250px flextube shell mod

[Joseki]

Really pleased with how this turned out (such a minor thing I know!), but thought I'd post a mini report here rather than one long one when it's finally finished (sometime next year I predict). Anyway, I recently bought the tube and truss assembly of a black skyliner 250px Flextube from Astroboot (arrived very quickly) to use with a 10" mirror I got - I really love the look of these scopes, the fact that its an open tube design and that this was about 4 time cheaper than buying a new closed tube from the guy I spoke to..

The only problem is my mirror is f6.4 not the f4.6 that the flextube is designed for (I knew this when I bought the tube).

Actually this hasn't been too much of a problem. The original truss tubes can be removed quite easily - screw thread inside the tubes + large black anodized nuts hold them on - easily removed (this was where I got lucky I guess). The tubes themselves are one inch outside diameter, approximately 3 mm thick. I bought three 1 metre plain anodized aluminium tubes (a little thinner) from Wickes as I was passing. I'm going to worry about finishing them later on or might just leave them as they are (easy grip black shrink wrap tubing looks good though). Rubber fixtures from the original tube are put on these, improving the look. Happily they fit nicely in the bottom and top sections of the OTA. A useful feature of the flextube design is I don't have to chop the tubes down to size just yet and they are adjustable (90 degree screws, tipped with rubber clamp the tube against what looks like teflon - also a spring mounted ball bearing though mine are currently living in a margarine tub).

The top tube section holds the tubes in place quite nicely. Most of the way through the horizontal plate is a 1 inch diameter hole but the last bit is smaller, stopping the tube. After I spent a long time looking on the internet for nuts - seriously I'm not paying $5 plus p&p per nut to import them from America, and some hints from the SGL archives, I realised/found out that what I need is "star fangled nuts" as used for bicycle headset. For the price I took a minor risk and ordered some which included the top cap, e.g. http://www.chainreac...px?ModelID=6278 (got plain black ones). What I should have done is buy the special tool for installing them (also a cycling component) - but that seemed excessive (12 pounds). Anyway, they came today and I managed to whack them into the tops of the aluminium tubing. Best of all the top caps fit very snugly, match the black of the tube and come with a rubber bung to insert into the allen key hole in the top of the bolt.

So phase one was a success. Now all I have to worry about is the extra torque from the extended pole length. I reckon I can cope with that by sticking with a lightweight focuser & finder, adding weight to the bottom or rebuilding the top section in a ultra lightweight design (if the other two options fail).

I'll post some pictures later on today.

p.s. I've got an optical supports cell (at 1/2 price from FLO) - it should fit okay. If not, IT WILL FIT (hammer time). Destiny curved vane spider on order, telescope tube rings ordered for mounting altitude bearings, FLO-type flocking material bought from Wilkinson.

[Joseki]

or the star fangled nuts for bikes come in two sizes so I had to make sure I picked the right ones

[Joseki]

or = oh

[Joseki]

well here it is, a flextube that is longer than the average flextube booboo!

[Joseki]

.. and it's actually quite rigid. I've got a cable mod in mind if it does prove to be too flexible - though it would make it more ugly since I'd have to keep them out of the optical path.

[JamesF]

To an owner of the "standard" 250 flextube, that looks very odd indeed

I built my own dob mount to fit mine and using the existing mounting points as centres for the alt bearings it's a little bit too top heavy. It will very slowly sink with my 32mm Revelation plossl in the focuser, even with a low-profile (and therefore relatively lightweight) focuser. I would be temped to mount the alt bearings using the existing mounting holes, but shift the rocker centres a fair way forward in your case, otherwise I think you're going to have to add quite a chunk of weight to the bottom as a counterbalance.

Here's mine:

http://www.tanstaafl.co.uk/2012/08/my-10-dobsonian-build/

James

[Joseki]

Hi, thanks for your comments. Sounds like Ive got a bit of DIY left to do. I would like to avoid hanging a lot of weight off the back if possible and I do ideally want a moonlite focuser.. I hadn't thought of offsetting the bearings so thanks. I might have a go with some wood rings on the poles also.

Thanks for the link too. I read about your project a couple of days ago, pretty encouraging for me, I think I'll try and do a similar mount (I have the book too).

Sent from my HTC Wildfire using Tapatalk 2

[Joseki]

Well I've been doing a bit of shopping so have made some progress on my little project. I've now got

JMI motofocus focuser (2nd hand)

optical supports mirror cell

Destiny curved vane (3 vane) spider, dew heater upgrade + screw upgrade.

All of which require drilling the tube to mount. (the mirror cell sticks out the bottom of the tube so I want to move it up a bit). and I'm very happy (but my bank balance isn't). The JMI focuser is smaller than the skywatcher ones as we all know, but for some reason I really didn't want to get the skywatcher dual speed upgrade. Motofocus is completely unnecessary for me .. but very fun. I'm thinking about trying to cast a base plate with resin. I don't really want to wait until I can afford to buy a secondary dew heater before mounting the secondary though given how damp my garden is it will be a high priority I'm sure.

Anyway, I also bought the cheapest plunge router I could find on ebay. The plunge action is rough but it works and cutting discs for the base and semicircles for the altitude bearings is suddenly very easy. I'm working with the nasty OSB board I've got to save money (may redo it in plywood later or may try and finish this off in gloss black with lots of wood filler that I've got spare from a "mend the front gate" project). This is what it looks like right now:

actually I've also added some vertical wood beams either side of the altitude bearings to stop it falling off. Plenty of supports to be added.

Once I cut the altitude bearings I saw that they were as long as the primary section of tube - so nothing much sticking out the bottom end to balance the secondary and focuser etc. As JamesF suggested (and similar to the method used by DobStuff) I'm mounting the altitude bearings up along the truss tubes. To do this, as can be seen in the photo, I'm using Rhino truss clamps http://www.visiosound.co.uk/stands-cases-c11/lighting-stands-c48/accesories-c49/rhino-lighting-multi-clamp-truss-t-bar-lighting-clamp-p154 pretty chunky, though in practice it impinges in the optical path ever so slightly so may get replaced later.

I haven't fixed it on, but it's actually sitting on two discs and a lazy susan bearing. I'll be screwing it to both sides (you just need to drill an inspection hole to access the screws on the 2nd bit of wood, which will be 1" in diameter so I can then plug it with a left-over tube insert I'll be buying). - so no central pivot bolt - seems to be okay in tests. Manual setting circle will be made for the az.

My only innovation in this ugly monster is that I plan on having an adjustable friction bolt - basically in the top part of the base I'll mount a t-nut and have a bolt that's accessible from above. This bolt will be attached to a disc of teflon (I've bought some 8mm teflon for this and the altitude bearings) and will push down on the ebony star type laminate disc below.

With all the packages arriving for me I may be responsible for all of the UKs cloud..

Interestingly I discovered that the think white end of the tube (i.e. top of the secondary section of the tube) weight 600g. Some low weight alternative (i.e. that stops the tube getting squashed easily), or simple car window edging would help balance the scope.

[Joseki]

oh, and to mount the bearings to the tube I bought a pair of 288mm tube rings. Now that I'm only using one the other will probably end up as a mounting point for the counterweights I might need (Tesco loose weights look about right for this).