I own a 15 inch UC f4.2 and it does suffer from collimation drift ( both secondary and primary mirror collimation ). In order to reduce collimation drift, I usually collimate when the scope is pointing at a 45 degree angle. I also asked someone to machine me an aluminium upper ring which has also reduced collimation drift . I was not able to machine a ring thick enough as it would unbalance the scope. These are my thoughts :
- Reducing the focal length from f4.2 to f4, would result in shorter poles. Which as a result would buckle less and reduce pressure applied to the ring. This might improve things
- My primary mirror collimation drift I think is caused by the primary mirror slightly lifting off when slewing the scope towards horizantal. I do not think its the mirror cell as the triangles seem to float without restriction, but it would be nice if I can get someone else to check it out one day for me. What I think it is is that the foldable bearings bend a tiny bit under the weight of the primary mirror and cause the mirror the lift off and also the poles to buckle. This is why probably obsession introduced those "stabilizer bars " that connects both points of the bearing ends. So once again, Ij think with the shorter poles, they would be stiffer and less buckling.
But overall I really enjoy the scope. With the aluminum ring, collimation drift are acceptable. And as mentioned by someone else in previous posts , when I do planetary viewing I also collimate right at the exact angle. Sometimes I also hang a weight on the focuser when Im using my autocollimator from cats eye. I think the autocollimator is too light, so I try to mimic the weight of paracorr/eyepiece combo as much as I can. With a relatively flexible upper ring, I think hanging a weight helps during collimation
Edited by azfar72, 16 August 2021 - 01:52 PM.