I will be coating a couple of small mirrors for one of my projects so if your mirror is not quartz over coated then I could do yours as well while I'm at it.
This thread has been quiet for a while so I'll share my current project. I'v just rebuilt my Bath Interferometer to make it simpler to operate. The first vesion had everything adjustable, which meant it was always going out of adjustment. This time I've fixed the mirror and beam splitter so I only need to align the laser and lens. The only issue is getting the extra laser dots out of the image as you can see in the picture.
I've also picked up a 365mm mirror I had started back in 2000 and put aside when it got scratched during polishing. It has kept getting scratches at 5 micron this time aroundthe end I finished at 9 micron and polished from there. At the moment I'm about halfway there. The laser test still has speckles and the mirror is bit zoney with a beaut turned down edge. I expect it will all come good by the time I'm finished. I've been using an oversized (400mm) lap as described by Mel Bartels, but I'm not happy with the contact so I'm going to make a new full sized tool before doing an more polishing. I've attached a ronchi image and the open fringe report which seem to agree in general terms.
found an easier and quicker method for beveling larger mirror blanks. Instead of taking what seems forever (hours), creating bevels on larger mirrors, (20"+ sizes), using knife sharpening stones, I decided to go hi-tech. Using a pair of belt sanders fitted with #120 silicon carbide belts was able to reduce time taken to a fraction that previously taken using the hand wielded sharpening stones.
Method: (Dry belt). Orientate grinder/sander at 45deg to top face/edge of the blank. I move the sander with approx 12" strokes clockwise, forward and back (not too quickly) a fixed number of strokes (I use 15 return strokes), then move approx 12" clockwise around the blank, and repeat the procedure until full circuit. Two (or three) circuits are completed, then you need to either rest the sander as it does get hot if run too long resulting in belt failure due to belt join glue melting or if you have a pair of sanders, then spell number one and do a couple of circuits with number two. About 5 (or 6) circuits of the blank will produce a neat 3mm to 4mm bevel.
Important Notes:
(a) As the work is by dry belt and glass dust can be injurious to health, lungs and eyes need to be protected, so nose mask and goggles are essential, also protection for ears (belt sanders are noisy).
(b) Very little pressure is needed with sander to glass as contact surface is very small, particularly at first.
(c) Best to do this work out doors. Dry conditions, preferably no wind.
Including a couple of pictures of equipment used and completed bevels (4mm) on my current 33" mirror project. (Over 207" of edge beveled).
newbie question i guess. i was thinking that if your only taking out 7 or 8 mm for a parabola on a small telescope could you not use two pieces of 10 mm thick glass glued together. i am guessing if your coating it the internal reflections from the joins shouldn't be a problem. the only problem i can think of is perhaps uneven cooling of the glass. i know rather simplistic view of things but i thought i would throw it out there. the other thing is i have no idea what you would use to glue glass together with however they do glue some glass together for some camera lenses so it must be able to be done
Not sure you should attempt this route. My experience with gluing glass goes back a year or two when I attempted a 25"f4.25 sandwich mirror project. I used a not too fast curing super strong epoxy glue. At first during hogging through to fine grinding, and even when part way through polishing all looked good, (Ronchi test was ok), but there was a particular point nearer the end of polishing when the Ronchi test showed that some of the glue joints at the junction glass pillars(7mm thick) to front and back plates(each 19mm thick float glass) were failing. At that point I laid aside this project. The only way to get two glass surfaces to successfully adhere long term is too fuse those surfaces together by heating to the required temperature in a glass or ceramic kiln. In the future I may take my sandwich project to a kiln and do the "fuse". I dont know what diameter mirror your thinking on working. My advice would be to use a single good quality 19mm float glass blank (for anything 14" in diameter and less). Larger will need a thicker substrate.
it was just a thought . i am actually thinking of buying a 4inch primary and a little secondary of ebay to make a telescope just to wet my appetite. mostly for terrestrial and moon viewing. granted it's not actually mirror making. however i spent ten years in a factory cutting granite slabs into kitchen benchtops and running a cnc to cut out the holes for sinks and hotplates in the end the dust was giving my lungs grief and i had to get out of the game. i would like to try making a mirror one day doubt i 'd go down the dry belt sander path. maybe try something similar with a wet polisher perhaps not sure at this point
Stephen, your gluing attempt and another mirror I've seen, has got me thinking about glass lamination. Not through gluing, but as you mentioned, fusing.
Not an exercise to reduce weight, but to make thicker blanks (yes, thicker not thinner), but also increase the rigidity of the resulting blank. I've seen a mirror, 16", which is laminated by fusion, in the same manner as your gluing attempt.
Starting with stress-free blanks, the heating and cooling cycles would need to be very long in order to not introduce stresses.
Gluing glass blanks won't survive the aluminising process, when it's put in a vacuum chamber. Most epoxies degas and will cause strains that will deform your figured surface. The blanks may even separate.
Only satisfactory way is to fuse the pieces in a kiln.
your gluing attempt and another mirror I've seen, has got me thinking about glass lamination. Not through gluing, but as you mentioned, fusing.
Starting with stress-free blanks, the heating and cooling cycles would need to be very long in order to not introduce stresses.
Interesting.
Hi Alex,
If you were interested in building sandwich type blanks, it would be ok (from my checking around the web), to use similar to my method in that epoxy is used to locate components, ie the two glass discs and glass pillars, then find a kiln, go through the fusing and annealing cycles and all should be good. It seems the epoxy is not a problem as it just burns away leaving a clean fuse.
Shouldnt be too difficult to find a suitable kiln close to your locality. Even in Oamaru (a few minutes drive from my place), I know of two kilns which could do the work, one of which I have approached, $100 for the work, but unfortunately was only able to take blanks to about 600mm, (my 25.2"[640mm] a tad too big). The other kiln, (which I havent visited as yet), can accommodate blank diameters to around 1 meter.
thanks for sharing your project. Any updates would be appreciated.
Best regards
Stephen.
I'm afraid any updates will be few and far between. I'm moving house in the middle of the year and this takes back seat to the preparations for that.
I put in a couple more hours with the old lap and it seems to have done the trick. The edge isn't perfect, but it's much improved, and I think the last bit will correct itself as I start figuring. The laser test is good and no more speckles are to be seen.
I've attached a comparison of fringe images from before and after the last session. You can easily see the hook at the edge before, but not no much now. I've also attached a Ronchi image - not perfect, but the zones are minute when I look at how much parabolising I have to do.
Hi all, after 9 months work, I have just finished a 19 inch mirror at f4.5. It appears to be very good on the foucault tester for the central 18 inches, then things go down hill outside of that. But that's OK as I treat the lost inch as a 1/2 inch support ring for the 18 inches remaining of the mirror. I will paint the edge black, and all will be well. I will report back with a pic after the scope build.
cheers, Frank
Hi all, after 9 months work, I have just finished a 19 inch mirror at f4.5. It appears to be very good on the foucault tester for the central 18 inches, then things go down hill outside of that. But that's OK as I treat the lost inch as a 1/2 inch support ring for the 18 inches remaining of the mirror. I will paint the edge black, and all will be well. I will report back with a pic after the scope build.
cheers, Frank
Is the edge rolled off on Figure XP. It can get a little confusing out there and when you do what you think is right it only seems to get worse - been there!
I've got my own little mirror project going on right now, an 18" f3.3 and it's been a challenge to say the least.
Hi Raymond,
yes the edge is rolled, but I always buy the glass with an extra inch of diameter so as not to get caught up in trying to achieve perfection. Fig XP does show it as rolled, more like a cliff really, than a gentle roll! Ronchi tests showed sever edge roll also, so I rebuilt my mask to reflect 460mm diameter and continued figuring until the results were OK.
I completed a 17" mirror a few years back, got reasonable numbers with 16" of it, and thought - I'll just try to go a little better. After another year I got back to where I was before I fiddled!! Don't want to repeat that as I am at 1/20 lamda at the moment.
Just built the lower box for the mirror to Plop specs, need to finish it for the Queensland Astrofest!!
As for your project, please post some details as things progress, I would like to make a shorter one as well. What thickness glass is your one? Mine started out as 25mm and ended up as 22mm due to more grinding due to an edge issue.
I just got a plate glass blank from Stephen. Its 460mmX35mm. It's currently about 1/6 wave with figure XP but it has a few zones i'm trying to sort. It star tests ok and jupiter is showing great detail but i want to try and get it as good as i can. The edge is very good.
I did a little bit more work on it tonight. Figure XP says its about 1/11 wave p/v. Its got a 20nm high zone from about 50% to 80% and I thinks it's a little rough from using a small lap so i would like to smooth things out without effecting it too much if possible.
The seeing in my shed wasn't that best for photos tonight, it's windy outside and it makes the large tilt door move causing all the air inside to move around.
4 Months and I'm done with my first ever mirror and it wasn't an easy one being 18" and f3.3
I'm sure Figure XP is more than optimistic but I've spent the last few hours going over and over and turning the mirror and i can't get any different results.
There is a little roughness an inch in from the edge but in fear of stuffing it up a don't want to do too much of anything else. I have run over the mirror with a larger lap to smooth things over a bit.
. My experience with gluing glass goes back a year or two when I attempted a 25"f4.25 sandwich mirror project. I used a not too fast curing super strong epoxy glue. At first during hogging through to fine grinding, and even when part way through polishing all looked good, (Ronchi test was ok), but there was a particular point nearer the end of polishing when the Ronchi test showed that some of the glue joints at the junction glass pillars(7mm thick) to front and back plates(each 19mm thick float glass) were failing. At that point I laid aside this project. The only way to get two glass surfaces to successfully adhere long term is too fuse those surfaces together by heating to the required temperature in a glass or ceramic kiln. In the future I may take my sandwich project to a kiln and do the "fuse". I dont know what diameter mirror your thinking on working. My advice would be to use a single good quality 19mm float glass blank (for anything 14" in diameter and less). Larger will need a thicker substrate. 
Don't want to repeat that as I am at 1/20 lamda at the moment.
