Sommers-Bausch Observatory - University of Colorado
What We Don't Recommend Buying ... and Why!
Actually, the ad itself isn't so bad. In fact, for the most part it's quite accurate in telling you what you'll get. It's just that you have to pay attention to what they're actually saying, because what sounds like one thing really is really another, or is just simple gibberish.
'Fer instance, there's all these references to extreme distance:
Deep Space 100-Billion Mile Telescope.
Let's see, if the telescope can see objects all the way out to 100 billion miles, that covers all the stuff in our own Solar System. Only. The nearest star is thousands of times more distant. Not very impressive, seems to me (but maybe they're just being honest!).
Bring ... Distant Stars... Even the Milky Way, into Full Close-up View. Wait, I thought the telescope only worked for the solar system. Now they're saying it can see things millions of times further that that! (Well, fine, it probably does. Just like your naked eye, which can also see those distant stars and the Milky Way.)
But what does FULL close-up view mean? The Milky Way wraps entirely around the Earth, there's no way to magnify it all at once. The implication is that because the Milky Way is quite distance, it's somehow a great engineering technical feat to bring it in "close-up". Nope, all things magnify equally.
Extra-Long-Range We're beating a dead horse here. They're banking on the novice who thinks that somehow an instrument can only "reach out" so far into space, and then runs into an optical brick wall; that a telescope goes out and "grabs" pieces of light from a distance source and brings them back to your eye. Nope, you can see to the ends of the universe already, the photons from both near and far are already arriving all the time. All a telescope can do is bring a bunch of them together in your eye to make a sufficient stimulus to your retina so that you detect "light".
Then how about all of that nonsensical astronomical gobbledegook which actually has nothing to do with their product (or even with reality!), but which they want you to associate with it anyway?
- Bring the surface of the Moon, Mars, Venus, etc. right into your living room. Not recommended. Peering with a telescope through window glass is a sure-fire way to make any telescopic image look crappy. But never mind that. The typical apparent diameter of Mars is about 10 arc-seconds as seen with the naked eye. Even using the highest magnification of 40X, that would make Mars look like it was 400 arc-seconds across. That's what a dinner plate looks like from a mile away. That's an awfully big living room!
Track comets streaking .... Quick! Look fast! It'll be gone from view in only a week or two!
See meteors flame through the skies. Meteor trails typically last for a half-second or so. In over a quarter-century of observing, I've never seen one pass through the field of view of my telescope. I challenge anybody to spot a meteor, then swing their telescope around so as to get it in view, focus, and admire its "flames" in that interval of time!
Be absolutely spellbound in your ringside seat as asteroids collide in fiery explosions. Wow. You betcha I would be, if only I could see such an event. So would any astronomer. Fact is, nobody on the face of the Earth, living or dead, has ever seen two asteroids collide. If that little telescope could manage to pull this off, it would be the greatest astronomical device ever invented!
Distant galaxies such as the Milky Way. Uh, the Milky Way isn't so distant. It's the nearest one. We live in it. We're part of it.
OK, so now let's take a look at those instrument specifications:
High-impact housing ... objective high-impact lens. Sounds just like "plastic" to me. But anyway, it doesn't say the the things actually resist high impacts, it just says you can hit things with them.
After reading all of those so-called specifications, we still have very little idea about what they're selling, and no information about the actual optical design (other than it's a refractor with plastic lenses). What's the lens diameter, or aperture (the most important parameter for light-gathering)? What's the focal length (for magnification)? What's the f/ratio (for optical "speed")?
Reflective lenses. A great example of truth-in-advertising! Unfortunately, lenses are not supposed to be reflective, mirrors are. A lens is supposed to transmit light, rather than have it bounce off its face. People pay extra bucks to have special lens coatings to prevent reflections. The advertiser is admitting a basic flaw and trying to make it sound like a virtue!
Tripod mounted. But doesn't say what kind of mounting. Well, from the picture we can see it's an alt-az mount, not the equatorial kind that is most useful in astronomical telescopes. But hey, what do you expect for $29.95.
- Interchangable eyepieces - 20X, 30X, 40X. Actually, these magnifications are probably rational and reasonable choices for what we believe the telescope is capable of achieving (see analysis below). Anything of higher power would almost certainly be useless. Nevertheless, a typical beginners scope is usually best used in the 50X to 120X range.
- Focussing controls for both primary and secondary body cylinders. Huh? Can somebody explain what they're talking about? Sure, there's a focus knob. It's supposed to be a telescope, after all.
- Multi element optical system for sharp image quality. Yes, multi-elements usually produce better images. Are we counting two lenses, one in the objective and one in the eyepiece as a "multi-lens system? I certainly hope not, but since they don't say, who knows? Is the objective a chromatic singlet or an achromatic doublet? What is the eyepiece design? Most likely they're 2-element Ramsdens, the kind that usually appears in junk department-store telescopes: state-of-the-art stuff in the 18th century, but currently the cheapest and poorest-performing design.
- Both exit pupil lenses and objective high-impact lenses .... Ah. All are plastic lenses.
- ... engineered for maximum transmissions. Clear plastic lenses.
- Total spectrum clarity. Colorless clear plastic lenses.
Well, we can make some intelligent guesses; for reference, see our Telescopes & Observing tutorial.
Virtually every cheap telescope that only advertises "power" (magnification) includes an eyepiece that will yield the highest magnification possible for the telescope; ie., one in the 2mm-8mm focal length range. Let's give the telescope every benefit of the doubt, and assume that the 40X eyepiece is an 8mm design (if it were 2mm, then everything that follows will be a factor of four smaller.
The objective lens focal length equals the eyepiece focal length times the resultant magnification. Thus, the telescope focal length is (probably at most) 8mm x 40 = 320mm. That means the telescope tube is about one foot in length.
We have a picture of the telescope, and discounting perspective effects, it appears that the width of the tube is about 1/8th to 1/10th that of the back-of-the-lens-hood to eyepiece-focal-plane distance. Let's give it the benefit of the doubt and assume it's the optically faster (and larger) f/8. The aperture would then be the focal length divided by the f/ratio, or 320mm/8 = 40mm in diameter: one-and-a-half inches.
The exit pupil diameter is a measure for how well designed the instrument is for night observing. The classical 50x7 binoculars produce an optimal 7 mm exit pupil of light, matching nicely the 7 mm entrance pupil of the dark-adapted eye. For our 40mm aperture telescope with its brightest available eyepiece (20X), the exit pupil is only 2mm, utilizing only about 1/12th of the photon-collecting capability of the human iris.
So, we have something that vaguely resembles a finderscope that you might find mounted piggyback on a basic beginner's telescope. It's about 12-13 inches in total length, with a diameter half the size (and with 1/4th the light gathering ability) of what most amateurs would consider the minimumal "starter scope". Not $199.95? I certainly hope not!
A Special Offer
The advertiser didn't really give us any useful information about their "instrument", and much of the foregoing is based upon probable assumtions ... but assumtions nonetheless. I'd really like to get my hands on one of these toys, to test and measure and see for sure; but I'm not inclined to support this kind of marketing by sending them my $29.95.
So here's my offer. If you were one of the lucky customers who purchased one of these things, and if (through lack of extreme customer satisfaction) you're willing to part with it again, I will personally send you $29.95 plus $5.00 for Shipping in exchange for that telescope (with all original accessories and instructions)! Sorry, this offer is only good for the first person to to take me up on it. I only need one piece of junk.
If you're interested, please contact me, Keith Gleason, via the information at the bottom of the front page of the SBO Website. And I suppose this institution would like me to put in the disclaimer that all views and opinions expressed in this article are those of the author only, and do not necessarily reflect those of the University of Colorado or its Regents. But I'll stand by my comments until you send me something that proves me wrong.