Friday, August 19, 2011

Why aperture matters

When choosing a telescope, one of the most important questions that has to be addressed is the aperture, or diameter of the objective lens or primary mirror. That determines among other things, the size, weight, portability,  resolution and light gathering power a telescope has to offer. The larger the aperture, the more expensive the telescope of any given telescope will be. Furthermore, some types of telescopes are much more expensive than others of the same aperture. If observing galaxies and nebulae is what your telescope will be used for, then a telescope that offers the most aperture, and hence light gathering power is the best choice when observing capabilities alone are considered. There is no nirvana when it comes to aperture, a bigger telescope can always show dimmer objects than a smaller one from the same site. However, your muscles are only so strong, your vehicle only so big, and your bank account only so fat. Henceforth the goal when choosing a telescope is finding one large enough to show you want you want to see, yet is still affordable and isn't too heavy and bulky to transport and use easily.

With all of these factors are taken into consideration, Dobsonians are the best option for folks who want a telescope that can take them to the realm of the galaxies, and yet be affordable, portable and still have excellent optical and mechanical performance. Below are two drawings I made of the same globular star cluster from a heavily light polluted area. One was made through a 6-inch, the other through my 15-inch.

The top drawing I made with my small 6-inch Dob from a light polluted area at 60X. As you can see, it is unresolved and does not look at all like a globular cluster. From my light polluted front yard, it looks more like a comet, and even at high magnification only a few stars around the edges appear.

This is the same object from my light polluted front yard through the 15-inch telescope at 250X, which fully resolved it into stars and revealed clearly the star like inner core of thousands of stars packed into a region a few light light years across. In both cases I had the glare of thousands of streetlights lighting up the sky all around me. As you can see, a larger telescope does more even from a light polluted area, but it will be of much greater benefit from a dark site than a light polluted one.

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