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Upper Ring Stiffening for Ultralights and UC 18"

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#1 Almagne

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Posted 10 August 2013 - 12:23 PM

The Obsession UC 18 is a great scope, but one small issue is that the upper ring should be a little stiffer. Some have noted this on threads here on Cloudy Nights and I've noticed it on my UC 18. This issue will be a danger for any ultralight telescope that uses a single ring at the upper end. The collimation will slightly change when the elevation is changed from 80* to 20*, even with the Glatter sling modification. The change is slight but it is there. A large heavy eyepiece along with a Paracorr will also cause a small amount of flexing of the ring at the focuser. In addition when moving the scope by the handle on the upper ring, one can actually see it flexing. The issue is small but noticeable. Some have tried to solve the problem by having an aluminum plate manufactured to replace or reinforce the upper ring. This is both expensive and adds weight to the upper end of the scope - exactly where you do not want it. The fix described here is specifically for the Obsession UC 18, but the technique would be beneficial for any ultralight.

A 3/4" wide 1/8" thick aluminum strap screwed to the edge of the upper ring will greatly stiffen it. The aluminum is only a few ounces and it can be easily applied. I was able to do this to my scope and the result was a much stiffer upper ring.

Here is how it can be done:

Purchase at local Home Depot one strip of 4' long 3/4" by 1/8" strip of aluminum. The circumference of the upper ring of the UC 18 is about 6'. I only added 4 feet of aluminum to the part of the ring centered on the focuser. This is where all the heavy items hang on the ring and where it is grasped to be moved. Also one box of Teks lath screws (size #8 by 3/4") were used to screw the aluminum to the ring. These screws have a wide head which helps hold the aluminum tight. Also some metal primer spray paint, one can of spray flat black, and a little solvent to clean the aluminum are needed to paint it. I began installing the strip before painting to make sure it would work. Once it was clear it would work, it was painted then installed. The early pictures of the install show the aluminum as being bare, then later it is painted.

Finally the only other equipment needed is a clamp, a cordless drill with a Phillips head driver, and two drill bits. One bit to drill the aluminum and one to drill a small pilot hole into the wood ring. Also a little sand paper is helpful to remove the drill burrs and round the corners of the aluminum ends.

For installation one end was screwed to the ring so that the strip would be centered on the focuser. The aluminum was easily bent along the circumference and the clamped. With a pencil the places to drill were marked, so that the screws were spaced about 3 inched apart. There were places the screws were only about 2.5" because of the need to avoid things on the ring like the focuser. At the two places where the poles connected, screws were placed one inch away on either side. Then with the locations marked the aluminum was removed and the holes were drilled. It was painted also, then installed. It took a two days to prime and paint it. All told it took only three or four hours of labor. The pictures will help show the process. I hope this helps solve some problems!

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#2 Almagne

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Posted 10 August 2013 - 12:24 PM

Photo

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#3 Almagne

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Posted 10 August 2013 - 12:25 PM

Attaching the strip to mark the screw locations.

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#4 Almagne

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Posted 10 August 2013 - 12:26 PM

Screwing the strip on.

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#5 Almagne

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Posted 10 August 2013 - 12:26 PM

Photo.

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#6 Almagne

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Posted 10 August 2013 - 12:27 PM

Another photo.

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#7 Almagne

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Posted 10 August 2013 - 12:28 PM

Another shot.

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#8 Almagne

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Posted 10 August 2013 - 12:29 PM

One more.

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#9 Almagne

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Posted 10 August 2013 - 12:30 PM

The end result.

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#10 Mirzam

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Posted 10 August 2013 - 03:18 PM

Great idea! Thanks for showing your approach.

JimC

#11 Dick Jacobson

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Posted 12 August 2013 - 09:49 AM

I took a completely different approach and made my focuser board an integral part of the truss on the 20" Newtonian that I just finished building. The 22" circular focuser board is permanently connected at the bottom to a pair of truss poles. The upper end of the focuser board simply connects to the secondary ring at a single point and does not put any bending stress on it.

For a lightweight yet extremely strong upper ring, I bent a 7/8" diameter aluminum tube into a circle. I used a homemade ring roller, but a much better tubing bender is now available from Harbor Freight.

#12 GeneT

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Posted 12 August 2013 - 06:50 PM

Good fix! Glad things worked out.

#13 jm510227

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Posted 12 August 2013 - 10:27 PM

This is impressive, creative, intelligent and time consuming analysis not to mention the work involved on a well documented problem with an expensive scope. Don't want to start a flame war here, but as a potential purchaser of an 18UC, I am interested to know if has anyone has expressed to Obsession the necessity to solving this problem at the source? Also would like to know of Obsession's response, if any.

#14 Papa Taylor

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Posted 13 August 2013 - 07:45 AM

I have an 18"UC with the ring flexure issue. The thread I started on the reflectors forum a month or so ago can be found here:

http://www.cloudynig...5898261/page...

It is long but I included quite a few pictures that illustrate the results of the ring flexure as indicated by the movement of the laser spot on the primary and secondary mirrors when the scope is tilted up and down and also when the focuser is racked in and out.

I contacted Obsession about this and their response was in essence that it is not an issue that needs to be adressed.

They recommend collimating the scope at 60 deg. since this is where we use our scopes the most and re-collimating when the scope is moved up or down significantly.

They also say that by using the barlowed laser technique, the scope will still be accurately collimated in spite of the wild movement of the laser spot due to ring and other component flexure places inherent in the design of a (lightweight) dob. And that in spite of these things, the views are still excellent.

And thank you, Almange for posting a link to this thread in the reflectors forum. I would not have seen it otherwise. :bow:

I have not made any modifications yet to my 18"UC to resolve the ring flexure issue and appreciate you taking the time to tell us what you did. Great write-up.

#15 Dick Jacobson

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Posted 13 August 2013 - 08:45 AM

They also say that by using the barlowed laser technique, the scope will still be accurately collimated in spite of the wild movement of the laser spot due to ring and other component flexure places inherent in the design of a (lightweight) dob. And that in spite of these things, the views are still excellent.

I agree, ring flexure is not as big an issue as it may seem. The purpose of collimation is to make the optical axis of the mirror pass down the center of the focuser. Inward and outward movement of the focuser board only puts the image out of focus, which is easy to correct. I experienced this with my 22" circular focuser board as mentioned in a previous post, which originally was not stiff enough and caused the image to go out of focus when I pushed on the focuser board.

#16 Almagne

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Posted 13 August 2013 - 05:38 PM

The purpose of collimation is to make the optical axis of the mirror pass down the center of the focuser. Inward and outward movement of the focuser board only puts the image out of focus, which is easy to correct.


The flexing is not inward or outward movement of the focuser.

The issue is that there is a torque or turning force on the focuser (and the entire upper end of the scope) caused by the Paracorr and large widefield eyepiece (like an E21) hanging from the focuser. The result, as I see it, is that axis of the focuser is not parallel to the optical axis of the main mirror and the diagonal.

In the UC 18, the upper ring bridges approximately 18 inches between the truss connections, and the focuser is in the middle of this run. So when the telescope is pointing to the zenith, the Paracorr and eyepiece are sticking straight out from the side of the upper ring. Gravity is trying to pull it downward and one end is attached to the focuser. Consequently there is a turning force on the focuser trying to rotate it. The upper ring resists this force but it slightly twists, because it is not stiff enough in this dimension.

When the scope is pointing lower in the sky the twisting force would be less, and therefore the deflection would be less. Consequently, according to Papa Taylor, Obsession recommends to split the difference and collimate at a 60* elevation to minimize the error and recollimate when the scope is moved up or down significantly.

Let's be clear, the amount of deflection and the amount of decollimation is small. When going from something near the horizon, say 25*, to near zenith, 90*, the view is really excellent. Nevertheless, a tweak of the collimation is necessary to get the most out of the scope. The reality is that it should not need to be recollimated.

When this modification was made, the need to recollimate disappeared.

#17 jm510227

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Posted 13 August 2013 - 09:56 PM

Nice work! I'm glad and relieved to know that you have solved the 18UC flexure riddle. In your testing of heavy wide field eyepieces + Paracorr combinations, did you have the opportunity to try the heaviest of the group, the 31T5 or the ES 30mm for flexure?

#18 jonathanCR

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Posted 13 August 2013 - 10:52 PM

Wonderful solution. Have you think about adding another ring in the inside or it is not necessary?

#19 Almagne

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Posted 13 August 2013 - 11:19 PM

In your testing of heavy wide field eyepieces + Paracorr combinations, did you have the opportunity to try the heaviest of the group, the 31T5 or the ES 30mm for flexure?


No, just a Paracorr Type II and a 21 Ethos were the heaviest set I was able to test.

#20 Almagne

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Posted 13 August 2013 - 11:30 PM

Wonderful solution. Have you think about adding another ring in the inside or it is not necessary?


I thought about it, but it does not seem necessary. Although I've only had it out once since the modification, it was clear from the start that it was stiffer. The collimation held very well throughout the evening from Sagittarius near the horizon to Cygnus at zenith.

#21 Jon Isaacs

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Posted 14 August 2013 - 05:50 AM

The issue is that there is a torque or turning force on the focuser (and the entire upper end of the scope) caused by the Paracorr and large widefield eyepiece (like an E21) hanging from the focuser. The result, as I see it, is that axis of the focuser is not parallel to the optical axis of the main mirror and the diagonal.



I believe this is what PapaTaylor was referring to, this is basically the axial alignment of the focuser versus the tilt of the primary.

The motion of the laser about the face of the primary mirror is a measure of the focuser axis alignment. According to my understanding of what Vic Menard and Nils Olof Carlin have calculated, the FAA tolerances for a 18 inch scope are something like 18inches/30 = 15mm without a Paracorr and 1/6th of that = 2.5mm with a Paracorr. This constraint concerns the tilt of the focal plane...

The tilt of the primary is more important and it is measured with the Barlowed Laser. The actual tolerance is a fraction of the diffraction limited circle, at F/4.2, the radius of the diffraction limited circle is 0.812mm, you want to be close enough to 0.8mm is large by comparison.

So, one needs to distinguish between the two...

I had a similar experience with my 16 inch Dobstuff, it is a three tube strut scope that uses a single ring instead of a upper cage. The ring itself is more robust than the Obsession UC but I still experienced collimation shift due to a number of factors, the stiffness of the upper ring was one of the more important ones.

My solution was to use a thicker upper ring. Since the stiffness in bending and torsion is related to the cube of the relevant dimension, (generally the thinner of the two), it doesn't take much to make a big difference, in my case I increased nominal stiffness by a factor of 4 with something like an increase of a pound of weight.

Eventually I ran down all the issues and now the scope exhibits zero collimation shift with the appropriate laser for both the axial focuser alignment and the primary tilt, no visible shift.

Jon

#22 wfj

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Posted 05 September 2013 - 06:54 PM

Often very little is required to stiffen a structure to make up for a bending moment on any thrust structure.

Keep in mind that its the geometry more than the weight or strength of the material. The surface of the object constrained against a given dimensionality (depth, diameter, curve, triangle) is the source. Which is why sparse structures like I-beams, honeycomb, and channel are often as strong or stronger than solid material.

In lightweight thrust structures, one works to reduce weight and gain stiffness through, not to buy stiffness through bulk and weight alone.


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