I scored a set of optics for a 22" F4.5 Newtonian recently and have just about finished the design work. Been a while since I've tried something of this size so I thought I'd chuck up a couple of CAD drawings for comment / Feedback.
The OTA truss framework will be expansion foam filled, welded 50 X 50, 100 X50 and 200 X 50 Aluminium box section with 3mm wall thickness. Primary will be in an 18 point flotation cell with 12 point wiffletree edge support.
Current 100mm secondary will provide about 150mm (approx.) of back focus. I'd like more, but such is life. Imaging focuser will be a PDF with OAG for guiding. No decisions on imaging gear yet. Third image is an interesting graphic showing the off-set of a secondary.
I'm toying with a bearing mounted rotating secondary that can select ports for imaging, visual and possibly a spectroscope. This will necessitate a different adjustment system compared to a standard newt with the secondary becoming the centre of the optical axis. I suspect that's gonna make for some fun and games getting the axis of symmetry right but I have a friendly engineer keen to do the work on this.
Estimated weight is about 75kg all up, without instrumentation.
Clincher is that it will be mounted on my PME 2 via a wide dovetail plate. Obviously observatory mounted.
If I can pull it off, it could make for an interesting system. Any thoughts, especially polite ones, most welcome.
And so it begins. here is the result of a lot of calculations and some very careful cutting. These are the lower trusses and central frame. Upper trusses are also cut. Hoping to have the end rings rolled in about a week. After that, welding. So far so good. Low res phone image... Please excuse quality
Okay,
a couple of design changes were forced on me due, in part, to a tube roller, who couldn't... and a rethink on the primary mirror mounting system.... but I'm making progress.
The OTA is in 3 sections at the moment but they should be a single unit in about a week. The primary mount is a jigsaw puzzle but seems about right. Still some work to do...
Fantastic build so far. Impressive looking bit of kit.
Was there any issues with warping from the welding process.
Aluminium absorbs a lot of heat when welding so its susceptible to warping if you are not careful. We've largely addressed this issue by building rigid templates in which individual components are mounted and then welded in situ. My fabricator is also a perfectionist on more levels than I care to list and so his "so so" welds are better than I could even dream of doing.
The major OTA components will be joined in the next week or so, (working on the primary mirror mount and its attachment plates at the moment), and I am confident that there will be less than a 2mm error over the 2400mm length. Better than I hoped for by a long shot.
I'm liking where this is going, I do have a thought but. Would it be possible to have the secondary mirror spin 360 degrees and have 4x focus's each at 90 degrees thus you could have you normal eye pieces in the focuses on the horizontal plane and on the vertical plane (Up & Down) you have your imaging equipment ie. the heavy "stuff" would be on top of the scope, it would also reduce torque on the scope. Have thought about getting a large newt and doing some mods to the tube to enable this. I have google searched it a bit and came up with nothing.
I'm liking where this is going, I do have a thought but. Would it be possible to have the secondary mirror spin 360 degrees and have 4x focus's each at 90 degrees
Cheers
Thats the plan. It has some interesting implications for the alignment process.... which I am exploring at the moment. The optical axis has to be centered on the secondary which has to align with each port as it rotates. Hoping for dedicated imaging viewing and possibly spectroscope ports... One step at a time though. At the moment. weight is the issue. She's got a biggish arse....
22 inch F 4.5 Newtonian
and a rethink on the primary mirror mounting system.... but I'm making progress. 
4x focus's on Newt
