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Is my OTA too long for equatorial mount?

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#1 benchacom


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Posted 10 May 2009 - 01:03 AM

I have an 8" f/7 reflector in a dobsonian setup. I've been thinking of taking the tube and mounting it on an equatorial mount. My concern is that the tube will be too long for such an arrangement to work, the tube is almost five feet long. Is it unheard of to have that long of a tube on an equatorial setup?

#2 Kaizu



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Posted 10 May 2009 - 04:12 AM

The fork mounting is not much higher than Dobson type.
We have that type in our Cosmological Observatory.
Of course we need stairs to see zenit. The newt is ca. 2,5m (8,5ft)long


#3 RAKing


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Posted 10 May 2009 - 07:44 AM

I have used my 8 inch reflector on my GM-8 for a while and it works fine.

Your main problem might be tube clearance against the tripod legs. I had to adjust the struts of my A-P Portable Pier to clear some "swing" room for the bottom of the tube. You will notice this more when the RA axis is horizontal and you are moving the scope on the Dec axis - like when the target is overhead and/or you do a meridian flip. My GM-8 can track past the meridian and I had to reset my safety limits to avoid colliding with the pier struts.



#4 Eddgie



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Posted 10 May 2009 - 08:37 AM

You may want to consider either one of the old Meade-type EQ mounts with the stubby pier, or a modified modern mount.

The problem will be finding a mount with a tripod that is short enough to still allow reaching the eyepiece when viewing closer to zenith.

I saw a 8" f/7 planetary Newt that someone had out in the field about 7 or 8 years ago on a GM-8. The very intersting thing about this mount was that he had replaced the legs with short wooden legs. The counterweight shaft and the rear of the OTA barely cleared the ground, but it put the eyepiece at a really superb height.

It might be possible to duplicate this with something like an Atlas and just make a set of short wooden legs. Alternatly, perhaps you could buy a pier for the Atlas and shorten it.

Another issue though is that the eyepiece will forever be getting into bad positions. Rotating the tube in standard clamshell rings is tedius. True rotating rings are VERY expensive, but I would never have another Newtonian scope on a GEM without them.

The other option is a tracking platform.

My own preference would be the GEM though.

#5 PJ Anway

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Posted 10 May 2009 - 04:30 PM


I use my 8" f/7 with a Celestron CI-700 mount on a short pier I made. It works very well for visual observing. Here is a pic:

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#6 Telescopeman54


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Posted 10 May 2009 - 04:39 PM

That's the way they used to come. Check the old Edmund and RV-8 scopes that were 8" f/8 on GEMs. The more expensive ones, such as Cave, etc. had rotating rings. The aforementioned ones were just bolted to the saddle or had felt lined rings to allow rotation. We did just fine with them.



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