Tube size 152 or 203mm???

[boomboom]

Ok
After more than a year thinking about this it's time to get on with building a refractor. BUT FIRST.... a question for you ATMers out there in Istar land.
I'm going to build a 6" f12 and have a choice of two locally available and affordable tubes. Both are aluminium
1. 152mm outside width 2,4mm thick, weight about 8kg
2. 203mm outside and 3mm thick weight about 11kg

Which one?

I'm leaning toward the 152 mm with a wooden counter cell arrangement for reduced weight up front.
Or it could be easy to build the cell into the 203mm tube.
Weight is a slight concern as this is going on an NEQ6 with an ATM wooden tripod and extension which I have built and general opinion is Its bomb proof.
Hoping to have the finished OTA weighing in around 15-17 kg with finders etc. Actually I'd like to end up with my own version of a Unitron scope.
Cheers
Matt

[Mike]

Matt, we sell a 204mm F9 and it has a 200mm OD tube. Fortunately I have a CGE and CGE Pro to handle it for testing. At F9 it's a large scope. At F12 you are going to have one big heavy tube. Granted the lens is the same weight either way. Can you think of an advantage of using the larger tube? The usable aperture is 150mm so looks like you'll lose an insignificant .4mm with the 152. Also 2.4mm is plenty of wall thickness on the 152 so no tube flexure. Aesthetically, unless you are using a 3 inch or larger focuser, the scope will look more proportional with a 2 or 2.5 inch unit. Rings will be less expensive unless you are fabricating your own. You'll have less counterweight on an equitorial. Less weight means more stability and easier on the back when setting up. Besides, the Unitron look, which I always liked, is long and skinny.







Incidentally, I replaced my homemade wooden wheeley bar with a JMI unit, with 5 inch wheels with brakes, that I picked up from a local guy. It is really nice. My concrete drive is on a grade so I have to be careful that it doesn't get away from me. It has adjustable threaded rods with feet so it can be leveled in a minute. I can shove my whole setup out of the garage and be ready to align in less than 5 minutes. I have marks on the drive and pre cut blocks so the orientation is the same every time. I used to think those wheeley bars were for lazy old men, but I was wrong.






Mike

[boomboom]

Nice set up Mike. I'd love to have a wheel in wheel out scope but I have a large backyard with grass...so its in and out of the shed in bits.One day the f12 will be on a pier in an Obs with a decent mount head, EQ8 perhaps.
First thing is to do a full ray trace and see how long the tube will have to be.Might decide then to go down to f10??? Depends on weight really. I have got down to a fine art the balancing of scopes f12 is my first choice. I'll be using my GSO Linear Bearing 2" focuser with a Rigel Systems electronic focus control which works beautifully. Looks like the 152mm is the way it will go. At worst I'll be in touch with Dan at Crawmach for front and rear cells if I need to.
Tube rings might do an ATM or Parallax 155mm.
I'll post a pic of the wooden mount with my f8 which should give an idea on height.
Matt

[bn1777]

Yes go the 152mm tube Matt , looking forward to some photos .

And Mike , I love that 8 inch set up , lovely looking scope .

Brian.

[boomboom]

Going the 152mm.... thanks Brian for the help thus far, it's gonna be fun
Another Istar is born down under!
Matt