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Why not a wedge for a fork mount instead of a GEM mount?

Started by Big Louie, 02/24/2021 11:15AM
Posted 02/24/2021 11:15AM Opening Post
Hi there:

I am a noob. Sorry. I cannot understand why a fork mount on a wedge would not work as well as a German Equatorial Mount. Seems like I am the only one with this question though it has probably been answered a thousand time. I just cannot find it.

Thank you.

Barry
Posted 02/24/2021 04:56PM #1
I believe the problem is field rotation. Look up that subject and I believe  you will find your answer.  
Joe g
Posted 03/03/2021 01:59AM | Edited 03/03/2021 02:06AM #2
Barry,

If the fork on a wedge is aligned with the North Celestial Pole (NCP) then you are fine with a fork on a wedge instead of a GEM. The problem with a fork NOT on a wedge, is field rotation. Think of the constellation Orion: When it rises, it is on its side, When it is high in the Southern sky it is upright, When it sets in the West it is on the other side. Fork on a wedge keeps the OTA oriented correctly like a GEM, Fork NOT on a wedge does not keep field rotated correctly.

One issue is that the fork needs to be stiff enough to keep from flexing from the load held "at arms length", so the forks are rated for a lower payload than a GEM which has the OTA closer to the axis.

Ed (aka eblanken)
Posted 03/03/2021 04:19PM #3
Originally Posted by Ed Blankenship
Barry,

If the fork on a wedge is aligned with the North Celestial Pole (NCP) then you are fine with a fork on a wedge instead of a GEM. The problem with a fork NOT on a wedge, is field rotation. Think of the constellation Orion: When it rises, it is on its side, When it is high in the Southern sky it is upright, When it sets in the West it is on the other side. Fork on a wedge keeps the OTA oriented correctly like a GEM, Fork NOT on a wedge does not keep field rotated correctly.

One issue is that the fork needs to be stiff enough to keep from flexing from the load held "at arms length", so the forks are rated for a lower payload than a GEM which has the OTA closer to the axis.

Ed (aka eblanken)
Posted 03/03/2021 04:24PM #4
Thank you Ed.

I figured the wedge would essentially make an alt azimuth mount essentially equatorial if it were placed on a wedge. Thank you for confirming. 

I think your observation regarding The strength of the forks is right on and would apply to any fixture on any mount of any kind. They all must have enough rigidity to hold the OTA steady, vibration free and in the same position regardless of the mount choice. I can see how that might be a problem with a flimsy fork.

Thank you for your response Ed.

Barry