Quote:
Originally Posted by Lyinxz
anyone got a simple way of helping me understand how to know/find the
Celestial equator?
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Before we describe what the Celestial Equator is,
let's start with Right Ascension (RA) and Declination (Dec).
Just as points on the Earth's surface can be specified in terms of Longitude
and Latitude co-ordinates, points in the sky can be specified in terms of
RA and Dec. In fact, if you were to project the Earth's equator (latitude zero
degrees) infinitely out into space, that line across the sky would
correspond to a Declination of zero, or what is known as the Celestial
Equator.
From our location here in Australia, we know that the Earth's equator
is north of here, running close to places like Singapore. So imagine
a friend in Singapore pointing a very bright laser pointer straight up
toward their local zenith and then starting to wave it back and forth
along a line in the sky exactly due East and due West of them.
If we were standing in Sydney and could somehow see that laser
pointer moving back and forth across the sky, it would be inclined
about 56 degrees above our northern horizon when it reached its highest
point in the sky and the line it followed would be the Celestial Equator.
If you are familiar with the three stars that form Orion's belt, they sit very
close to the Celestial Equator as well.
This Wikipedia article on the Celestial Sphere might be helpful
as well if you are just coming to grips with celestial
co-ordinate systems -
http://en.wikipedia.org/wiki/Celestial_sphere
Now let us describe where the meridian is. As you know on Earth, lines of
longitude run North-South from the North Pole to the South Pole.
Likewise, lines of Right Ascension run from due North-South as well.
Imagine a line in the sky that runs from due North to due South
and passes directly overhead (i.e. through the zenith) at your location.
That imaginary line that arcs directly overhead and runs North-South
is known as the meridian.
I assume you want to know about the Celestial Equator in order
to help establish your Argo Navis encoder direction senses?
If so, the intersection of the meridian, which runs North-South,
and the celestial equator, which runs in a line north of your location
from roughly North-West to North-East is the point you should point the
scope to begin the SETUP AZ STEPS test. You don't need to
be exact about it. Just point the OTA roughly at that point is fine.
So if you are standing by the telescope as if you were an artilleryman with
a canon, you will be facing initially due North, as will the telescope,
and it will be inclined in Altitude about 56 degrees above your northern
horizon. It is now pointing at the intersection of the meridian and
celestial equator. Now DIAL up MODE RA DEC and swing the telescope
from that point, pointing northward, to a spot on your North-East horizon,
which will be on your right-hand side. As you do so, observe the left-hand
displayed value which should increase. It will wrap at 23:59.9 back to
00:00.0 but in general the left-hand displayed values should become larger
(the right hand values will change too, but ignore it). If RA decreases
rather than increases, reverse your SETUP AZ STEPS sign and repeat the test.
One you have established the signs, try the following test as well.
Firstly, go to MODE SETUP, SETUP GUIDE and set the number of GUIDE
DECIMAL to 2 DECIMAL PLACES. Perform a FIX ALT REF and two-star
alignment. GUIDE back to one of the original two alignment stars. Note
that the display shows close to 0.00 0.00. Wait 15 minutes and GUIDE
back to the same star. Does the display still show close to 0.00 0.00?
Now GUIDE to a third star, well away from the original two in some other
part of the sky, and check that the angular error residual of this third
star is small.
As mentioned in the User Manual, irrespective of which way the
arrows point in GUIDE mode, always push the scope in a direction
that makes the GUIDE angles closer to zero. One can later change the
convention of which way the arrows point using SETUP GUIDE as a
matter of personal preference for your particular installation.
Best Regards
Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au
Ra/Dec, Celestial Equator and the Meridan - a refresher
