First Telescope - On To Mounts

A different subject from my usual. This is based primarily on my
experience in my "First Telescopes - Experimentation" thread where a
borrowed a friends small GoTo telescope and had alignment headaches.
As you all know, I have no great need for GoTo but will get it if the
budget permits, largely because the difference between a dual axis
drive and GoTo looks to be under $150. In the discussions below, the
primary mounts I'm talking about are the ones that come with the GT
and non-GT versions of the Celestron Advanced series.

First, how I plan to use my telescope even if it has a GoTo mount. I
plan to find my star hopping, discovering on my own. Heck, I've found
with my binoculars that I frequently find interesting things in the
sky when I was looking for something else. Then I get the enjoyment
of trying to figure out what I'm looking at. To me, that is fun.

If I had a GoTo capable telescope, I would use it more at Star Parties
or public viewing events to quickly switch between objects that people
usually want to see or, for my own use, to quickly jump to my base
star on my star hopping journeys. I remember a Local Outreach event
to look at Saturn on 1/1 of this year. It was a cloudy night and one
guy had a 10 inch telescope with GoTo. He was rapidly able to
reposition when various targets appeared through the clouds while it
too the other people a bit longer. But, I'm moving off of my intended
subject rapidly now.

My real question here is mount alignment. To summarize what I think I
know:

Aligning a non-goto GEM mount: You level the tripod and then set the
rotational axis of the equatorial axis to point at the north star
(there appear to be nice spotter scopes that you can buy that fit into
the mount and make this a snap). Once you do this, the elevation
scale on the mount should match your latitude. If done accurately,
your equatorial axis should now be in plane with the celestial equator
and changes in this axis will be pure changes in RA. The declination
axis should move directly towards and away from the North Star. If
properly aligned, stars should stay centered in the eyepiece with only
adjustments to the RA axis (most planets will probably stay aligned as
well).

Now, I was reading some of the manuals for the various telescopes I'm
intending to buy and was confused about the alignment procedure for
the GoTo versions. These seem to be the same regardless of the mount
type (fork or GEM). They have easy align, 3 star align, 2 star align,
etc. No effort seems to be made to level or polar align the GEM's.
In addition, there is a separate polar alignment procedure that you
would do AFTER performing another alignment.

This seems inefficient to me. I realize that a computer (or a human
for that matter) can track an object using any 2 perpendicular axes.
However, the beauty of the GEM design is that you really only need one
motor (and thus less power required) if the mount is polar aligned
first. Am I missing something? Why don't you polar align the mount
first? Should I be doing it anyway?

Now, to really add to my confusion, I tried to look up how a GPS
receiver would help you out since these mounts have the capability to
accept one (hey, I'm an engineer). I figured that the GPS would give
a nice, accurate time (I realized how critical this was when I
borrowed the telescope) and position, but that you would still need to
align the telescope. Much to my surprise, it seems that the GPS
allows the computer to completely align and level the telescope
without any user input at all! Again, am I missing something and why
wouldn't you want to polar align the mount first?

Finally, a quick question that I'm a little confused about. Why do
you need to level the mount and then set the elevation scale on a GEM?
It seems to me that if you have the equatorial axis properly aimed at
the north star, you are aligned with the celestial axis and it
shouldn't matter if the base is absolutely level and the elevation is
set exactly to your latitude. I can see how leveling and using the
elevation scale might make the job of polar aligning easier,
especially if you don't have one of those neat little polar alignment
scopes, and I can see how it will make the base a little more stable
by more evenly distributing the weight on the tripod, but I don't see
how it is absolutely necessary. Again, am I missing something?

Thank you to everybody for your continued help. I'll be at a large
star party on Saturday and your responses have already given me a lot
to ask about and look at. I hadn't quite realized what a big jump
moving from Binoculars to a Telescope

Edward Smith wrote:

Aligning a non-goto GEM mount: You level the tripod and then set the
rotational axis of the equatorial axis to point at the north star
(there appear to be nice spotter scopes that you can buy that fit into
the mount and make this a snap).

Well, for a "quick and dirty" aligment for purely visual use (non-Go-To), this
would work, but for things like Go-To and astrophotography, the alignment is a
little more tricky. The north star is not quite at the location of the north
celestial pole (about a degree away), so you have to carefully tweak things a
bit to get that last little bit of accuracy required for the Go-To system to
work well. Usually, the 2-star alignment will get things close enough so that
the polar axis of the scope is pointed very close to the true celestial pole.
In some cases, the Go-To's computer can compensate for a slight misalignment
by putting in small corrections based on the 2-star alignment process. In the
case of altazimuth telescopes with Go-To, they can track along the two axes
using the on-board computer and alignment system to keep the target in the
field of view.

Now, I was reading some of the manuals for the various telescopes I'm
intending to buy and was confused about the alignment procedure for
the GoTo versions. These seem to be the same regardless of the mount
type (fork or GEM). They have easy align, 3 star align, 2 star align,
etc. No effort seems to be made to level or polar align the GEM's.
In addition, there is a separate polar alignment procedure that you
would do AFTER performing another alignment.

This seems inefficient to me. I realize that a computer (or a human
for that matter) can track an object using any 2 perpendicular axes.
However, the beauty of the GEM design is that you really only need one
motor (and thus less power required) if the mount is polar aligned
first. Am I missing something? Why don't you polar align the mount
first? Should I be doing it anyway?

Yes, you should try and get the telescope's polar axis as close to the true
celestial pole as possible. If you could get it perfectly polar aligned, then
you would only need one motor, but in point of fact, this is rarely completely
possible. With a Go-To system, you already have two motors, so it is logical
to employ both to compensate for minor aligment mistakes. A good polar
alignment can reduce the current drain on the system, since the declination
motor doesn't have to run very much of the time (in altazimuth Go-To's, both
motors are running all the time). You don't really need to level the tripod,
but only get that polar axis parallel to the rotation axis of the Earth. Any
residual errors can be handled by the computer in the Go-To system via its
alignment process where it determines what the error is and compensates for it.

Now, to really add to my confusion, I tried to look up how a GPS
receiver would help you out since these mounts have the capability to
accept one (hey, I'm an engineer). I figured that the GPS would give
a nice, accurate time (I realized how critical this was when I
borrowed the telescope) and position, but that you would still need to
align the telescope. Much to my surprise, it seems that the GPS
allows the computer to completely align and level the telescope
without any user input at all! Again, am I missing something and why
wouldn't you want to polar align the mount first?

You don't have to, as again, the computer can often align and track for almost
any minor axis deviation. The main thing the GPS receiver gives you is that
you don't have to input your location and the time before you align the
telescope. Once the GPS receiver gets the location and time to the
telescope's computer, the computer will roughly calculate where all the
alignment stars in its database are and which ones are above the horizon.
During a GPS alignment, the scope does a 2-star alignment, and the scope will
slew itself towards each of the locations of those stars to "get you in the
ballpark". Then, *you* have to tweak the hand controller a bit to center each
alignment star in your field of view of first your finderscope and finally,
the telescope itself. Once that is done for both stars, alignment will be
about as accurate as you can get and the scope will go-to and track about
anything in its database that is above the horizon. The closer you get the
polar axis to the true pole when you set up the scope, the closer the scope
will get to the two target stars when doing its initial alignment. It isn't
very hard at all, and its kind of fun to watch the scope do its thing with
only a little input from the user.

Finally, a quick question that I'm a little confused about. Why do
you need to level the mount and then set the elevation scale on a GEM?

Well, if you want to use the elevation scale to determine roughly how high the
polar axis should be tilted relative to the horizon, then the mount base
should be level. Otherwise, you can ignore the scale (and the leveling) and
just concentrate on getting the polar axis pointed as close to the celestial
pole as possible. Clear skies to you.





--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

"Edward Smith" wrote in message
...

If I had a GoTo capable telescope, I would use it more at Star Parties
or public viewing events to quickly switch between objects that people
usually want to see or, for my own use, to quickly jump to my base
star on my star hopping journeys. I remember a Local Outreach event
to look at Saturn on 1/1 of this year. It was a cloudy night and one
guy had a 10 inch telescope with GoTo. He was rapidly able to
reposition when various targets appeared through the clouds while it
too the other people a bit longer.

Even though this "sounds" right, my personal experience has been that at
public star parties people (especially kids, but also seniors who are not so
steady on their feet) have a bad habit of grabbing, pushing or pulling on
the OTA, which means I have to go through the alignment process all over
again. After doing this several times one night, I always use a Dob for
public star parties. Interestingly, I find that people are as impressed
that we can fairly quickly find objects in the sky as they are with
computerized GOTO.