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Finding Chart Foibles

• ORIENTATION: Unless specified otherwise the charts have conventional astronomical orientation, North to the top, East to the left.

• SCALE: the scale is given by the diameter of the central cross which is always one arcminute in size.

• MAGNITUDE LIMIT: this can vary, and depends on the plate source material. The limits of the commonly used plates to within 0.5 magnitude are: UKST B=22; UKST R=21; POSS1 E=20, POSS1 O=21.5. Not all faint images at the plate limits will be real.

• COORDINATES: The position given on the chart is the position of the centre of the cross-hairs. The Equinox can be B1950.0 or J2000.0 and the positions of the objects are as measured at the Epoch of the source plate material for the chart. The plate epoch is only likely to be relevent in the case of high-proper motion stars.

• PLOT CONVENTIONS: Stellar-like, unresolved objects are plotted as filled symbols. Non-stellar objects are plotted as open symbols. Merged images are considered to be non-stellar. The size of the plotted symbol is based on the size of the image detected on the original plate material and hence is not necessarily proportional to the brightness of the objects.

• CAVEATS: While the charts are easy to use there are a few properties that may be confusing. The APM data is stored in files corresponding to the original plate source material. These plates are finite in extent. If the target object is close to the edge of the measured area substantial portions of "blank sky" can appear on the plot.

The classification of images into stellar, non-stellar is an automated process. There are some important limitation of this procedure: When the lowest measured isophotes of two or more images touch the images get treated as a single 'merged' image. Merged objects are plotted as single open ellipses with the size of the ellipse proportional to the isophotal size at the lowest measured isophote. Many cases are easy to interpret but sometimes one component of a merged image can be quite faint whilst the combined image is still significantly elliptical. On very rare ocassions objects are 'missed' by the APM since the background tracking algorithm follows the diffuse halos that appear around bright objects. Because of this it is possible for large complex objects such as bright galaxies, scratches and diffraction spikes to be 'broken up' into a swarm of many smaller images.