I’ve been using a 10” TS Optics ONTC Newtonian as my primary imaging scope for a few years now. Overall it’s a great scope and I’ve been very pleased with its performance. It is, however, not without some issues. One issue which has plagued me from day one is changes in collimation during imaging, particularly when slewing between objects on opposite sides of the sky. At first, I thought this was due to the primary shifting in its cell. Although this was occurring it was easily fixed by adjusting the fit of the mirror cell to the primary. (TS Optics ships the scope with a rather loose primary mirror fit.) When this failed to solve the issue I assumed the secondary was moving, it wasn’t. Then I noticed that the nuts and bolts securing the focuser to the tube were somewhat loose. I tightened them, recollimated, and thought the problem was solved.
Over the next several months, however, the problem slowly got worse again. I checked and tightened everything I could and then I noticed something odd: When putting light lateral pressure on the focuser the inner carbon fiber liner of the tube bowed inwards. It turns out, to save weight, the ONTC uses a sandwiched carbon fiber tube with a foam core. The outer carbon fiber layer is fairly thick and resists compression, but the inner lining is relatively thin and easily bends allowing compression of the foam core. Since the nuts that secure the focuser are fairly small, this puts a substantial point load onto the inner lining whenever the focuser is in any orientation other than pointing straight up or down. Over time, the foam core fatigues and the focuser begins to noticeably lose collimation when slewing. There are a number of possible solutions to this: A metal backer plate in the inside of the tube, a metal insert that goes through the bolt holes and is firmly fixed to the outer tube, or removing the foam core around the bolt hole and replacing it with a load bearing material. I opted for the last solution.
I removed the focuser and primary mirror cell then covered the secondary mirror. I used a jigsaw blade to remove the foam core from an area about 1/4” around the bolt holes. I then cut four short lengths of 1/4” doweling and coated them with candle wax. I mixed some structural epoxy that doesn’t require filler and drew it up into a 10mL syringe. (Probably overkill, but I knew I couldn’t undo this process.) I then fitted a blunt needle to the syringe and injected the voids in the tube with the epoxy. Painter’s tape was used to hold the epoxy in place until it had cured enough to hold its own weight. I trimmed any excess with a sharp knife and allowed it to cure fully.
Using an autocolimator to check deviations in collimation, I was able to verify that the tube now flexes less than the mechanical flexure in the focuseer (which is minimal). I haven’t yet field tested this and I have no data on how well this will hold up over time but the initial results look promising. I feel that TS Optics should reinforce this area if the scope is being used for full-frame imaging as the loads are too much for the tube to handle as shipped. It can be very difficult and frustrating to nail down this kind of problem since there are so many possible culprits in a Newtonian that all have similar appearance. (Ie. tilt, focuser sag, primary mirror cell flexure, tube flexure, secondary mirror holder flexure, spider vane flexure, etc.)
Special thanks to @flyonthewall for his assistance!
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