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.

The Bad Ad 'Fer instance, there's all these references to extreme distance:

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?

OK, so now let's take a look at those instrument specifications:

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")?

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.