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Q: how to use a reflector telescope? ( Answered 5 out of 5 stars,   0 Comments )
Subject: how to use a reflector telescope?
Category: Science > Astronomy
Asked by: juantxo-ga
List Price: $4.50
Posted: 21 Aug 2002 17:27 PDT
Expires: 20 Sep 2002 17:27 PDT
Question ID: 57156
I have a Tasco Galaxsee, 375x, d=114 mm, f=500 mm, and I want to know
how to use it (proper adjusting from a fixed foint in az/el, use of
the declination knob, etc)
Subject: Re: how to use a reflector telescope?
Answered By: alienintelligence-ga on 21 Aug 2002 23:47 PDT
Rated:5 out of 5 stars
Hi juantxo


The Tasco Galaxsee
Model 46-114375
114mm aperture
500mm focal length
f/4   focal ratio

Maximum theoretical magnification
based solely on math and not real-
world situations, 375x

[ ]
[ ]

Maximum theoretical magnification
based on typical optical performance,

Real-world max magnification, 100x-150x 
100x = (5mm) eyepiece @ 500mm F/L
150x = (3mm) eyepiece @ 500mm F/L

Realistic, Optics-limited magnification,
50x = (10mm) eyepiece {supplied}


General Telescope optics information:
[ ]


This is an excerpt from the
Bushnell manual for an equatorial
mounted scope of theirs, the info
is the same for your scope. It also
has a part detail for the mount, so
you know what parts are being referred



The Equatorial Mount is designed to move in any direction. It
can be set to allow manual controls to track the movements of
celestial bodies across the sky. This is referred to as diurnal
movement; movement of celestial bodies in the direction opposite
to that of the earth's rotation and is around the earth's axis.

By aligning the telescope's polar axis at celestial North, you
will place the telescope in parallel with the earth's axis and
thus be able to locate stars in the sky based on star atlas
information. To compensate for your position on earth, the polar
axis is set in this way:

ˇ Set up the telescope at night. Loosen the Declination Lock
Knob and rotate the telescope around the declination axis until
the arrow on the declination scale points to 90 degrees. Tighten
the Declination Lock Knob. The telescope is now roughly in
parallel with the polar axis.

ˇ Loosen the Horizontal Axis Lock Knob and turn the telescope
until the objective end faces due north. This can be done by
approximating the location of the pole star (Polaris or North
Star) or by the use of a compass. True North is then found by
directing the telescope at Polaris, as magnetic North is
slightly away from true North.

ˇ Look up the latitude of your area in any geographical atlas.
Loosen the Latitude Lock Knob and set the latitude scale to the
correct latitude for your area. Aim the Finderscope at Polaris.
You will probably notice that Polaris is not dead center in the
Finderscope's field of view. This is probably because your
telescope is not absolutely level with the ground. Loosen the
Horizontal Axis Lock Knob again and turn the telescope so that
it is directly aimed at Polaris. Tighten both the Horizontal
Axis Lock Knob and Latitude Lock Knob. Polaris is 1 degree from
the North celestial pole. Therefore, the sighting of stars will
have to be slightly adjusted as you locate them in the heavens.


This page has some great diagrams:
[ ]

Align With the Celestial North Pole

 The final step is to align the telescope with the celestial
pole. First, rotate the telescope in declination to 90 degrees.
This is the declination of the celestial north pole. Second,
rotate the telescope in right ascension until the optical tube
is sitting directly above your north-pointing leg. The dec axis
should point directly at this same leg. Now, look into your
finder scope. Hopefully, the view will be similar to the
illustration at left.

Polaris is flanked by two fainter stars. Together, this group
forms a triangular pattern with Polaris being the brightest star
along the short leg. The triangle spreads across two degrees of
sky. The cross hairs in the diagram are centered at the location
of the celestial North pole. Notice its position with respect to

If you don't see this when looking into the cross hairs, don't
be alarmed. You just have a few minor adjustments to make. Do
not move the telescope in right ascension or declination to
finish the alignment. The mount may have a lock screw that, when
loosened, allows you to move the whole mount around the horizon.
If not, you'll just have to grab a couple of the mount legs.
Rotate the mount around the horizon until Polaris is visible
near the cross hairs. Make sure the declination still reads 90
degrees and the optical tube is sitting directly above that
north-pointing leg. If the cross hairs are centered on a point
close to the illustrated position, then the polar alignment is
accurate enough that a clock drive will move the telescope to
track objects at high magnification for several minutes at

This process may take 15- to 30-minutes the first few times you
go through it. However, polar alignment will soon become second
nature. I spend about five minutes doing this procedure and
often am able to use my Newtonian's clock drive to track planets
at nearly 400X for 30 minutes or more. If you need extremely
accurate polar alignment, then visit my page with instructions
on how to use the declination drift method.


The aforementioned Declination Drift method:
[ ]

The declination drift method allows you to perfect the polar
alignment of your telescope. When finished, your telescope will
track as accurately as possible. That's why this is the method
used by many astroimagers. Even the slightest drift due to
misalignment can ruin an astrophoto. This method allows you
eliminate declination drift, leaving only periodic error and the
resulting right ascension drift to correct during exposures. The
declination drift method requires time and patience. Plan to
spend at least an hour the first time you attempt it. At
minimum, this method requires 30 minutes. If you haven't
performed a rough polar alignment, do so before moving on to the
first step.


This page has basically the same setup info for the mount, but
goes on to describe how to use the setting circles. They are
used to locate the objects you are seeking in the sky.

[ ]

USING SETTING CIRCLES: used to locate objects in the sky - the
telescope MUST be polar aligned .

The Easiest way to use the RA and DEC circles is to calibrate
them from a known object.

1.Find an object in the sky of known RA and DEC (consult a star

2.Place this object in the eyepiece (use a low power one) on your

3.Set the DEC axis setting circle to it's value for declination

4.Set the RA axis setting circle to it's value for right

5.Now move the telescope to the RA and DEC values of the required
object and you should see it in the eyepiece

*Some telescopes have fixed DEC circles and there is no need to
adjust this. It assumes the telescope to be Polar aligned.


An additional setup page, with good photos:

Using the Mount to Locate Objects

Now that we are all set up, balanced and Polar Aligned, we can
use the mount to find objects by their "Celestial Coordinates".
First, you will notice that the Declination Setting Circle does
NOT rotate. It is set at the factory, and needs to remain where
it is. The Right Ascension Setting Circle DOES rotate, and is
changed when we use it to find objects. To begin, we must first
set the scope to a star with a known address. We will use a
"circumpolar star", or one that is visible all year long from a
Latitude of 40 degrees North... Beta Ursa Major, or Merak. This
star is the bottom right star of the bowl of the Big Dipper. Its
Celestial Coordinates are..11 hours, 1 Minute of R/A.. so.. we
get this star in the center of our eyepiece, and quickly set the
R/A setting circle to 11 Hours. The numbers on this circle
represent hours and minutes. on my mount, the divisions are in
10 minute increments. The declination of this star is +56
degrees.. 23 minutes. This should already be where the
declination circle is, if you have set up the mount properly.
Now, moving quickly, because as the stars move across the sky,
the R/A moves with them, find the coordinates of an object you
wish to find, and move the scope on its two loosened axis' until
both the R/A setting circle, (without touching the R/A circle)
AND the Declination circle read the star's address. If you have
done everything correctly, the object should be in or near the
center of the eyepiece in the scope. You MAY need to move the
scope around just a LITTLE to find the object, but it will be
very near to where you are pointing the scope.


Astro Note 13: Using Setting Circles 
[ ]

This note presupposes an equatorial mounting, properly aligned
with the Earth's axis of rotation. If you are using a portable
mounting, the accuracy with which your setting circles can point
your telescope is directly proportional to the accuracy with
which you have aligned your mounting to the pole. A few extra
minutes spent achieving proper alignment can pay big dividends
if you plan to use setting circles

Two techniques to polar align before you begin observing. 
[ ]

A template for a form you can use to record your observations. 
[ ]

The various ways that an object's position in the sky can be stated. 
[ ]


Once you have gotten everything setup, these
are some things to look for:
[ ]

During the years from 1758 to 1782 Charles Messier, a French
astronomer (1730 - 1817), compiled a list of approximately 100
diffuse objects that were difficult to distinguish from comets
through the telescopes of the day. Discovering comets was the
way to make a name for yourself in astronomy in the 18th century
-- Messier's aim was to catalog the objects that were often
mistaken for comets.
Fortunately for us, the Messier Catalog became well known for a
much higher purpose, as a collection of the most beautiful
objects in the sky including nebulae, star clusters, and
galaxies. It was one of the first major milestones in the
history of the discovery of Deep Sky objects, as it was the
first more comprehensive and more reliable list: Only four
objects were initially missing because of data reduction errors,
which could be figured out later though. Today's versions of the
catalog usually include also later additions of objects observed
by Charles Messier and his collegial friend, Pierre Méchain, but
not included in his original list. The study of these objects by
astronomers has led, and continues to lead, to important,
incredible discoveries such as the life cycles of stars, the
reality of galaxies as separate 'island universes,' and the
possible age of the universe.


Here are some good books if you get into checking
out the gems of the night. One called "Star Hopping"
by Robert Garfinkle is really big on the concept
of finding celestial treasures by moving from one
to the other in small jumps or "hopping" as it is
[ ]

This month some nice sights are 
M2, M15, M57, M81 and M82, to name a few
[ ]
[ ]
[ ]
[ ]
[ ]


Lastly... don't get discouraged when everything
you see looks like a dim cottonball at a mile
away in bright moonlight. Unfortunately our eyes
aren't dedicated for such low light viewing that
the actual colors that exist will be lost to us.
Reds will suffer the most, with greens and blues
being most visible to us. Dark acclimation of your
eyes (at least 20-30min total darkness before
observing) and the 'darker the better' skies will
help you get the most from the scope. Don't use
white lights when looking at charts, etc... get
a red light. These are my favorites:

Also, if you can... next 'new moon' goto a
local "star party". I've been a tourguide for
plenty of those, they are a great way to get a
chance to see what those fuzzballs look like 
thru different scopes. It's a great place to
ask questions and get good answers.

Hope this is the info you needed. If you
have a question about what I posted, please
ask for a clarification.


Clarification of Answer by alienintelligence-ga on 22 Aug 2002 02:14 PDT
Hi again juantxo

I forgot to include the search method
for your answer. Aside from being an
avid astrogazer, I searched google
for the terms below for your reference:

-search techniques-
"equatorial mount" setup
[ ://

tasco galaxsee 375
[ ://

"how to use" "equatorial mount"
[ ://

use setting circles telescope
[ ://

messier list
[ :// ]


Request for Answer Clarification by juantxo-ga on 22 Aug 2002 05:11 PDT
Wow...! you give me more info that I can imagine... I haven't had
enough time to read it all, but a brief look at some of them assured
me of having what I need.

However, answer if possible, I was looking al Venus last night and I
see it fuzzy (small city, almost full moon, some ligth around); is
that normal, or my telescope are malfunctioning...?

Best regards,

Clarification of Answer by alienintelligence-ga on 22 Aug 2002 06:00 PDT
Hello again juantxo...

To be sure of the source of the
fuzziness, you have to make sure
that the mirrors in your telescope
are properly aligned and centered.
This is called scope collimation.
[ ]
[ ]
[ ]
[ ]

[ ]
"If you have a long focal ratio of over f/6 your collimation is
much more forgiving. If your scope is faster than f/6,
collimation is much more important. Once you get below f4.5,
collimation is very critical and must be done with great

You're at f/4, so it will be important to make sure you are
"on" with the collimation.

If everything is aligned, you have to take into consideration
the "seeing"
[ ]
[ ]
[ ]

If your skies are clear and the seeing is fine... are
your optics clean and free of smudges, oil, dust, etc?
What eyepiece did you have? The 20mm or the 10mm? If
the 10mm is fuzzy, and the 20mm not... that means you
have exceeded your "seeing limit" and passed the limit
of magnification. Moving to a true star should allow
you to tell if focusing to a point is possible. If the
point source of light gets drawn to any side, that could
indicate the collimation as I stated above. Did you take
a peek at the moon and see if the moon limb and craters
came into clear view?

This page says Venus is in "half phase":
[ ]
"That the near full Moon passes Uranus obviously means Uranus
too is opposite the Sun. On Monday the 19th, the planet, now in
full retrograde motion, will pass its formal solar opposition.
Not to be outdone by such a dim planet, Venus reaches its
greatest elongation east, when it is 46 degrees to the east of
the Sun, the same day as the full Moon, on Thursday the 22nd,
allowing a grand view of the evening sky both to the east (for
the Moon) and to the west (for Venus). This time of year,
however, the ecliptic, near which the planets move, lies rather
flat against the northern hemisphere western evening horizon. As
a result, Venus will not be all that high in the sky even though
it is at its greatest angular separation from the Sun. At that
time, the telescope will show Venus in a "half phase," that is,
we see half the daylight side, half the night. Though the Sun
will now begin to catch up with Venus, the planet will continue
to brighten for another month as it gets closer to the Earth,
reaching greatest brilliancy on September 26. During this time
the planet is seen telescopically as a continuously slimming

Venus and the Moon provide us with subtle and little-known
lighting effects. The blue sky is caused by the blue component
of sunlight scattering off the Earth's atmosphere. The Moon is
illuminated by reflected sunlight, and in very clear sky away
from artificial lighting, you can see the sky take on a bluish
cast. Once the Moon is out of sight, Venus is so bright that in
a fully darkened sky, trees and other objects will cast Venusian
shadows on the ground."

Here are some pages with what you can expect to 
see of Venus:
[ ]
[ ]
[ ]

Venus, being covered entirely by clouds will never
appear truly sharp. If it's as fuzzy as those...
your scope is probably fine. If it's much worse,
it might be optics.

Interesting page that quantifies the limiting magnitude
of your optical system:
[ ]
Um, in English, that means you can tell how dim of
an object you can observe

I learned after I got into astronomy... that as
with most hobbies, the accessories are as expensive
as the "main equipment" My eyepiece collection now
exceeds the cost of any one of my smaller telescopes.
It was a learning experience about the quality of
that final bit of glass before your eyes. Once you
look through an eyepiece that has an 82 degree
field of view like the amazing Nagler eyepieces
[ ] on a scope
of "larger size" 20" or bigger aperature... you will
be ruined ;-)  Stay away from them... lest the fever
hit you.

clear skies!
juantxo-ga rated this answer:5 out of 5 stars

Superb answers, excellent service, and FAST...!

alienintelligence must work in an observatory... he'll predicts your
next question...

Keep on the good work.


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