Hack10.Equip Yourself for Urban Observing

Hack 10. Equip Yourself for Urban Observing

Choose the perfect grab-'n-go scope for observing at a moment's notice.

You can, of course, use your primary (or only) scope for urban observing if you are limited to doing so by budget or inclination. But to take advantage of the spur-of-the-moment aspect of urban observing, many serious amateur astronomers keep a special telescope dedicated to that purpose. It's called a grab-'n-go scope or quick-look scope, which describe its purpose perfectly. A grab-'n-go scope is always set up and always ready to use on a moment's notice. You can carry it out the door and start observing immediately. Figure 1-13 shows our grab-'n-go scope, a 90mm long-tube refractor that we leave set up and ready in our library. (We caught it with its pants down; ordinarily, this scope has a Telrad unit-power finder attached.)

Figure 1-13. A grab-'n-go scope, set up and ready to go

Here are some considerations for choosing the best grab-'n-go scope for urban observing:


Portability is critical. The ideal grab-'n-go scope should be light enough to pick up with one hand and carry out the door, and should be small enough to be stored unobtrusively in any handy corner or closet. A light, portable scope will be used often. A heavier, less portable scope will sit in the corner gathering dust. The differences can be substantial. For example, our 90mm refractor came with an equatorial mount and weighed more than 30 pounds. We replaced the equatorial mount with an alt-azimuth mount, reducing the total weight of scope and mount to about 10 pounds. That difference may sound minor, but it's not, particularly after a long day at work.

Fast set up and tear down

Urban observing is often done on the spur of the moment. You take the dogs on their evening walk and notice that the sky is particularly clear that night, or driving home from a party, you notice that Mars is ideally placed for a quick look. If your grab-'n-go scope is set up and ready, all you need to do is carry it out the door, set it down, and start observing. Conversely, if you know it'll take 10 or 15 minutes to set up the scope, polar align it, align the finder, and so on, chances are you'll let that observing opportunity go by. The same holds true, in spades, for tear down time. When you've stayed up past your bedtime watching the dance of Jupiter's moons, the last thing you want to face is another 10 or 15 minutes of tearing the scope down and packing it away.

Fast cool down

When you carry a telescope from a warm house to a cool backyard (or vice versa), the optics need time to equilibrate to the outside temperature (for some reason, astronomers invariably call this process "cool down," even when it's warmer outside than indoors). A scope can't produce its best images until it is fully cooled. Until that happens, the scope produces mushy images. An uncooled scope may produce acceptable images of dim DSOs (Deep-Sky Objects), but for bright, detailed objectslike Luna, the planets, and multiple starsa properly cooled scope is essential. Because most urban observing targets fall into the latter category, fast cool down is important for a grab-'n-go scope.

Different scopes have different cool-down characteristics. Large scopes generally take longer to cool down than smaller scopes. Refractors cool down faster than reflectors, reflectors cool down faster than SCTs (Schmidt-Cassegrain Telescopes), and SCTs cool down faster than MCTs (Maksutov-Cassegrain Telescopes). Depending on the temperature differential between its storage place and the outdoor air, a small or mid-size refractor may be ready for use immediately, or at most in a few minutes. Larger scopes may take literally hours to cool down sufficiently to provide sharp, high-contrast images of Lunar and planetary detail.

Sufficient aperture

Small apertures contribute to portability, but it's easy to go overboard and end up with a scope that's too small. Many astronomers choose 70mm or even 60mm scopes for urban observing, but we think that's a mistake. Although even a 60mm scope gathers sufficient light for Lunar and planetary observing, light gathering ability is not the only consideration. Aperture also determines maximum usable magnification and the ability to resolve fine detail.

The general rule of thumb is that small, high-quality scopes can use up to 50X to 60X per inch of aperture. Some observers push this as high as 100X per inch, but that requires world-class optics and results in an exit pupil so small that diffraction effects, "floaters" in your eyes, and other issues reduce the amount of detail you can see. Observing Lunar and planetary detail requires using magnifications in at least the 150X to 225X range, so any scope you plan to use for that purpose should support that amount of magnification. A 90mm to 100mm scope spans that range nicely, while a smaller scope may be pushed to and beyond its limits to reach even the lower end of that range.

Aperture also determines the finest detail that can be resolved, with larger scopes able to resolve finer detail than smaller scopes. Small scopes, such as 60mm models, simply cannot resolve sufficiently fine detail to satisfy most serious observers. A 90mm scope resolves 50% finer detail than a 60mm model, and a 100mm scope 67% finer detail.

On that basis, you might think that an even larger scope would allow you to see more detail. That's true in theory, and sometimes in practice. However, there is another consideration. Even an optically perfect large scope is limited in the amount of detail it can resolve by how turbulent the atmosphere is, a characteristic that astronomers call seeing. On a perfectly steady night with superb seeing, which in most locations is extremely rare, even large scopes may be aperture limited in terms of their ability to resolve fine detail. On more typical nights, any scope larger than 6" to 8" may be seeing-limited. On very turbulent nights, which are common in the city, scopes larger than 90mm or so are often seeing-limited.

Low price

Depending on your means, this may not be an issue. Some observers use $3,000 apo refractors as their grab-'n-go scopes. But for most of us, price is definitely an issue. Fortunately, there are many suitable candidates for a good grab-'n-go scope that sell for $150 to $500. At that price, many observers will find it worthwhile to dedicate a scope to urban observing.

1.11.1. Excellent Choices

Here are some scopes we think are excellent choices for an inexpensive grab-'n-go scope:

90mm to 100mm long-tube refractor on an alt-az mount

We think a 90mm to 100mm (3.5" to 4") long-tube refractor on an altaz mount is the ideal grab-'n-go scope for most urban observers. The aperture is sufficient for viewing Luna, the major planets, and most other objects suitable for urban observing, including the brightest DSOs. A good quality 90mm or 100mm scope can be pushed as high as 200X to 250X, which suffices for viewing Lunar and planetary detail.

Our favorite scope in this category is the Guan Sheng 90mm f/11 refractor, which is the model we use. Orion used to resell this model as theirSkyView Deluxe 90mm Refractor, but they have since stopped reselling Guan Sheng models and shifted entirely to the less expensive and lower-quality Synta models. The Guan Sheng 90mm refractor is still available from smaller astronomy retailers, usually under their house brand names.

The newer models provide a 2" focuser instead of the 1.25" focuser supplied with our older model. Unfortunately, the Guan Sheng 90mm optical tube is usually bundled with the Guan Sheng EQ (equatorial) mount. It's a decent EQ mount, but for a grab-'n-go scope we much prefer a lighter alt-azimuth mount, such as the Synta AZ-3 sold by Orion. If you can find a Guan Sheng 90mm optical tube for sale by itself, buy it. Otherwise, buy the scope with EQ mount, replace the EQ mount with an alt-azimuth mount like the AZ-3, and resell the EQ mount on Astromart (http://www.astromart.com). Or, do what we did and resell the EQ mount to a member of your astronomy club. The mount itself is quite good for a small, inexpensive EQ mount. The problem is that it's too heavy for a grab-'n-go scope.

It's often claimed that a smaller scope is better than a larger scope for urban observing. That's a myth. More aperture always shows you more, even under light-polluted skies. However, it is true that a smaller aperture is less subject to bad seeing conditions, which often predominate in urban environments.

6" Dobsonian

If we couldn't have our 90mm long-tube refractor, our next choice for a grab-'n-go scope would be a good 6" Dobsonian reflector. It's not quite as light or portable as the 90mm refractor, but neither is it heavy or clumsy. One of our regular correspondents lived in a New York City apartment building until recently. He routinely carried his 6" Dob up several flights of stairs to observe on the roof of the building. A 6" Dob is generally f/8, which allows it to provide good images with inexpensive eyepieces. The f/8 focal ratio also allows 6" Dobs to use a small secondary mirror, which contributes to higher contrast and good Lunar/planetary performance.

There are many good inexpensive 6" Dobs available, including models from Orion and Guan Sheng (under several brand names, as usual). Our favorite of this group, albeit the most expensive model, is the Orion 6" IntelliScope. The optics and mechanicals of the IntelliScope aren't quite as good as those of the Guan Sheng Dobs, but the IntelliScope has the inestimable advantage of including digital setting circles (DSCs), which Orion refers to as their Object Locator System. Under light-polluted urban observing conditions, DSCs can allow you to find your target object in seconds rather than spending frustrating minutes looking for it manually.

Depending on circumstances, we might also consider an 8" Dobsonian, which is slightly heavier than a 6" model, but not much larger. The tube of an 8" Dob is of course 2" larger in diameter than that of a 6" Dob, but they are generally the same length because most 8" Dobs have an f/6 focal ratio (a 6" f/8 scope has the same focal length as an 8" f/6 scope). And, while a 6" aperture is a bit small if it is to be your only scope, an 8" scope can serve both as a grab-'n-go scope and a general-purpose scope.

If we could afford only one scope, we might well settle for an 8" Dobsonian. If we could afford multiple scopes, we'd buy a dedicated grab-'n-go scope for urban observing and a 10" or larger scope as our primary instrument.

Orion StarBlast 4.5" Dobsonian

Although we call the StarBlast a Dobsonian, it isn't, really. The StarBlast tube is secured using a Dob-like bearing to a single vertical arm that connects to a baseplate. The baseplate rides on a ground board, again using a Dob-like bearing. So, although the StarBlast is not technically a Dobsonian, it operates much like one, with all of the advantages of stability and smooth motion that implies. Orion markets the StarBlast as a kid's scope, but the truth is that it's a capable observing instrument in its own right. Many serious amateurs keep a StarBlast as a secondary scope, even though their primary scopes are expensive SCTs or giant Dobs.

The StarBlast has some real advantages for use as a grab-'n-go scope, not least its $169 price tag. At 4.5" (114mm), the parabolic StarBlast mirror delivers some serious aperture for a grab-'n-go scope. At only 18" tall and 13 pounds, size and weight are not an issue.

The 450mm focal length of the StarBlast is a two-edged sword. The short focal length allows very wide fields of view, even though the StarBlast has only a 1.25" focuser, but it also makes it difficult to reach higher powers. For example, using a 24mm Tele Vue Panoptic eyepiece (which costs nearly twice as much as the scope itself), the StarBlast provides a 3.6°+ true field of view. But to reach 150X magnification requires a 3mm eyepiece or eyepiece/Barlow combination, and 225X requires the equivalent of a 2mm eyepiece.

The f/4 focal ratio means that a 28mm eyepiece, which provides a 7mm exit pupil [Hack #8], is about the longest useful focal length, even for young observers. Older folks will probably find 20mm to 24mm to be the longest useful eyepiece focal length. The f/4 focal ratio also means the StarBlast is hard on eyepieces. Even premium eyepieces are being pushed at f/4. Inexpensive eyepiecesincluding the Explorer II Kellners supplied with the StarBlasthave very soft edge performance.

Finally, StarBlast quality control seems variable. Some StarBlast scopes are excellent, easily reaching 150X or higher with good image quality. Others are mediocre, turning in mushy performance even below 150X. It seems to be the luck of the draw which type you'll receive. Fortunately, Orion has very good return policies. Although it's inconvenient to do so, you can simply keep sending back unacceptable StarBlasts until you get one you're happy with.

1.11.2. Bad Choices

There are also some scopes that we think are poor choices for urban observing, although they are popular for that purpose.

Inexpensive, small go-to scopes

The idea is attractive. Build a computer and some drive motors into a small scope and let the computer find your objects for you automatically. No matter how bright your skies and how hard it is to locate objects manually, you can simply zoom to your target automagically. Or so the marketing literature says.

In practice, it's different. Although go-to scopes work better nowadays than did models from a few years ago, they're still a very poor choice. The fundamental problem with inexpensive go-to scopes is that too much of the price is going to the computer and drives and too little to the optics and mount. If you buy one of these scopes, you end up with an instrument that can put you on target quickly, but won't let you see much when you get there. The optics are too small (and usually of mediocre quality) and the mount is flimsy and shaky. Not to worry, though. The electronics and drives are cheap and fragile, so they'll probably fail soon anyway.

If you really want a go-to scope, be prepared to spend at least $1,800 for what we would consider a minimally acceptable model, which is to say an 8" SCT on a mainstream go-to mount.

Small refractors

As we said earlier in this section, refractors much smaller than 90mm are generally poorly suited as grab-'n-go scopes because of their optical limitations. There are exceptions, of course, but not among inexpensive scopes. If your budget allows you to pay $500 to $2,000 or more for a scope and mount, you may be happy with one of the 80mm StellarVue models, the Tele Vue TV-76, or even the Tele Vue 60. But unless you're on a champagne budget, give small refractors a miss.

Large refractors

If a 90mm or 100mm long-tube refractor is good, then a 120mm or 150mm long-tube refractor must be better, right? Wrong. The problem with larger refractors is weight and portability, both of the scope itself and its mount. If you get far beyond 100mm aperture, you'll find that refractor tubes become long, heavy, and difficult to manage, and their mounts become much too heavy and awkward for grab-'n-go use.

Also, the problem of false color increases with increasing aperture. For example, our 90mm f/11 achromatic refractor produces very little false color, even on very bright objects. To get the same low level of false color with a 150mm (6") achromatic refractor, it would have to be about f/18, which would require an optical tube nine feet long. No one makes such a refractor, of course. Instead, large refractors usually have short focal ratios to allow reasonable tube lengths of a meter or so. A typical 120mm refractor, for example, might have a focal ratio of f/8.3, and a 150mm refractor f/6.7 or thereabouts. The false color from such instruments is intense.

Finally, a longer, heavier optical tube requires a larger, heavier mount for stability. A stable tripod and alt-azimuth mounting head for a 90mm or 100mm refractor is light enough that the scope and mount can be picked up with one hand. Mounts for larger refractors are correspondingly larger and heavier. Even Mills Darden, the 7'6", 1,000 pound lumberjack who was the real-life prototype for Paul Bunyan, would have had a hard time carrying a properly mounted 6" refractor around with one hand.

Short-tube refractors

Short-tube 80mm and 90mm refractors are immensely popular, although we've never understood why. Although they are compact and lightweight, they have at least two severe drawbacks as a grab-'n-go scope for urban observing. First, as short focal-ratio achromats, they produce hideously bad false color on bright objects, including Luna and the planets. Since Luna and the planets are the most common targets of urban observers, we'd rule out short-tube refractors on that basis alone. Second, with the exception of premium models, which are quite expensive, short-tube refractors generally have poor quality objective lenses. Most produce mushy images at anything much over 75X, and none of the inexpensive Chinese models we've seen will support the 200X+ needed for observing Lunar and planetary detail with anything like reasonable image quality.

SCTs and MCTs

Schmidt-Cassegrain Telescopes (SCTs) and Maksutov-Cassegrain Telescopes (MCTs) in the 127mm (5") range are popular with some urban observers, but we think they're less than ideal for that purpose. The main problem is cool-down time, which for an MCT may be an hour or more. The only time we'd choose an SCT or MCT for a grab-'n-go scope is if we could store it in an unheated garage or outbuilding where it would always be acclimated to the ambient outdoor temperature.

In short, there are a few good choicesand many bad choicesfor a grab-'n-go scope. Choose carefully according to your budget and needs, and we think you'll be happily surprised with just how much a good grab-'n-go scope will let you see from your own backyard.

    Astronomy Hacks
    Astronomy Hacks: Tips and Tools for Observing the Night Sky
    ISBN: 0596100604
    EAN: 2147483647
    Year: 2005
    Pages: 112
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