Carbon tube for 10" OO mirror

[Robert72]

Just about to start a carbon tube to house the 10" OO mirror, cell, spider and secondary I recently bought from Stu over in the classifieds zone.  I realise help and advice at this stage is crucial, so if anyone has any experience, feel free to give advice etc.  I have already considered using the sonotube as a mandrel with vacuum infusion, but don't fancy my chances of releasing the finished tube.  Using a female mould would aid release, but then the lay-up seems to be impossible with dry cloth flopping all over the place in the female.  I already have a 295mm OD Sonotube, and my intention is to use this as a male plug to create a female mould.  From this I will mould a carbon tube using pre-preg and a home made oven!  That is the plan anyway, but if anyone knows a way of simple lay-up combined with achievable release please let me know!

[Stu]

Good luck Rob! Watching with interest, will be great to see the mirror in use again.

[NGC 1502]

 

Although I’ve constructed several Newtonians from parts, I’ve never made the tube, I’ve used plastic tubing from builder’s merchants etc.   Nor have I ever ground and parabolised a mirror.  ( I realise you’re also using a ready made mirror ).

But of course there’s lots more that needs sorting out, including the mount.  A great source of relevant information can be found at-      stellafane.org    

After having acquired or made the tube, the next thing is to sort out the primary cell, secondary vanes and focuser positions so that the telescope comes to focus.   A good  idea is to get the focus position at mid focuser travel. As most focusers have generous travel, this gives a fair margin of error if your calculations are not spot on. The focal length of the primary mirror equals the distance from its aluminised surface to the middle of the secondary then out through the focuser hole to the focal plane of the eyepiece.  Sounds a bit daunting at first, especially working with mirrors that have easily damaged optical surfaces. But go steadily and carefully, you’ll get there. You can give yourself a bit more wriggle room by having elongated holes to fix the primary cell. This means you can shift the cell towards or away from the secondary a bit.  It’s a lot easier shifting the primary than  moving the secondary, because you’d also have to move the focuser.

Have a good look at Stellafane, Cloudy Nights DIY section, or of course not forgetting folks on SGL ?

Enjoy the build and please let us know how it goes, good luck, Ed.

Edit - it’s a good idea to make a full sized drawing.  The back of a roll of wallpaper is ideal for that. A drawing helps to sort out the primary, secondary mount and vanes, and also focuser positions already mentioned.

 

 

 

Edited January 18, 2019 by NGC 1502

[fwm891]

Wrap the sono in polythene tightly first that will give you a smooth barrier between the sono and the CF. Or I think TS do a CF tube for 10 inch optics.

https://www.teleskop-express.de/shop/product_info.php/info/p10113_TS-Optics-Carbon-Composite-Tube-Upgrade-for-GSO-254-1250-mm-Newtonian.html

They do others

[Robert72]

Ed - Thanks for the advice, I like the one about the drawing on wallpaper. Gives me a chance to "build" the telescope without damaging drilling or cutting anything first.  I have a copy of Richard Berry's "build your own telescope" which is not bad, and I have a GT NEQ6 mount that I bought from Neil27 over in the classifieds zone.  I also like "Newt for the Web", seems quite handy.

Francis - If I use the sono male/poly wrap route, will it not be almost impossible to remove? The reason I ask is that if I am doing a wet layup then I will want to do vac infusion, and ideally I would like to remove the finished moulding without too much grief and/or destroying the plug.  Buying the tube or telescope is an option, but where's the fun in that?

[fwm891]

If your saving the sono tube then yes it would be difficult, might do it adding multiple layers as poly sheet will slide against itself quite easily, or put a cloth layer round the sono, then a poly layer...

[Robert72]

Ed - Yes I will trial it first with the rear cell unfixed, the focuser mid travel and the mirror in collimation with adjustment available.  When I am happy with the position I will fix the rear cell.  Am I correct in saying that the focal plane (point?) of the primary and the eyepiece must coincide for the image to be considered in focus?  And should I take any future requirements for imaging into account or is this just a matter of adding extensions to what is already there?

[Robert72]

Thanks for the update Francis.  I am not against destroying the sono, just seems a bit wasteful, and I know how difficult it can be especially with a 1+ m tube, always more difficult than it seems, I have found this out from a previous unrelated project.  The idea of poly on poly sounds good, and may yet go down that route, however I am starting to convince myself that i need to build a plug and female mould, just for the experience for future parts/projects.

[Robert72]

1 x Sonotube 295mm OD x 3000mm

 

 Is now 2 x Sonotube 295mm OD x 1500mm

 

The one nearest the camera is the one I will use as a plug / mandrel.  Next step is to router up a couple of end formers out of MDF or chipboard and stick a 30mm spindle through.  You may notice the dent on the right hand side.  I will soon sort that out and then laminate the surface with GRP.  Probably not much progress for a bit till I get a router though and some chipboard, and some decent weather (my workshop is the back yard).

 

The Sono still in wrapper will go up for sale in the classifieds zone once I have submitted over 50 posts.

Edited January 19, 2019 by Robert72

[NGC 1502]

7 hours ago, Robert72 said:

Ed - Yes I will trial it first with the rear cell unfixed, the focuser mid travel and the mirror in collimation with adjustment available.  When I am happy with the position I will fix the rear cell.  Am I correct in saying that the focal plane (point?) of the primary and the eyepiece must coincide for the image to be considered in focus?  And should I take any future requirements for imaging into account or is this just a matter of adding extensions to what is already there?

 

 

Sounds like a good plan, but make sure you are focusing on a distant object before fixing the primary cell.  I’m not an imager, but I think you’ll find that to reach focus for that, the focal plane needs to be further away from the tube than for an eyepiece.

Yes, the focal plane of the primary must coincide with the focal plane of the eyepiece to achieve focus. With simpler eyepieces you can often see where the focal plane is if you look into the chrome barrel.  The field stop is normally at the focal plane.  With more complex eyepieces it often happens that the focal plane is between the lens elements.   TeleVue eyepiece specifications tell you where that is in relation to the top of the chrome barrel.

There’s all sorts of ways to correct mistakes, as already mentioned you can reposition the primary, pinch a bit by more fully tightening or slackening the collimation adjustments, add spacers between focuser and tube....etc.

That’s one of the great things about Newtonians, a tinkerers’ delight ?

Ed.

 

Edited January 19, 2019 by NGC 1502
dodgy spelling

[Rusted]

Tubes?

One arrangement is to saw several supporting disks in half, then remove an inch across the middle for a batten so that they can be knocked out.
The tube is split lengthways with 1" missing and a batten between each long cut to resurrect the tube.
It can all be held together with ratchet straps until you can wrap it with poly. The entire mould can be collapsed inwards. None of this is remotely easy!

You don't need a router to make circles. I've cut 24" disks of 3/4" ply or 3&4" Formica covered chipboard by hand using a coping saw and fretsaw.

I've made GRP tubes over a male mould and the results were cosmetically hideous.
The pretty side wants to be on the outside. Making a pretty female mould is tedious and very demanding.

You can fiberglass over the cardboard tube. Again you have the rough side out.
A tube can be laminated inside by rolling the wetted out matt/cloth over a pipe and unrolling it inside the tube as it is gently rotated.
Wet glass mat wants to disintegrate so it could be very messy and very expensive!
Best done in split, half cylinders.
Sanding GRP is a horrible and unhealthy job. I did a kit car body once. Never again! CF is worse!

Do not leave cardboard tube lying down. It will go oval before you know it. Stand it on end. DO IT NOW!
Mine wouldn't go round again even when I forced 3/4" plywood rings into it.

My favourite tube is laminated birch marine/aircraft ply. It can be costly unless you can source affordable full sheets.
Usually 5'x5' in Olde Money. Boat and canoe builders might be able to help. Or not.
I used 2 layers of 1.5mm but three layers would have been better in a larger tube.
Preferably with precision cut, birch ply baffle as supporting rings.

All tubes of the same strength weigh much the same whatever you make them out of. Steel, Ally, GRP, CF, resin rolled paper, or steel.
Carbon fiber is only light if you make it thin enough or a sandwich over foam. Then it's brittle and vulnerable without great expertise.
I've compared lots of material over the years. Only a truss is lighter because it has more air in it. Provided you make the rings thin enough.

[Robert72]

Thanks for the reply Ed. Yeah I will probably set up using a TV eyepiece.

Rusty,  have considered the collapsible mould concept, but seems like a bit awkward as you mention.  Have considered using a jigsaw for circles, but i watched a video on YT about cutting circles with a router using a wood radius, it looks ridiculously easy and the results are superb.  Plus i've been meaning to get a router for some time now - this will force me to take the plunge ?. After I tidied the ends of the tubes, yes they went into the spare room on their ends!  I thought this might be an issue, and they take up less space.  I could imagine laminated thin ply would be superb for tubes, it is also good in the model aircraft world in terms of S/W ratio.  As for other materials, I like the look of the phenolic tubes that chap in Germany makes.

[Rusted]

I looked at the phenolic tubes, too, but there was no weight saving over what I had.

Though they do make a nice, hard and stiff tube. Not cheap though.

Only the foam cored CF tubes are a bit lighter but even more expensive!

I need a 12" Ø tube myself for my 10" f/8 planetary Newt. 

After wasting a year, or two, on building abject failures I have obtained a 30cm x 2m steel, ventilation duct.

Another roundtoit, I'm afraid.

[Robert72]

I do like the idea of foam/honeycomb core, but I believe there are issues where you need holes.  I am considering 1x 210g surface ply and 2 or 3x 416g backing ply, 2 is probably sufficient.  I would really like to add carbon baffles as well at some point to add stiffness.  But that is way off in the future.  Have just ordered an Arduino, 16x4 display and display serial backpack to control the oven.  Will be tinkering with that for a bit, while I accumulate tools and materials.

[Robert72]

Just a quick update.  Unfortunately I think the oven project is out of the window for now, due to oven space requirements, hi temp mould cost etc.  Simpler and cheaper to start off with a resin infusion project for now.  But seriously considering the implementation of a nomex honeycomb core as a three shot vacuum process - outer skin, core, inner skin.  No doubt this will change again, but for now seems like a feasible goal.  Hopefully if the weather holds up I can cut the chipboard formers this weekend.

The main reason for posting this update though was to pose this question:  Is there an optimum distance from the centre of the focuser to the front edge of the tube?  Is it just to stop background light spoiling the image, or is there a more fundamental calculation based on optical requirements/design?  I would imaging it is probably more a rule of thumb thing, please feel free to educate me on this, the mirror is 250mm and the tube OD is 300mm.

[tonyowens_uk]

Robert re tube design and baffling see here:

http://www.download82.com/download/windows/newt/

It gives internal baffles and their effects too. It does assume you have an accurate figure for the FL of the primary...

 

Tony

[Robert72]

Hi Tony, thanks for the reply.  I can't use that link that as I don't have Windows.  I have been using Newt for the Web, and it asks for "focuser to front of tube".  This is the only number I am unsure of.

[tonyowens_uk]

OK I see. Well there is no real rule for tube extension length beyond the focuser that I'm aware of apart from the matter of adequately baffling the background visible at the eyepiece around the secondary mirror. There are two extremes.

1. Dematerialised to just a potato chip-shaped flocked disk opposite the focuser like in many ultralight truss Dobs

2. At the other extreme (Intes Micro Mak-Newt's) it can be a set of close-pitched knife-edged baffle rings stationed along the tube directly opposite the focuser hole, and the provision of an external internally-baffled tube extension (dewcap/baffle tube) of around a 1.5 times the scope aperture.

Your choice will reflect convenience, weight, hassle and the quality of baffling you need which depends on what you want to do with the scope!

 

MN86 from Intes Micro. Note huge dewcap/baffle

...with internal knife edge baffles

...and yet more baffles behind the secondary. The scope is pointed a few degrees away from the Sun (light entering from the LHS) but the off-axis light is fairly well suppressed even thought this is a Newt

This is the tube and optical layout for an unmodified MN86 Mak-Newt (no dewcap/baffle in the model). It is almost identical to an 8" F5.9 regular Newt in baffling and layout.

 

Food for thought perhaps?

Tony Owens

[Robert72]

Tony, superb example of the effect of baffling, and interesting to note that nothing is visible forward of the spider.  Thanks.  In Newt I have been using a distance of 125mm, which seems reasonable.  I am limited by cloth width to 1250mm for the overall tube length, which works out just about perfectly with the dimensions I have been putting in to Newt.

[tonyowens_uk]

That figure sounds fine to me Rob. Its easy to add a front baffle later to kill off-axis lighting during critical lunar imaging or similar.

 

Tony

[Robert72]

Just a quick update.  Managed to get the plug formers fabricated today out of 18mm chipboard.  Please excuse the crudity of my workmanship.

Now in plug with 30mm steel spindle.  No more work for a few weeks, as I am off to BAE to do a course in Virtual Machines.  I have started to draw up the plug and mould jig in FreeCAD though, and may upload a few pics in a week or two.

[Rusted]

Neat! 

What was your method of indexing the holes around the circles?

Did you use a hole saw for the larger ones?

 

I forgot to mention I saw an add for a CF tube on U.K. Astronomy Buy & Sell in Reading.

Edited February 3, 2019 by Rusted

[Robert72]

Rusted good question.  I obtained a piece of 610 x 610 x 18 chipboard.  Using a meter stick I drew lines (A) from corner to corner.  Then I used a set square to draw horizontal and verticals lines (B) through the point of intersection of lines (A).  Then I used a meter stick to draw lines (C) joining the end of lines (B).  Then I used a set square to draw lines through the intersection of lines (A) and (C).  This left me with four quadrants to cut out, with each quadrant having an 8 point star, the points of intersection being the centre.  No measuring required.  I then measured 100mm from the centre along each line to give the 8 hole positions.  The centre hole is 30mm cut with a flat bit.  The 8 large holes are 38mm cut with a flat bit.  The small 8 holes are 10mm cut with a wood bit.  The 8 large holes are to allow easy rotation of the plug on the upcoming plug bearer, when rollering etc.

That carbon tube looks like a bargain for someone.

Edited February 3, 2019 by Robert72

[Alan White]

So how is progress of the tube build going?

[Robert72]

Just restarted Alan. Please excuse the tardiness of my reply, I didn't seem to get a notification. So yes, after work commitments and a minor project or two, the build has recommenced. I have started making the mould tool and plug cradle. Tonight I quickly assembled the mould tool to check the mechanical integrity and fit. Seems pretty good, stronger than I thought so no bracing required. For now, here is the kit of parts for the mould tool and plug cradle, I will try and upload a pic of the assemblies on Saturday.