3.5 Finding Charts

The ability to identify your target by pattern matching is essential. Hours have been wasted observing the wrong object because of misidentifications or while the GO pondered identification of the target.

For stars fainter than 6th magnitude, the GO is advised to bring finding charts. This is true for an offset star as well as for the target. The FES can view a generally circular field of up to 16 arc minutes in diameter. A 10.8 arc minute square field is standard. Finding charts in the form of photographic prints or transparencies are recommended; slides or hand-drawn charts may be adequate. Scales of 6 to 12 arc seconds/mm are most easily compared with the FES image. Charts should show stars as faint as 12-13th magnitude. Transparencies are convenient because they can be rotated and flipped to match the orientation of the image produced by the FES.

As part of the new TOCS system, the HST Guide Star Catalog is now available as part of the offline software. While this has quickly become an invaluable tool, especially with the FES scattered light problem, the observer should not completely rely on it as the only means of identification.

Please keep the following problems in mind:

1. While maneuvers are usually accurate to several arc minutes, occasionally the errors may be as large as 10 to 15 arc minutes, especially for slews with a large beta angle change. There is no guarantee that the bright star nearest the center of the field is your target.
2. BD, SAO, or AAVSO charts are not adequate for targets of eighth magnitude or fainter because there may be field stars nearly as bright as the target that were not included in the catalog.
3. The relative brightness of stars in the FES field may be distorted slightly due to non-uniform reflectivity of the aperture plate.
4. The FES gives no color information. A target which is readily identifiable visually by its unique color must be identified using star patterns.
5. Identifying stars by magnitudes is risky, especially when using magnitudes from the SAO catalogue or a comparable source.
6. Finding charts are recommended for offset stars if they are fainter than about 6th magnitude.
7. Finding charts are occasionally needed even for bright stars which may have optical doubles in the field or may be members of a small open cluster.