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Test of Atlas Mount w/Extension Tube vs. Legs Out

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#1 John Miele

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Posted 09 January 2009 - 09:02 PM

I pinged this forum and other groups about the difference in stability of an Atlas mount with a C11 on top using the tripod extension tube vs. extending the legs. I got quite different answers. Some saying the pier makes the telescope less stable and some saying more stable. Before I go further, I realized my choice of the word “stability” was not a good one. What I really was interested in was the vibration damping time. Some people may have misinterpreted my use of the word stable to mean how hard it is for the tripod to overturn. And there is no question that the tripod’s footprint is wider with the legs extended and it is more stable in that regard. Anyway, I concluded I’d just have to order one and see for myself. So I went ahead and purchased Orion’s 8” long tripod extension tube and ran two tests. For both tests, I set up my scope (a C11) with all normal accessories and centered a star at medium power (130x) and rapped the tube. Using a stopwatch, I measured the time from the rap to when the star image stopped moving. I took the average of 10 measurements in each configuration. I made sure that all knobs and connections were tight and the scope was in the same approximate orientation. For the record, I’m using an ADM D-series dovetail plate and an ADM Dual saddle and a set of Celestron VSPs. So you can see I’m doing everything I can to reduce overall flexure and vibration as much as possible. For test 1, the tripod legs were almost fully retracted. (about 1 inch was showing just to allow tripod leveling) and the extension tube was bolted in place. The average rap test damping time was 2.2 seconds. For test 2, the extension tube was removed and the legs were extended about 14 inches. The average rap test damping time was 4.7 seconds. The scope was much “springier” in test 2. I was very surprised at the amount of stiffness reduction from extending those tripod legs. I’ll definitely be sticking with my extension tube. I also drilled a hole through the extension tube adapter plate and drilled and tapped a mating hole in the top of the Atlas tripod and used a bolt to prevent the extension tube from rotating on the tripod. If you need some extra OTA height and you want to minimize vibration damping time with a C11 on an atlas, the extension tube is better than extending the legs out.

John Miele

#2 AlexN

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Posted 10 January 2009 - 03:23 AM

Good write up mate...

I always use my C11 with the legs low and no extension tube, but mine is purely for imaging...

The extension tube I have gets used occasionally for the refractors at star parties just to stop peoples heads from walking through frame...

#3 Mert

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Posted 10 January 2009 - 08:22 AM

That's very well described John, thanks for that!

#4 Doug76

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Posted 10 January 2009 - 09:29 AM

I found the same thing to be true on my CG-5ASGT mount, using the longer extension (16") post for it, with a C6R refractor aboard. Definitely a quicker vibration abatement time than with the legs extended, which was a good thing, as you really need the extension to use that refractor without the tube hitting the legs.
But the extension is too high for me to use my SCT on, and as the extension is longer than needed even for the refractor, I'm considering cutting off about 4" of it. This will put it at the perfect height for the SCT, and hardly any lower for the refractor. I have measured it, but haven't done the deed yet.

#5 John Miele

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Posted 10 January 2009 - 11:05 AM

I found the Orion extension puts my C11 just high enough to where my observing seat at its lowest height is still useful when the EP gets low.


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