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gustavo_sanchez
sage
Reged: 12/30/10
Loc: Puerto Rico, US
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Max Load on CG-4 (latest version)
#4360123 - 02/03/11 09:08 AM
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Hello,
I have read a lot about the Omni 120 refractor being undermounted on the CG-4. If the CG-4 load capacity is 20lbs. and the 120 OTA is about 12 lbs. and then the accesories are about 3 lbs., then why it is undermounted? Just a curiosity. 
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dandabson
super member
Reged: 10/15/08
Loc: Dover, DE
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Re: Max Load on CG-4 (latest version)
[Re: gustavo_sanchez]
#4360230 - 02/03/11 10:25 AM
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I have mounted a 120/F8.3 on the same class of mount, an older Orion EQ3. I found it to be usable. The mount you are using is an excellent no frills mount. If it works for you go for it. Whatever gets you out under the sky is good for me.
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mcoren
Pooh-Bah
Reged: 01/11/06
Loc: Virginia, USA
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Re: Max Load on CG-4 (latest version)
[Re: dandabson]
#4360369 - 02/03/11 11:23 AM
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"Undermounted" or "properly mounted" is subjective. To paraphrase what Dan said, if it works for you and you enjoy using it, don't let other peoples' opinions take away from that.
Weight rating alone only gives half the picture. A short OTA like an SCT or a fast refractor or newt will be more stable on the same mount than a long focus refractor or newt with the same weight. The reason is because most of the weight is in the lenses or mirror, and if they are farther away from the pivot point of the mount, they will generate a longer moment arm. In other words, they will act as a longer lever and exert more force on the mount due to vibrations, wind, etc.
Again, it's subjective. What you might be perfectly happy with in terms of vibration damping and wind pickup, someone else might find completely unacceptable. Whether you're doing just visual observing or photography with the mount also has an effect. Most astrophotographers seem to prefer higher capacity, sturdier mounts for the same payload.
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Trev
scholastic sledgehammer
Reged: 12/18/05
Loc: Fort McMurray, AB
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Re: Max Load on CG-4 (latest version)
[Re: mcoren]
#4360446 - 02/03/11 11:58 AM
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The biggest problem with refractors is the moment arm.
Newt's and Cat's are more compact and sit closer to the pivot points for the mount, wheras a refractor sticks out much further creating more torque to start moving it and to slow it down. When dealing with refractors on EQ's think of it more of a question of torque than mount capacity.
If you think of a figure skater spining in a circle the closr their arms are to the body the faster they spin, putting the arms out slows them down
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