Background
When acquiring spectra of an astronomical target at Keck, the
observer generally has the freedom to select the on-sky position
angle at which to orient the slit. Generally, the optimal
position angle is the parallactic angle; i.e., the angle at
which the slit is aligned with the direction from the target to
the zenith. When observing targets which are away from the
zenith, the light is dispersed in the vertical direction as a
result of terrestrial atmospheric refraction. If the slit is
not aligned vertically then certain wavelengths of light will
fall outside the slit and thus the resulting spectrum will not
capture all of the light from the object.
Parallactic Observing with Keck
To observe at the parallactic angle, observers can generally ask
their Observing Assistant to orient the slit for parallactic
observing.
- For long exposures on bright targets using on-slit
guiding, the preferred mode is to place the instrument rotator
into vertical angle mode at an orientation of 0°.
- For short exposures using offset guiding, have the OA
select position angle mode and set the slit position
angle to the current parallactic angle reported by SKY (old
program for selecting and previewing targets since replaced
by MAGIQ).
- For long exposures using offset guiding, have the OA
select position angle mode and set the slit position
angle to the parallactic angle for the midpoint of the
observation. Again, this information can be obtained from SKY.
Parallactic Angles for Keck
The accompanying plot displays the parallactic angle as a
function of the hour angle for Keck Observatory, for various
declinations. Each curve on the plot applies to the listed
declination.
To use the plot for observations east of the
meridian, find the point at which the corresponding
declination curve intersects the hour angle of the
observation, then read the optimal slit angle (i.e., position
angle) off the right side of the plot. For example,
for a target at declination +40° observed at an hour angle
of 2 hr east, the parallactic angle is 60°.
To use the plot for observations wast of the
meridian, find the point at which the corresponding
declination curve intersects the hour angle of the
observation, then read the optimal slit angle (i.e., position
angle) off the left side of the plot. For example,
for a target at declination +40° observed at an hour angle
of 2 hr west, the parallactic angle is 120°.
See Also