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Equipment Discussions >> Mounts

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Falcon-
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Reged: 09/11/09

Loc: Gambier Island, BC, Canada
Counterweight question new
      #3588757 - 01/28/10 04:39 AM

I have a CG-5 ASGT mount that I have been using for astrophotography for a while now with just a DSLR mounted and a 300mm lens. Now, at long last, I have acquired an imaging scope to go between the camera and the mount - a 6" f/6 MCT.

Naturally though since I have been using just a DSLR till now the never-used 11lb counterweight is still sitting in a box on the mainland so I can not simply take it out and TRY the thing!

So to finally get to the actually question, the scope itself weighs 15lb and I will likely add another 2lb to that with the camera/guidecam. Will the single 11lb counterweight be sufficient if at the end of the shaft, or will I need a second counterweight?

If I do need a second weight I am thinking that another 11lb weight would be a bit much, would anyone have a recommendation for a source for a smaller counterweight?

Thanks

- Sean


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RTLR 12
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Reged: 12/04/08

Loc: The Great Pacific NorthWest
Re: Counterweight question new [Re: Falcon-]
      #3589182 - 01/28/10 10:52 AM

Sean,

Maybe someone has the 'yes no' answer to your question, however this may help to establish your own opinion.

Stan

http://www.robincasady.com/Astro/WeightCalc.html


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Falcon-
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Reged: 09/11/09

Loc: Gambier Island, BC, Canada
Re: Counterweight question new [Re: RTLR 12]
      #3590744 - 01/29/10 02:31 AM

Thanks for the link.

Ok, so given I do not have immediate access to measure my CG5 I am going to have to guess at the A, B, & C values as referenced at that page... I am going to guestimate A & B at 6" and C at.... say... 12".

The scope itself has an OD of 200mm (7.9inch) and I am going to guess that mounting hardware is about 1.5", so distance from tube COG to axis is 11.45", call it 11.5" with finderscope. Tube weight is 7kg, or 15.4 lb, bring that up to ~17lb with DSLR and guidecam.

That is 195.5 inch-pounds. To get that with a single 11lb weight it would need to be at 17.8" out from the axis, B+C is 18inches.... so it COULD work.... *IF* my guesses for distances are correct.

So then, two new questions arise:

1) Can someone sanity check my distance guesses with reality on a CG5 please?

2) Given that this is intended to be used for imaging would it be better to have a single 11lb weight way out at the end of the arm or have 22lb of weight way in at the start of the counterweight bar? In other words, should I avoid extra weight or extra moment-arm?

Thanks

- Sean


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Luigi
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Reged: 07/03/07

Loc: MA
Re: Counterweight question new [Re: Falcon-]
      #3590910 - 01/29/10 07:18 AM

I added a 2.5 lb dumbbell weight to the standard 5 kg weight with 3M industrial double stick tape. It works with my ~16 lb OTAs + 2" diag + big heavy EPs + finder, with some room to spare. I expect it would be sufficient for your setup too. You could add a 5 lb dumbbell weight if not. I added a 5lb weight the same way to the 10 kg CW of my G11.

The tape is available at hardware stores, K-Mart, Walmart, etc, and has grey foam with a red release layer. I used this stuff to fasten a bilge pump in my Zodiac RIB and it's still there after a number of years of immersion in salt water. It comes off only with great difficulty.

My preference is to use less weight and longer arm, within reason. Others advocate the other way around. You're mainly adding radial load to the suffering bearings and weighing down the tripod which will lower the resonant frequency of the setup and lengthen damping time, IMO.


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Eddgie
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Reged: 02/01/06

Re: Counterweight question new [Re: Falcon-]
      #3591173 - 01/29/10 10:44 AM

Vixen counterweigts come in small incremental sizes. Expensive, but allows you to keep the weight as low as possible.

Orion sells a 7.5 lb counterweight. Not as expensive as Vixen, but not bad.

You can also buy counterweight bar extension from different vendors and if you are only short an inch or two for balance with your existing weight, this might work.

Finally, if you are only short an inch or so, you can get a local machine shop to drill the existing weight out on one side to a depth of about 1" and a diameter sufficient to clear the safety nut. The Vixen weights are made this way. It allows the weight to overhang the safety nut about 1.5". I love this about the Vixen weights.. Just too expensive though.

Regards.


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Falcon-
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Reged: 09/11/09

Loc: Gambier Island, BC, Canada
Re: Counterweight question new [Re: Eddgie]
      #3591448 - 01/29/10 12:50 PM

I guess that means the skyview pro has the same diameter counterweight shaft as the CG5 then?

Still the question remains if I *should* use a single weight at the end of the bar or not for reasons of moment-arm on a guided imaging platform...


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BlueGrass
Carpal Tunnel
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Reged: 07/25/09

Loc: Wasatch Front, UT
Re: Counterweight question new [Re: Falcon-]
      #3592638 - 01/30/10 12:27 AM

Falcon,

There are two 'schools' of thought on the weight placement. Is the torque required to move the mount the same regardless of weight placement? Does distributing the weights along the shaft dampen vibration quicker? I asked a similar question a month or two ago and there were a number of views about this. I'm still not 100% convinced either way although the dampening aspect is critical to good AP and I plan on using that suggestion as a starting point while balancing.

Edited by BlueGrass (01/30/10 12:32 AM)


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Falcon-
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Reged: 09/11/09

Loc: Gambier Island, BC, Canada
Re: Counterweight question new [Re: BlueGrass]
      #3592706 - 01/30/10 01:44 AM

George:

Thanks for sharing your thoughts.

I had not considered vibration dampening as part of the weight placement question. That may well be the most important aspect, especially with my lowly CG-5! I do not quite know though what configuration would be best for vibration damping though - any suggestions?

Quote:

Is the torque required to move the mount the same regardless of weight placement?




I did some google'n to see if I could figure that out. As far as I can determine it *IS* the same so long as balance is reached. 22lb at 6" is identical to 11lb at 12", as is 18.5 lb at 7.135". In other words any weight/distance that does balance the OTA will of necessity apply the same amount of torque. http://easycalculation.com/physics/classical-physics/moment.php

Feel free to correct me if I am wrong, but as far as that part of the question goes it *seems* like it is a non-issue. So if my guesses at counterweight arm length are correct then the single 11lb weight would be the way to go as it would put the least total weight on the bearings and should be identical in torque to a 22lb closer in on the shaft.

- Sean


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BlueGrass
Carpal Tunnel
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Reged: 07/25/09

Loc: Wasatch Front, UT
Re: Counterweight question new [Re: Falcon-]
      #3592718 - 01/30/10 02:16 AM

Sean,
I also was under the same assumption, i.e. one set amount of counter weight used and its placement was identical to another different setup for purposes of torque required as long as balance was maintained. However, when extending the counter weight bar to use less weight, a practical limit is reached. For my current setups, I purchased an Orion 7.5 lb that I use as a 'slider' weight between either the CGEM 17 lb or the CG5 11 lb. This allows me to achieve better balance control while keeping the primary weight mass at a fixed point. Plus it functions as the dampener and is much easier to move when you need to shift the balance east or west heavy. Good luck with your mount though. I look forward to your results and feedback.


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vinnie
Minister of Misinformation . . . Oh Yes
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Reged: 01/05/08

Loc: Queensland, Australia
Re: Counterweight question new [Re: BlueGrass]
      #3593302 - 01/30/10 12:07 PM

Quote:

I purchased an Orion 7.5 lb that I use as a 'slider' weight between either the CGEM 17 lb or the CG5 11 lb. This allows me to achieve better balance control while keeping the primary weight mass at a fixed point. Plus it functions as the dampener and is much easier to move when you need to shift the balance east or west heavy. Good luck with your mount though. I look forward to your results and feedback.




Like the small weight in the photos here? http://www.astroholic.com/showthread.php?t=242

Its a home made job and yep the idea of putting the big weight low down and a small "slider" in place above it works really well


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Luigi
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Reged: 07/03/07

Loc: MA
Re: Counterweight question new [Re: vinnie]
      #3593329 - 01/30/10 12:22 PM Attachment (36 downloads)

CG5 CW with 2.5 lb addition.

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Falcon-
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Reged: 09/11/09

Loc: Gambier Island, BC, Canada
Re: Counterweight question new [Re: Luigi]
      #3593505 - 01/30/10 01:43 PM

Luigi: Interesting - how is the smaller weight secured to the original?

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Falcon-
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Reged: 09/11/09

Loc: Gambier Island, BC, Canada
Re: Counterweight question new [Re: Falcon-]
      #3601495 - 02/03/10 02:42 AM Attachment (32 downloads)

So I finally have a reality check from the real world! Turns out that with my Canon XT (350D) attached to the scope and the single 11lb counterweight all the way out it *ALMOST* balances. It is close enough that the scope will not move when the clutch is loose, but if you tap the OTA side it rotates a few degrees while when you tap the counterweight side it does not move.

I actually got a perfect balance by attaching some old hard drive magnets from a couple of ancient full hight SCSI drives I took apart some years back. I will I think do something similar to what Luigi apparently did for a final solution once I have the guide cam on hand and know the final weights.

Below is a picture of the first time the mount and scope scope have come together..... including hard-drive magnet counterweights

- Sean


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bardo
scholastic sledgehammer
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Reged: 09/13/09

Loc: US
Re: Counterweight question new [Re: Falcon-]
      #3601567 - 02/03/10 05:49 AM

i unscrew the CW shaft a little and use the locking bolt to tighten it. it gives up to about an inch extra length.

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Falcon-
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Reged: 09/11/09

Loc: Gambier Island, BC, Canada
Re: Counterweight question [Re: bardo]
      #3603156 - 02/03/10 07:39 PM

bardo

Thanks, I had not considered having the counterweight shaft anything but fully threaded in. I will give it a try


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