You should have come to this page from an article describing my experiences with my
Meade ETX-70AT telescope. If you have not read that page, here is a link to the page.
One of the problems I have with the ETX-70AT involves finding objects in the sky.
The scope is a computerized GOTO scope -- that is, its handheld computer controller
contains a database of several thousand astronomical objects. To use the GOTO
function, set up and align the scope -- the first few times you do this it seems complex
but it's really fairly simple. Once the scope is aligned, find the object you want
to view in the computer controller, press ENTER, then press the GOTO button and the scope
moves to the object.
My experience is that while alignment is a simple process, it can be frustrating if the
scope does not find the alignment star -- mine does not always put the alignment star(s)
in the FOV, leaving me to search for them. However, once the scope is aligned, when
I tell it to GOTO an object, it does so without error -- align, GOTO Jupiter, and there is
Jupiter dead center.
I believe there are two reasons for a finder scope on the ETX-70AT:
- Very helpful with alignment.
- Need a finder scope if you use the scope without the GOTO function.
Before we proceed, let's review how to align the scope
- Set up the scope with the tube level and pointed toward celestial north. Celestial
north is the North Star -- Polaris. So, when the scope is set up, before turning it
on, the tube should be level and pointed at a point on the horizon below Polaris, not
tilted up in the sky pointed to Polaris.
- Turn on the scope's power, forward through the warning, and the scope will ask for date,
time, daylight savings time ON or OFF. Assuming you have already told the scope your
location, it will now go to ALIGN.
- Select EASY, ONE STAR, or TWO STAR alignment and press ENTER.
- Let's say you have selected TWO STAR alignment. The scope asks you to select the
first star -- for example, last night I selected Dubhe as the first star. The scope
slews around to where Dubhe should be -- the controller then tells you to CENTER DUBHE
THEN PRESS ENTER. (After finding and centering Dubhe, I selected Sirius as my second
How the finder scope helps with alignment
Unless you have aligned the telescope perfectly -- and have a bit of good luck -- your
first star will not be in the field of vision of the scope -- or it will be on the edge of
the FOV. You need a finder scope to help you find the stars for alignment.
Without the finder scope, you are left to manually slew the scope up, down, left,
right, all the while sighting down the tube or looking through the eyepiece, trying to
find the alignment star. With a finder scope, you have a wider FOV through which to
look as you move the scope around. And, the star may well be in the FOV of the
finder scope allowing you to center the star in the finder scope crosshairs, then go to
the main scope to refine the location before pressing ENTER.
Ditto for the second star -- once you set the scope on the first star, it asks for the
second star then slews to it and asks you to center the second star and press ENTER.
Again, my experience is that I always must search for the second star -- and a
finder scope makes searching much easier.
Using the scope without GOTO
If you want to explore the stars without the GOTO function, you will definitely need a
finder scope. The wide FOV of the finder scope allows you to get the object in the
FOV of the telescope, then you can refine the view with the high powered scope.
Which finder scope do I use?
Mike Weasner's Mighty ETX Site has an excellent discussion of
finder scopes with contributions from many ETX owners about their experiences with finder
scopes. Check out the various finder
scope ideas on Mike Weasner's site.
Two types of finder scopes
There are two types of finder scopes in use: (1) small telescopes, low-power with
wide fields of view, and, (2) reflex viewers. Generally, finder scopes mount onto
the tube of the main scope. Some are on mounts that are built into or screwed onto
the main tube. After-market sights, especially the reflex viewers, mount with
double-sided sticky tape.
Boresighting a small telescope type finder scope
Either type of finder scope must be "boresighted" with the main scope -- that
is, when an object is in the center of the finder scope, it must also be in the center of
the main scope. Finder scopes will have adjusting screws on them that move their FOV
up, down, left, right. You can boresight the scopes one of two ways.
Distant terrestrial object
In daylight, find a distant point -- 1/2-mile or more away -- the top of a power pole,
church steeple, prominent point on a ridgeline, some unmistakable and small point.
Get that point centered in the main scope FOV. Now, look through the finder
scope -- the object should be visible. If not, adjust the adjusting screws to pull
the object into view in the finder scope -- do not move the main scope.
Your finder scope will have a crosshair -- set the crosshair on the object. Now,
go back to the main scope and see if the object is still centered. Go back and forth
between the two -- finder scope and main scope -- to refine the adjustment.
Aligning the finder scope with a star means that you must chose a star that does not
move -- and that means Polaris. Set up the scope and center Polaris in the main
scope. Then, align the finder scope as you did for the terrestrial object -- you
want the star centered in the main scope and in the finder scope.
Now that the finder scope is boresighted with the main scope, find an object in the
finder using the main scope's up, down, right, left movement -- set the object in the
crosshair and it will be in the center of the main scope.
Boresighting a reflex viewer
Reflex viewers are different from small telescopes -- they do not magnify, do not have
crosshairs. A reflex viewer has a small lens through which you view the sky as it is
-- no magnification. Inside the reflex viewer is a small light source that projects
either a red dot or a set of concentric circles on the lens. As you look through
this lens, you see a red dot or a set of concentric circles -- a bullseye -- on the sky.
Just move your scope until the object you want to see is under the red dot or
centered in the bullseye.
The most common reflex finder is the Telrad -- it's sold by many telescope dealers and
is an excellent piece of equipment. BUT -- the Telrad is BIG. You really don't
want to mount something this big on a small scope like the ETX-70, -90, -105, or -105.
There are other reflex viewers -- many of them are discussed on Mike
Weasner's site. (AstroTom's site
has an excellent discussion of the Telrad.)
Boresight the reflex viewer the same way as the scope-type finder scope -- put an
immovable object in the center of the main telescope FOV then use the adjusting screws on
the finder to move the object into the center of its view.
My finder scopes
So, which finder scope to use -- a reflex/red dot/Telrad-type viewer, or, a finder
scope that is a small telescope? I took the easy way out and put both on my ETX-70AT
-- here it is.
The reflex viewer finder scope:
Mounted on the front of the scope is an Orion EZFinder reflex viewer finder scope.
Note the tube of the ETX-70 has two diameters -- the rear 2/3 of the tube is a
slightly smaller diameter -- the front 1/3 is the focusing element -- when you turn the
focus knob, this part of the tube moves in and out. Thus, if you mount a reflex site
on the rear part of the tube, as the focusing part comes to the rear, it will encounter
the reflex viewer mount and stop moving, thereby no longer focusing. So, I mounted
my red dot viewer on the movable part of the tube. Look at Mike Weasner's site to
see how other people have mounted their reflex/red dot finders.
This finder comes with a mount and a piece of double-sided foam tape. Once the
tape is applied, it will not let go. Be very careful to get the mount aligned so it
is parallel to the main tube. The mount is molded to fit a rounded tube so it's easy
to align it.
I have a Telrad reflex viewer on my big scope -- an 8-inch Dobsonian reflector -- and I
prefer the Telrad with it's bullseye over the EZViewer with its tiny red dot -- but -- the
Telrad is 1/3 the size of the ETX-70 -- there is NO WAY it could be mounted on the ETX-70
-- don't even try.
The normal finder scope
Note at the rear of the ETX-70 is a small right-angle finder scope. This is the Meade model 827 8X50 finder scope
made for the ETX-70AT -- it comes with a mounting bracket. This photo shows the 8X50
finder scope more clearly.
While it does not show perfectly here, the 8X50 finder scope comes in a mount that
rests on the ETX-70 tube the bends down over the back of the tube. There are two
holes in the back of the ETX-70 to the left of the eyepiece mount -- one has a screw in it
that holds the back of the scope in place, the other is just a hole. The finder
scope mount has a plastic protrusion that sticks into the empty hole and a hole for the
screw. The finder scope comes with an Allen wrench -- remove the screw from
the back of the main scope, stick the mount in place, and put the screw back in -- then
boresight everything. That's it.
There are two adjusting screws -- you see one sticking up from the top and you can just
make out the second one sticking toward you from the rear of the finder scope. These
are the screws that control the up, down, right, left of the finder scope -- use these to
boresight the finder with the main.
I added these two finder scopes to my ETX-70AT on February 21, 2003 -- we were at the
end of 10 days of rain and snow and the first night I was able to use the scope with the
finders convinced me I had done the right thing. I always do a two-star alignment
and was constantly frustrated at having to search for the alignment stars -- no more --
when I selected the first star, the scope slewed toward it, stopped, and told me to center
the star then press enter. The star was not in the FOV of the main scope but there
is was in both the reflex viewer and the 8X50 -- simply used the arrows on the hand
controller to center the star in the 8X50 crosshair and there it was.
I hope this is useful to other ETX-70 users.
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