With peter_4059's help, (I think) I'm on the home stretch of tightening up the stars in my images with the 10" Newtonian. Just the top right corner stars now are a bit elongated. I noticed to tube flexing a bit while playing with the focuser the other day and can't help but think it's the weight of camera that's making the OTA buckle a little and causing the sensor plane to tilt.
Plan is to stiffen up the OTA wall around the focuser by bolting some 3mm thick rolled aluminium plate inside. I've been searching around here for ideas but can't get a hit on previous threads. Maybe using the wrong search terms?
Anyone done something like this? What plate thickness? Dimensions? Photos?
Thanks mate. What I got from that was 1.2mm thick rolled sheet and contact adhesive it on the inside. Hadn't thought of that, but would certainly be neater and less room for error than bolting etc.
Hi Troy. maybe clamp an additional tube ring around the OTA near the focuser. if it works OK, bolt it on permanently. I used half a tube ring on mine, attached with bolts + builders bog filler to get a perfect fit and it helped a lot - not real pretty, but it works well. I recall reading a similar suggestion elsewhere on IIS, but I can't find the post to give credit. regards Ray
I remember reading James added a 3mm Al plate to his 12" OTA.
have a look at the video toward the bottom. illustrates the flex very well http://deepspaceplace.com/at12in.php
Want to post a couple of basic photos showing the focusser and another showing the room around the Focusser and maybe the inside of the OTA and the dovetail.
Here are some photos. Measured the outside circumference and it was 908mm. So what's that make the internal diameter? Around 296mm or so? I think with some aluminium plate on the inside is the trick, maybe 150 wide and third to half the internal circumference of the tube.
A good concrete band about 50 cm wide and 7.238 cm wide should stiffen it sufficiently from my calculations. But the main flex would probably be due to that garish focuser inducing flex as the tube tries to get way from it. Shoulda got a red one or at least a green one.
I don't have the tools to bend a thick piece of aluminium to the perfect shape - so mine looks a bit rough around the edges. Even with the reinforcements, I can still see some movement, but I've come to the conclusion that no matter what you do there will always be **some** movement, the trick is to get it within the tolerance you need to hold focus and collimation.
I'm happy to report my scope is now reasonably stable and even keeping focus well through the night (this was the hard part).
I changed so many things, I'm not exactly sure what fixed the problem in the end. Probably a combination of things in the end...
Your other issue is the actual tube flexing between your mirror and the focuser but thats another thing all together!
if you where to put one OTA ring around near the focuser and actually screw the focuser base plate to the OTA ring it will distribute the weight over a far bigger area.
stress = Force/Area.
If you are going to use contact adhisive to attach your strengthening plate under the focuser on the inside of the OTA that will be as useless as using clag glue to hold on your bonnet of the car... Contact adhesive while it holds on like !@#! to fur is flexible/ductile.
You are trying to laminate two layers of material together. for them to work in composite actions you need to make sure there is no give between the two layers. Using bolts tightly fixed will semi allow this action to occur. bolts with a epoxy cement will take it to the next level. But once again stress = F/A but this time its the material that is able to take more stress as it has more depth and hence more leaver arm to resist the moment couple, that and the fact it is now applying the force to the OTA itself over a much larger area.
Oh if you want to get a piece of Ally to fit within the tube take it down to your local fab shop, they will have a roller in the workshop and for 10 bucks would roll it for you in about 3 minutes to suit the ota, far easier and much more professional!
Thanks Brendan. Some of what you said makes sense. 20 years as a structural engineer does give me some understanding of forces and stresses
I'm not going with the glued approach. Decided to use bolts. The focuser baseplate will bolt through the OTA and also through the stiffening plate. Then I'll have some other bolts at the ends of the stiffening plate as well. Haven't done careful analysis or anything like that. And I'm not chasing composite action. I reckon the plate alone will stiffen things up enough, so it doesn't really bother me if there's no interaction between the OTA tube and the plate, although there will be some interaction there anyway.
I measured it all up last night and will be sending off the specs to a local metalworker today. Plate will be 3mm thick, 150mm wide, 300mm long (about a third of the tube circumference), and have a 60mm diameter hole in it for the focuser. I'll drill and tap the holes myself... ok, Peter will do it, but I'll be watching.
I suppose we could re-engineer the whole tube, but my 2 cents would be move your Telrad, one tube ring on the inside side of the focuser, the outside side has the tube end piece close so you shouldnt get too much movement there. Then you could get a piece of rolled Al in the inside near the focuser, cut with a whole for the focuser to sit through, bolt it to the focuser and the tube ring. Your rolled Al would need to be at least 1/3 the circumferance of the tube, as per Brendans formula.
You could also get the gear to do it carbon fibre
For the price ($60), I would start with the tube ring and see if it improves, you will find the flexure under weight is distorting the whole tube, not just the focuser area.
A good concrete band about 50 cm wide and 7.238 cm wide should stiffen it sufficiently from my calculations. But the main flex would probably be due to that garish focuser inducing flex as the tube tries to get way from it. Shoulda got a red one or at least a green one.
I was going to make a smart comment about the sparkly OTA, but realised, I have a BD too now. LOLOL
Well I got the plate rolled, and dropped around Peter's place on Friday to do some drilling and tapping. Must say I really appreciate his help, and his knowledge/experience with this metalwork stuff is way beyond mine.
So here's some pics. As I said in a post somewhere above, the plate is 3mm aluminium plate, 150mm wide, 300mm long (about a third of the circumference). The edge of it closest to the front end of the scope I kept just below the secondary spider connections, that's why the focuser hole is offset. Got the metalwork shop to just provide the plate, put a 65mm dia hole in it, and roll it to the right curvature. They did a pretty good/accurate job.
When we first put the plate on the inside, there were some issues where we didn't think they'd rolled the plate to the right dimensions. Then we realised that the original finder bracket connection point has a thin stiffening plate around it that we hadn't seen because I'd covered in with flocking a long time ago. Duh.
First shot shows the outside of the OTA. Just added the 2 holes on the left there for one end of the stiffening plate, the 4 around the focuser hole are original. There's another 2 for the other end of the plate.
Second shot shows the tube side of the plate. The holes around the focuser are just holes (ie not tapped) so the screws that connect the focuser mounting plate will fix through it, clamping the focuser to the stiffening plate. The holes in each corner of the plate are tapped so that the screws can be bolted directly to it. We also oversized some holes for the finder mounting.
Third shot just shows me holding the plate on the inside, painted matt black but will be putting flocking over it as well.
Fourth shot shows the plate connected just via the outside 4 screws for now. I need to order longer ones for the Moonlite mounting plate, because they're about 3mm too short now due to the plate thickness. You can see I didn't make the plate as wide as the original one on the OTA. This was to minimise openings in the plate, maximising stiffness. It's just enough to allow the focuser drawtube through it plus minor tolerance.
Final shot just shows it from the inside, waiting for the focuser to be attached, flocking to be added.
Perfect set up mate! I may even look at doing this easy strengthening job too as it should be quite easy to do with the right tools! I have a spare ring sitting there so I might just also add that to the system aswell and see how i go!
One other thing that i slowed down the tube flex by getting the OTA rings a decent distance apart. the top dovetail plate runs in at 450mm and has the effect of deviding the OTA into thirds. With the tube optimised for AP and having close to 200mm out of the OTA as the focal point the tube itself is approximately 800mm in total. that may be something to look at too
Look forward to see some results after testing Troy.
