GEM vs. fork mount
Matthew Gelhaus
mounts vs. fork mounts for SCTs? I am aware of the following:
GEM strengths:
less weight to lift at once (can be carried in more pieces)
GEM weaknesses:
tracking across meridian can be awkward
fork strengths:
less overall weight
no counterweights to balance
fork weaknesses:
poor eyepiece access when pointing toward pole
How do the two types of mounts compare in terms of stability
and vibration dampening? I realize it will depend largely on
the specific mount and how bulky it is, so what I am really
looking for are rules-of-thumb.
I pose my question because I am looking at inexpensive 8" SCTs,
and I noticed the Celestron G8 costs less than the basic
Celestar 8. While the Celestar 8 adds a built-in tracking
motor, my guess would be the CG-5 mount of the G8 would be more
stable and be more versatile than the Celestar 8's fork mount.
I am shopping for a new "first" scope to replace a crappy
department-store scope I received as a gift several years ago.
I am primarily interested in visual deep-sky observation, but
would like the flexibility to try my hand at limited film
photography. If I decide to get serious about imaging, I could
deal with upgrades later. Portability/storage are important to
me right now, too.
Thanks for your advice,
Matthew Gelhaus
--
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MATTHEW J. GELHAUS
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mgel...@proaxis.com
http://www.gelhaus.net
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Michael A. Covington
The best GEMs are steadier than forks, which in turn are steadier than cheap
GEMs. Price is a good indication of what to expect.
Rod Mollise
>mounts vs. fork mounts for SCTs? I am aware of the following:
>
HI:
How stable are fork mounts...? Well, imagine a _tuning_ fork. Add to that the
need to tip one over in an inherently unbalanced way to polar align it for
equatorial use.
And yet...and yet...I've used forks for years and continue to. Why? Well they
are actually, if well done, surprisingly workable despite the above. Sure, big
GEMs are great...but they are big, heavy and inexpensive. What about the
smaller GEMs like the GP and the CG5? Sadly, they are, in my experience
somewhat less stable than fork mounts. Given a budget fork and a budget GEM,
I'd probably choose the fork. Yes, observing the far north is a witch with a
capital "B", but luckily the far north doesn't contain as much of interest as
the southern latitudes.
Peace,
Rod Mollise
Mobile Astronomical Society
http://members.aol.com/RMOLLISE/index7.html
The Home of _From City Lights to Deep Space_:
Rod's Guidebook for the _Urban_ Deep Sky NUT!!
*********************************************************
Phillip
> How stable are fork mounts...? Well, imagine a _tuning_ fork. Add to that
the
> need to tip one over in an inherently unbalanced way to polar align it for
> equatorial use.
>
> And yet...and yet...I've used forks for years and continue to. Why? Well
they
> are actually, if well done, surprisingly workable despite the above. Sure,
big
> GEMs are great...but they are big, heavy and inexpensive. What about the
> smaller GEMs like the GP and the CG5? Sadly, they are, in my experience
> somewhat less stable than fork mounts. Given a budget fork and a budget
GEM,
> I'd probably choose the fork. Yes, observing the far north is a witch with
a
> capital "B", but luckily the far north doesn't contain as much of interest
as
> the southern latitudes.
My experience is as follows:
Forks: Can be a pain to balance well, especially if you are hanging a
camera and a bunch of astro-photo gear off your scope.
Are a pain to look at anything further north than the zenith,
which does include a few nice objects.
More difficult to accuratley polar align.
Tend to keep the eyepiece in relativley the same area when moving
the scope to different areas of the sky, this is a good thing.
Are easier for beginners to use.
GEMs: Easy to balance. Can easily be pointed to any area of the sky. They
do have have to be 'flipped' when they cross the meridian. Easy to polar
align. The eyepiece can move over a large area when changing from one part
of the sky to the other, especially with longer scopes like refractors which
isn't a good thing.
Generally speaking and this is only my opinion, forks are better for visual
observing and GEMs are better for astrophotography. Most forks are less
expensive than good GEMs, mostly because they come as package deal with the
OTA. Some fork mounts have electronic packages as good or better than some
nice GEM's. An example would be the Celestar Deluxe, now that it's PEC is
reportedly fixed, or the LX-200.
Phillip
Dan Cook
<mgel...@grizz.gelhaus.net> wrote:
>fork strengths:
> less overall weight
> no counterweights to balance
A fork does require the addition of couterweights to balance in situations
where with a german equatorial mount you can just slide the OTA in its
dovetail and/or slide the couterweights on the counterweight shaft. For
instance, if you use a big eyepiece with a fork, you have to balance that
with a weight bar. Or if you use a fastar camera with a fork, you have
to stick a big dummy weight in the eyepiece holder.
Forks also have to be long and spindly if you want to be able to swing
the OTA between the fork arms. The Meade 12" in equatorial fork mode
(as opposed to altazimuth fork mode) hits its base at low declinations
in the South. Not a problem with a german equatorial mount.
GOTO capable fork mounts tend to be quite a bit less expensive than GOTO
capable german equatorials.
- Dan
Bill McHale
The one advantage to GEMS that no one has mentioned so far is that they
are independent of the Telescope being used. The fork to an 8" SCT can
only be used with an 8" SCT, and probably one from the same
manufacturer. In contrast, if you combined a C8 with a Vixen GP-DX, you
could also reasonably expect to use the mount for a 4-5" refractor, a
6-8" Newtonian, 6-8" Maksutovs and if you don't expect too much
photographic use a C9.25 OTA. Further you can start with a basic set up
using slow motion controls and upgrade later to GOTO or whatever.
Personal opinion is that while a Fork mounted SCT is cheaper initially, in
the long run a good GEM will justify its higher initial cost.
--
Bill
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Phillip
Bill McHale <wmc...@umbc7.umbc.edu> wrote in message
news:8kke42$m2$2...@news.umbc.edu...
>
> The one advantage to GEMS that no one has mentioned so far is that they
> are independent of the Telescope being used. The fork to an 8" SCT can
> only be used with an 8" SCT, and probably one from the same
> manufacturer. In contrast, if you combined a C8 with a Vixen GP-DX, you
> could also reasonably expect to use the mount for a 4-5" refractor, a
> 6-8" Newtonian, 6-8" Maksutovs and if you don't expect too much
> photographic use a C9.25 OTA. Further you can start with a basic set up
> using slow motion controls and upgrade later to GOTO or whatever.
>
> Personal opinion is that while a Fork mounted SCT is cheaper initially, in
> the long run a good GEM will justify its higher initial cost.
Yes, I knew I was forgetting something, just couldn't remember at the time.
(Lightning struck as I was writing the post and killed the power so I had to
re-write my message.)
Phillip
John Ford
> of the sky to the other, especially with longer scopes like
refractors which
> isn't a good thing.
>
> Generally speaking and this is only my opinion, forks are better for
visual
> observing and GEMs are better for astrophotography.
I would beg to differ on these points...
Eyepiece movement is a small price to pay for the ability to actually
have easy access to the eyepiece in many orientations.
Visually, the fork is a pain and often cause people to curse when doing
carefull star-hopping (a lost art, it seems) because it is very
difficult to get the OTA to stay put while looking at the chart without
needing a third arm to "lock the DEC". This is particularly true if the
scope has been accessorized with diagonals, piggy-backs, dew shields,
etc.
If there are no GEMs at the site, people tend to accept the drawbacks
of forks. Otherwise, people at our local sites tend to realize they
made a mistake when they see the smoothness and ease of movement of the
GEM (yes, even the CG-5!!!).
Of course, some folks have successfully used both mounts for a number
of uses in spite of the ups and downs.
--
Clear skies,
John Ford
South-Eastern Michigan
jf...@inac.net
Sent via Deja.com http://www.deja.com/
Before you buy.
David Nakamoto
In general, it is easier to make weight adjustments and balance
a scope on a GEM verses a fork. This is not to say you cannot
balance the weight on a fork mount, but it takes more equipment
that usually does not come with the mount, whereas the GEM
is designed to shift weights around to balance things out. This
is critical when imaging objects, or even photography when done
through the main scope.
Matthew Gelhaus wrote:
> What are the relative merits & weaknesses of German equatorial
> mounts vs. fork mounts for SCTs? I am aware of the following:
>
> GEM strengths:
> less weight to lift at once (can be carried in more pieces)
>
> GEM weaknesses:
> tracking across meridian can be awkward
>
> fork strengths:
> less overall weight
> no counterweights to balance
>
--
------------------------------------------------------------------
A man does not insist on physical beauty in a woman who builds
up his morale. After a while he realizes she is beautiful --
he just hadn't noticed it at first.
--- Lazarus Long
Name: David Nakamoto
E-mail: d...@blankreg.jpl.nasa.gov
--------------------------------------------------------
RKBerta
both are "OK" but you have to quantify that with the good,
the bad and the ugly. First I assume you are talking about
good fork mounts and better quality GEMs. The very low end
GEMs are usually not much better than worthless.
Also...those who say that vibration is a problem in either
mount may have a point unless you do something about
it...for instance...most of the lower end fork mounts can
be significantly made more vibration free and rigid with
things like a set of Celestron vibration reducing
pads...these are a MUST for any tripod mounted
scope....they are very effective. You can also fill the
legs of a tripod with sand to increase damping whether GEM
or fork mount. Size of the OTA and weight on a given type
of mount is very important. While some mounts are fine with
a given size scope...other larger scopes may really overtax
it. But even this can be worked around. I have a 6"
refractor mounted on a Celestron CG-5 GEM. While the stock
legs on the tripod are fine for a smaller scope they are
undersized significantly for something like a 8" SCT or 6"
refractor. But all is not lost...just replace the stock
legs with home made wooden legs and the mount can EASILY
deal with the big refractor.
I have a Celestron 8" SCT Deluxe with Fastar optics and CCD
camera also. The stock setup is quite good...heavy duty
tripod, nice wedge, etc. But I was able to fill the lower
legs with sand and use a set of the vibration pads for a
very stable and vibration free setup....final upgrade was
replacement of the stock wedge with Celestron's extra heavy
duty wedge...now it is a VERY fine mount....less effected
by vibration and wind than some extremely expensive and
huge mounts I have compared it to.
The other thing to compare is useability. I find a Fork
Mount to be superior to a German mount for ease of use and
they seem to be a little more precise when used with a DSC.
Of course other than a SCT will rule out the fork mount.
The one big advantage of a German mount...is that you can
attach any type of scope to it.
Bottom line....either works fine but when it comes time to
scrimp...don't scrimp on the mount. It would be far better
to really get a very good mount and scrimp
elsewhere...especially if you ever intend on doing any
photography through the scope....for that you really need a
very good mount that has accurate tracking, is vibration
free and allows for fine increments of movement via a hand
controller pad.
Bob Berta
* Sent from AltaVista http://www.altavista.com Where you can also find related Web Pages, Images, Audios, Videos, News, and Shopping. Smart is Beautiful
Phillip
> I have a Celestron 8" SCT Deluxe with Fastar optics and CCD
> camera also. The stock setup is quite good...heavy duty
> tripod, nice wedge, etc. But I was able to fill the lower
> legs with sand and use a set of the vibration pads for a
> very stable and vibration free setup....final upgrade was
> replacement of the stock wedge with Celestron's extra heavy
> duty wedge...now it is a VERY fine mount....less effected
> by vibration and wind than some extremely expensive and
> huge mounts I have compared it to.
I can testify to this. I used to own a Celestar Deluxe and I too upgraded
the wedge and used the vibration supressors. I was actually dissapointed to
find out after I sold it that it was more stable and tracked better than my
current Losmandy GM8. Of course the GM8 is *much* easier to polar align and
balance and can be used with other OTA's
Phillip
Frank Widmann
NJP GEM. I did this for 3 main reasons: 1) The NJP is a
superior quality mount for astrophotography; 2) You can't point
the fork mounted telescope too close to the north without camera
or CCD equipment running into the base; 3) Lifting and carrying
the OTA and fork mount any distance is a killer, while the OTA
can be easily removed from the GEM. The fork mount's goto and
high speed slewing is a great feature, particularly for visual
observing, but you have to be careful because every now and then
the scope loses the encoder count and goes where it shouldn't.
It is pretty hard to find that little shutoff switch in the dark
before either your CCD or the OTA is driven into the base.
In my experience, the bottom line is that if you are looking for
a scope 8 inches or less for general observing, fork mounts are
pretty good. They are also okay if your scope is larger but
permanently mounted. However, if you have a 10 to 14 inch scope
that you want to move around or a scope you will use primarily
for astrophotography, then the GEM is probably the better
choice.
Frank
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HAVRILIAK
>the OTA and fork mount any distance is a killer, while the OTA
>can be easily removed from the GEM.
Boy, do I agree. Ihave the Meade 12in OTA mounted on a Losmandy G-11 mount.
The OTA is heavy enough to move around, and mounting to the G-11 is pretty easy