How to choose a telescope: Magnification revisited…
- Filed under: Buyer's Tips, telescopes
- Date: May 22,2008
We’ve realized that in all of our ‘how-to-choose’ articles, we make a concerted effort to warn prospective buyers of optical instruments against magnification hype. And for good reason too. We still see people around us falling for that trick and ending up with instruments that have either poor optics or insufficient aperture or both. Hopefully our readers do not fall prey to the marketing machine anymore. Just to be sure that our newer readers are aware of this trap, we will quickly revisit it here (and expand on it a bit):
While the hyped-up power ratings often seen are not necessarily fraudulent claims (the instrument will usually go to the stated magnification), there is a limit to usable magnification. The quality of the lens, mirrors, eyepieces, etc play a huge role in how much of that power one can actually use. At high magnification, the image and also imperfections in glass are also magnified.
The same goes for movement. At high magnification, movement is also magnified and the slightest movement renders the image completely unusable, and in the end all you can see is a blur! So this is where the usable magnification is clearly determined in part by the quality of the optics and also the quality of mounts and/or tripods.
Lens diameter is a key factor in the amount of usable power and the general rule of thumb, called the sixty rule, states that the upper limit of a telescope’s power is determined by its lens diameter (in inches) multiplied by 60. For example, a 4″ telescope has a maximum usable power rating of 240X. In our experience, if you take 15% off that, you have a more reliable maximum power rating that does not depend on perfect viewing conditions. that And this upper limit pretty much is assuming you have an extraordinarily calm and clear night sky.
So, if you’re after power, then choose a scope with largest diameter lens that you can afford. Large diameter lenses capture more light too, so you are able to view much dimmer objects (Note: a 4-inch scope has four times the light gathering power of a 2-inch scope, and not double).
Optical quality and mount stability are not even a consideration - never compromise on those - its all about usable power, remember? If the image is blurry, its blurry. And that’s that - your first date with your new scope (and maybe even a new hobby) is ruined before the romance could even really get started…
Note
Often a larger diameter scope in the same series is only slightly more expensive than the one below it. These Celestron scopes are a good example. If budget allows, we always recommend higher apertures…
Celestron NexStar 4 SE Telescope
Celestron NexStar 5 SE Telescope
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