Date: February 4, 2011

Title: Junk Telescopes

Podcaster: Stephen Uitti

Organization: Ford Amateur Astronomy Club – http://boonhill.net/faac/

Description: A typical very poor quality telescope is reviewed. Where not to buy one, what makes these scopes so poor, and what might have been done better by the manufacturer.

Bio: Stephen Uitti is a Mechanical Engineer, computer programmer, and amateur astronomer. He belongs to two astronomy clubs, and contributes to the Astronomy For Everyone TV show.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Neil Christie. Those who can Podcast, those who can’t donate.

Transcript:

Junk Telescopes

So, you’re at the grocery store, or a toy store, a department store, or one of those discount chains. And you see telescopes that are pretty cheap, and even have a brand name you recognize. You’re thinking that stars are hot and planets are cool, and maybe the kids will learn something. My advice is to keep walking. These scopes are so frustrating to use that they will turn you and your kids off from astronomy, maybe forever. The best way to buy a scope is to join an astronomy club first.

I recently bought a junk telescope from a vendor on Ebay. Don’t feel sorry for me. I knew it would be junk before I bought it. And, I already had a first rate telescope, which i’d been using for years. I get asked what kind of telescope should be bought all the time, and so I’m very interested in the low end. I wanted to do a review. I wanted to find out first hand what makes these instruments so poor, and maybe get an idea what could be done about it.

Now, when shopping for this scope, i already knew that most of these scopes advertise and deliver useless magnification. I’ve seen scopes advertised as “675 power”. I’d even gotten a chance to experience 675 power on one of these scopes. I didn’t need to experience that again. The deal is that a 50 mmm scope – that’s the diameter of the biggest lens or mirror, here about 2 inches, does not support 675x. Technically, it delivers the magnification, but the images are dim. There isn’t more information delivered than at much lower magnification. So, the images are grainy. The common rule of thumb is that the maximum useable magnification is about twice the diameter in millimeters. So a 50 mm instrument should support about 100x. My own rule is worse: one to one. So, in my experience, a 50 mm scope can support 50x magnification. In addition to high power dimming the view, the field of view narrows, and that makes finding objects harder, and it amplifies any instability of the mount.

So I didn’t buy one that boasted huge magnification. There’s no need to mention brand names here. The scope I bought is a 40 mm refractor on a tripod that you place on the floor. It comes with two eyepieces. The high power gives 48x. Now 48x is a bit above my rule of thumb, but not terribly high over it. So, this should be usable.

The low power eyepiece gives 24x magnification. This is what I’d expect to use most of the time in this scope. The scope also comes with a 90 degree diagonal. This is good. It means that you can look comfortably down into the scope, instead of having to get down under the scope, lean back and look up. This is most important when looking nearly straight up.

The scope comes with a 2x barlow. A 2x barlow doubles the magnification that the eyepiece gives you. The instructions say that it comes with a 3x barlow. So the company must have gotten word that useless magnification is useless, and recently changed the barlow but not the instructions. When this barlow is used with the low power eyepiece, it makes the magnification the same as what the high power eyepiece gives. That’s not very exciting. When used with the high power eyepiece, it gives 96x. that’s too much for my tastes, but at least it comes close to the general rule of them. That rule would max out the scope at 80x for this 40 mm instrument.

But the instructions fail to mention that to use the barlow, you can’t also use the diagonal. So i had difficulty getting it to work at all. One of the club members knew about this wrinkle. Thanks Marty!

I got the 2x barlow working. Without the diagonal, it’s really uncomfortable looking at anything at elevations much higher than the horizon. For anything high in the sky, it’s nearly impossible to use. But it’s ultimately useless. I’d delete it from the package. It’s astounding that a manufacturer can produce something that works exactly as advertised, isn’t defective, and yet makes you feel like they’ve comitted fraud. These instruments deliver to their specifications. It’s the specifications that are wrong.

My scope came with a 6×30 finder scope. It magnifies things 6 times more than naked eye, and it is 30 mm in diameter. It attaches to the side of the main scope with a thumb screw. You are expected to put it on the scope when you set it up, and take it off the scope when you’re done. if that weren’t the case, the packaging would have space for it while attached to the scope.

So, you put it on the scope. You point the scope at a bright object, and adjust it so that both the finder and the main scope point to the same place. You can get close by pointing at a telephone pole. But even the few inches between the two scopes introduces a paralax error that you won’t have looking at the Moon. If you’re doing this during the day, using a tree or something, pick one that is as far away as possible. If the Moon is up, which is often the case even during the day, then you can use some feature – a crater or something,

But be very careful. These finder scopes often go out of alignment when you touch them. Some are so bad that touching them with your eyelashes is enough to move them.

There’s another problem. These finder scopes often have a very poor field of view. The wider the field of view, the easier it is to find things. But often, the field of view isn’t much better than that in the main scope. And think about this for just a second. The finder scope looks tiny, but 30 mm is 3/4 of 40 mm. It’s of comparible same size. I’d be happier without a finder scope on a scope this size.

As an aside, about 400 years ago, Galileo’s first scopes were similar to this finder scope that I’m saying should be omitted. The magnification is similar, and the diameter is similar. Today’s optics are probably superior. And with his first scope, he discovered the phases of Venus, the moons of Jupiter, and the fact that something very strange was going on at Saturn.

Next comes the mount. My junk scope came with a full hieght tripod. The legs extend enough so that an adult can look through the scope comfortably. The legs can be shortened for the smallest of children. They shrink marvelously small for transport. Everything looks good. Except that they aren’t very stiff. That means that the scope will never be very steady. And when you’re looking through a telescope at 24x magnification, any little jiggle moves the image a long way. And once it starts moving, the whole thing acts like a big spring, and it bounces around for ten seconds or more before settling down. If there’s any hint of wind, it’s hopeless. It never settles.

Between the tripod and the main scope is the head. This bit often has a handle that lets you point the scope. It has two adjustments that lets you set how tight the movements are. If it’s too tight, then when you move the scope, the whole tripod moves. The tripod isn’t very heavy. If it’s too loose, then the scope doesn’t stay where you pointed it. One adjustment is for rotation, and the other is for the tilt. On my model, and several others I’ve tried, there is no happy medium. If the scope stays where you put it, then it sticks while it moves. If you have it any looser, then it moves, but won’t stay where you put it. so it’s very difficult to get it to stop where you want it to. And worse, the head has lots of play, so the scope is free to wobble around quite a bit. If you show the scope so much as a picture of a tree swaying in a breeze, the scope moves around, and your object leaves the field of view entirely.

Here’s a typical observation experience, paraphrased from my log. I brought the scope to my front porch, so that it would be protected on two sides from any wind. I used the Moon to align the finder. Venus was up and clearly in view, so i found Venus with the finder. But the alignment had already been knocked out of place, so it wasn’t in the main scope, even though i was using low power. I realigned the finder, but soon gave up. I pointed the main scope in the area of Venus, and hunted for it for about ten minutes. The image danced around wildly, passing through the field of view. I grabbed the eyepiece with my hand to stabilize it. My pulse was evident in the movement of the image, but it still behaved better than it did while free. After a few more minutes of struggle, i was able to confirm the crescent phase of Venus. A half hour had elapsed. I was done. Two objects logged. The Moon and Venus.

The mount is the hardest of the design problems to solve, and the one that produces the most frustration in cheap telescopes. Just for comparison, I also own a ten inch (254 mm) Dobsonian telescope. The mount’s settle time when bumpted is much less than a second, and the scope behaves well in high wind. My astrophotographer buddies with mounts costing over $6k are packing it in before I even notice. My lowest power eyepiece produces about 31x magnification. I usually view at 48x or 120x. The 9×50 finder scope attaches quickly, and aligns in seconds easily. Mostly, it needs no alignment.

In 2008, my considered opinion was that there were no telescopes on the market that were worth owning that cost under $200 US. In 2009, the International Year of Astronomy, the Galileo scope was introduced. It was astonishingly cheap – around $20, and they solved many of the problems with cheap scopes. There is no finder scope, but it includes a sight that is part of the plastic tube. It’s sort of a gun sight. It works. The eyepieces give reasonably low magnification. Now, it should be pointed out that one of the eyepieces is of the design that Galileo himself used. By today’s standards, it’s awful. But it does produce an upright image. I believe it was included for instructional purposes. It also comes with a more modern design eyepiece.

The Galileo mount problem was partly solved. The scope comes with a quarter-twenty nut, which allows it to be used with any standard tripod. That’s all right for me. I already have a high end photography tripod. It’s very strong and stable and has a very smooth acting head. But it cost me $250 in around 1990. Lately, I’ve seen similar models for over $400. I’m still looking for a low cost high quality mount. Perhaps an easy do-it-youself project. The Galileo scope is assembled by the owner. So you can use it to learn how these things work, and teach your kids too.

Also in 2009, a low end 60 mm table top reflecting dob came on the market. It was under $100. All of the parts are usable. The mount rotates and the scope tilts. It’s very stable. You need a table to use it. I’ve used the trunk of my car, and it was OK. The scope is small and extraordinarily portable. This was very, very exciting when I saw it for the first time.

In 2010, a 100 mm (four inch) table top reflecting dob came on the market for right around $100. The views are excellent. I’ve used one to look at detail on the Moon, the bands on Jupiter, the great cluster in Hercules, and was always impressed. The mount is stable, and it also mounts on a tripod, which worked very well with my beefy tripod. It comes with a red dot finder. This finder doesn’t magnify, but is easy to align to the scope, and it’s easy to use. The eyepieces are the same 1 1/4 eyepeices that I use with my more expensive scope. So I was able to try my more expensive eyepeices, barlows and filters. The scope performes very well to 100x magnification. This scope is about 6 pounds (2 kg). Very portable. My club bought one for outreach efforts.

So, with one of the new cheap scopes in hand, the next problem is what to look at. While there are some web sites, like skymaps.com, that have free maps and lists of objects to find, the easer way to do this is to find a local astronomy club. Sky & Telescope magazine’s web site has a list of organizations. You can search the list by where you live. Most clubs are happy to assist you in using your scope, and will show you stuff they’ve found in their own scopes. It doesn’t get much easier than that.

End of podcast:

365 Days of Astronomy
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