Found out what the annoying little tag on the front of the secondary holder in my GSO f4 8inch Newt is for - it stopped the secondary mirror from falling into the tube when the double sided tape securing the mirror lost it's stickiness. The mirror remained pretty much in place, but could move by small amounts in arbitrary axes as the attitude of the scope changed.
results - minor scratching on secondary mirror surface, lots of low quality subs.
symptoms - collimation wander, variable strange star shapes (worse with MPCC in place), variable differential flexure when tracking.
Looks like the secondary may have been loose for months, while I was chasing MPCC/camera alignment, focuser slop, tube flexure, primary mirror mounting, collimation etc. etc. Refractors have a lot going for them.
It could have been worse, though - it could have dropped down and chipped the primary! Secondaries aren't that expensive to replace compared to a primary mirror, either.
Hi Jacqui and Fred
thanks very much for your comments - yes it could have been worse and I have now fixed it properly (I think). The scope is great when it works properly, but this problem did turn out to be a diabolical waste of time and a greyer of hair.
kind regards
Don't you just hate it when you've wasted time on xyz and it turns out to be a simple abc! I tried to get my secondary out of its holder once to clean it. It was stuck on well and truly. Ended up leaving it there and washing the whole unit.
I tried that, but the mirror holder has a shroud around the edge of the glass and the silicone would not set in the enclosed airspace. Used some ds tape designed to hold bathroom fittings onto walls. Has stuck properly and I will check for astigmatism to make sure it has not introduced any stress when I can get out under the stars.
