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  1. Please bear with me. This is my first substantive SGL post. This contrived dialogue (with yours truly as the interviewee) is intended to illustrate the benefits of Synta/Orion's Intelliscope magnetic encoder DSC system available on some of their Dobsonian telescopes. I also realize that some of the telescopes mentioned in the piece may be available under different brands in the UK. Prices, too, are in the incorrect currency. Nonetheless I hope this is useful to the members. If the format is inappropriate or the content off base, I apologize. If folks find it interesting, I'm delighted. Without further adieu... To Intelliscope or Not to Intelliscope? That is the question. It seems as if weekly I find myself recommending a 6" to 10" Dob to a beginner asking about a first scope or first *real* scope. I thought it might be helpful to collect my views in one place, and encourage others to critique my views and to offer alternative suggestions. Question: Jim, in your opinion what is the absolute best first telescope design for someone just starting out, and why? Answer: I'm glad you asked. I believe the solid tube Dobsonian is the absolute best design for a first *real* telescope. Here's why: 1. They are affordable: Solid tube Dobs offer the best aperture-per dollar ratio in the hobby. The primary purpose of a telescope is as a light collector. Your pupil at full dilation is around 7mm aperture, if you're lucky. The telescope supplants your eye's maximum aperture with its own aperture. The larger the aperture, the bigger your scope makes your eye and better the scope satisfies its primary function. Compared to other designs, solid tube Dobs deliver the most aperture per dollar spent. For example, let's compare three decent entry level telescope systems, the Zhumell Z8 Deluxe, Celestron Nexstar 8SE and Celstron Omni XLT 102. The Zhumell Z8 Deluxe is a well-equipped 8" f/6 Dob without digital setting circles or motorized tracking for $380. The Celestron Nexstar 8SE is an 8" f/10 Schmidt-Cassegrain on a motorized tracking mount with computerized object location (GOTO) for $1200. The Omni XLT 102 is a 4" f/9.8 achromatic refractor on a non-motorized equatorial mount for $480. Each of the 8-inchers have 4x the light gathering of the 4-incher. (Remember, light grasp is based on area of a circle, not diameter. While an 8" circle has 2x the diameter of a 4" circle, it has 4x the area). In aperture per dollar, these three scopes stack up as follows: Z8: $1.90 per millimeter of aperture ($1.90/mm) 8SE: $6.00 per millimeter of aperture ($6.00/mm) XLT 102: $4.70 per millimeter ($4.70/mm) 2. They cool quickly: Solid tube dobs cool quickly for their aperture. When you deploy a telescope from a warm house or automobile to a cold outside environment, the telescope's optics begin cooling. While mirrors and lenses cool, they change shape. Also, while optics are radiating stored heat, they contribute to air currents in and around the telescope's tube assembly. To operate at their best, telescope optics need to be precisely figure. While optics cool they are changing shape. Accordingly, and optic in the process of cooling is not operating at its intended figure. Until the optic is cooled, image quality suffers. You often hear that refractors cool quickly. In absolute terms this is generally true, though some refractor designs cool more quickly than others. What is often left out of this statement is the notion of aperture. Refractors larger than 5" to 6" in aperture are exceedingly rare. Dobsonians and Catadioptrics (like Schmidt-Cassegrains) under 6" are likewise rare. While it is true that a 4" refractor cools more quickly than a 6" Dob or Schmidt-Cassegrain, a 6" refractor doesn't cool as quickly as a 4" refractor. The larger the mass of glass that needs to cool, the longer it takes. Per inch of aperture, Dobsonians cool relatively quickly. They cool a little more slowly than like apertured refractors, and much more quickly than like apertured catadioptrics. But why? Think about the schmidt-cassgrain design from a thermal perspective. You have a thick, heavy glass mirror, nested in a thick aluminum casting, housed in a thick aluminum tube, capped with a glass window, with a long baffle tube stuck through its center into the tube. Think about how heat stored in that mirror must exit the tube? It's a slow, painful process. Look then at the 8" Dob. The thick, heavy glass primary is open to the air in pack and in front. The Dob cools quickly per inch of aperture meaning you'll be observing with the optics at the intended figure more quickly than the main catadioptric alternative, the Schmidt-Cassegrain. Question: Okay, I get the dollar-per-inch and cooling-rate-per-inch aspects, but you've specified "Solid tube Dob" several times. Why? What's wrong with those cool strut, truss and collapsing pole Dobs I've seen? Answer: Yes, I specifically recommend solid tube Dobs for a variety of reasons. In addition to cooling, proper alignment of optical components is crucial. A system where the optical components are correctly aligned is said to be "collimated". Any telescope that is not collimated will produce images of lower quality than it's best potential. Unlike a solid tube Dob where the secondary mirror assembly remains ruggedly fixed relative to the primary mirror cell, truss and strut dobs are less likely to maintain the relationship between the secondary mirror and the primary mirror cell. That is, with a trusser or strutter, you will likely need to adjust both secondary and primary tilt each session (rather than merely adjusting primary tilt with a solid tube Dob) in order for the optics to be collimated and perform to their potential. Solid tube Dobs also tend to be cheaper at a given aperture than trussers or strutters. For folks starting out, cheap is good I think. Question: So earlier you seemed to be saying an 8" f/6 Dob was a really good deal. What's wrong with a bigger Dob like a 10" f/4.7 or 12" f/4.9? Also, what's wrong with a smaller Dob like a 6" f/8? Answer: Actually, there's nothing wrong with any of the Dobs you mention. Choosing the right aperture and focal length for a given observer, however, requires a little knowledge about properties of different apertures and focal ratios, as well as a little knowledge about your primary observing site conditions and the kinds of targets you think you'll enjoy observing most often. Here's how I would break down these considerations: 1. Faster Dobs have more aberrations: The smaller the f-ratio number, the faster the Dob. Faster Dobs have more steeply curved parabolic primary mirrors. This results in more off-axis coma (that is, stars at the edge of the field of view don't look like points but instead look like little comets). This also results in off-axis astigmatism (that is, stars that have non-point shapes and/or shift in shape on each side of focus). Coma is the same at a given f-ratio no matter what eyepieces are used. Astigmatism is usually greater in simpler, less expensive eyepieces. More costly, complex eyepieces are more costly and complex because of the materials used (different glasses have different light bending properties) and the way those materials are shaped (crazy, difficult to figure curves help correct for edge of field astigmatism). By combining expensive glasses in larger numbers and figuring those elements in complex ways, expensive eyepieces reduce astigmatism visible when used in a fast scope. To get the most out of a faster Dob, you may need to buy a device for correcting coma, more costly well-corrected eyepieces, or both, to get the most the scope is capable of delivering. 2. Larger aperture Dobs are generally heavier: While the length of a solid tube Dob's tube is generally determined by the scope's focal length, larger diameter mirrors are heavier, require larger diameter, heavier tubes and a larger dimension, heavier base. An 6" f/8, 8" f/6 and 10" f/4.7 Dob each have a 1200mm focal length, so the tubes are the same length. The 8" is heavier than the 6" and the 10" is heavier than both. Before choosing a Dob, you need to figure out how long and how heavy are too long and/or too heavy for you. 3. Slower Dobs have a bigger sweet spot: This point is tangentially related to the first point above about aberrations. Dobs with larger f-ratios have a much larger well-corrected field than Dobs with smaller f-ratios. F/8 and f/6 Dobs have much less coma than f/5 and faster Dobs. An f/8 or f/6 Dob will also produce more acceptable images when slightly out of collimation than will a faster Dob. For example, a 1/64th turn of a collimation bolt on a perfectly collimated f/8 Dob will take it slightly out of collimation and slightly degrade image quality. That same 1/64th turn of the bolt on an f/4.7 Dob will take the Dob significantly out of collimation and be more damaging to image quality than the same tweak would on the slower Dob. 4. Optimal lunar, planetary and double star performance requires a balance of aperture and precision: Targets like the moon, bright planets and double stars have fine details that require a combination of good optics, perfect collimation, perfect cooling and aperture to reveal. Aperture delivers resolution. That is the larger the aperture, the smaller detail a scope can POTENTIALLY show, all else being equal. However, a large aperture scope that is miscollimated, not cooled, or both, won't be able to deliver the level of resolution promised by its aperture. As discussed earlier, faster scopes are more demanding of precise collimation and take longer to cool than smaller, slower scopes. While a 10" f/4.7 is certainly capable of showing more fine detail on Jupiter, for example, due to its aperture-provided greater resolving power, in practice, unless that 10" is cooled and perfectly collimated, the faster cooling, more forgiving 8-incher might actually show more detail on the same target. Each observer must decide for himself or herself what the right balance of aperture, focal ratio and light grasp is for his or her observing. 5. Deep sky observing requires aperture above all else: The factors that might make a bigger scope less desirable than a slightly smaller one (at least for folks just starting out), are pretty much irrelevant when your observing menu consists of a large percentage of deep sky objects like galaxies, nebulae, globular clusters and the like. While all targets will look better if the scope is cooled completely and collimated perfectly, on these classes of targets, capturing more of their faint, distribute light is much more important than being able to resolve fine detail. If you fancy yourself budding galaxy hunter, go big or go home. Question: Okay, so let me summarize what you've told me so far. Dob over other designs due to aperture-dollar and cooling-rate-per-aperture. Solid tube Dobs over other Dobs due to more rugged alignment of optics and lower price. Understand the benefits and liabilities of f-ratio and aperture, and also have an idea as to what aperture and f-ratio is most in tune with (i) how strong I am, (ii) where I live and (iii) the stuff I think I want to observe. Got it. Now the $20,000 question; which Solid tube Dobs are best for beginners? You seemed to be really positive about the Zhumell Z8 Deluxe... Answer: Hold it right there! Actually I put the Zhumell Deluxe Dobs in a distant "second place" to the solid tube Dobs I recommend. Let me explain. While there are many other brands of Dobs, most affordable solid tube Dobs these days come from one of two sources in Asia. A company called GSO in Tawian or another company called Synta with facilities in both Taiwan and mainland China. Zhumell Dobs (and Meade Light Bridge truss Dobs) use optics and components from GSO. Orion (solid tube and presumably truss Dobs) use components from Synta. Quality of Synta vs. GSO optics is a wash today. Both are very good, especially for the price. Think about this: you can get an entire 12" Dob with Synta or GSO optics, eyepieces, finders, focuser, etc., for less than $1000. A premium custom made 12" f/5 mirror alone would cost ~$1,700 from someone like Zambuto Optics. I recommend Orion Skyquest XT Intelliscope Dobs. There I said it. I'm showing my Orion colors. But before you brand me a "shill" let me explain why... In a word, "Intelliscope". You're probably wondering what is Intelliscope? Intelliscope is an electronic (or more precisely, magneto-electronic) system that lets the scope know where its pointed in the sky, and guides you to objects you want to view. Specifically, there are magnetic encoders installed on both the azimuth (left-right motion) and altitude (up-down motion) axes that are linked to a small hand computer. When you turn on the Intellsicope system, you point the tube straight up, hit enter, move the scope to the first alignment star, hit enter, move the scope to the second alignment star, hit enter, and then you have a tool that is able to coach you to move your scope to point at any object capable of being seen in the scope at the time. There are many such systems available aftermarket using optical encoders and hand computers, but they cost between $600 and $900 depending on excoders, mounting hardware and brand of hand computer, plus require some DIY skills to install. So you could add a $700 Sky Commander digital setting circle (aka "DSC") system to your Orion XT Classic or Zhumell Deluxe Dob, but would you? I mean, that's almost 2x the cost of the scope! Enter the Intelliscope advantage. The Intelliscope system, when purchased with the Orion scope (it's not available separately, in fact) adds about $200 to the price. That is, an Orion 8" XT Classic is about $200 less than an Orion 8" XT Intelliscope Dob. Given its reliability, affordability, utility and simplicity, I rank the Intelliscope system among the absolute best, "must have" deals in amateur astronomy. (interrupting) Question: Wait a minute. Why do I need Intelliscope at all? My friend Bill who has been an amateur astronomer for over six years and invited me to a star party at my local club says that you're a wimp unless you star hop. There are all kinds of things that help you find stuff using a Dob without DSCs. I could use a Telrad or a scope-mounted green laser pointer, or the finder scope that comes with the Dob to find stuff, and I don't want other astronomers to think that I'm a wimp. Besides, an extra $200 is a lot of money when you're talking about $400 item. Why is Intelliscope better than those other object finding tools? Answer: Let me break it down for you. First, I hate to be a hero killer, but odds are your pal Bill is a complete !@&(*%. Sorry, but it's true. Second, there's absolutely nothing wrong with buying and adding a Telrad, using the finder scope that came with the scope, or adding a green laser pointer to help show where the scope is pointed; it's all good, really. But ultimately if you have a few hours out of a busy week to spend observing through your telescope, wouldn't it be preferable to observe say a dozen targets in a two hour session rather than four or five? (interrupting again) Question: But Bill seemed to be able to find objects pretty quickly. He showed me M42 in Orion, M45 in Taurus and M41 in Canis Major in short order. It probably didn't take him more than a few minutes to get each centered in the eyepiece for me. That seems plenty easy to me. Answer: You are aware, of course, that all three of those targets are naked eye visible? In fact, the Pleiades are so bright as DSOs go, I can even feel discreet heat coming from their stars when I observe. Next time you're out with 'ol Star Hoppalong Bill, ask him to show you NGC 2261, NGC 2403, Neptune and Hind's Crimson Star. Bring a book and a red flashlight though. You'll want to kill some time while Bill finds them on his chart, and then grumbles and curses as he hunts them down in the sky. Better yet, have Bill bring his scope to your suburban back yard with magnitude 4 skies at zenith instead of you going to the club's rural observing site, and ask him to find the same targets. I suspect you'll learn very quickly that there was a reason the astronomers of old developed analog setting circles and a celestial coordinate system. Really, they weren't wimps, no matter what !@&(*%...er...Bill says. So, where was I. Yes, it's all about seeing stuff. The easier you make it, within your budget, to see stuff, whether in choosing a scope aperture, an observing site, eyepieces or finding aids, the better you've made your observing life. Star hopping is a fine way to learn the sky, and I'm not against it by any means. In fact, for objects I know well when I use a Dob I usually star hop to them. However, it is inevitable that at some point you'll want to observe something that is not bright and easy to find, or to observe at a site where it's a challenge to even find stars for star hopping. The moment that situation arises, you'll either praise you wisdom for being smart enough to invest the extra $200 in an Intelliscope system or rue the day Bill convinced you that you should spend the $200 on the used Nagler eyepiece he sold you instead. Best of all, the Intelliscope system is invisible. You want to star hop? Don't use it. Heck, don't even turn it on. But when you need it, use it. It really is primarily about seeing stuff and not how you find the stuff to look at. There's a reason most folks drive cars from the east coast to the west coast rather than ox-drawn Conestoga wagons. The folks that opt for cars aren't wimps. Rather the folks that opt for Conestogas are, well, at a minimum a bit off, less generously "odd", and truthfully, nuts. Question: My head hurts. I'm feeling buried with information. Just tell me, all things considered, the BEST solid tube Dob for a beginner, if you had to recommend just one. Answer: But that would be telling wouldn't it? I mean, if I just told you the answer, right here, in black and white, you wouldn't be doing the thinking; I would. Question: Aren't you being a bit hypocritical? Answer: What do you mean? Question <smiling>: Well it's all about looking at stuff, right? Answer: Well, yes... Question: And you advocate for use of DSCs over star hopping in many cases, because it will help me see more in the limited time I have, right? Answer:...yes.... Question: So if you make it easier for me, and just tell me which scope to get, won't I be out observing and seeing stuff sooner than if I sit here and wade through your preening pontifications? Answer: (Ouch!) Well...er...yes. The best first *real* scope for a beginner is the Orion Skyquest XT Intelliscope 8" f/6 Dob.
  2. Hello. My name is Jim Barnett. I am an avid amateur astronomer living in Sonoma County, California. I've contributed to a number of other astronomy-related forums, including publishing several equipment reviews. I've noticed that SGL forum posts seem to come up often when I search for astronomical information on the web, so I have decided to become a community member and participate. One thing that I would like to see is a "Reviews" section for publishing illustrated gear reviews and astronomy-related articles. Perhaps the SGL blog feature would be appropriate for publishing such material? In any case, I look forward getting to know other community members and hope to encounter some new perspectives on the hobby. Regards, Jim
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