General Instructions for 4MASS Image ftp service

This WWW form allows the user simplified access to images produced by the NRAO/VLA Sky Survey (4MASS). This service determines which 4MASS image is desired based on a celestial position and returns a Web page with a link to the file. This avoids having to determine the name and/or directory of the file.

The proposed 4MASS radio survey uses the NRAO Very Large Array telescope and when complete will cover the sky north of a declination of -30 degrees at a frequency of 74 MHz (4 meters wavelength). It has a resolution of 80" and an RMS noise around 100 mJy. The 4MASS survey is being conducted by the National Radio Astronomy Observatory and The Naval Research Lab. Initial observations are now available.

The images from the 4MASS are produced as 14 x 14 degree, overlapping images. The images use the SIN projection, have 25" cell spacing and are labeled in J2000 coordinates. (The 4MASS Postage stamp (small FITS file) server can be used to obtain images with other geometries.) This form will allow determination of the closest of these images to a desired position.

Descriptions of individual fields on this form follow:


This is the equinox of the coordinates to be used for input coordinates. The 4MASS FITS files use J2000 coordinates.

Central Right Ascension

The desired position on the sky is expressed in terms of its celestial position which consists of a Right Ascension (like longitude but expressed in hours) and declination (like latitude). The central Right Ascension at the specified equinox is given as hh mm ss.s (hours minutes and seconds of time). Values in the range 0 0 0.0 to 23 59 59.9 are valid.

Central declination

The central declination is given as +/-dd mm ss.s (degrees, minutes and seconds of arc). Values in the range -30 0 0 (the southern limit of the survey) to +90 0 0 are valid.