Sunday, February 20, 2011

 

Getting Set II


Where were we? We’d at least got started on that steep-looking stairway to heaven that is amateur astronomy. We’d got you a telescope, probably an 8-inch Dobsonian telescope. We tried to keep our spending down to 500 big ones in order to leave sufficient money, at least a couple of C-notes, for accessories. By “accessories,” I mean vital accessories, things you cannot live without. Two-hundred will get you going there, but you must choose wisely.

Eyepieces

Numero Uno when it comes to necessary accessories is eyepieces, oculars, or, as some newbies like to call them, “eye-lenses.” Your new telescope probably came with one or two, but that’s not enough to provide the range of magnifications needed to take on a wide variety of objects. You’ll want at least three eyepieces or at least three magnifications: low (but not too low if your sky is light-polluted) for finding and for large objects, medium power as your bread and butter eyepiece, and a high power for small objects and for the Moon and planets.

Eyepieces are simple things; all they really are is sophisticated magnifying glasses. Their purpose is to take the image formed by your telescope’s optical system and enlarge it to a size practical for detailed inspection. Long focal length eyepieces deliver lower powers and short focal length ones deliver higher powers.

What’s a good eyepiece? There are many, including wide field (68-degree AFOV range), ultra-wide field (82-degrees), and mega-wide field (100-degrees) eyepieces like Televue’s Panoptics, Naglers, and Ethoses respectively. Alas, Al Naglers’s wonders are for tomorrow. Today, they don’t just break our budget, they decimate it. Any one of these will cost more than what remains of our pot.

What exactly is AFOV, “Apparent Field of View,” anyhow? Many beginners bandy this term about without really understanding what it means. It’s purty simple, muchachos. True field of view, TFOV, is the actual expanse of sky shown by an eyepiece. If you can see the whole Moon, the eyepiece’s TFOV is at least .5 degrees. The apparent field of view of the eyepiece, the angular size of the circle you see when you look in, is different. For a Plössl eyepiece it is about 50-degrees in extent. Clear as mud, huh?

The difference between a narrow AFOV eyepiece and a wide AFOV eyepiece is somewhat analogous to the difference between watching Jersey Shore on a 50 inch big-screen TV and a 12-inch portable. You see the same thing (Snooki), but the big TV’s image is more expansive and immersive. This analogy is not quite perfect. Objects are larger on a larger TV screen, but a 12mm 100-degree AFOV eyepiece and a 12mm 50-degree AFOV eyepiece deliver the same magnification. The Moon is the same size in both; you just see more of it in the 100-degree jobber-do. Maybe a better analogy is looking at a scene through a small windowpane as opposed to a picture window.

So, no Ethoses for us. What then? There are less expensive high quality wide field eyepieces like the William Optics Uwans (a.k.a. Orion “Megaviews”), but even one of these will cost too much. Our old reliable will be Plössls.

The Plössl eyepiece (sometimes also known as the Symmetrical or Super Plössl) is a good design that’s far more able and sophisticated than the basic eyepieces of Unk’s day, the Kellners and Ramsdens. It is composed of (usually) four lens elements. It offers decent if not generous eye relief. “Eye relief” is the distance you have to hold your eye from the lens to see the whole eyepiece field, and Plössls generally have about 5 – 8mm of that depending on eyepiece focal length. Bottom line? If you wear glasses, you will have to remove them to see the whole field in a Plössl. These eyepieces also deliver a relatively aberration-free field if done right. Mostly what attracts us to Plössls, though, is their price. A little shopping will turn up plenty of Plössls that cost less than 40 dollars.

How many eyepieces do you need? Three or four to start. Which focal lengths? That depends on your telescope’s focal length. As you may already know, the magnification produced by an eyepiece is equal to its focal length divided into the telescope’s focal length. An 8-inch f/6 scope has a focal length of 1200mm, so a 10mm eyepiece will give us 120x in this telescope (1200/10). If you followed by my advice, most of you will have purchased 1200mm focal length scopes—both an 8-inch f/6 and a 6-inch f/8 have focal lengths of 1200mm. If your telescope focal length is different, just adjust the following eyepiece focal length recommendations to give you the suggested magnifications.

So which eyepiece focal lengths do I recommend? You probably received a 25mm, which yields 48x in a 1200mm scope, a low power useful for object finding and framing larger targets. Most deep sky objects cry out for more power rather than less, though—something many beginning and not so beginning amateurs don’t realize. So, you’ll want a higher power ocular to go with the 25. A 6mm will yield 200x. That is good for planets and smaller deep sky objects. If a 6mm is not available in your eyepiece line of choice, a 7mm is just about as good.

I also strongly suggest you glom onto either a 10mm (120x) or 12mm (100x) eyepiece. This will be your workhorse eyepiece most of the time. Not only does this magnification range show off many objects to good advantage, if you live where the sky is compromised by light pollution a 12mm will spread-out the background sky glow, helping the target pop out without darkening the object beyond redemption.

How about a low power eyepiece? Sure, why not if you have the moola to spend? A 32mm or 35mm focal length eyepiece (37x and 34x, respectively) can frame larger objects nicely. Let this be the bottom-priority eyepiece, though. If, like most of us, you live where light pollution is a factor, these powers will deliver sky backgrounds bright enough to make some deep sky objects disappear.

OK, three – four eyepieces. Maybe a 32mm, a 12mm, and a 6mm in addition to the 25mm that came with new baby. Where do you get them? Plössls seem to be everywhere being sold by everybody. Which to choose? This is a good time to buy Plössls. The Chinese factories are churning them out with abandon. Even better, practically any Plössl sold by an honest-to-god astronomy vendor (i.e. not cotton picking eBay) will be just fine.

In fact, most Plössls I’ve used have been more similar in performance than different. The single exception I’ve noted is TeleVue’s Plössls. They are a cut above the rest of the pack both optically and mechanically. Alas, they cost twice as much as the competition and will bust our budget. They go for about 80 bucks a pop, and unless you are willing to up the accessory budget by another 150 or so, it’s best to compromise.

Compromise does not mean “bad eyepieces.” Orion’s Sirius Plössls, for example, are mucho cheapo at $45.95 for most focal lengths (the 32mm is $51.95), and they are not the only loss leaders. One of my low price faves, of which I have several that I use in my binoviewer, is Handsonoptics’ GTO Plössls line at $22.95 to $32.95 depending on focal length. Celestron, Meade, Skywatcher and others sell nice imported Plössls, too.

How about an eyepiece kit like those sold by Orion, Meade, Celestron and others? These kits usually include four or five eyepieces, a Barlow (see below), and some other small accessories like colored filters in a nice box. Worth it? Maybe, if one fits your budget and the eyepiece focal lengths are ones that you will really use. A 4mm eyepiece won’t get in the scope much unless your seeing is very good, for example.

One question I get frequently from the wet-behind-the-ears brigade is “Uncle Rod, do I need 2-inch eyepieces?” They sure are pretty, but my answer is, “No, not to start with.” The advantages of 2-inch eyepieces are two-fold. First, they allow wider fields. The 1.25-inch barrel restricts the amount of true field an eyepiece can deliver, no matter what its focal length. But, if, like me, you do most of your observing from light pollution, the very low powers and very wide true fields of longer than 32mm focal length eyepieces are not very useful.

What else can the 2-inch format do? Some sophisticated wide field eyepieces are only available in the 2-inch barrel size. That shouldn’t worry us now, as we are not after sophisticated wide field eyepieces, anyway.

Let's talk Barlow lenses. Some beginners are suspicious of Barlows. It just seems too good to be true; the elusive Free Lunch: a lens that doubles the magnification of your eyepieces, virtually doubling your eyepiece inventory and sometimes making those eyepieces better. For once there really is such a thing as a free lunch. Good Barlows really do do these things.

Better? How? The narrower light cone produced by a Barlow is easier for less expensive eyepieces to handle well. The field edge of a Barlowed eyepiece may be sharper than an unbarlowed eyepiece of the same design and final magnification. Also, since longer focal length eyepieces generally deliver better eye relief—you don’t have to mash your eye up against the eyepiece to take in the whole field—a 2x Barlowed 25mm is often more comfortable to use than an unbarlowed 12mm of the same type.

How do you use a Barlow? Simple: you place the Barlow in the telescope focuser and your eyepiece goes into the Barlow. If you want to use a filter, you can thread one onto the end of the Barlow (most are threaded for this purpose) or thread one onto the eyepiece before inserting it into the Barlow.

One caveat about Barlows: if you plan to buy one, choose your eyepiece focal lengths wisely. If you buy a 2x multiplying Barlow (the usual value), there’s no point in having a 25mm, a 12mm, and a 6mm eyepiece. With the Barlow the 25mm comes close to 12mm and the 12mm becomes a 6mm. A 25mm, a 16mm, and a 6mm would be a better lineup.

Which Barlow? You are very lucky here. Almost all today’s Barlows are well made optically, even the inexpensive Chinese numbers. If they have a deficiency, it’s usually mechanical, not optical, like that the interior is not sufficiently blackened to reduce stray reflections. Me? I’ve used one of Orion’s 1.25-inch Shorty Barlows for years and years, including for planetary imaging, and have been as happy as a bird with it.

Wide Fields on the Cheap

Just because you are thrifty (or, like Unk, stingy), does not mean you have to stick with the 50-degree apparent field of view delivered by most Plossls. Big AFOV eyepieces are just more comfortable and involving. Al Nagler calls it the “spacewalk experience.” Luckily you do not have to pay the bucks for one of Uncle Al’s creations to at least get a taste of spacewalking.

Orion Expanse

I’ve got the complete set of the 66-degree AFOV Expanses (made by Synta), 20mm, 15mm, 9mm, 6mm, and like them very much. If you have an f/6 or slower (larger f-number) telescope, I bet you will too. The 15mm is the prize, but all deliver wide AFOVs and sharp images over most of their fields in higher focal ratio telescopes. If there is a bring-down, it’s that the 9mm and, especially, the 6mm are prone to internal reflections caused by the Barlow lens element they use to achieve their shorter focal lengths. This is usually only a problem when looking at the Moon, however. At $47.96 each, the Expanses are a real value; I love them.

Owl Astronomy Products

Owl has a series of 80-degree AFOV eyepieces that, while far from perfect in fast telescopes, do fairly well at f/6 and slower. The prices are certainly right, with the most expensive one being the 20mm at $99.95, and the rest of the line being considerably less expensive. The longer focal length members of this “Ultra-black Ultra-wide-angle” series do require 2-inch focusers, but that’s OK; most if not all new 6 – 8-inch Dobs now come with 2-inch Crayford focusers now.

Filters

Not long after buying their first scope and a few eyepieces, most novices get curious about filters. Do they need any? How many? What kind?

First off, let’s dispense with Moon filters. You don’t need them. These filters, usually green-tinted, are designed to reduce the Moon’s brightness. The Moon, however, is not really that bright. After observing Luna for a while, you will find her silv’ry glow not at all uncomfortable. And in case you are wondering, there is no chance she will damage your eyesight. Even the light of a full Moon is way too weak to hurt you. In reality, the surface of the Moon is close to the color of asphalt blacktop. Her seeming brightness is a matter of contrast and, again, is not dangerous.

“But Uncle Rod, Lunar observing is still uncomfortable for me.” If you simply must reduce Luna’s glare, do it with a color “planetary” filter or a polarizing filter. A colored filter or a polarizing filter may even enhance the details you can see. The usual “Moon filter” won’t. Also, in my experience, these green filters are often of poor optical quality. My usual solution? If the Moon is uncomfortably bright, I increase the magnification.

How about colored filters for the Solar System? Some can help. An 80a (its “Wratten” designation) blue filter can make Jupiter’s bands easier to see. A yellow filter can bring out details in Mars’ atmosphere. The improvements brought by color filters are subtle, however, and I rarely use them. You can at least move colored filters to the bottom of your Must Buy list.

Then there are the light pollution filters. No filter can make deep sky objects brighter, but LPR (light pollution reduction) filters can darken the background of the field, increasing contrast between object and sky. Light pollution filters come in three basic flavors, mild filters with wide passbands (Skyglow, Deepsky), medium with narrower passbands (UHC, Ultrablock), and “line” filters with very narrow passbands (OIII, hbeta).

Should you get one? If your budget will allow, yes. Otherwise, put a light pollution filter at the top of your "buy next" list. An LPR filter can make the difference between seeing and not seeing a nebula in light pollution, and can allow you to see more under even a good sky. Which one should you get? Stay away from mild filters. They might be somewhat useful for imaging, but don’t do enough to help visually if your light pollution is in any way serious. Stick with the UHC type medium filters. When the time comes to add another LPR, get the OIII, which works well on many objects, rather than the hbeta, which only works on a few. And by “objects,” I mean nebulae. The only deep sky objects improved by LPR filters are nebulae.

Whose to purchase? Lumicon’s filters are very good. So are Orion’s at somewhat lower prices. I particularly like Thousand Oaks filters, and have found those from Baader (often sold under the Celestron brand name) also outstanding. I did not like the single TeleVue LPR filter I owned, an OIII; it was too mild, not much like an OIII at all.

Star Charts

“You can’t find the stars’ homes without a map!” That is as true in the sky as it is in Hollywood. The simple monthly star charts in the astronomy magazines can help you find some of the brightest objects, but beyond that you’ll need more details—a larger map scale.

What do you get? That’s simple. Wil Tirion’s Sky Atlas 2000. It is available in various formats, but any of them are very useful for novice astronomers, and will contain more than enough stars to make objects easy to find if you are star hopping, and enough deep sky objects to keep you occupied for many years, maybe even forever.

More and more amateur astronomers have given up print star atlases for computer programs. What’s the advantage? Computer programs can show far more objects than any print atlas, even the legendary Millennium Star Atlas. They can also be customized to show as many or as few stars and deep sky objects as desired. They can be zoomed and unzoomed at will. They can even display photos of the objects you are hunting.

Which computer star atlas? There are many, but I suggest you get your feet wet with freeware. There are two programs, Cartes du Ciel and Stellarium (shown at left), that despite their freeware status will do just about anything the most expensive programs will, and will easily meet the needs of any novice. Stellarium delivers a beautiful rendition of the night sky, and is perfect for checking “what’s up.” It can also be useful for actual observing outdoors, but Cartes du Ciel is probably a better choice for use with the scope. It’s not as pretty as Stellarium, but it can go far deeper, and has features working observers will find very useful, including the ability to download object pictures from the Digitized Sky Survey.

Don’t have a laptop to use in the field? A computer program may still be a better choice than a print atlas. Most current programs can print outstanding, nearly typeset-quality charts, which will be fully customizable, making them more useful than the best book.

Flashlights

You need a red flashlight to protect your night vision and that of the people observing with or around you. Back in the day, amateurs used everything from a standard flashlight with a couple of layers of paper from a brown paper bag over the lens, to a real “astronomer’s flashlight” with a red lens. Today, what you want is a red LED flashlight (with a brightness adjustment; too bright a red light can ruin your night vision). LEDs, unlike filtered incandescent bulbs, produce pure red light that is especially easy on the eyes. You can even get a red light equipped with white and blue LEDs to help you find your way when you are off the field, but be sure you know which switch or switch position is red and which is white, or you will get yelled at at a star party.

Flashlight brands? I like to recommend American products, but the Rigel flashlight I bought has disappointed me, having a tendency to fall apart. I spent quite some time at a star party hunting the guts of my brand new light after it came to pieces while I had it hanging around my neck via its strap. The import red lights sold by Orion and Celestron are fine.

Everything Else

Just a few more things. One is a table for all your astro-junk—charts, computer, eyepieces, etc. I used a card table for years, and that was perfectly satisfactory, but strolling through Walmart’s sporting goods section one evening, I found something better (as you can tell, I buy a lot of “astronomy” accessories at the dadgum Wal-Mart), a camp table that folds in the middle and is the size of two card tables when set up.

After a few months in the astronomy game, you’ll find you are accumulating lots of small things, and that all these things need to be contained. Especially eyepieces. They need to be protected from dust and moisture. You need an eyepiece case. Cardinal rule? Don’t get a small eyepiece case or you’ll soon be buying another eyepiece case. I have had one of Orion’s 17x12-inch aluminum cases for years and have been very pleased with it. HOWEVER…you can often find the very same thing—an aluminum case with “pluckable,” customizable foam—for about half the price at home improvement and sporting goods stores as a “tool attaché” or pistol case.

One last item. If you followed my advice and bought a Newtonian in the moderate f/8 – f/6 focal ratio range, collimation, alignment of the optics, is not overly critical. Still, you will eventually have to collimate, and for a Newtonian you’ll need a tool to help you do that. Your telescope probably came with a "collimation cap" for that, but if you have 50 bucks left over in your budget, you can do better. You can get fancy-schmantsy lasers, but a simple Cheshire-cum-sight-tube like those sold by Orion and Celestron will be almost as simple to use as a laser and will often deliver better collimation.

So, that’s it—for a little while, anyhow. Now try to forget about buying astro-stuff for a while (hard, I know), and get out there and start using it. A lot. Before buying anything else, get plenty of experience with what you have, which will ensure you know what you really need when the astronomy-buying frenzy hits again.

Next Time: The most wonderful optical instrument you own, your eyes.

Comments:
Where were you 4 years ago.... Had I read this blog entry a few years back, I may be several dollars richer.

Lots of great information which will come in handy as we continue to grow with this wonderful hobby.

Thanks again, Uncle Rod. Your words of wisdom are truly appreciated!!!
 
When doing astronomy out in the field, it is good to have all of the accessories you need to make observing easier and more enjoyable.
A laptop computer is nice, but will work better with a transparent red acrylic face filter over the display.
 
Have you tried the Denkmeier "Hi-Def" filters? And can any filter be used in the Denkmeier filter switch?
 
Haven't tried their filters, but if they are as good as there other stuff, they should be great. I haven't found a filter that couldn't be used in the 'Switch.
 
uncle rod what do you think of zhumell eyepieces i have a zhumell 8inch dobsonian with 3 lenses 30mm 14.5mm with long eye relief and 9mm plus 2x barlow i live on long island ny bright skyies i want to see deep sky objects so what lenses do i need
 
I think the Zhumells are good values for the money. If your sky is bright, it's best to stay away from long focal length, low magnification eyepieces. Higher powers tend to spread out the background skyglow.
 
Great advice on the eyepiece case - pistol cases are much cheaper.
 
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