Re: what is the best mount?
- From: "Steve Paul" <smarshallpaul@xxxxxxxxx>
- Date: Wed, 9 Sep 2009 14:24:08 -0400
MikeToms wrote:
GEM's suck with newtonians and not that great with SCT/MAK type scopes
either since when you add eyepieces you change the balance and have
to move the dovetail which can be a challenge.
So does the wedge win out?
You must be using some pretty heavy eyepieces. If so, the answer is, "not
really".
Shifting weight (that much) on that axis will have an effect on scope
balance with any mount. For forked Cats (wedge or no), you need to add
(hang) weight on the front end. In a GEM, you deal with sliding the scope,
as it is, forward and back. No additional parts needed.
Most ready-made Dob mounts these days have some sort of tension adjust on
the altitude axis to compensate for variances in eyepiece weight (to a
degree), and the fork mounts have pretty good grip for handling similar
loads.
For larger differences, TeleVue makes (or made) a (brass?) weighted 2" to
1.25" eyepiece adapter to compensate. Pull out the 2" eyepice, and drop in
the weight compensator adapter, then drop in your 1.25" eyepiece. Keep the
scope set to the heavy eyepiece position on the mount.
Since I'm on a break, let me add some more commentary....
GEMs are useful for observing with refractors and small Newtonians, and for
imaging with several different scopes, including SCTs and Maks. If you are
going to buy just one scope and use it for the foreseeable future, there are
other choices available, that may make more sense..
For larger aperture portable observing, the Dob mount Newtonians and
non-wedge forked GoTo Cats just can't be beat. Two components to setup, the
base/tripod, and the OTA/Cat fork assembly. If you have balance issues with
a Cat, dump those eyepieces that are causing you grief, or get the TeleVue
compensator. For a Dob, just add velcro strip and a weight or on top of the
OTA at the mirror end, that can be moved fore and aft.
No matter what scope and mount you choose, there are then those "equalizing
extras" that demand effort, accessories like a battery and cable for the
GoTo mount and dew prevention systems for a Cat corrector, an observing
chair, and eyepiece holder/case. If a Dob setup, you will likely have a
table with maps, although if you use DSCs on your Dob, you can also dispense
with map and table, and use just an pre-arranged itemized NGC list (don't
forget your red LED flashlight to read the list).
If you value widest field large aperture viewing (open clusters, large
nebulae, star fields), the shorter focal length of the average focal ratio
Newtonian is superior, while suffering nothing at the high power end with
the use of a good barlow lens. Of course, high power observing begs for a
driven mount, which the Dob is not, without additional components.
If you are primarily a high power observer (planets, single stars, double
stars, small open clusters, globlular clusters, planetary nebulae, most
galaxies), the longer focal length of the GoTo fork mounted Cats is no
disadvantage, although it is not unreasonable to argue that contrast issues
will be perceptable to the skilled observer with these OTA designs.
For imaging, the GEM wins overall, because it wins the versatility category
(mulitple scopes, one mount). By a good one, and you can experiment with a
myriad of different telescope designs and focal lengths. That said, the
equatorial fork is considered superior by some in that there is no meridian
flip, and imaging through the meridian means imaging through the least
amount of atmosphere (for the most part). A good fork mount, with additional
balance weight, can also handle a piggy backed wide field refractor or small
Newtonian allowing both high resolution and wide field imaging, each scope
acting as the guidescope for the other as needed. Not only, but for remote,
unmanned imaging, there's less concern with a fork, once properly setup.
That's where the meridian flip of the GEM is a more serious weakness,
comparatively.
Personally, for imaging with even a limited number of scopes, I still prefer
the GEM, but I don't have an observatory. When I was imaging, I used a
ScopeBuggy to wheel everything out to the back yard, and kept the entire
setup assembled in the corner of the garage.
Lastly, a good GEM comes with a GOOD polar alignment scope, and good polar
alignment is about 60% of the effort in setting up for good imaging once the
buggy is out in the yard. The faster you can do that, the sooner you'll be
imaging. The remaining effort is divided in varying amounts between getting
on target, focusing and setting up the autoguider, getting the camera into
focus, and setting up the software to fire off the exposures.
--
Steve Paul
.
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