Sunday, August 01, 2010

 

Uncle Rod’s Telescope Academy: Mounts

Helluva weekend a couple of weekends back. I don’t mean that in a good way, either. It was hot, humid, and cloudy. Much as I wished I was, I sure wasn’t away from it all, lounging around on some tropical south Pacific beach enjoying the Solar eclipse, Questar 3.5 at my side. I did hold out a little bit of hope for Saturday evening observing-wise; there were at least a few semi-blue squares here and there on the Clear Sky Clock. I even went so far as to lug our ETX 125, Charity Hope Valentine, and her tripod down to the front parlor ‘bout midday. I had no illusions of continuing The Herschel Project, but maybe I might catch a few cool deep summer sights with humble Charity?

Ha! By 6pm I’d received an email from one of my fellow PSAS members telling me it was already raining just to the west of the city, not far from our dark site. A quick consultation on the phone with good buddy Joe, who lives out thataway, led me to conclude “fuhgeddabout it.” I was a little put-out, sure, but admittedly not over-much. Poor Miss Dorothy was having a bad weekend, the dang chemotherapy has thrown her for a loop, and I decided it was best to just stick ‘round the ol’ Manse. I pulled out Season One of Battlestar Galactica (the new one, natch), broke out another bottle of the Yell, and settled in for the night.

With no observing expedition to tell y’all about, it’s a good time to start a new series I’ve been thinking about bringing you for a while. It came to me as in a dream that some of y’all reading this are prob’ly purty green. You’re wet behind the ears newbies or almost. Novices who don’t know much else other than that you want a look at the beautiful night sky. So, I’m a-guessin’ that at least a portion of the junk you read here is about as decipherable as a sermon in Sanskrit. I doubt explaining the technicalities of amateur astronomy will make my pitiful prose much clearer, but it can’t hurt, can it?

What I’m gonna do this morning, and intermittently for a while, is tackle the basics of our ever more complicated avocation. The short and the sweet of what you need to understand to get off the ground in amateur astronomy. First time out, we will not discuss telescopes. Telescopes are actually fairly easy. What usually hangs up newbies is what you put the scope on, that most confusing question in amateur astronomy, “what kind of mount?”

When you come right down to it, you have just three choices: fork mount, German equatorial mount (GEM), or alt-azimuth (altitude-azimuth, alt-az) mount. Dobsonians? They are, technically, alt-az, if a little different from traditional alt-azimuth designs. Anyhoo, let’s set those big, loveable telescopes aside for the moment. A Dob’s mounting is almost always an integral part of a telescope package, and we’ll deal with telescopes, including Dobsonians, another time. Oh, and before the curmudgeons out there start mumblin’ under their breaths and shakin' their canes at me, yes, there are choices other than fork-GEM-alt-az. But they are almost always homebuilt, or at least custom-made. For today, we’ll only consider what y’all novices can order on the web this afternoon.

Fork Mounts

Hokay. Forks. A fork mount is just what it sounds like, a big metal fork that holds the telescope between its tines. If you are using a modern, computerized fork, and the mount is working in alt-azimuth mode, the tube moves up and down (altitude) between the fork tines, and swivels (azimuth) on the fork mount’s base (“drivebase”), which is attached directly to the tripod head. If you have the fork set up in equatorial mode, either because you are taking pictures (long exposures require an equatorially aligned mount) or because you are using an older fork mount scope built before the telescope computer revolution, or just because you are some kinda masochist, you’ll have the fork and drivebase mounted to a wedge instead of directly to the tripod.

A wedge is, yep, a wedge-shaped hunka metal (usually) that tips the fork over at an angle so the fork’s tines point at Polaris (or, more properly, the North Celestial Pole). In equatorial mode, what was formerly the azimuth axis becomes the right ascension axis (east-west, R.A.), and the altitude movement is now declination (north-south). Sound complicated? It’s not. It just means the movements of the scope now follow the celestial lines of latitude (declination) and longitude (right ascension). What The telescope can track the stars with one simple movement at one speed on one axis, the R.A. axis.

“Why should we care, Uncle Rod, why?” Because of field rotation, younguns. The alt-az-mode fork must stair-step along, going up/down right/left as necessary to follow the stars, and changing speed depending on where it is in the sky. That’s a complicated bidness, which is why it didn’t become possible till telescopes gained computers. Sadly, computers or not, this stair-step tracking is useless for long exposures; the stars will trail no matter how well the alt-az computer drive works.

It’s easy to understand why. Go out on a late fall evening and take a gander at Orion rising in the east. He’s lying on his right side, ain’t he? Go back inside and watch a few episodes of Jersey Shore. When you reenter the darkness, ol’ Orion is standing upright as he crosses the Local Meridian (the imaginary line that cuts the sky into east and west halves), when he is due south. If you go back outside just before dawn, Orion’s in the west reclining on his left side. That is field rotation. An equatorial mount or a fork mount set up in EQ mode follows this rotation exactly, an alt-azimuth mount can’t.

Whatever, right? What are the strengths and weaknesses of the fork? Its strength is in its adaptability. Are you a visual observer? The computer-driven fork set up in alt-azimuth fashion is about as good as it gets for visual workers. With a Cassegrain type scope or a refractor, you can sit and look at anything in the sky. Think you might want to take pictures? Despite my dire warnings about field rotation, an alt-az mode fork is quite capable of taking short exposures; especially with that new-fangled Faststar/Hyperstar stuff (more on that soon). A Stellacam or a Mallincam deep sky video camera can go insanely deep with brief exposures, Hyperstar or no. And you really don’t need anything but alt-az mode for planetary work.

Want more, as in “3-hour exposure of NGC umptysquat”? You can use computer software to stack many short exposures into the equivalent of one long one, but the result will never be quite as good as you’ll get with longer “integrations.” That’s OK. A carefully guided fork set up in EQ mode is easily capable of tackling the deep sky despite the sniffing of GEM fans. You will need that wedge, of course. Unfortunately, Meade and Celestron, about the only purveyors of fork mounts, haven’t done very good vis-à-vis wedges of late.

To put it plainly, neither Meade nor Celestron makes a decent wedge. Even Celestron’s Heavy Duty wedge ain’t heavy duty enough. And their wedges’ altitude and azimuth adjusters aren’t good enough for the precise polar alignment that will be needed for long exposure imaging. Luckily, there are several purveyors of aftermarket wedges, like Mitty Industries, who can sell you a wedge that works right for not much more money than you’ll pay Meade or Celestron for their not-good-enough best.

That’s the good of forks, what’s the bad? The main bad about fork-mounts used to be getting them. For years and years, Meade and Celestron refused to sell their forks separate from their telescopes. Thankfully, that is finally changing. Celestron has just begun selling its single-arm NexStar forks without telescopes. The mounts feature full-go-to, computerized pointing that is, and are equipped with Vixen-style dovetail brackets, making it easy to adapt most telescopes to them. Given the Keep Up with the Jones competition between Meade and Celestron over the past three decades, I wouldn’t be surprised to see Meade follow suit “soon.”

And how about single-arm forks. Both Meade and Celestron sell ‘em. Ain’t they too shaky? Surprisingly, no. As long as scope tube length and weight is kept within reason for the heftiness or lack of it of the single-arm fork in question, these mounts seem just about as stable as dual tine jobs. Which shouldn’t be a surprise to you old timers. One of the better commercial fork mounts over the years was the single arm model OTI did for its “Questar killer” (well, they tried) Quantum Maksutovs.

To hear GEM fans tell it, the forks are guilty of two mortal sins: “hard to balance, and not as good for imaging as German mounts.” There is a certain amount of truth to the former. A GEM is easier to balance. That does not mean a fork can’t be balanced sufficiently, however. Sliding weights on the tube and small weights mounted to the fork arms with Velcro can do the job—more or less.

No good for imaging? Forks have proven themselves in that arena over the 40 years since they (mostly married to SCTs) became popular. Frankly, a fork may actually perform better than a comparably priced GEM. Its R.A. gear is usually considerably larger than the GEM’s. If properly polar aligned and on a sufficient wedge and balanced, a fork is most assuredly at least as good as a similar-grade German mount for taking pictures. Are some GEMs better at taking pictures than Celestron and Meade forks? Sure. A fork can compete well against an EQ-6, a CGE, or a G11, but GEM fanciers can throw money at the imaging problem.  They can move up into AstroPhysics-Takahashi-Bisque price and quality territory where Meade and Celestron have feared to tread with their fork mounts.

Finally, with the exception of the Celestron NexStar SE series of forks, you are out of luck if you want to use different telescopes with your fork mount. The NexStar SEs are attached to the mount with Vixen-type dovetails and are easily removed. That is not the case with most Celestron and all Meade models, though. Oh, they can be removed and replaced on their forks, but the process is neither easy nor recommended.

There’s also the issue of the fork’s basic design. What does a fork mount look like? A big tuning fork. What do tuning forks do? They vibrate. Setting a fork up for equatorial use exacerbates vibration problems. Take another look at a fork on a wedge. What you have is a mass, the telescope/fork/drivebase, tipped over and with its center of gravity at least somewhat offset from the tripod head. No matter how you slice it, a commercial Meade or Celestron fork is going to be shakier than a comparably built German mount. In equatorial mode. Few mounts are as naturally stable as a fork set up directly on the tripod head for alt-az use.

German Equatorial Mounts

What makes a German equatorial mount, a “GEM,” German? Usually only the nationality of the man credited with its invention, Joseph von Fraunhofer. To a beginner’s eyes, a GEM looks frighteningly complex. Look past the wires, computer drive motors, control panels and other modern frippery, though, and what you have is a very simple machine.

All German mounts, big and small, follow the same basic design, a “T”. You’ve got one axis, the upright of the T, that’s tilted on the tripod head to point at Polaris (or, better, the North Celestial Pole), and which forms the Right ascension axis. And you’ve got the T’s crossbar on the “sky” end of the upright, which forms the declination axis, the north-south axis. The telescope is mounted on one end of the declination crossbar, and a weight is on the other end to counter balance the scope (which is why that weight is usually referred to as a counterweight). When polar aligned, with the T upright pointed at the North Celestial Pole, the mount will track the stars with just one simple motion, rotation on its RA axis.

Why do some people (who often describe themselves as “advanced” amateurs, whatever the hell that means) like GEMs so much? Well, for one thing, until recently unless you wanted a Celestron or Meade SCT, a GEM was the only equatorial game in town. But that’s not the GEM’s only strength. If, like yours truly, you are approaching broken-down-hillbilly-hood, a GEM is much easier to lug around than a fork. I can still manage my 66-pound NexStar 11 GPS and its big fork, but I am not sure how many more years I’ll be able to do that. With GEMs, the sky is almost the limit. The fact that you can break a German mount into its components: tube, mount head, tripod, counterweights, makes a 12-inch or even a 14-inch SCT at least conceivable. Even for moi.

There’s the quality factor, too. A medium-range GEM, an Atlas, a G11, or a CGE, is somewhat better than a run-of-the-mill Meade or Celestron fork. But if that is still not good enough for you, you can kick it up a notch or a couple of notches. German mounts are readily available to handle any payload with any tracking accuracy any amateur will require. And will cost as much as even the more affluent among us will be happy to pay, with the top-of-the-line AstroPhysics 3600 mount, which is not the most expensive GEM out there, settin’ you back a cool $26,700 for the most advanced model—before you begin to buy options like a pier.

It's undeniably easier to properly balance a GEM for astrophotography: slide the scope back and forth in its cradle to balance in declination; move the counterweight up and down its shaft for Right ascension balance. Which is one of the major reasons a lot of astrophotographers consider GEMs better for imaging. Making the mount just a little east heavy, so it tracks its best, is trivial with a GEM, but challenging with a fork mount. The fact that the load (the scope tube) of a GEM is closer to being centered above the tripod head also helps the GEM deliver the best tracking its gears will allow—and means it’s less shaky than a comparably built equatorial mode fork.

So GEMs are perfect? What is? Depending on what you want to do, a GEM may not be the best choice. A German mount will take considerably longer to assemble than a fork mount, whose setup consists of “plunk tripod down, put scope on tripod.” There’s also no denying that for visual use a fork (in alt-az) is more comfortable than a German mount, which can place the eyepiece in some fairly awkward positions. Still, there’s little to criticize with German mounts. Except for the exclusively visual observer, a GEM is just a better solution, especially since just the right mount for scope and task and budget can be chosen by you, not by the telescope maker.

Alt-azimuth Mounts

For the longest time, the only non-driven, non-computerized alt-azimuth mounts you could buy were shipped with beginner scopes. Often the very worst sort of beginners’ scopes; the kind you find lurking right outside the optical department of the pea-pickin’ Wal-Mart after Thanksgiving. Which was a shame, since the plain, ol’ alt-azimuth up-down-right-left mount has some advantages. For a grab ‘n go scope, a telescope designed for at-a-moment’s-notice use, nothing beats the point and shoot simplicity of an alt-az. No alignment. No batteries, just, yeah, point and “shoot.”

Somebody finally listened; in fact, a bunch of people listened. Today, there are plenty of alt-azimuth-only telescope mountings available that can accommodate sizeable instruments. These mounts range from the high priced spread, represented by the beautiful mounts produced by Half-hitch, to the inexpensive but functional AZ series by Synta sold under the Skywatcher and Orion badges.

At the top of the line, you’ll get fine machining and the option for adding digital setting circles. At the lower end, in the sub-two-hundred buck category, you don’t get DSCs, but you do get a pleasing simplicity that in this old boy’s opinion maybe better suits the alt-az idea. As I found out when I was looking for a mount for my newly acquired Celestron Orange Tube C90. Sure, I coulda just slapped a Vixen-style dovetail bracket on the little girl and used her on my Celestron CG5 mount, but what fun would thata been? Reasonable as the CG5 is weight-wise, it’s still a handful. And needs at least a middlin’ polar alignment. And some kinda go-to alignment. That don’t spell p-o-r-t-a-b-l-e.

The answer turned out to be Synta’s AZ-4. I’ll give y’all the full rundown on the mount some Sunday, but I will say right here that it is a remarkable value, and so much better than the (expensive) photo tripod I was using with the C90 initially. Somebody at Synta realized you could take the Teflon bearing surface idea of the Dobsonian and make one hell of an alt-az for not much moola. It is an absolute joy to pan across the sky with the C90 – AZ-4 combo, and the mount even accommodates my ATM 6-inch Newt for low power scanning.

Which don’t mean an alt-azimuth mount is for ever’body. One is not very well-suited for longer focal length telescopes like MCTs and SCTs. Yeah, you can find an alt-az hefty enough for your C8 or maybe even your C11, but these long-focus scopes really need a drive. Smooth as alt-az mounts can be, nudging a sizeable f/10 or f/15 CAT along is not my idea of fun. The alt-azers don’t really strut their stuff until you put a fast refractor or a small CAT on ‘em—then they are heaven.

Actually, there is a driven alt-az mount available now: the Ioptron Cube and its descendents, the Minitower and Minitower Pro. Not only will they track objects, they offer full go-to, and the top of the line model can support surprisingly heavy payloads. Not a bad idea, but the Ioptron mounts have been plagued with QA problems from the beginning. If the company can get their act together, though, these “alt-az PLUS” mounts could be just the thing. Unk further notes that there is now another go-to alt-az, the “Eklipse,” being sold in the UK and Europe, so maybe this is an idea whose time has come.

So which should you choose? Only you can decide that, muchachos. But here are some general guidelines. If you are a purely visual observer who wants a Meade or Celestron SCT, you could do way worse than one of the companies’ fork mount SCT packages. If you are serious about imaging or think you will be soon, or want to use multiple telescopes with the same mount, or just want to accentuate the Quality Factor, consider a GEM - OTA (Optical Tube Assembly, “telescope”) combo you assemble yourself. Finally, if you, like me—at least a part of the time—consider a Short Tube 80 refractor a Big Gun, or just like “light and simple,” consider one of the 21st century alt-azimuth jobs.

So, that’s it. Not so bad, was it? Admittedly I didn’t talk about the infamous “go-to or no go-to” question. There’s plenty to be said about that, and I believe it deserves a blog article all its own. Which will happen eventually. The short and sweet of it, though, for me anyhow, is that I star-hopped for 35 years. Now that I have go-to, I rarely do. At this time in my life, I am more interested in seeing than hunting, which may or may not describe you. I know my intention is to see as much as I can in the years remaining to me.

Spurious Book Review: Double Stars, the Story of Caroline Herschel

Padma Venkatraman’s Double Stars is classified by most booksellers as a “juvenile,” I believe, but don’t let that fool you. This is a very readable bio—for anybody, from sprout to broken-down hillbilly—of the first professional woman astronomer. It actually has more insight into the characters of both William and Caroline Herschel than you’ll find in some more stodgy biographies of the pair. Despite being hampered, as all biographers of Caroline have been, by their subject’s propensity to burn letters and journal entries concerning any even slightly controversial incident or period in her life, Ms. Venkatraman does an excellent job of putting two and two together to clarify the reasons behind some of Caroline Herschel’s otherwise inexplicable behavior.

Anything I didn’t much like? Astronomy is not Padma’s field—she is an oceanographer—but she does an admirable job of handling most of the astronomical ideas and issues presented. Still, sometimes the astronomy does get a little muddled—but just a little. What else? Since the book is at least nominally aimed at the young set, direct quotes from the Herschels and their contemporaries have unfamiliar/antique words “translated” in parentheses, which may be annoying for some adults. Finally, whoever chose the illustrations for the book didn’t always choose well. Some of ‘em are just a little off the wall, including one, for example, that’s supposed to illustrate the design of the Gregorian Telescope. What’s there is a schematic of a modern, probably spaceborne, instrument.

Despite those few carps, Double Stars is an attractive and well-written telling of Caroline’s life. It maybe did more to help me understand the woman’s character than anything I’ve read, including Caroline’s own memoirs. Recommended.

Next Time: I did get out to our dark site in the wilds of Tanner-Williams, Alabama for a while, but the sky, which had looked OK at sunset, rapidly devolved into haze and downright clouds. That was OK. When I took a look at the forecast Saturday morning, it was pretty clear the Herschel Project would remain in its holding pattern. What I could do was drag my Atlas mount out and give him a spin.

I bought the Atlas (pictured above) nearly three years ago to help me make some astrophotos for the book I was writing a the time. The mount did admirably, but has mostly sat neglected since then. It's just so much easier to drag out the CG5. That was my perception, anyhow. What was the reality? Ah, muchachos, that is a tale for next Sunday...

Comments:
Great article for us Newbies!
I have the Mallincam bug and am saving up for the telescope on a gem mount and what will set me up later to use the mallincam.

It would be a year or so after I get the telescope and gem and advice regarding vital gear to help set that up would be interesting too.

Any advice and wisdom to help those like me to help point us first with any additional visual gear advice and help getting to the ccd equipped stage be great as well.
I'm sort of a see it with my own eyes and want to image with the mallincam user.

What appeals to me about going the mallincam route is that you can share the viewing experience on the internet as well as a outreach showing folk on a monitor what we can observe.

I just realize to get there it will be a good investment of money.

I enjoy reading your current book choosing and using a new cat. And following the yahoo group as well.
My biggest struggle is do i go all out for the aperture or be more conservative and upgrade as I learn. The price difference currently form a 8 inch to a 11 is sorta small.
 
Hi Chris:

Getting started with a Mallincam is fairly simple. A good mount is always a good thing, but video astronomy is a little more forgiving on that score than long exposure imaging. A nice combo I use for video is a C8 + f/3.3 reducer + Celestron CG5 mount.
 
would also be a interesting blog article to give us your perspective on what is "necessary" to do video astronomy from what is the icing?
For all us newbies.

For my example Id first need to get the cat and mount and then use it a while before I could afford the minimal equipment necessary to operate for video astronomy.

I see alot of mallincam users on nightskiesnetwork and a few stellacams. Rock Mallin is on there a lot and he uses the mallincam mainly as a optical aid.

However I was wondering with the faster focal lengths can you with the right equipment get and stack images like the guys that do the longer exposures?

I would assume also that video cammeras like these need a gem or fork mount rather than say what the Nexstars and Meade's lightswitch 8"?

I am working with a 10" meade dob but finding anything presently has been a royal PITA.
 
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