Cahill Rooftop Observatory

Site Info

Cahill Observing Deck

Location: 34° 8′ 7.6″ N, 118° 7′ 35.3″ W, 235 m (770 ft) ASL

Unobstructed Sky:

Seeing: to be determined, estimated 3-5″

Sky Brightness:

General Advice and Resources

Choosing and Finding Targets

Sky & Telescope's Celestial Objects page has expert advice for observing everything from the Moon to distant galaxies. When in doubt, check there. provides monthly printable star charts, together with a list of worthwhile targets. This site is more geared toward beginners than S&T is.

The computer in the CRO storeroom has Starry Night Pro installed on it. It's a planetarium program that, among other features, can give you a visualization of where things are in the sky. S&T's Interactive Sky Chart does the same thing in less detail.

We maintain a list of worthwhile targets, organized by object type.

Light Pollution

If you can choose your observing time, observe late at night — there will be less light pollution once people start going to sleep. You can also avoid most of the glare from the Caltech athletics fields by waiting until after 10:00 PM.

For visual observing, the CRO inventory includes an Orion SkyGlow light pollution filter, which blocks all light blueward of Hβ and all light between [O III] and Hα. Most artificial lights are found in these two bands, so the filter blocks a disproportionate fraction of the sky background. The SkyGlow filter is great for nebulae because most of their emission is in the Hα, Hβ, and [O III] lines, which are all transmitted.

We don't recommend the SkyGlow filter for stars, or any other continuum source, because it will block a good bit of the source light along with the light pollution. We likewise don't recommend the filter for CCD observing: it lets through nearly all the near-infrared light pollution, making it much less effective.

Sky & Telescope has published an online article on objects that are visible through the eyepiece from a city. A large number of them are binary stars, but there's something for everyone. Our list of suggested targets is in part based on S&T's list.

While we do not have a suitable light pollution filter for CCDs, light pollution is only a serious problem for low surface brightness objects. You can get reasonable images of many brighter targets just by integrating long enough to overcome the background noise. The telescope calculator includes the effect of light pollution when calculating exposure times.