Paramount GEM User Guide
19 | 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 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 compared to sea level.
• Refraction is roughly inversely proportional to absolute temperature, so at –5 C the refraction is
about 10 percent more than at 20 C.
• 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.
Refraction also becomes an important consideration when taking unguided exposures at modest or longer
focal lengths. It effectively alters the “sidereal tracking rate” and, left uncorrected, produces star
elongation. The Paramount-specific ProTrack™ feature uses the TPoint pointing model to update the
telescope’s position to product pinpoint stars.
• TPoint’s Super Model feature and Polar Alignment Report automatically determines the position
of the refracted pole and gives recommendations on how to proceed, including how much to
rotate the altitude and azimuth knobs when adjustment is necessary.
• TPoint 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™, based on the point calibration data (page 160), continuously updates the
mount’s position in both hour angle and declination to compensate for tracking errors that are
caused by refraction, tube flexure, and other systematic errors.
Polar Alignment Basics
For optimal tracking performance, the polar axis of a GEM should be aligned to the
refracted pole to within about 100 arcseconds.
Without a TPoint model and ProTrack™ engaged, 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; more quickly when polar
alignment is particularly poor.
• TheSky Professional knows where the mount is pointing through a process called
pointing calibration (or calibration for short). Telescope calibration is the process
of slewing the mount to different mechanical positions and recording the pointing
error at each position. Once at least six calibration points are collected, a TPoint
model can be created to correct the telescope inherent pointing errors. With no
calibration, the mount’s position will be wrong, or “skewed” in part by the amount
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