Frequently Asked Questions

The Sloan Digital Sky Survey's goal is to image half of the northern celestial hemisphere (one quarter of the total sky), as well as a smaller portion of the southern sky. The portion of the universe that the survey will observe is represented by our vaguely parachute-shaped logo. We are at the center of the logo, at the vertex of the parachute strings. The elliptical background evokes the shape of a galaxy, the celestial object of greatest interest to the Sloan Survey.

Because the total solid angle (area) of the survey will be 10,000 square degrees, or pi steradians, the logo is sometimes jokingly referred to as "pi on the sky".

Yes, with a few restrictions:

• You must maintain the image credits if you publish or reproduce the image for any purpose other than personal enjoyment.
• You must not use the image in such a way as to imply endorsement for a product or service.
• If you reproduce these images on a web page, we ask that you provide a link back to our site.

Please direct further questions about image usage to the helpdesk.

You are welcome to visit APO during daytime hours. However, there are presently no public tours.

The "Sloan" refers to the Alfred P. Sloan Foundation, one of the major funding sources of the survey. You can learn about Mr. Sloan and his philanthropic foundation at http://www.sloan.org. More information about the survey members is also available on our site.

The problems of man-made satellites streaking across the images are well-known to most astronomers. The Iridium satellites may pose an additional challenge. Many observers have reported "flares" lasting several seconds, during which reflected sunlight makes an Iridium satellite appear almost as bright as Venus!

In general, detecting and removing satellite tracks from the images is a fairly easy process for a few reasons:

• Because the satellites are orbiting very quickly, they will show up as thin straight-line streaks across the entire CCD image. This is a very distinctive feature.
• The Iridium satellites, like many non-geostationary satellites, will travel in polar orbits. The SDSS mosaic camera has 30 CCDs, 5 filters in an east-west direction, 6 CCDs wide in a north-south direction. So a typical polar satellite track will be seen in only one color, another distinguishing characteristic.
• Satellite tracks in one image will not be present in overlapping images taken at a different time.

Rich Kron, survey director, adds: "We've also looked into the possibility of predicting the worst Iridium 'flares' and then ensuring that we choose a scan that won't intersect it. It is not yet clear if it is worth the bother (the coincidence of a flare and our pointing won't be that common), but we have thought about it!"

Bruce Gillespie, site operations manager for Apache Point Observatory, responds: "Light pollution is a big issue here, and considerable time and effort are spent on it. Lighting ordinances have been enacted for Alamogordo and Cloudcroft. We are working to get the entire county, state, and federal installations (e.g. Holloman Air Force Base) similarly cloaked. The skies here are still pretty dark, so the main concern is holding the line against sprawl (which is pretty slow out here). Enforcement of existing codes and educating the public are also important elements of our efforts. In order to work multiple fronts, we employ the time and efforts of many of the local amateur astronomers in lobbying and enforcement activities. We are members of the International Dark Sky Association and make good use of their advice and educational materials."

The first images were taken in the region of the constellations Serpens and Ophiuchus (so about 16h right ascension, and a declination of 0 degrees).

Throughout our galaxy there are clouds or regions of stuff which we collectively call the interstellar medium (ISM). It's made of gas (atoms, ions, molecules) and dust (small grains of ices, metals, silicates, carbon, etc.). The standard lore (Mihalas and Binney, Galactic Astronomy) divides gaseous regions into five basic types:

• cold, dense molecular gas (T = 20 Kelvins, number density > 1000 particles per cubic centimeter): these clouds contain most of the mass of the ISM, but occupy a very small relative volume because they're so cold and dense.
• cold gas (T = 100 K, n = 20 per cc): these clouds of primarily neutral atomic gas occupy about 3 times the volume of the colder clouds above.
• hot neutral gas (T = 6000 K, n = 0.3 per cc): this gas surrounds the colder clouds and occupies about one fifth of the total ISM volume.
• hot ionized gas (T = 8000 K, n > 0.5 per cc): this gas surrounds hot stars and consists mainly of ionized hydrogen (i.e. protons).
• very hot medium (T = 10⁶ K, n = 0.001 per cc): this medium, heated by shock waves from supernovae, occupies most of the volume of the ISM.

A device that produces a representation of a spectrum is called a spectrograph. So for example, a simple prism is a spectrograph. It produces a spectrum by refracting (bending) different frequencies of light by different amounts. An ordinary compact disc is another kind of spectrograph called a diffraction grating; the closely-spaced ridges of the grating diffract different light frequencies at different angles. You could even say that a collection of rain droplets in the atmosphere forms a spectrograph, because of the rainbow they produce!

The Sloan Digital Sky Survey uses a spectrograph that combines the ideas of a diffraction grating and a prism into an instrument appropriately named a "grism." The SDSS spectrograph is designed so that the light from hundreds of galaxies and quasars can be turned into spectra at the same time. The light travels from the telescope's focal plane through optical fibers and then gets split into spectra by the grism. The hundreds of spectra are then photographed by a digital camera. For an example of this, you can look at images and graphs showing the results of Sample Spectra from Early Release of G star (of same type as our sun).