Re-Silicone of GSO Newtonian
It’s seems that I rather overdid the silicone on the mirror last time I did this and it may have been contributing to triangular stars due to the mirror being a little pinched in the cell. I’m not 100% convinced but the general opinion was that all is needed is 3 pads of silicone for the mirror to sit on – not, as I had previously, the whole cell being covered in silicone!
Anyway, tonight I set about removing the old stuff and replacing it was said ‘blobs’……. Getting the stuff off was not too bad, got a craft knife set for £1 (from Poundland, of course!) and it worked a treat, I sliced thought the old stuff, removed the mirror, then used a flat edged blade and it just came right off the mirror when slid under the remaining silicone – All in all it took about an hour to get it back to ‘normal’
So above you can see the mirror cell and the 3 cork spacers it previously sat on, this is where the silicone blobs will go….
Next was to work out how to create a little space between the cell and mirror so that when placing it on the silicone pads it would not just squash them and sit back on the mirror cell itself. After pondering for quite a while about what to use to create this spacer I glanced upon the rubber mirror clips – these are like a little L shaped piece of rubber, so now there is 3 little spacers and the mirror sitting drying on to the 3 blobs of clear silicone. When it’s dry in 24hrs time, I will remove the spacers and stick them back on the mirror (I had actually done away with them when I had previously siliconed the mirror to the cell but think that as I now only have 3 blobs of silicone, it’s best to have them back in for safety!). The only (slight) concern is that the mirror is now sitting higher in the cell than it was before and does not have as much lateral support, I have slid 3 cardboard spacers in beside the cork spacers to tighten then up but am wondering whether to place a further 3 blobs of silicone to give more lateral support to the mirror????
Above, the mirror cell with the silicone ‘blobs, also shown is the rubber spacers (aka mirror clips)
The mirror back in the cell, you can also see the cork spacers for lateral support – not decided whether to add silicone here or not…
A couple of other potential areas for problems were also addressed as part of tonight’s re-silicone….
First up, the length of the collimation screws
The collimation screws supplied are just a bit too long and if tightened right up will actually touch the mirror surface, pushing the mirror forwards in the cell and further up against the mirror clips. Too tight mirror clips cause triangular stars, that’s bad enough but having a metal screw touching my glass mirror, something had to be done! I bought a pack of washers and added 4 to each screw, now when in as tight as they go, they still don’t touch the mirror – Job done! (BTW, the springs supplied with this scope are inadequate, I have already replaced them with stronger ones from ‘Bob’s Knobs’)
Finally, the centre spot – I have heard from a few people that the centre spot was not erm… centre! I found a cake tin exactly the same size as the mirror, drew round it on to a sheet of A4 paper and cut it out, folded it in half, then half again and cut a little from the centre to make a circle. I placed this on top of the mirror and it seems that my spot is, spot on!
Checking out the centre spot position
So… Now I just need to wait on it drying and then re-collimate and hopefully I will have some nice round stars!
After re-doing the collimation twice I think I have just about cracked it! Here is a 10sec 100% crop of Aldebaran