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16" equatorial Newtonian build

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#1 hickeydp

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Posted 05 September 2012 - 07:08 PM

So this is my first ATM build and being me I've decided on something probably quite difficult - a 16" f5 Newtonian that will ride on my G11.

My aim is to get something solid that will come in at about 20kg (44lbs). I have a friend who is a carpenter and between us we came up with a design for the tube made up of plywood rings held together with strips of wood and wrapped in a flex-ply sheet. The whole thing will be glued and pinned together. He assures me that it will be very stable. Attached is a picture of the completed frame with me next to it. I'm 5'9", so you will see the tube is going to be big. The obvious bend in the frame from top to bottom is an illusion caused by a wide angle camera phone lens. It's a lot straighter than that.

This is a complete seat of the pants experiment as the build is costing me very little so if the tube doesn't work out, I won't be broken hearted. I'm more than a bit worried about flexure in the tube.

As you will see in the picture, I offset the wooden strips for each section. The strips in each section are 120ยบ apart. I'm thinking that may turn out to be a bad plan but it seemed to make sense at the time. Without any empirical evidence I decided this would spread the strain over the tube better as the sections wouldn't be acting as one unit if that makes sense. Like I said, seat of the pants.

The two centre rings are where the cradle that will hold it to the mount will grip the tube. This will avoid pressure on the ply shell.

The cell is a work in progress. I have a design in mind and if it goes pear shaped, I'll buy one. I'm hoping to build it out of the same 3/4" ply doubled up. Plop is telling me that a 6 point cell would be best but I'm having a hard time believing that for a 16", 1.5" thick mirror. I'll probably address that in a separate thread when I get around to it.

The optics I bought 2 years ago and are a 1/4 wave f5 set.

I'd be very curious to hear what your thoughts are on the project. I'll post here as it progresses. I'm hoping for first light in November as a birthday present to myself ;)

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#2 Guyonthecouch

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Posted 05 September 2012 - 09:55 PM

Wow! I like the ambition of it all! I am in the process of research for a build soon and have really been looking at a double truss system. Take a look at serrurier truss scopes, I think it is right up your alley if this doesn't work out. If you decide to do the serrurier, let me know! It would benefit both of us for design ideas as mine is a 16 f4.5.

#3 polaraligned

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Posted 06 September 2012 - 06:10 AM

That is going to be a little rough on a G11 because of the size. It is the long "moment arm" that will cause the bad wiggles. Unfortunately, there is no economical mount that will handle a scope this size except a home built one.

#4 Jon Isaacs

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Posted 06 September 2012 - 06:50 AM

I'm more than a bit worried about flexure in the tube.



That would be my concern. My guess is that as it sits with the bottom squarely on the floor, you can wiggle the top without too much effort. Each ring with the short connecting columns represents a flexure.

Jon

#5 hickeydp

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Posted 06 September 2012 - 07:07 AM

@polaraligned: Moment arm is the other big concern especially on the G11. I'm hoping that by keeping the weight down I can reduce this to a minimum.

@Jon: You're absolutely right. As it is now it would be a disaster. There is plenty of wiggle in it. I've been assured by my carpenter friend that when it is glued and pinned into the flex-ply wrap it will become very solid once cured.

#6 Mirzam

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Posted 06 September 2012 - 07:41 AM

If the G-11 doesn't work out, you can always re-group and go with a dobson mount + equatorial platform.

I wonder if anyone has tried your "skin" approach with a lightweight internal Serrurier truss?

JimC

#7 hickeydp

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Posted 06 September 2012 - 07:52 AM

Hi Jim,

The Serrurier truss was one of the designs I looked at and may go back to if this idea bombs. We just figured the design we're using would be the lightest. A dob mount is also on the cards as an option. Might do that anyway so I can use the 16 visually while I'm imaging with other scopes.

#8 TxStars

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Posted 06 September 2012 - 09:34 PM

The build reminds me of the old "Lightspeed" scopes.
You can find out about the 16" by searching the archives "16" F-6 Lightspeed"

#9 polaraligned

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Posted 07 September 2012 - 06:13 AM

With the rings getting skinned with plywood, I see no stiffness problems with the tube at least at this point. The rings and structure you show in the picture are just a framework to construct the plywood tube on.

When you find the wiggles with your G11, look into a Hargreaves strut. The long focus refractor guys use it and it makes a big difference by all accounts.

#10 hickeydp

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Posted 07 September 2012 - 06:19 AM

Interesting idea, will definitely keep it in mind. Thanks :)

#11 cheapersleeper

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Posted 07 September 2012 - 08:18 AM

Looking forward to following your build. Do you think it can possibly end up 44lbs? The mirror alone should be around 23lbs.

Regards,
Brad

#12 TopherTheME

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Posted 07 September 2012 - 09:01 AM

You're greatly weakening the structure of the tube by offsetting the wood strips/struts 60* apart from each other. For the best structural efficiency you want them all to line up with each other and ideally they would be one big piece with the rings fitting between them. Sort of like the fuselage of an airplane.

By having that type of structure you are placing unnecessary moments on the wooden rings that form the tube.

#13 hickeydp

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Posted 07 September 2012 - 09:01 AM

Cheers Brad,

The mirror is only 12lbs. I made a mistake in my opening post, the mirror is only 1" thick.

I'm guessing with the cell it will all come in at about 18lbs with the mirror when done. The tube will be about 25lbs. By the time I add the spider, secondary and focuser it will probably be close to 50lbs realistically. But these are guesstimates.

My G11 is going to hate me.

#14 hickeydp

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Posted 07 September 2012 - 09:05 AM

@TopherTheME: I had a feeling this would be the case, unfortunately after the fact. With it all glued and pinned, I'm not sure it will be easy to dismantle and reassemble. But if I can, I will.

#15 cheapersleeper

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Posted 07 September 2012 - 12:33 PM

Cheers Brad,

The mirror is only 12lbs. I made a mistake in my opening post, the mirror is only 1" thick.

I'm guessing with the cell it will all come in at about 18lbs with the mirror when done. The tube will be about 25lbs. By the time I add the spider, secondary and focuser it will probably be close to 50lbs realistically. But these are guesstimates.

My G11 is going to hate me.


That is very good to hear. The only reason it made me curious is that I have a 12" 1 1/2" mirror in one of my scopes and it ended up being quite heavy. :lol:

Good luck!

B

#16 Scott Watson

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Posted 07 September 2012 - 05:38 PM

As it is, that thing is going to bobble like a weeble. I would seriously consider a thin wrap glued around the whole tube.

#17 Ed Jones

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Posted 07 September 2012 - 07:24 PM

I think the tube could be fairly rigid but what I would worry about is the attachment points where the tube mounts in it's cradle. If the rings would match up to where the cradle attaches you would get the best rigidity.

#18 m. allan noah

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Posted 07 September 2012 - 07:43 PM

Man, some of you guys cannot read...

allan

#19 Pinbout

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Posted 07 September 2012 - 08:14 PM

I have a friend who is a carpenter and between us we came up with a design for the tube made up of plywood rings held together with strips of wood and wrapped in a flex-ply sheet.



we make structures like that all the time in the exihibit business.

the only thing I would have done is turn the ribs toward the center and added more of them to glue and staple the wiggle wood, bendy board or whatever you guys call your flexible plywood.

#20 kfrederick

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Posted 08 September 2012 - 05:59 AM

Wonder how aluminum skin would work .Be a nice telescope .Thanks for posting

#21 hickeydp

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Posted 08 September 2012 - 06:01 AM

@Allan: Good point ;)

@Ed: That's my plan to attach the cradle around the centre two rings as I doubt the flex-ply would be strong enough to be stable.

@Danny: I think I will be adding more ribs to correct the mistake of offsetting each section. Turning them to make another attachment point for the skin is a good idea, thanks.

#22 hickeydp

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Posted 03 October 2012 - 09:54 AM

I've been neglecting this thread and it's about time I updated. Most of the bit and pieces have been made up at this stage and now need to be fitted and assembled. Here are the pics, apologies in advance for the photography. It was done on the fly with my iPhone 4.

First image is the frame, now painted with preserve and blackboard paint on its inward facing parts...

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#23 hickeydp

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Posted 03 October 2012 - 09:55 AM

Here's the tube half wrapped...

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#24 hickeydp

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Posted 03 October 2012 - 09:57 AM

Tube fully wrapped. As you can see from this image and the last, we got the circumference measurement off. Lesson learnt, use the same tape to measure each time, not two different tapes.

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#25 hickeydp

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Posted 03 October 2012 - 09:59 AM

We cured the above problem by attaching a length of 2"x1" along the seam, which we will use to help anchor it into its cradle. Here's a shot of the interior.

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