…continuedChoosing Your Telescope's Magnification
Before selecting a viewing magnification, consider carefully what it is you are going to be looking at. If you want to see small and faint galaxies, globular clusters, and faint stars, there is no substitute for aperture. Years ago, big heavy mirrors and long focal ratios were the rule. Eyepieces back then couldn't handle fast f/ratios well. Today, with modern, highly corrected eye-pieces and coma correctors, large and compact Dobsonian telescopes can perform as never before, and they're really portable. With 13- to 25-inch Dobsonians, you can use all the power the atmosphere and optical quality will permit. Subject brightness is rarely a limit.
A subject's contrast is sometimes as important as its brightness. Often small refractors outperform larger reflectors because of superior contrast. Increasing the magnification of any telescope will reduce the size of the exit pupil and darken the background sky. This is why the faintest stars are always seen best with moderately high magnifications. The contrast of extended objects such as galaxies and nebulae is fixed relative to the sky background and only looks better as you boost magnification because details become more visible. In general you can increase the magnification to darken the sky (the field stop is a good reference for "black") as long as there is still sufficient sky showing around the object of interest to provide contrast. This appears to contradict the old adage about using big exit pupils when viewing nebulae. Don't worry; trust your eyes and experience.
Resolution how much do you need? Most large reflectors exhibit better resolution when used with an off-axis aperture mask. This is because you can wait with frustration for those magic, fleeting moments when the atmospheric seeing allows high-resolution glimpses with a large aperture, or you can reduce the aperture and trade off some resolution for much more time when the view is satisfying. Once again, a small aperture gives a sharp image that jumps around in bad seeing, while a large aperture often averages the image into a fuzzy blob.
If you consider astronomical viewing as a supremely rewarding, aesthetic experience, then the universe is your painting and your telescope is the palette. Frame the subject properly. Open clusters in particular can be blown out by too much power you may not even recognize what you're viewing. I don't think seeing Alcyone and a few stars in the Pleiades at 300x compares with a good sharp view at 20x to 60x. Leave plenty of breathing room around the subject so it appears in context with its surroundings. One advantage of a short-focal-length telescope is that you have field to spare for all your framing needs. You can always go up in power with these instruments. Long-focal-length telescopes, on the other hand, are limited when wide fields are needed.
Low-power subjects are those 1° or more wide. Open clusters, large galaxies, diffuse nebulae, and the Milky Way star fields are examples. The Beehive Cluster is 1° across, the Pleiades about 2°, and the Hyades 5°. The Veil Nebula is great at low and high power, but the North America Nebula needs at least a 3° field to show its distinctive shape.
Now comes the question of how low you can go. First, consider the exit-pupil limits of refractors and reflectors. The 7-mm diameter of the dark-adapted eye's pupil seems to be a popularly enshrined value among astronomers. It is promoted by the 7-mm exit pupil of so-called night binoculars and corresponds to the exit pupil of a telescope used at a magnification of 3.5x per inch of aperture.
What we can physically fit into our eye as an exit pupil and what is appropriate may not be the same. Furthermore, they differ for reflectors and refractors. A refractor has no limits on how low the power can go and how large the exit pupil can be. This idea is heresy to many, so let me explain. Consider a 4-inch f/4 refractor with a 55-mm eyepiece. The exit pupil has a diameter of about 14 mm. Since you can use only about 7 mm, some would say that half the aperture is wasted and are you really using a 2-inch telescope. They would say you are wasting light and wasting resolution.
However, the truth is that while you are wasting potential aperture, you are not wasting light because your eye is fully illuminated, and you have the brightest possible image that you can ever have at that low magnification. Think of using 7 x 50 binoculars in the daytime when your eye's pupil is only 3.5 mm. Does the image appear dimmer than it does in 7 x 25 binoculars, which have a pupil that matches your eye's? Of course not. Also, the resolution reduction for a 2-inch scope compared to a 4-inch is totally invisible at that magnification.
If a 14-mm exit pupil at 8x doesn't cost you anything in brightness or resolution, does it have any benefits? Sure. At 8x, a 2-inch eyepiece will produce a true field 6° or more in diameter. If you want that large a field to view the Milky Way, for example, why not have it! I'm not arguing that it is particularly wonderful to have an 8x scope, but the concept is valid.
Does the same argument hold true for reflecting telescopes? No! The central obstruction that exists with conventional reflectors places a much stricter limit on the situation. Central obstructions run from less than 20 percent of the objective diameter for some Newtonians, to 45 percent or more on some Cassegrain telescopes. A 14-mm exit pupil on the latter would have a black spot in its center more than 6 mm in diameter. While this is an extreme case, it points out the value of reflectors with small secondary obstructions and keeping the exit pupil to about 7 or 8 mm. Large secondaries also limit your visual performance by blacking out the center of your eye's pupil, which is the sharpest part.
High-power subjects include the Moon, planets, globular star clusters, planetary nebulae, small galaxies, small open clusters, and double stars. Here the power is limited by the atmosphere, telescope aperture and optical quality, the quality of your eyepieces and Barlows, and the stability of the telescope mounting.
A steady atmosphere is a prerequisite for effective high-power observing. Look for a minimum of star twinkling and try to observe subjects high in the sky. Well-made apochromatic and fluorite refractors produce excellent planetary images, and so do traditional long-focus refractors and reflectors with relatively small diagonals. Telescopes of fast focal ratio require complex (and expensive) eyepieces and good-quality Barlows for best results. Barlows can improve image quality and provide more eye relief for comfortable, relaxed high-power viewing.
Also, don't neglect the telescope mounting's rigidity or the smooth drive that is necessary for high-powered observing. A shaky mount can ruin the benefits of an optically excellent instrument. Dobsonians are inherently stable, but they must be moved frequently at high magnifications. This situation can be minimized by using wide-angle eyepieces, which extend the viewing time before having to reposition the instrument.
When magnification gets too high; subjects become dim and lose contrast. They are also more affected by atmospheric seeing and any misalignments and defects in the optics. When using high power, use the "lowest" high power possible.