Reference Coordinates and Standard Fields

Bright Stars for Setting Circles

Standard Photometric Sequences (Landolt et al.)

Stetson Photometric Sequences

LONEOS Quasi-Standards

These are bright stars useful for setting telescope setting circles and synchronizing a telescope with the sky. This is an Acrobat file in PDF format.

These are fields which contain standard stars for BVRI photometry. Observations of such standards are required to derive standandard magnitudes and colors. These fields may also be used to determine the faintest magnitudes recorded in an image (limiting magnitudes). The above link is to an Acrobat file in PDF format which lists the coordinates for the fields, the field sizes, and the numbers of standards in each field. Many of these fields are based on the Landolt standards. This file is only a partial listing of the available Landolt fields. Finding charts for the stars and tables of the standard magnitude and color values are available from the literature references indicated in the PDF document. The SSU Observatory also maintains a ring binder of copies of all the appropriate articles. (Ask the Observatory Director for information about this data.)

The following table lists some examples of some Landolt fields. The standards in these fields are on the UBVRI system of Landolt (Cousins-Johnson).

The links on the field names are to finding charts on which the standards are identified. In these charts north is up and east is to the left. Unfortunately, the scale is slightly different for each chart. A guide to the scale and some comments for each of the finding charts is given below.

The sizes of many of the Landolt fields are technically about 60 minutes of arc, but the standards tend to be concentrated in smaller regions within the field. A field size of 20 minutes of arc can generally be positioned to accomodate a large fraction of the total number of standards in each field. The coordinates that are given in the table were selected to be in the central region for each concentration of standards. Several of the Landolt fields are small at only about 5 minutes of arc on a side. These smaller fields typically only have about 5 stars, but they often cover an excellent range in color.

The "data" link under the name of the field will display the data for the standard stars identified in the field. The data available includes precise coordinates for 2000.0 along with the magnitudes and colors for the standards. The detailed description and format for this data is available in the Landolt Data Format file.

Landolt has published several articles related to this project.

Landolt, A. U.; AJ 104, 340-371 and 436-491 (finding charts) 1992

This article also describes the technical specifications that define the instrumental magnitude system used by Landolt. This defines the Landolt system, although the published magnitudes and colors have been transformed to the standard Johnson-Cousins UBVRI system. The "Landolt system" is now the defacto standard for UBVRI photometry.

Landolt, A. U.; AJ 78, 959-981 and 989-1020 (finding charts) 1973

This is the first paper in the series. This paper defines the primary Landolt fields, but only includes data for UBV photometry.

Peter Stetson and his collaborators have expanded and extended the Landolt standards to fainter objects in small fields more appropriate for CCD images. Faint CCD sequences have been established in some of the Landolt fields, and new standard sequences have been established in the vicinity of some interesting objects such as clusters and galaxies. All these new standards have been carefully tied to the original Landolt system. This information is available on the web from the Canadian Astronomy Data Center. Click on Stetson under Photometric Standards to access this data. The information available includes finding charts (GIF format images from the Second Generation Sky Survey, plus FITS images), precise astrometric positions for the stars, and the photometric indices. Stetson will continue to update this data and add new standards.

The finding charts do not include specific identifications for the standard stars so identification of the standards in locally obtained images will be somewhat more difficult than for the Landolt standards. The Stetson fields are normally smaller than 20 arcminutes and can contain hundreds of standards in each field. The Stetson standards are typically fainter than 14th magnitude, but in the Landolt fields data is also included for the Landolt standards.

Stetson has published a description of this project (Stetson, P. B.; PASP 112,925-931, 2000).

Note that the above link is to an FTP server that will download the LONEOS Catalog to your browser. The catalog is simply a list of star names, precise coordinates, and reported photometry. The list is sorted by coordinates. The literature references on which the catalog is based are also available

Brian Skiff of Lowell Observatory continues to compile published (and unpublished) observations which could be considered "secondary standards" or quasi-standards. This is the LONEOS Catalog. Observations are merely compiled from the literature. Data is taken from published papers as reported by the original authors as being used as secondary standards in the fields where they were working. This catalog is obviously not necessarily on a homogeneous photometric system such as provided by the Landolt and Stetson standards. However, if you are desperate (and lucky) you may find that someone has already attempted to establish standard magnbitudes and colors for a star that happens to be in your field. A local copy of the LONEOS Catalog is available.

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For possible addition... here or elsewhere...

The stars contained in the standard ServoStar database.

The non-stellar objects contained in the standard ServoStar database.