The star test is often performed in the field on a real star. However, for critical testing, you will find that an artificial source of light is convenient and less variable. Artificial sources are preferred for a number of reasons:
1. You can, to some extent, control the brightness.
2. With bright sources, the color can be readily altered and adjusted using filters.
3. The close-in nature of the test allows less of the turbulent atmosphere to intervene.
4. Since the source is fixed with reference to the telescope, it requires no tracking. If required, you can clamp the telescope down rigidly.
The beautifully steady pattern that results from use of an artificial source will spoil you. There are some disadvantages, however:
1. The use of an artificial star usually demands a nearly horizontal telescope position. This places maximum demands on the optical mounting cells. Unusual astigmatism, misalignment, or warping may derive solely from the vertical position of the cell.
2. Some of the recommended source placement distances listed in Table 5-2 demand that you look through horizontal spans of 1600 feet (about 0.5 km) although most entries in the table are much less.
3. You need to make a point-like source of light. You must know the diameter of the pinhole to ensure that it is smaller than the resolution of your instrument. Unlike real stars, the source is not always guaranteed to be small.
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