Paramount User Guide
20
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P a g e
Except low down in the sky, the refraction goes roughly as the tangent to ZD, so at ZD = 70 degrees,
or, at 20 degrees above the horizon, it is up to 165 arcseconds. (It reaches a
1800 arcseconds
, or
0.5 degrees at the horizon.)
Refraction is proportional to pressure, so at high-altitude sites the refraction comes down
significantly. For example at Mauna Kea, 4,205 meters above sea level, the refraction is about 60
percent of the sea level amount.
Refraction is roughly inversely proportional to absolute temperature, so at –5C the refraction is
about 10 percent more than at 20C.
Humidity has little effect in the optical, though it matters a lot at radio wavelengths.
The color of the observed object matters, blue being refracted a few arcseconds more than red
at ZD 70.
When you use your Paramount to take long, unguided exposures at modest or longer focal lengths,
refraction becomes an important source of tracking error that is not taken into account in the standard
sidereal tracking rate.
TPoint can be employed to take care of the atmospheric refraction details for you.
TPoint’s Super Model feature and Polar Alignment Report automatically determines the position
of the refracted pole and give recommendations on how to proceed, including how much to rotate
the altitude and azimuth knobs when adjustment is necessary.
It accounts for and applies refraction when acquiring telescope calibration data as well as corrects
the telescope’s position when the mount is slewed.
TPoint’s ProTrack™ feature can be used to apply tracking corrections to the mount based on the
point calibration data.
Polar Alignment Basics
For optimal performance, the equatorial axis of a GEM must be aligned to the
refracted pole
to within 100 arcseconds.
If the mount’s equatorial axis is not “closely” aligned with the celestial pole…
Stars will drift in of the field of view, mostly in declination; quickly when polar
alignment is particularly poor.
When you tell TheSkyX Professional Edition where the mount is pointing, through a
process called “synchronization” (page 25), the synchronization results will be
incorrect, or “skewed” by the amount of the polar alignment error. For example, if
the polar axis is ten degrees “low” in altitude, then the resulting synchronization
positions computed by TheSkyX Professional Edition will be off by this same amount
in declination. The result is that the mount will not point with any accuracy or
repeatability.
