warm bodies under the light path to your telescope ) also can affect the local seeing , stirring it up . Lots of seeing effects occur in the first 200 feet [ 65 m ] of the air above your head and almost all of it is below 50,000 feet [ 15 km ]. Avoid being in the air flowing downslope from a mountainside at night . Set up your telescope early enough that it can reach the ambient air temperature by the time you plan to start observing . This reduces air currents in the tube , called internal seeing , that can also make in-focus images fuzzy . Refractors and other sealed-tube telescopes are less likely to suffer internal seeing but they are not immune . Chromatic dispersion and differential refraction are the words used to describe color visible in a magnified image caused by Earth ’ s atmosphere . Except straight overhead at the zenith , the atmosphere behaves like a weak prism and creates a spectrum ( a little rainbow ) out of each object in the sky ( Figure 2 ). It is much more apparent for bright objects seen low in the sky , like Mars ( and is related to the green flash [ not Disney ’ s cinematic version ]). The simplest solution is to observe everything near its meridian crossing , not when it is low . The trouble is that some objects are always close to the horizon when you want to observe them , and that is the case for Mars this year . In north temperate latitudes , Mars will barely clear 30 ° altitude at its maximum on the meridian during the night of opposition . The most dedicated observers make or buy an atmospheric dispersion corrector . These are made with a pair of Amici ( wedge ) prisms that rotate with respect to each other to provide adjustable counter-dispersion to reduce the color separation and its layering on an image .
Eyepieces
Good eyepieces , of which you
Sky ’ s Up
COURTESY OF Stephen Edberg FIGURE 3 : These examples of screw-in filters for eyepieces are medium blue , orange , and neutral grey . They are suitable for use observing Mars , though a red or deep red will provide better contrast between lighter and darker geography than orange . Accessory filter holders are commercially available and allow easy switching between filters without removing the eyepiece .
should have a range of focal lengths ( equivalent to magnifications ), will make a difference but they don ’ t have to be super-wide angle . They need to provide pinpoint star images across the field of view . If you get the chance , do comparisons between your favorites and someone else ’ s favorites and see which present more pinpoint stars and crisper views of lunar craters and mountains on the terminator ( sunset / sunrise line ) across the field of view before you purchase new ones . Especially if you need to wear glasses for observing , make sure the eyepieces have long “ eye relief ,” 20mm or more , so you can get close to the eyepiece and see most or all of the field of view with your glasses on . ( Some manufacturers offer an accessory lens for their eyepieces that can correct for your astigmatism .) A binocular eyepiece holder is a good value for serious observers . Using both eyes at the same time is much more natural and does allow you to see more . They are relatively expensive and require pairs of eyepieces with the same focal length for each magnification you intend to use . The large physical diameters of the very wide field eyepieces may not fit into the binocular holder close enough to match the separation of your eyes . Check before doubling up on those eyepieces .
Filters
Colored-glass filters work by absorbing some colors while letting
21