Paramount MX User Guide
16 | Page
If everyone reckoned time based on the Sun crossing the meridian, then each longitude on earth would
have a different time of day. That means noon, or the time the Sun crosses the meridian, would come a
few minutes earlier for someone living 60 miles to the east. Only those people living at the same
longitude would share a common time.
In the late 1800’s time zones were established to minimize the problem of having different time in
populous regions. The time within these zones is called zone time. Zone Time places all locations on
Earth into various time zones. By definition, time zone zero is at zero degrees longitude, and increases
by one each 15 degrees in longitude (more or less).
The Paramount MX relies on TheSkyX Professional Edition having the correct time zone for your
observing site. When TheSkyX Professional Edition’s time zone is not correct, the position of the
telescope cross hairs will be offset by the time zone hour error when viewing horizon-based Sky Charts.
Atmospheric Refraction
The effects of atmospheric refraction on the position of celestial objects, as well as its effect on the
sidereal tracking rate, are often overlooked or even ignored by many amateur astronomers.
The refraction nuisance (and other system errors like tube flexure) means the “sidereal tracking rate” is
simply not good enough to precisely track objects. Refraction also displaces the position of the celestial
pole and makes precise polar alignment more difficult.
Some other interesting and significant facts about how refraction affects an object’s apparent position
include:
• For a sea-level site, the refraction at 45 degrees zenith distance (ZD) is about 60 arcseconds
(one arcminute).
• 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 MX 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.
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