DFOSC Parallactic Angle Calculator
The DFOSC slit can be aligned with the parallactic angle, though care is needed as the DFOSC can only be rotated in the range -100 to100 degrees, and it is necessary to rotate by +/- 180 degrees if the parallactic angle is outside of the allowed range. This tool will calculate the parallactic angle required for your observation, and willalso tell you the DFOSC rotator value to be used.
Differential refraction of light can lead to slit-losses inspectroscopy. For example, at an airmass of 1.5 the differential refraction between 4000 and 6000 Angstroms is 1.08 arcseconds, whereas at an airmass of 3.0, it is 2.75 arcseconds!
Since differential refraction occurs in a direction perpendicular to the horizon, the ideal slit position angle is also perpendicular tothe horizon. In other words when the object is on the meridian, the position angle of the slit should be 0° or 180°. This angle,called the parallactic angle, will change as the object's distance fromthe meridian changes. As the object is usually close to the zenith onthe meridian the effect is generally small. When observing at largehour angles, however, where the objects typically have a high airmass,the angle will be close to 90° which is why the default setting ofthe instrument is with the slit in the East-West direction.
A fuller discussion of differential refraction and the parallactic angle can be found in: Filippenko, PASP 94, 715.