[gaseous (Patrick)]

In a moment of profound boredom last week I decided my 16” dob mirror needed a clean – big mistake. I'm now trying to recollimate it after noticing some visual nastiness, and I’ve hit a snag.


Using a collimating cap (which I rarely use), I can align the primary mirror centre spot with the hole in the collimating cap, and align the primary mirror reflection in the secondary, so everything looks pretty nicely aligned looking down the focuser. However, when I put a laser collimator in to check, it’s out by a pretty wide margin (laser spot 2-3 inches away from the primary centre spot). I know the laser is pretty accurate, so after collimating the secondary and primary with it, I recheck the alignment with the collimating cap again, and things are all over the shop – cap hole not centred on mirror spot, and primary badly misaligned down the focuser - still completely visible, but touching one edge.


Should I be relying on one method over the other, or is something more fundamentally amiss with my setup? Just star test with each method and see which is better? Any help would be greatly appreciated as I’m exhausting what I thought was a pretty extensive vocabulary of profane language.