The NexStar User's Guide II - Excerpt from
Choose Stars with Good Separation and Best
Placement in the Sky
Most of the alignment methods require you point at two or more alignment
stars or planets. StarSense and SkyPortal allow you to select several additional
alignment points but the NexStar hand control is limited to only two. (SkyAlign
requires three but only the two most widely separated are used to build the sky
model.) There are a couple of general rules to keep in mind when choosing the
Selection of stars varies slightly depending upon whether you are using the
scope in alt-azimuth mode (mounted directly on the tripod) or equatorial mode
(mounted on a wedge). Here are the main selection criteria:
- In alt-az mode, if you are in the Northern Hemisphere, using Polaris as an
alignment star is fine, but, on a wedge, it is important to avoid using Polaris
as an alignment star.
- Choose alignment stars that are well separated. I recommend a separation of
between 90 and 135 degrees. In general, consider your options and choose the two
that are farthest apart.
- Avoid alignment stars below 20 degrees in altitude (angle above the horizon).
Stars too close to the horizon are displaced optically by the effect of
refraction through the Earth's atmosphere.
- In alt-az mode, avoid stars above 70 degrees in altitude. Aligning on stars too
high in altitude exaggerates inaccuracy in centering the star due to the high
number of motor turns required to move small increments in azimuth. The
equivalent rule when wedge-mounted would be to avoid stars above 70 degrees or -70 degrees