Sky & Telescope - June 2019 - 66

Imaging Tips

bearings in the focuser, or by replacing it with an after-market
model that can better support the camera's weight.
You can distinguish the effects of miscollimation from
other problems by comparing the stars in short and long
exposures. Its appearance in images is unaffected by exposure
length, which can help to eliminate other issues such as poor
tracking or ﬁeld rotation.
An additional concern with deep-sky imaging is ensuring a camera's sensor isn't tipped within the focal plane of
the imaging telescope. Large sensors are more sensitive to
these tiny misalignments that mimic the effects of poor
collimation but aren't corrected by adjusting the optics or
the focuser. Images affected by sensor tilt appear the same
regardless of where the telescope is pointed. Some astronomical cameras equipped with large detectors include a push-pull
adjustment plate on the front of the camera that permits you
to tweak the alignment of the sensor.
Most refractors and Maksutov-Cassegrains are collimated
at the factory and don't require collimation by the user
(although the camera and focuser may require adjustment as
noted above). See articles on collimating a Schmidt-Cassegrain (S&T: Feb. 2018, p. 28) and Newtonian reﬂectors (S&T:
April 2019, p. 68). Specialized collimating tools such as those
offered by Hotech (hotechusa.com) permit collimation of
Newtonian and Cassegrain optics during the day.

such as the length of the exposure, the target's location in the
sky, the telescope's focal length and pixel scale, and even the
difference in pointing angle between the guide star and the
imaging target. Richard Hook published equations concerning
polar alignment in the February 1989 issue of The Journal of the
British Astronomical Association. One of these equations tells us
how well-aligned a mount needs to be for a given setup:
E=

In the equation, E is the maximum permitted polar alignment error in arcminutes, and S represents the tolerance for
ﬁeld rotation in microns. D is the declination of the target,
while T is the exposure duration in minutes. Focal length (in
millimeters) is represented by F, and A is the angle in degrees
between the guide star and the opposite edge of the ﬁeld.
This equation conﬁrms that short exposures and short
focal lengths combined with larger pixels are more tolerant of
imperfections in polar alignment. In other words, the higher
your pixel-per-arcsecond ratio is, the longer you can expose
with less-than-perfect polar alignment.
If you know how accurately polar aligned your mount
is, the equation can be rearranged to answer the question
"Given my current polar misalignment and equipment,
what's the longest exposure I can achieve before stars appear
elongated?" like this:

Polar Alignment
Even the ﬁnest equatorial mount won't produce images with
round stars if it isn't properly aligned. Polar alignment is
when the polar axis of the telescope is made parallel with
the rotational axis of Earth. Our planet rotates around this
axis, making stars appear to spin around the north or south
celestial pole. As an object moves
through the sky from east to west, a
properly aligned mount will cancel
out this movement, keeping the
object centered in the telescope.
Misaligned equatorial mounts (and
alt-azimuth Go To mounts) can
keep an object centered for visual
observation but will slowly introduce ﬁeld rotation, which makes
such setups unsuitable for longexposure astrophotography.
So how accurate does polar
alignment need to be for tracked
images to have round stars? The
answer depends on several factors,
u AUTOGUIDING Most astrophotography rigs require autoguiding to keep the
telescope pointing exactly at the target
throughout the entire exposure. This setup
includes a 10-inch ASA f/3.8 Newtonian
astrograph that has a piggybacked 80mm refractor for autoguiding.

J U N E 2 019 * SK Y & TELESCOPE

45,000 × S × cosD .
T×F×A

45,000 × S × cosD .
E×F×A

When shooting with an image scale of 1 to 4 arcseconds
per pixel, polar misalignment by as much as 3 arcminutes is
adequate for exposures up to about 15 minutes long.
Some telescope-control software and autoguiding software
programs include tools that make
precise polar alignment relatively
simple. Hardware tools such as
the QHYCCD PoleMaster are also
available (S&T: July 2018, p. 62).
The tried-and-true manual method
of drift alignment takes longer but
works well even if you don't have
a clear view of the celestial pole.
Numerous drift alignment tutorials
are available online.

Autoguiding
While good polar alignment, tracking, and collimation can improve
the roundness of the stars in our
images, many telescope mounts
require a little help staying on target. This is because most mounts use
gears that produce a repeating error
known as periodic error (PE), which

http://www.hotechusa.com

Sky & Telescope - June 2019

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