"Setting Circles" (RA/DEC...another newbie question)
Fran Maurais
week....
however, I have a question about "right ascension setting circles".
In my "instruction manual" for Celestron 60 EQ Refractor, it discusses the
celestial coordinate system, RA/DEC coordinates, polar alignment, then
setting circles, and I'm paraphrasing from it:
"The RA setting circle must be aligned. Choose a bright star on a chart,
note it's RA/DEC coords, find the star in the finder, then the scope, then
rotate the RA circle to match the coordinates of the star with the indicator
on your scope".
Ok, fine...I did this for one of the Big Dipper's pointer stars (just to try
it)...
Then it goes on to say:
"The RA setting circle does not move as the telescope moves in RA, than thus
it must be aligned each time you want to use it to find a new object.
However, you don't need a bright star each time. Instead, you can use the
object your observing at the time"
This is confusing to me....on my scope, when I adjust RA, the RA circle does
move with the scope (or maybe I don't understand what is meant here... I'm
not ruling this possibility out!) You have to re-adjust the RA circle for
each new object? This doesn't make sense to me, and would seem to defeat
the purpose.
Could someone clear this up for me? (Sorry for the silly "newbie"
questions)
_________________________________
Fran Maurais, LOGICON Syscon, Inc.
fmau...@mrburns.npt.nuwc.navy.mil // fmau...@logicon.com
Andrew Roscoe
should not move. I assume your scope is not motorized. Therefors, as you
manualy move the scope to follow the star, the setting circle of RA will
"lag" and therefore, to re-establish it's accuracy, you must reset the
setting circle to read the coorect RA of the star. Now you may find
another object by its coordinates by "dialing 'em up because at leat for
a few muinutes, your setting circle will be accurate. In a motor driven
scope the RA circle should rotate along with the scope drive to maintain
the accuracy of the readout. But with your scope, they will remain
stationary so must be rest after you are finished viewing your object.
1.Find a star and check its Ra coordinate.Might as well make sure your
DEC vernier scale is calibrated as well.
2.Calibrate RA circle to match stars coordinates.
3.Now, quickly move scope in RA and dec to find another object by is
coordinates. Your RA circle must not move during this procedure or you
will need to tighten it down or stabilize it in some way so it remains
stationary.
4.Hopefully if you are polar aligned, the object will be in or near the
eyepiece field. Observe it for however long and then befor you move on,
set the RA reading to this objects coordinates and now find the next
object by RA and dec.
Good luck,
Russ
dar...@shore.intercom.net
paull
With what the manual said, I'm not surprised you're confused :)
Once you've set the RA circle to the proper setting for a known star, you
can then use it to put you in the neighborhood of other objects by going to
their RA as indicated on your setting circle. The setting circle itself
DOES and SHOULD move right along with the RA axis as you move your scope
around.
One thing to keep in mind...RA is a factor of time. If you do not have a
clock drive installed on your mount, it won't be long before your RA setting
circle is off once you've set it to a known star. This is because the earth
is rotating, but your mount is not rotating to keep up with it. You can
probably get in the neighborhood of an object after setting your RA circle
on a known star for 10-20 minutes, depending on the new object's location,
but much after that your circle will start to get behind the advancing time,
and you won't even be close. Thus, without a clock drive, you'll have to
continually set on a known object, find a new object, then set on a known
object before finding another one, etc. It gets kind of old :) An accurate
clock drive, or better yet, being able to "star-hop" your way around the sky
without using the setting circles, will make it much easier for you to find
things.
Good luck,
Paul
Fran Maurais wrote in message <79n1b3$95b$1...@winter.news.rcn.net>...
Sidney Lee
>With what the manual said, I'm not surprised you're confused :)
>Once you've set the RA circle to the proper setting for a known star, you
>can then use it to put you in the neighborhood of other objects by going to
>their RA as indicated on your setting circle. The setting circle itself
>DOES and SHOULD move right along with the RA axis as you move your scope
>around.
I'm not 100% sure what you are trying to say here but I wouldn't agree with
what you wrote. RA scales generally come in 2 "flavours":
On many (but not all) scopes with motor drives the RA scale "tracks",
meaning once the motor drive is engaged the RA scale is carried along with
the motor's tracking motion. This arrangement results in the RA scale
reading remaining constant while the scope is tracking. If the scope is
properly polar aligned and the RA scale has been set once on this type of
scope it will indicate the RA pointing direction of the scope as long as
the motor drive is turned on. On this type of system the RA scale moves
only as it follows the motorized drive. Moving the scope to repoint it by
releasing the clutch does not move the RA scale but rather changes the
reading indicated on the RA scale. Thus, once the scope is properly set up
and the RA scale reading set correctly the RA scale may be used for the
remainder of the session to point the scope at objects using the published
coordinates.
On other scopes (some motorized and many that are not) the RA scale does
not move with the motor or slow motion adjustment controls. In this
arrangement the scale is simply mounted on the mount in a manner that
allows it to be adjusted by turning it manually. Using this scheme the
indicated RA reading on the scale for a target being tracked will change as
ths scope pointing direction is slowly adjusted to track the target while
the scale sits still. Repointing the scope also causes the indicated
reading on the RA scale to change but it will not indicate the correct RA
reading unless the scale itself has been repositioned manually just before
the move so that it indicates the correct RA reading "at that time". In
practice this can be done during an observing session by either pointing
the scope at a known target and setting in its RA just before each
retargeting activity or by adjusting the scale to read the correct RA of
the current target just before retargeting.
Tim Rudderow
I am also confused. The fact that the RA circle does not move is in bold
print. The RA circle moves manually, and then has a locking pin. Is this
all wrong?
Tim Rudderow
BillFerris
>The fact that the RA circle does not move is in bold
>print. The RA circle moves manually, and then has a locking pin. Is this
>all wrong?
The instruction manuals that come with many low to moderately priced telescopes
are not well written. Most telescopes in this category are imports from Asia.
The manuals are often English translations of Japanese, Chinese or Korean
originals. It is not uncommon for something, namely clarity, to be lost in the
translation.
In any event, there are good reasons for the RA setting circle to have two
settings, one where it moves and another where it remains stationary.
The RA circle is manually adjustable so you can calibrate it. A star's RA and
Declination remain pretty much constant. This is usually where the first hand
goes up, "How can that be? The stars constantly move. Their positions
change."
From your point of view, the stars do move. The question is, why? Today, we
know the east-to-west motion of the stars is caused by Earth's orbit about the
Sun and its axial rotation.
If you think of the night sky as a celestial sphere, a shell of stars
surrounding Earth, the coordinate system begins to make more sense.
The grid of RA and Dec lines is fixed against the starry background. The Earth
moves. So, your position with respect to the fixed celestial sphere is
constantly changing.
As a result, you need to regularly calibrate the RA setting circle.
The RA circle is calibrated by manually rotating it until the RA indicated
corresponds to the RA of a star centered in the telescope's eyepiece. Of
course, if the telescope has not been polar-aligned, this calibration is wasted
time.
Telescopes with slow motion controls are usually designed so that the RA
reading changes when using the slo-mo to move the telescope through RA. Why?
This allows you to dial-in objects.
How does it work? Imagine, you've just calibrated the RA circle. There's a
galaxy 30 minutes east in RA. Using the slow motion control, you move the
telescope that distance. The change in RA typically is indicated in one of two
ways.
Find the pointer that indicates RA on the circle. This is often attached to the
mount in a fixed position. Since the pointer is fixed, the RA setting circle
needs to move with the telescope to indicate changing RA.
If the pointer is affixed to the RA axis of the telescope and, therefore, moves
as the telescope is rotated in RA, the circle needs to remain in place in order
for changing RA to be indicated.
In either case, moving the telescope with slow motion controls indicates
changing RA.
After all this, the operation of your equatorial mount is probably still pretty
murky. A well-illustrated description of the celestial sphere and celestial
coordinate system will probably go a long way towards clearing the murk.
Unfortunately, that's not really possible in a newsgroup.
As I recall, _Turn Left at Orion_ has a nicely illustrated discussion of this
topic. It's a good book for users of small to moderate aperture telescopes,
particularly refractor owners.
Clear skies,
Bill Ferris
Flagstaff, AZ
Bert Harless
it's supposed to. First thing I did was pull it off an clean off some
of the waxy grease they put all over everthing. This crap was so stiff
on mine I couldn't even turn it by force. I did manage to get it off
and cleaned it up a bit. I think the way it's supposed to work is you
aim it at a star of known coordinates and manually turn it to the right
numbers, then slew the scope to the object you are trying to find by
using the RA and Dec numbers for this object, at this point the RA dial
should move with the scope, so you move it to the desired RA and Dec and
find your object of desire, then tighten the screw on the RA circle.
Now when you use the slow mo knobs to keep the object centered for
however long you want to look at it the RA circle should stay at the
same number so that when you want to move to the next object, you loosen
the screw and slew to it. On mine the grease still has a little too
much friction and the screw has a little too little friction this means
that I have to reset the RA dial back to the right number before I slew
to a new object.
If you are even close to polar aligned the RA and Dec numbers for any
object stays the same, no matter how long you track it. If you don't
lock the RA dial as you track, you have to reset it to the correct
number before you move to the next item. Also there are two sets of
numbers on the RA circle the ones closest to the vernier scale are for
northern users and the ones on the outside are for southern users, so
you only need to use the appropriate set and disregard the other. Once
you figure out how the vernier scale works you can get pretty accurate
RA settings from this setup.
I hope this clarifies things a bit. Enjoy your new scope.
Bert
Tim Rudderow wrote:
>
> I have read the same manual as I am setting up a new c102 HD. As a newbie,
> I am also confused. The fact that the RA circle does not move is in bold
> print. The RA circle moves manually, and then has a locking pin. Is this
> all wrong?
>
Tim Rudderow
Thanks for the info. I figured out last night exactly what you describe. I
have another question about the 102hd. As the scope came out of the box, if
you align the OTA with the polar axis, the dec read 82 instead of 90. I
assume I have to adjust the alignment. Did you make a similar adjustment?
Also, is your altitude adjustment very stiff?
Thanks
Tim
Bert Harless wrote in message <36C398A7...@henge.com>...