hurting myself, but for now, I figured that some good could come of the unintended comparison between a 10" and an 8" scope of the same model.

Brightness and Resolution: The 10" wins, of course, but by how much? The differences are subtle, but they are not hard to see at the eyepiece. There are two places where this difference is most obvious to me: resolving globular clusters (especially tight ones), and, oddly enough, seeing colors in stars. The 10" shows more vibrant colors in stars than the 8", and that actually is perhaps the most obvious difference between the two scopes in my eyes.

Still, the differences are subtle: the 8" still provides outstanding views, and the heavens are full of colorful stars when using it. But the 10" goes a bit deeper, breaks globular star clusters apart a bit better, and turns up the saturation on star colors a bit more. You will notice the difference, but you won't be put off by the 8" in my opinion.

Now, I must admit that my skies are very light polluted, thus I did not have very many opportunities to compare threshold objects in the two scopes, but it stands to reason that objects that are just faint smudges in the 10" may be invisible in the 8" scope. This may not matter too much if you live under severely light polluted skies since you'll be focusing on brighter objects that provide more impressive views in both telescopes.

Sharpness of Images: The 8" wins this one, interestingly enough. Now, this may be caused by differences in mirror samples, but I suspect that the focal ratio differences are key - F/5.9 for the 8" vs. F/4.7 for the 10". This difference manifests itself in two ways.

First, the 8" has more depth of focus or focal range (I think one of those is the right term). By that, I mean that each turn of the focuser knob changes the focus less in the 8" scope than in the 10" scope. This means that it is easier to fine tweak the focus to get really crisp stars without over or undershooting proper focus. I think this is because of the difference in focal ratios.

Second, is that the stars just appear more crisp, especially around the edges, in the 8". Again, this may be caused by the differences in the focal ratios, or perhaps the 8" has a better mirror, or maybe the 10" wasn't quite collimated as well as the 8" (which makes sense since the 8" is easier to collimate than the 10" scope because of its higher focal ratio.) Another cause for this may be coma, which I will discuss in more detail later.

This difference between the scopes is also subtle, but it was noticeable. This is almost surely the result of differences in focal ratios or collimation (the ease of which is dependent upon focal ratios.)

A Note About Coma: The reduction in coma in the 8 F/5.9 vs. the 10 F/4.7 could be the cause of the sharper images in the 8" scope. What is interesting about this is that the coma in the 10" was not very noticeable to me while using it and I actually had to look up the definition of coma and how it appears in the eyepiece before I really saw it. For the most part, it seems that my mind ignored the coma while using the 10" but I was aware of its absence when using the 8" - rather interesting.

Portability: The 8" definitely is more portable than the 10" since the tube is about 12 pounds lighter and is not as wide, so it is easier to get one's arms around. However, the 10" is probably not that hard to move for most people, but the back problems and stairs made it annoying for me; each person has to figure out their own limits when it comes to how much telescope they want to move, and don't overbuy since a huge scope that is not used is less useful than a small one that sees use most clear nights.

Other Differences: The 8" cools down quicker, though adding fans to the 10" can help. Of course, one could also put fans on the 8" scope, so it still has a lead in this area. Also, the 8" that I own has smoother azimuth movements since less weight is riding on the azimuth pads. Note that this can be adjusted and may vary from sample to sample, though it makes sense that the 8" would move easier in that direction than the 10" since it weighs less.

Which Scope is Better? Not surprisingly, there is no clear-cut answer here, but the comparison of the two scopes was very educational. To the new observer, the differences between the scopes that will be most noticeable are: increased object brightness on the 10" (especially star color, at least from what I saw) vs. the 8" scope being noticeably lighter weight and easier to move around. The other differences are, at least to me, subtler, and only revealed themselves because of my observing experience with both scopes in such a short amount of time.

So, which one do you get? If you can move around the 10" and don't mind working around the longer cool-downs and trickier collimation, go for it since extra aperture is always nice. On the other hand, if you want a scope that is less fussy - lighter weight, quicker cool-downs, and more forgiving collimation - get the 8" scope. Both scopes provide very similar images and both will provide you with an astounding number of objects to look at in the sky. Don't let aperture fever ruin your back, but if you can go big (and move it) do so!

The Other Choices: 6" and 12" Dobs: In most cases that I've seen, people who are undecided about which Dob to get are comparing the 8" and the 10" scopes, perhaps because they are rather similar and both provide great views without being too hard to move. However, for the sake of completeness, I felt I should mention the 6" and 12" scopes from this line of Dobs as well.

6" Dobs: The 6" scope is far closer in portability to the 8" scope than the 8" is to the 10" scope. Yes, they are all similar in size, but the 8" weighs only 5 pounds total more than the 6", while the 10" weighs 15 pounds more than the 8, and most of that (about 12 pounds) is in the tube.

Considering the generally insignificant weight and size differences and the big gain in going from 6" to 8" one could ask, why get the 6" Dob?

My answer to that is based upon cost. First, the 6" Dob costs less than the 8" and can still show nice views of the sky (far better than most introductory 60mm to 4.5" scopes). However, the cost savings comes into play a second time when one considers that the 6" scope operates at F/8. Suddenly, all the low-cost widefield 1.25" eyepieces are acceptable performers, which is probably not the case in the 8" F/5.9 Dob, and definitely not the case in the 10" F/4.7 Dob. In short, one can save money on the scope and on the eyepieces, making the 6" F/8 Dob a great scope for a person on a budget. Interestingly, that would also make it a good travel scope since one does not need to pack Naglers or other high cost eyepieces - same idea with using it at a public star party, where one might be concerned about damage to costly eyepieces.

So, the 6" Dob could be a great travel scope, star party scope, or introductory telescope for a person on a budget. However, do note that it does not come with a 2" focuser and in most cases the 6" Dobs have closed mirror cells, so they don't cool down any faster than an 8" Dob with an open mirror cell. Still, for the price, they are good buy.

12" Solid-tube Dobs: The 12" Dob is a monster, both in terms of physical size and light gathering power. I can think of no other way that an astronomer can get so much aperture for so little money. While a 12" solid-tube Dob is an excellent deal from the viewpoint of price and aperture, please, PLEASE make sure you can move it around before buying it. The tube alone weighs as much as an entirely assembled 10" Dob, and at 55 pounds and about 5-feet in length is a chore to move around. There are also going to be cool-down issues and, from what I hear, the bases on these Dobs often need some work because of the sheer weight of the optical tube.

Because of its size and need to properly cool down before being used, this type of scope seems ideally suited for folks who can just roll it into and out of a garage or storage shed. The key thing to realize is the size and weight of these Dobs - be sure to see an example scope in person and try to move it around before buying one. While they cannot be beaten for sheer amount of aperture at such a reasonable price, the cost to pay is in portability.

Anyway, I hope this comparison based on my experiences is of value to the folks considering buying one of the standard, solid tube Dobs! As always, keep in mind that good accessories can make the difference between loving astronomy and being frustrated by it. Make certain you have a comfortable and useful finderscope to starhop, even if it means upgrading to a right-angle finderscope and/or a unity finderscope like a Telrad or Rigel, get a good star atlas and planisphere to learn the sky, and be sure to observe from a comfortable chair with clothing that is appropriate to the weather, especially during the winter where cold can be a real danger.

Whatever telescope or accessories you buy, keep on looking up at the stars!

Article submitted by: Matthew Hannum
Username: ForgottenMObject
Date: 11/29/2005

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