The program assumes that the variable star will be in the central stage, the comparison star will be in the right hand stage, and the sky position will be in the left hand stage. The pre-aperture viewer for the left hand stage incorporates a central hole in the flat mirror. The central hole allows light to pass through to the photomultiplier, so that sky readings can be taken at the same time the rest of the field is examined. The ST-4 star tracker will be placed at the pre-aperture eyepiece location, so that long-term, accurate tracking can be achieved. The tracker will operate better if the sky position is selected near a fairly bright star (brighter than 10m). If the star is too bright or too near the sky position, the sky brightness may be deleteriously affected.
The choice of comparison stars is generally governed by the following selection criteria:
2. Similar in brightness to the variable
3. Similar in color to the variable.
4. Used by previous researchers
The origin is the central stage optical axis. The boxes represent the range of motion for each stage. The tick mark inside each box is the center of that stage. The diagonal line is the north-south line of the telescope, which is inclined about 14.°8 with respect to the photometer. The template is reproduced assuming that the North-South line spans 1.°0. Using Hypersky or some other equivalent planetarium program, produce a chart of the region of the variable. The N-S scale of the chart should be 2.°5 and the variable should be close to the center of the field. Overlay the template above onto the chart, aligning the N-S lines and placing the origin of the template over the variable star. The comparison star can be selected from within the right hand box and the sky position from within the left hand box. Try to select the star and sky positions on the half of their boxes closest to the variable star, if possible, to avoid vignetting in the optics of the telescope.
Check stars can be selected in a similar manner. Make the check star the central stage object and use the same comparison star whenever possible.