Location: '34 South' Young Hilltops LGA, Australia
Posts: 1,323
Ok so your edge tiles are down to a bit more than half thickness. I would keep grinding for now. It gets tricky. You don't want to run out of glass near the end of the 80 grit because then if you have to retile, you will be grinding just to grind the resurfaced tool to match the mirror. You definitely don't want to run out of glass on the edge during the fine grind or you'll have to go back to coarse grind to resurface the tool. Just monitor the situation. If it looks like you won't make it, resurface the tool with fresh tiles when there is maybe 3mm left on the edge. Then you'll have an even 8mm all over to finish the job.
Hi Barry, I assume your using #80 grit. I would most definitely suggest "re-tiling" at this point in your grind to ensure you "break in" the new tiles as getting them to conform to the mirror curvature would take too much time once finer grades are employed, in fact almost impossible at say #220 and beyond.
Thanks, ordered dental plaster to make a new one & I'll wait until it arrives. Will also build a bit of weight into the cast as I found the current one worked much better with an additional 3kg.
Progress is slow, most days I get about an hours work. Currently fine grinding and was amazed at how quickly the rough grind marks were removed from the centre of the mirror. The edge however is a different storey, it is obviously turned down to about 1.5mm in from the outer edge but is slowly coming out. Probably 8-10 hours more fine grinding which should clean up the edge.
Sagitta is at 5.5mm and seems stable, can't see any pits at 10x over the majority of the surface.
Guess it is time to make some test gear.
Progress is slow, most days I get about an hours work. Currently fine grinding and was amazed at how quickly the rough grind marks were removed from the centre of the mirror. The edge however is a different storey, it is obviously turned down to about 1.5mm in from the outer edge but is slowly coming out. Probably 8-10 hours more fine grinding which should clean up the edge.
Sagitta is at 5.5mm and seems stable, can't see any pits at 10x over the majority of the surface.
Guess it is time to make some test gear.
Pitting at edge.
To fix up the edge, it would be much quicker to just go back to coarse grit.
Still polishing & beginning to test but would like clarification.
I have measured the sagitta at 6.02mm which gives an F of 2.6 and ROC of 1298mm.
When the mirror is in the test stand the reflected light comes to a tiny circle at approx 1254mm giving a differential of 44mm. Yellow paper not sitting flat but was necessary to get an image.
My conclusion is that the reflected light measurement is correct and that my sagitta measurement is wrong.
The light distance measurement was correct. More polishing, ROC is 1290 mm, Ronchi is showing straight lines across the centre but a TDE. A bit over a month before I get back to polishing.
Re-coated 25" mirror (ready for Winter sky &Planetary oppositions)
Coating had pretty much expired on my 25" mirror (silver coating approx 13 months old). So using my collection of "garden sprayers" etc and "spray silver chemicals", did the re-coat. Actual chemicals used cost less than US$20.
Surprisingly previous DIY coating lasted quite well. As a comparison have attached a couple of pics. First is of the 25" mirror freshly coated. Second pic of my 33" mirror and 12+ months old silver coat (rough [after a clean or two] but still usable [just]).
AngelGilding: see pics of what you need. Enough chemicals to do approximately 6-8 25" mirrors.
Stephen
Hi all, been a bit busy for a few years so I haven't recoated my 19" mirror for awhile. It will be the third Qld Astrofest with same silver coating. I will admit it is a little "dusty" now.
Couple more mirrors ready to polish.Will begin work on the 33"f4 first. Its the second 33 I have attempted. The first is "in service" but needs more work also. Its ok for low power work (115x-200x). Goes deeper than my 25" with over 70% more light gathering power.
i have a build thread here 
, Watching with interest.
