GEM Questions

What are the disadvantages of using a German Equatorial Mount?

- Are portions of the sky unreachable?

- Can a GEM track across the meridian?

- GEM owners seem to fuss a lot over polar alignment 'scopes. How
(briefly) does one align a GEM with sufficient precision for
astrophotography? Does drift alignment work?

- What else don't I know?

TIA

Davoud

--
usenet *at* davidillig dawt com

"Davoud" wrote in message
...
What are the disadvantages of using a German Equatorial Mount?

- Are portions of the sky unreachable?

- Can a GEM track across the meridian?

- GEM owners seem to fuss a lot over polar alignment 'scopes. How
(briefly) does one align a GEM with sufficient precision for
astrophotography? Does drift alignment work?

While I am too much of an amateur to answer the first question, I do have
experience with the latter 2. Drift alignment is an option. In fact, it is
what a lot of people use who observe deep sky objects. It allows the scopes'
drives to be accurate for hours at a time. If you are observing planets, on
the other hand, a quick alignment with Polaris is almost always good enough.
I have heard of people getting as much as 15 or 20 minutes of tracking
ability with a simple Polaris alignment. I would say 10 minutes is more
easily achieved.

Have fun!

Davoud wrote:
What are the disadvantages of using a German Equatorial Mount?

David,


Too many to list here :^)


- Are portions of the sky unreachable?

No, and it doesn't matter how much equipment you have hanging off the
focuser either. No fork arms to clear.


- Can a GEM track across the meridian?

Most can easily go 2-3 hours past the meridian. Surprisingly enough I
hear that the Millinium won't. But in a CCD only world that's probably
not a big deal.


- GEM owners seem to fuss a lot over polar alignment 'scopes.

Not me. I have two GEM's and one has the PAS and I never use it.

How
(briefly) does one align a GEM with sufficient precision for
astrophotography? Does drift alignment work?

Drift alignment always works. And *even* if you use a PAS to get close
Drift Aligning can polish it up and get very close.
It's a simple process that takes roughly an hour once you get the hang
of it. And you can start long before it's dark enough to open a shutter
too.


- What else don't I know?

You dare to ask that on s.a.a. ???? wow .... courage !

You'd probably get better info on the Losmandy list since I seem to
recall you are thinking about a G-11 ?


http://www.celestial-images.com/testing.html

G-11 with 75 pounds of junk hanging off of it and a few sample images.

Regards

Bill

--

William R. Mattil : http://www.celestial-images.com

Tom wrote:
While I am too much of an amateur to answer the first question, I do have
experience with the latter 2. Drift alignment is an option. In fact, it is
what a lot of people use who observe deep sky objects. It allows the scopes'
drives to be accurate for hours at a time. If you are observing planets, on
the other hand, a quick alignment with Polaris is almost always good enough.

Drift alignment is used by a lot of people who *image* deep sky objects.
I assume that is what you meant (in any event, it should be what you
mean g), but it was unclear.

For visual use, you need no better alignment with a deep sky object than
you do with the planets. In fact, if anything, since planets are often
observed with a smaller true field of view than DSOs, you need better
tracking for observing planets than you need for observing DSOs, not the
other way around.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Davoud wrote:
What are the disadvantages of using a German Equatorial Mount?

They're a bit ungainly for some beginners, and I've seen a couple of
them whack themselves in the arm due to an incomplete understanding of
the balancing system, but probably you're far along enough that you'll
be able to figure the main issues out.

Otherwise, they have no significant *inherent* liabilities. (Of course,
you can always make a poor one.) They wouldn't be as common as they are
if they did have.

- Are portions of the sky unreachable?

Not intrinsically. If the mount is made poorly, you may be unable to
point directly at the pole, or in some small region around the pole.
But the same problem exists with respect to the zenith in a Dobsonian.

- Can a GEM track across the meridian?

They might not be stopped right at the meridian, but they won't make
it all the way across the sky.

- GEM owners seem to fuss a lot over polar alignment 'scopes. How
(briefly) does one align a GEM with sufficient precision for
astrophotography? Does drift alignment work?

I personally never found much use for a polar alignment scope. Either
you're using the scope visually, in which case a couple of degrees from
the pole is entirely acceptable, or else you're imaging, and a polar
alignment scope won't be reliable enough. (At least, I wouldn't risk a
four-hour exposure on a polar alignment scope alone. If I were an
imager. Which I'm not.)

- What else don't I know?

Loads. But that's true of all of us.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

"Brian Tung" wrote in message
...
Tom wrote:
While I am too much of an amateur to answer the first question, I do
have
experience with the latter 2. Drift alignment is an option. In fact, it
is
what a lot of people use who observe deep sky objects. It allows the
scopes'
drives to be accurate for hours at a time. If you are observing planets,
on
the other hand, a quick alignment with Polaris is almost always good
enough.

Drift alignment is used by a lot of people who *image* deep sky objects.
I assume that is what you meant (in any event, it should be what you
mean g), but it was unclear.

Actually, I meant both observe and image. People at star parties oft times
use their drives to keep something in alignment, so their friends can take
turns looking without losing the object.


For visual use, you need no better alignment with a deep sky object than
you do with the planets. In fact, if anything, since planets are often
observed with a smaller true field of view than DSOs, you need better
tracking for observing planets than you need for observing DSOs, not the
other way around.

I stand corrected on the visual vs image usage. You are right. But if
imaging was the objective, then DSOs would definitely require better
alignment.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Tom wrote:
Actually, I meant both observe and image. People at star parties oft times
use their drives to keep something in alignment, so their friends can take
turns looking without losing the object.

It's unusual for a social observing session to involve keeping the
object in view for much longer than 10 or 15 minutes, but it does
happen, and when it does, yes--you do need better alignment. I
suppose a polar-alignment scope could come in handy in that case.

I stand corrected on the visual vs image usage. You are right. But if
imaging was the objective, then DSOs would definitely require better
alignment.

Yes, though for a different reason--hardly anyone needs to do long-
exposure imaging of a planet. Even the new tenth planet won't need
anything like an hour of tracking.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

On Fri, 07 Oct 2005 21:55:19 -0400, Davoud wrote:

What are the disadvantages of using a German Equatorial Mount?

Theoretically, GEMs are less stable than forks because of their two
cantilevered axes. In practice, of course, stability is a function of
engineering quality, and aftermarket GEMs are usually of higher quality
than mass-produced forks from Meade and Celestron. That is why good GEMs
are frequently acquired by serious imagers.

- Are portions of the sky unreachable?

Not unless something is wrong with the design.

- Can a GEM track across the meridian?

Yes, but in most cases you can't track from horizon to horizon. At some
point you reach a position that requires flipping the scope. This is a
big disadvantage if you are an imager, since it means interrupting your
exposure and probably finding a new guide star.

- GEM owners seem to fuss a lot over polar alignment 'scopes. How
(briefly) does one align a GEM with sufficient precision for
astrophotography? Does drift alignment work?

Aligning a GEM is identical to aligning a fork. The techniques are the
same, the level of difficulty is the same.

- What else don't I know?

While forks are better for imaging because they don't require an axis
flip, they also can be problematic since there is limited distance
behind the OTA that can be used before your equipment won't clear the
base. GEMs don't have this problem.

IMO the ideal fixed mount for an imager with an observatory is an
aftermarket fork, such as those made by Mountain Instruments. But you
will pay more for one of those than for a good GEM.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Howdy Davoud,
I'd like to comment here, and make a point, in which others here have
said otherwise about the inherit qualities of a GEM.

Yes, depending on what type of OTA you are using, there can be the
possibility of a portion of the sky that cannot be easily accessed, and
that might be in the instance with a long FL Refractor, and trying to
point to the Zenith.

The long OTA could possibly interfere with the GEM's Tripod Legs. With
SCTs, this shouldn't be much of a problem.

I have this problem myself with one of my Refractors on a Byers 58
GEM/Meade GFT Tripod.

If you are considering a G-11, and then getting the Losmandy Polar
Scope, be ready for a 3-5 month wait for the scope. Evidently, these
items come, and go from staying in available stock, and there currently
is a waiting list for them I understand.

None the less, for decent polar alignment for visual use, and getting
fairly accurate use from the setting circles, one can easily sight
through the Polarscope Bore on the Mount, center Polaris as best one
can, and get reasonably good results. Mark

"Brian Tung" wrote in message
...
Tom wrote:
Actually, I meant both observe and image. People at star parties oft
times
use their drives to keep something in alignment, so their friends can
take
turns looking without losing the object.

It's unusual for a social observing session to involve keeping the
object in view for much longer than 10 or 15 minutes, but it does
happen, and when it does, yes--you do need better alignment. I
suppose a polar-alignment scope could come in handy in that case.

I didn't mean a viewing session of more than 10 or 15 minutes. But to keep
something in the FOV for more than a matter of seconds requires decent
alignment. Certainly the drift method wouldn't be needed, just aligning with
Polaris real quick would do the job fine.

I stand corrected on the visual vs image usage. You are right. But if
imaging was the objective, then DSOs would definitely require better
alignment.

Yes, though for a different reason--hardly anyone needs to do long-
exposure imaging of a planet. Even the new tenth planet won't need
anything like an hour of tracking.

Oh, I agree. Many people imaging Pluto make exposures from 1 to 5 minutes.


--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt