Appendix C-Polar - Backyard Astronomer`s Guide

A P P E N D I X
C
Polar Aligning Your Equatorial Telescope
When a first-time purchaser of
it takes to plunk the telescope
an equatorially mounted tele-
mount down with its polar axis
scope reports that “the clock
aimed in the correct direction.
drive doesn’t work—nothing
The time exposure photo at
stays centered,” the trouble
right shows how the mount’s
probably lies not in the drive
fixed polar axis points north to
but in the user’s skill at setting
Polaris. Note that the telescope
up the telescope.
itself is aimed south, toward
Equatorial mounts with clock
drives can provide hands-off
turns around Polaris (as shown
tracking of celestial objects.
by the star trails), the mount
However, for tracking to work
turns about its polar axis (as
as promised, the mount must be
shown by the blurry telescope).
aligned to the celestial pole.
As the sky turns
a target of interest. As the sky
Ah, but how to find Polaris?
NOTE: Go To telescopes
The Big Dipper’s Pointer Stars
on fork mounts can be set up
aim at Polaris, sitting due north
to track objects without polar
and at an altitude above the
alignment, just an initial align-
horizon equal to your latitude.
ment on two stars. See p. 326
To polar align a scope you don’t
of The Backyard Astronomer’s
need compasses, GPS receivers,
Guide for advice on this process.
or calculations of magnetic devi-
through the night it
Polar aligning traditional
ation or sidereal time—just aim
revolves around Po-
equatorial mounts carries the
the polar axis at Polaris. That’s it!
laris, the North Star,
undeserved reputation for
so that Polaris barely
being complex and intimidating
Hemisphere have it easy—they
Indeed, for the perfectionist it
have a bright “North Star” to
night. Polar aligning
can be time consuming. But for
aim at. Polar aligning in the
is a simple matter of
the vast majority of backyard
Southern Hemisphere takes
stargazers, polar alignment need
more skill as there is no conve-
take little more time than what
nient “South Star.”
moves through the
aiming the mount’s
polar axis at this star.
Observers in the Northern
Up the Axis
The yellow arrow shoots
through the polar axis of
this German equatorial
mount. This axis needs to
be set at an angle equal to
your latitude on Earth, then
at night aimed due north to
Polaris (or due south if you
live in the southern hemisphere below the equator).
Aligning a Fork
This SCT is set to 90°
declination so the fork
arms and main tube both
aim to the celestial pole.
With the scope set like this,
use the wedge’s altitude
and azimuth adjustments
(or shift the tripod legs)
to aim the finderscope,
and therefore the main
telescope, at the pole. It
is important to adjust the
finder first so that it indeed
does point to exactly the
same place in the sky as
do the main optics. Note
that aligning a Go To forkmounted scope in this manner is necessary only if you
wish to take longexposure astrophotos.
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The Quick Way
to Align
More Accurate
Methods
Rigorous, time-consuming methods of
precise polar alignment are necessary only
for advanced astrophotography. For general
observing, snapshots of the Moon or wideangle piggyback exposures, an alignment
within one or two degrees of the celestial
pole will be adequate. This is accomplished
in a few seconds by aiming the polar axis
toward Polaris, the North Star, as closely as
possible.
Schmidt-Cassegrains can be aimed by
peering up one of the fork tines and raising
or lowering adjustable tripod legs to achieve
approximate alignment. Precise leveling is
a waste of time for casual point-and-look
viewing. The polar axis of German equatorial mounts can be eyeballed toward Polaris,
as shown at left and on the previous page.
For demanding applications, the telescope’s
polar axis should be within five arc minutes
of the true celestial pole. The north celestial
pole is conveniently near Polaris, the end
star in the handle of the Little Dipper. To
be exact, the true pole lies 0.9 degrees from
Polaris in the direction of Alkaid, the end
star in the handle of the Big Dipper.
For observers in the southern hemisphere, locating the south celestial pole is a
little more difficult. It lies one degree from a
5.4-magnitude star in Octans called Sigma
Octantis, a barely naked eye star.
The finder charts included opposite and
on page C7 should help you zero in on the
celestial pole, north or south. With charts in
hand, the next step is to aim the telescope’s
polar axis at the pole.
North Celestial Pole Finder Chart
The Dipper’s Pointer stars aim at Polaris. Then, use this finder chart to help you aim your telescope’s polar axis toward the precise location of the North Celestial Pole. Keep in mind that simply centering Polaris will provide sufficient accuracy for most visual purposes.
Photography, use of analog setting circles,
and precision Go To aiming with computerized
German equatorial scopes all require aligning on the
true pole. To find it, imagine its location amid a triangle
of stars with Polaris forming one vertex of the triangle.
This helpful triangle lies on the side of Polaris opposite
another faint star just off Polaris. This true pole position
lies away from Polaris toward the direction of Alkaid,
the end star of the Big Dipper’s handle. The yellow circle
marks the field of a typical 6°-wide finderscope. Charts
courtesy Starry Night Pro®/Space.com.
NORTH
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POLAR-ALIGNING FORKMOUNTED TELESCOPES
Calibrating the Declination
Circle
To locate the pole with
the finderscope, the declination setting-circle reading must be accurate. In
other words, when the telescope is set at 90 degrees
declination, the tube must
be aimed at the same spot
that the polar axis is. Declination circles can slip, so
a setting of 90 degrees may
not in fact be 90 degrees.
Swing a fork-mounted
telescope in declination so
that it parallels the forks
as closely as the eye can
judge. For German mounts,
move the instrument to
bring its tube parallel to
the polar axis. Look into
the eyepiece of the main
telescope at low power, and
watch the stars (any stars)
as you rotate the telescope
around the polar axis. Do
the stars circle the center
of the field? If the telescope
is truly set to 90 degrees
declination, they will.
If not, move the telescope
slightly in declination to see
whether the situation improves. Keep adjusting the
declination until the stars
move in concentric circles
when the instrument is
rotated in right ascension.
Now, loosen the declination
circle (they are designed to
move), and set it to show
90 degrees. Once tightened, the declination circle
should not need calibrating
like this again.
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Which one is the polar axis? In forkmounted telescopes such as SchmidtCassegrains, the polar axis is the one around
which the forks revolve. The other motion,
which swings the tube up and down
through the fork arms, is the declination
axis. To be polar aligned, the polar axis, and
therefore the fork tines, must be aimed at
the celestial pole. This requires a “wedge,”
usually optional on Go To models.
1. First, adjust the altitude setting on the
wedge to an angle equal to your latitude.
From a latitude of 40 degrees North, set
the angle on the latitude scale to 40 degrees. This can be done at any time, even
indoors.
2. At the observing site, place the telescope
so that its forks are aimed northward.
Roughly level the telescope tripod if you
wish, but precise leveling is not necessary.
3. Swing the tube so that it reads 90 degrees
declination on the circles on the side of the
tube, and lock it there. This should put the
tube parallel to the forks. For Go To scopes
lacking circles use the software routines in
the controller to aim at Polaris.
4. Move the telescope left to right to cen-
ter the pole in the finderscope (or for rough
alignment, centering Polaris will suffice). Do
this by moving the whole tripod or by using
the fine azimuth adjustments on the wedge.
Do not alter the telescope tube’s declination
or right ascension.
South Celestial Pole Finder Chart
Locating the South Celestial Pole is a challenge. It lies in one of the blankest regions of sky, with no bright “South Star” nearby. Alpha
and Beta Centauri (called the Pointers) aim at the Southern Cross, while the upright of the Cross points across the sky to the Pole.
5. Move the telescope up and down to cen-
ter the pole in the finderscope. (For this to
work the finder must be aligned so it points
precisely where the main optics point, an
adjustment you can perform in the daytime.) This may mean raising or lowering a
tripod leg (it is usually best to have a tripod
leg pointing south for this) or using fine
altitude adjustments on the wedge. Again,
do not move the declination axis.
6. It may be necessary to adjust the azimuth
and altitude a few times to refine the aim
point. With practice, you’ll find that the
process takes only 5 to 10 minutes. To aim
at the north celestial pole, move the entire
telescope so that the finderscope cross
hairs are 0.9 degrees from Polaris along a
line toward the end star in the Big Dipper’s
handle. If that star is not visible, use a line
joining Polaris and Epsilon Cassiopeia, the
first star in the distinctive W shape, but still
offset toward the Big Dipper’s handle.
To locate the South Celestial Pole, pointer
lines drawn from Achernar and Canopus help,
as well as imagining the pole forming a triangle with the
Large and Small Magellanic Clouds. The field lies off
one side of the faint triangular pattern of Octans. Fifth
magnitude Sigma Octantis is the brightest star in the
finder field near the pole. Look for a semi-circle of stars
that contains Sigma. Nearby, a smaller and fainter circlet helps nail the precise location of the pole. The yellow
circle marks the field of a typical 6°-wide finderscope.
Charts courtesy Starry Night Pro®/Space.com.
SOUTH
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Polar Scope Option
Many modest and premium
German equatorial mounts
(such as this Chinese-made
EQ3 model) have polar axis
sighting scopes available
as worthwhile options. (The
lowest cost entry-level
mounts lack hollow polar
axes.) While it is possible
to roughly polar align by
sighting through the empty
polar axis, a sighting scope
like this makes it easier
to zero in on the true pole.
The large bolt seen here
moves the mount up and
down in altitude (needed
when changing latitude).
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The main problem with this method is
that it can be difficult to find the correct pole
location, since it lies in a blank area of sky.
Moreover, it is easy to move off the pole star
by the required amount but in the wrong
direction. In straight-through finderscopes,
the sky appears upside down, in rightangle finderscopes, the sky is right side up
but flipped left to right. Most finderscopes
have a field of view about six degrees wide,
which means that when the true celestial
pole is in the center, the pole star (Polaris or
Sigma Octantis) is about one-third the way
from the center to the edge of the field.
ALIGNING GERMAN
EQUATORIAL MOUNTS
The finderscope method described above
can be applied to all telescopes on German
equatorial mounts. The declination axis on
such mounts has the telescope on one end
and the counterweight on the other. The polar axis—the one the clock drive turns—has
the declination axis attached to it and is the
part of the mount that must be aimed at
the pole.
First, set the angle of the polar axis to
your present latitude with the adjustment
at the base of the mount—usually a large
bolt with a graduated dial showing 0 to 90
degrees. Extra care should be taken, though,
because the whole mount can flop down
when this bolt is loosened. If the telescope
has a graduated dial, set the latitude and
tighten the bolt. The latitude adjustment
should be made only once, when the instrument is purchased, unless the telescope is
transported north or south to a new latitude
(traveling east or west makes no difference).
If the equatorial mount does not have a
graduated circle for a local latitude setting,
follow the steps in the next paragraph; otherwise, skip ahead.
Latitude adjustment: At the observing
site, place the telescope so that the polar
axis aims as close to Polaris as possible using
the eyeball method. Adjust the tripod legs
to level the base of the mount. (This is one
case when you do have to level the mount.
Some mounts have bubble levels for this
purpose.) Swing the tube in declination so
that it is at 90 degrees as read on the declination circle—the circle nearest the tube or
the counterweight. The tube is then parallel
to the polar axis and is pointed in the same
direction. Lock both axes. Carefully loosen
the bolt that clamps the tilt of the polar
axis, and adjust it until Polaris is seen in the
finderscope midway between the top and
bottom of the field (not necessarily centered, just midway). Now, tighten the bolt,
and that should set the latitude angle. This
procedure is necessary only once.
After the latitude adjustment is made
and the telescope is leveled, the polar axis
will be at the correct angle if it is aimed toward Polaris. On subsequent setups, with
the tube at 90 degrees declination, use the
fine altitude and azimuth adjustments on
the mount to move the telescope left and
right and up and down to center the pole
area in the finderscope. If your telescope has
no fine adjustments, alter the height of the
south-pointing tripod leg and nudge the
tripod left or right.
Most German equatorial mounts with
Go To computers also need to be polar
aligned in order to find objects well. This can
be done with the old-fashioned method (i.e.
sighting up the polar axis). However, software routines can automatically swing the
scope to where Polaris should be. You then
use the mount’s altitude and azimuth adjustments to center Polaris to polar align.
Polar Alignment Sighting Scopes
Polar-alignment scopes in the polar
axis have reticles that show how far
to offset from Polaris in order to center on the
true celestial pole. (Other marks help in lining up the South Celestial Pole.) This is the
pattern in one type of polar scope, but others
are similar, with spots for key “guide stars”
or at the very least a crosshair to show you
the line from the Dipper through Polaris, to
the true pole, and on to Cassiopeia. Don’t fuss
with dialing in sidereal time—simply rotate
the polar scope or entire polar axis so that the
guide star slots or Dipper-to-Pole line coincide
with the real sky. Note that polar scopes invert
the image so the true pole appears here to be
offset toward Cassiopeia from the pole star.
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More Precise
Methods
Serious astrophotographers require stars
to stay within a few arc seconds of their
intended spot for an hour or more. This
requires high-precision alignment.
Changing Latitudes
The adjustments at
the base of this and most
equatorial mounts are suitable for the fine positioning needed to polar align.
After a major move north
or south in latitude, you
may need to loosen the
main bolt(s) on the altitude
axis to raise or lower the
mount by a large amount to
set it for your new latitude.
Traveling east or west but
staying at the same latitude
makes no difference to your
mount’s adjustments. For
much more information on
basic setup and operation
of beginner’s telescopes,
see Chapter 6 of The Backyard Astronomer’s Guide.
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THE SINGLE-STAR METHOD
This technique was described by Dennis
DiCicco in the December 1986 Sky & Telescope. With practice, it takes only 10 minutes.
First, follow the steps in the previous section
to align the mount with the celestial pole.
Then aim the telescope at a bright star near
the celestial equator whose right ascension
is known, preferably in coordinates for the
current year (see the table opposite).
With the star centered in the eyepiece,
rotate the right ascension setting circle so
that it displays the star’s right ascension.
Now, swing the telescope back until the
circles show the coordinates of Polaris
(2002: R.A. 2h 34.5m Dec. +89° 16’). Move
right ascension first, then declination. When
swinging the mount in declination, be sure
to stop at the first 89-degree setting. Do
not go past the 90-degree mark to the 89degree mark on the other side. Lock the
mount in right ascension and declination.
Don’t worry if Polaris is not in the field.
Using the mount’s fine altitude and azimuth adjustments, move it until Polaris is in
the center of the field of a medium-power
eyepiece. Do not move the declination or
right ascension motions. Once Polaris is in
the center, unlock the telescope and swing
it back to the calibration star. Adjust the
right ascension circle again if necessary.
Repeat the procedure. Each repetition
should require fewer and fewer adjustments. If the starting position was fairly
close, only a couple of iterations should be
needed to zero in on the pole. As DiCicco
points out, the method also works in the
southern hemisphere with Sigma Octantis
and its coordinates (2002: R.A. 21h 10.6m
Dec. -88° 57’).
If this technique is used often, keep the
pertinent coordinates handy in a logbook.
Or if the telescope is set up in the same spot
every night, mark the ground so that the
tripod returns to the same orientation.
THE TWO-STAR METHOD
This procedure is more time-consuming but
for perfectionists, it is the method of choice.
When setting up a permanent site or a backyard observatory, it is also the best way to
achieve the final alignment of the mount.
First, use a simpler method to roughly
polar-align. Then aim the telescope at a star
on the celestial equator due south. If possible, put an illuminated-reticle eyepiece in
the telescope, and align the cross hairs so
that they run parallel to the lines of right
ascension and declination motion. Ensure
that the drive is running. Now, watch the
star carefully. Ignore any drift it makes east
or west in right ascension, but watch for a
drift in declination, that is, north or south. It
may take a few minutes to show up.
• If the star drifts north, the polar axis is
aimed too far west (it is to the left of the
actual pole in the northern-hemisphere).
• If the star drifts south, the polar axis is
aimed too far east (to the right of the pole).
Be careful. Make sure you know which
way north is in the eyepiece. Move the
mount in azimuth in the appropriate direction, then go back to the star, and watch
again. Has the drift improved? No drift
should appear even after 20 minutes.
Once this stage is satisfactory, point the
telescope at another star on the celestial
equator, but one that is rising in the east.
Observe it for a while, again ignoring any
drift in right ascension.
• If the star drifts north, the polar axis is
aimed too high (it is above the pole).
• If the star drifts south, the polar axis is
aimed too low (it is below the pole).
Adjust the altitude of the polar axis accordingly. As long as the initial setup was
good, only a small adjustment should be
required. Now, go back to the east star,
and watch again. The drift should improve.
Repeat all the steps. If this is done in the
southern hemisphere, substitute south
everywhere we have said north, and vice
versa. Clearly, such a tedious procedure is
best reserved for permanent setups or for
times when only perfection will do.
Calibration Stars for “Single-Star” Polar Alignment
Star
Name
α Andromedae
α Arietis
Alpheratz
Hamal
α Tauri
α Canis Minoris
R.A.
(2003)
Dec.
Sky
00h 08.6m
02h 07.4m
+29° 07’
+23° 29’
Northern Autumn
Northern Autumn
Aldebaran
Procyon
04h 36.1m
07h 39.5m
+16° 31’
+05° 13’
Northern Winter
Northern Winter
α Leonis
α Bootis
Regulus
Arcturus
10h 08.6m
14h 15.8m
+11° 57’
+19° 10’
Northern Spring
Northern Spring
α Scorpii
α Aquilae
Antares
Altair
16h 29.6m
19h 51.0m
-26° 26’
+08° 53’
Northern Summer
Northern Summer
α Eridani
α Carinae
α Cruxis
α Centauri A
α Piscis Austrini
Achernar
Canopus
Acrux
Rigel Kentaurus
Fomalhaut
01h 37.8m
06h 24.0m
12h 26.8m
14h 39.9m
22h 57.9m
-57° 13’
-52° 42’
-63° 07’
-60° 51’
-29° 37’
Southern Hemisphere Sky
Southern Hemisphere Sky
Southern Hemisphere Sky
Southern Hemisphere Sky
Southern Hemisphere Sky
Alignment, Go To Style
Some computerized Go To
telescopes, such as the
fork-mounted models, do
not require polar alignment to find and track
objects. However, they do
need to be initially aimed
at two stars, a process
also called “alignment.”
However, Go To scopes on
German equatorial mounts,
such as Meade‘s LXD55
and Celestron‘s CGE and
Advanced Series, do require at least rough alignment of the polar axis to
the celestial pole in order
to find and track objects accurately. Meade‘s Autostar,
Celestron‘s NexStar, and
systems from other manufacturers contain software
routines to aid polar alignment that are computerized
versions the “single-star”
method described opposite
— they automatically aim at
where Polaris should be.
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