Carbon 12" Newt

[Hedworx]

I am turning my skywatcher 12" f5 dobsonian into an imaging newtonain.

My primary goal for this particular scope will be as a long range planetary weapon. With my ASI183gt and a televue 4X powermate I hope to get it ready for next planet season. I have done some planetary and lunar imaging with this on the skywatcher base but the results were sub-optimal as the 15 yr old mount just wasn't up to the task.

I now have a TTS Panther 160 alt-az mount which performs very well. With the addition of the rotator atop I can now achieve sensational results which I was unable to get in the skywatcher dob base. The TTS mount head capacity is 20 kgs which means I need to trim some weight off the OTA. The heavy collapsible stock tube is far too cumbersome for the mount.... enter carbon fibre rebuild.

I had made a carbon tube previously for this scope purely for aesthetics but in the collapsible hardware still, for an imaging newt a full tube is in order. I had a piece of signwhite metal rolled to the correct diameter and made a mandrel to which I can wrap my carbon.

To prevent the epoxy sticking to the mandrel I wrapped it in plastic tape, a test with some epoxy initially came out positive with no sticking.

Two layers of 600gsm carbon were applied with West Systems epoxy with a wet layup application. To compress the carbon I then wrapped the whole lot tightly with more of the same tape. The tape wrap applied uneven pressure so I pulled it off and wrapped the whole lot in peel-ply instead.

[Hedworx]

To mount the tube on the TTS Panther I have decided to use a plate direct mounted instead of tube rings to save some weight.

I epoxy glued a 6mm plate made of laminated marine ply and carbon. Another layer of 600 gsm carbon goes over the top so I need the transition from tube to plate to be comprised of gentle curves. Automotive bog was used to achieve this, I used this technique a lot when I used to work at a boat repair workshop.

This solid and flat mounting area gives the losmandy plate somewhere to screw on, hopefully it is long enough at 550mm to achieve balance.

[Hedworx]

To complete the carbon part I wrapped a third layer around the tube and plate base, I also added a 4th strip at the front for extra stiffness to minimise any flexure around the spider area. In hindsight trying to compress the tube with the plastic tape was a terrible idea, it left high and low areas along the length of the tube as there was no way to ensure even compression. I sanded as much as I could before the third layer went on, turns out it wasn't enough, still some low spots. More sanding and I have had a gut full... it is what it is.

Being somewhat artistic I opted to paint part of the tube, a red plate and red eaf means that red is the colour... also every scope goes faster if it is red. The pinline tape I used to mask off the carbon areas gave me an idea, it was blue and didn't look half bad with the red. More tape before the clear goes on would be the plan, plus it breaks up the line and gives it a bit of something extra.

I used marine grade 2k clear, the first layer revealed a bunch of pin holes in between the twill weave... more sanding was needed. I hate sanding carbon!

You can see in the photos the darker and lighter shades of carbon as a result of the tape compression. The clear helped somewhat.

[Hedworx]

The second coat of clear filled the pin holes somewhat, the satin finish turned out quite well I think. Overall I am not 100% satisfied with the finish, those pin holes irk me to no end. There comes a time though when enough is enough, it works and looks acceptable enough.

If I were to do it again then a proper vacuum setup would have solved most of the issues I encountered... you live and learn.

[Rod]

Looks great Luke. I would like to truly carbon fibre one day so it’s good to read how you went about it.

Rod.

[Hedworx]

So it turns out that the plastic tape wasn't the best to thing wrap the mandrel with. The stuff stuck to the epoxy dammit.

I spent over two hours and was only able to peel off about 100mm at the front of the tube. I have since cut the excess off with a sharp blade and painted over it with black paint. I will most likely flock the inside of the tube at some stage to enhance the blackness and contrast.

[Leo.G]

I wanted to play with some carbon some years back but I was soon warned off with mention of having to have the vacuum set-up to achieve good results and as such I've never bothered. The image with you holding the tube it appears you have certainly shaved considerable weight off the unit.
Very interesting project, I hope it all works out well for you!

[Hedworx]

It seems to have made a massive difference so far, the tube itself is 6.5kg. Once I put in the new mirror cell, plate, spider, focuser etc. It will of course be heavier, my aim is to get the whole thing at around 15kg instead of the 22kg that is was before.

[Hedworx]

The spider vanes were looking pretty sad, bent and rusty.. An hour down the shed sanding, straightening and painting and they turned out much better. The painted interior over the plastic tape seems ok also. I will flock the ends of the tubes to neaten up appearance more.