A Guide to the Sky, Telescopes, and Telescope Programs
What can you See?
Once you know what's up in the sky this evening, what will be able to see? Not everything. Two major limiting factors here: your observing site and your equipment.
Darker is better.
Where you observe from makes a huge difference in what you can see. The darker your site, the better your sight. Much like the sun's light washes out everything in the sky, man's light washes out the fainter objects in the sky. There are many objects that simply cannot be seen from cities and suburbs. The brighter your sky, the less you can see. It doesn't matter how big your scope is if your sky if very bright. Set up a giant scope in the daylight or twilight and you see nothing, regardless of how big your scope is. Similarly, a dark sky lets a little scope see really deep (one of my favorite nights was with a $100 4-inch scope in the very dark skies of Mammoth Mountain, CA).
If you have a nearby safe place to set up a scope that is dramatically darker than your backyard, it is often worth it to set up there. Most astronomy clubs have a "dark" site where members and the public congregate to dramatically increase what they can see with their equipment. The San Diego Astronomy Association, for example, maintains a site 80 minutes east of San Diego where members can set up their scopes in one of the darker skies around San Diego.
Street and house lights can wreck your night vision. If you need to set up in the back yard or the nearest vacant lot, set up in a spot that affords a good view of the sky, but is shielded from local stray light.
While the observing site is the major limiter to what you can see, equipment will also make a difference. My wife asked me once after I snuck another telescope into the house "Why do you need more than one telescope?" "Because they all do different things."
A large apochromatic refractor may be the most versatile scope, but they are very expensive. A giant "light bucket" may allow you to see the faintest objects, but they are not very portable (and can be very expensive). Catadioptric scopes, like the popular Schmidt-Cassegrains and Meade ETX Maksutov-Cassegrains, are portable, have large apertures and high magnifications, but have narrow fields of view and are susceptible to having their lenses getting wet (and opaque) with dew.
A rule of thumb is that the bigger aperture (diameter) you have, the fainter objects you can see. Small scopes are only able to see faint objects under very dark skies. Remember that under urban or bright suburban skies, adding aperture does not necessarily allow you to see fainter objects.
Focal length sets a limit to how high a useful magnification you can reach with a scope (as does aperture) and limits how wide a section of the sky you can see at low magnification. Both ends of the spectrum are very valuable. Large Star Clusters, such as the Pleiades and the Beehive, are best viewed with a wide field of view. Wide fields also allow you to see multiple objects in the same eyepiece such as the Galaxies M81 and M82 and the Leo Triplets. Wide fields also make it easier to find objects through the eyepiece.
Short Focal Length = Wide Field
Long Focal Length = High Magnification
What will you be able to see?
It depends on your sight, your equipment and your Target Type. Click this page for what you may see from San Diego: Urban Viewing
Questions or comments? Email:Jeff Martin