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UTA Adding Weight

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#1 LateViewer

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Posted 11 October 2013 - 06:58 PM

I have been modifying my f/6 String-Truss-Dob to an f/5. The focal length going from 75" to 60" dropping the EP by 15".

No more step stool.

The down side of this reduction in height is that the UTA is no longer in balance with the Mirror Box by 3 to 4 pounds. Two weeks ago I first brought this new mirror to focus and realized how significant this change was. I filled a pouch with some heavy items and hung it off of the upper ring of the UTA to balance it out. In the future I am going to make a new UTA and will make a series of modifications that will make the UTA heavier.

But in the short run I am looking for a nice way to add weight without hanging this bag. Tacky.

I have thought of adding a finder scope, I now only use a Telrad now, to add weight.

Any other ideas?

Al

#2 cpr1

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Posted 11 October 2013 - 07:50 PM

Is this a single upper ring scope? Pic? It would help to figure out what can be done.

Can you adjust the bearings down on the mirror box to change the COG? Or move the teflon pads further apart to provide more friction.

As far as adding weight to the upper it would depend on the design of it. If it is a traditional upper cage you can add weight inside the upper cage struts.

#3 Mirzam

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Posted 11 October 2013 - 07:56 PM

Orion sells some nice heavy brass focuser knobs... (I have actually used these as counterweights believe it or not).

Can you make the LTA lighter? Maybe by changing steel to aluminum or by drilling some holes?

JimC

#4 LateViewer

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Posted 11 October 2013 - 08:19 PM

It is a double ring traditional UTA. I have thought about filling the UTA struts with steel pellets?

If I move the bearings lower I move the EP higher and that defeats the whole purpose of the change.

Truthfully I would have built the mirror box differently if I was starting from scratch. But I am not.

I have a WO 66mm Zenith Star that I thought of adding but it is too heavy. The just right finder scope would be cool. Or maybe some other accessory I have not thought of.

If not then I will figure something out.

#5 cpr1

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Posted 11 October 2013 - 08:29 PM

If you have to add 3 or 4 lbs. filling the struts with lead shot or steel would work and could be fine tuned easy.

You could add some epoxy after to keep the shot from rattling around or that expansion foam stuff. Or make your lower lighter like Jim said.

#6 Mirzam

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Posted 11 October 2013 - 08:47 PM

If you don't already have a dew heater for the secondary you could add one of these with the power supply battery to the UTA. Also, a Paracorr would add some weight although at f/5 it is not really essential.

JimC

#7 RossSackett

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Posted 11 October 2013 - 09:20 PM

When I re-engineered an 18" so I could use it with a camera and powermate, I trimmed it 5 lbs light on the top end. When I use it visually I just use a barbell weight to balance it.

#8 a__l

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Posted 12 October 2013 - 03:57 AM

Steel rings on my UTA.

#9 LateViewer

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Posted 12 October 2013 - 11:03 AM

I have been doing a little web prowling and telescope tinkering since my last posting.

When I put the lycra shroud and Telrad on the UTA and use a 1 pound EP. That's a light EP. I am 2 lbs out of balance. If I add a Stellarvue right angle 60mm finder and rings I am right there at 24oz with rings etc.

Maybe that is the way to go.

#10 derangedhermit

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Posted 12 October 2013 - 10:50 PM

If I move the bearings lower I move the EP higher and that defeats the whole purpose of the change.

If you give back a few of those 15", I think it will make little difference in use, and keep things light and simple. You could get closer to balance that way, and reduce the gap you need to close.

Maybe you want a larger secondary now? There's several ounces.

#11 LateViewer

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Posted 13 October 2013 - 10:01 AM

The secondary has already been changed. 2.6 vs 1.8.

I decided to add a Stellarvue 60mm finder scope.

Among the modifications I will make in a future UTA rebuild will be to make a thicker, 1", ring. This is a string truss design with the strings being an inverted V between the struts. The force of the strings pulling down on the lower ring while the struts push up causes the current ring to deform. Making this ring thicker will also add some weight.

All in all everything is turning out nicely.

And the new Lightholder mirror is excellent!

Al






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