Notes on using APM digital finding-charts

APM image parameter catalogues can be used to produce computer generated finding-charts from APM measures of Schmidt plates or other plate material. These APM charts are simple to use and interpret and are normally generated with a set of default properties:

Orientation: Unless specified as 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/POSS2 Bj=22.5; UKST/POSS2 R=21.0; POSS1 E=20.0, POSS1 O=21.5 Not all faint images near the plate limits are 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 and noise-like images are also plotted as open symbols. The size of the plotted symbol is based on the size of the image detected on the original plate material and hence, although related to, is not proportional to the brightness of the objects.

Caveats: While the charts are easy to use there are a few properties that can be confusing at first.

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" may appear on the plot.

The classification of images into stellar, non-stellar is an automated process. There are some important limitations of this procedure. When the lowest measured isophote of two or more images touch the images get treated as a single 'merged' image. Merged objects are plotted as single open ellipses. The size of the ellipse is proportional to the isophotal size at the lowest measured isophote. Many cases are easy to interpret particularly stellar classified elliptical objects. On very rare ocassions objects are 'missed' by the APM since the background following algorithm uses a fixed scale size in order to track the diffuse halos that appear around bright stars or large galaxies. In addition large complex objects such as bright galaxies, scratches and diffraction spikes of bright stars may be 'broken up' into a swarm of many smaller images. These cases are usually obvious.

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