I have been a while polishing and I found the glass is reasonably flat. It is difficult for me to take a photo, but it should be able to be seen on the attached photos. I have polished two sessions on the turntable and every time the fringes look a little different. I thought that it was not polished out, because I could see very small glittery flecks on checking with a laser light. That is the reflection on top of the glass. After each session of half an hour, I hose down the glass with town water, wash down with distilled water in a squeeze spray bottle and then with 70% alcohol from the spray bottle. I let this dry in the air and put it down flat. I use lens clean from my eye glasses supply and wipe it down with a very soft cloth, like velvet. I could see not difference in the polish after the last time. I tried shining the laser on the glass and wing it with a lens tissue and found that the flecks can be shifted with the tissue, they move when wiped. So I have lint after all the things I did.
Could you tell me how to get rid of the lint to see if the glass is indeed polished out. What do you use to wipe down the glass?
Wash it with plenty of running water and stand on its edge to drain and dry
away from dusty drafts like you would a mirror . I lean mine against the tile wall on a laminate block resting on tissues or in the bath.
Quote:
Originally Posted by legswilly
I have been a while polishing and I found the glass is reasonably flat. It is difficult for me to take a photo, but it should be able to be seen on the attached photos. I have polished two sessions on the turntable and every time the fringes look a little different. I thought that it was not polished out, because I could see very small glittery flecks on checking with a laser light. That is the reflection on top of the glass. After each session of half an hour, I hose down the glass with town water, wash down with distilled water in a squeeze spray bottle and then with 70% alcohol from the spray bottle. I let this dry in the air and put it down flat. I use lens clean from my eye glasses supply and wipe it down with a very soft cloth, like velvet. I could see not difference in the polish after the last time. I tried shining the laser on the glass and wing it with a lens tissue and found that the flecks can be shifted with the tissue, they move when wiped. So I have lint after all the things I did.
Could you tell me how to get rid of the lint to see if the glass is indeed polished out. What do you use to wipe down the glass?
If your finding lint after the wash/rinse etc, the lint is coming from your lens cleaning cloth. I use well launder cotton, washed several times with no detergents. Keep them in a container so you don't pick up containments from benches etc.
Are you using warm water on your polish? Both the blank and mirror as well as the polish bottle water etc need to be warmed to help with contact during polishing. For hand polishing you would simply use ToT to move the polish out to the edge - this has a greater chance of scratching as the lap channels no longer take away the larger particles from the glass.
Steve
Ps. I cut up old tea towels and towels, anything that's was good cotton and made a lot of them. If you put a cloth down in the wrong spot, or touch it with dirty hands etc... throw it in the wash basket and get another one...
I washed down the glass this morning and most of the specks are gone. I do it at the shed which is about 100m away from the house, that's where the boss wanted it. I don't have a warm water tap, I could heat water in a pot, but that seems a little too much. So I will just let it dry by itself. I could buy a hairdryer, that might work. What do you think. I need one anyway when I silver the mirror.
At the moment, I think that it needs at least another 3 sessions of polishing to get it fully polished out. The fringes are not really straight, I will see if they are getting worst after another session. Before I start polishing again, I will cut the hole in the centre. Plan to use a little dam of blue tack for the water and cover the glass with a couple of layers of glad wrap.
Just take a kettle out with you, no need to take it to the boil though. Keep topping up your wash basin prior to rinsing etc, to get the water warm. Store the polishing powder slurry bottle in the wash basin as well, you don't want the glass contracting/changing shape with cold water.
Good luck on your drilling, use a pilot and don't jump size too quickly. You might want to keep some 80 grit at the drill bit once the well is started, that will help at low speed.
Steve
Give the side of the mirror contact as well to help with vibrations.
Did not think of a kettle. Have to get my mind around that and see how I could set this up. At the moment I built a washstand, just a tilting board with two wooden pegs and I wash it outside the shed. Hose it down.
I cut a 40mm hole with a diamond cutter, cheap on ebay. Did it on the drill press with a weight on the handle to apply even pressure while cutting. The entry on the polished side looks good and I will chamfer it a little. As it cut through, it broke off some small bits. They will be on the back of the mirror, so it wont matter. Gave it one more polish session and it is now drying standing up until tomorrow.
I have been a while polishing and I found the glass is reasonably flat. It is difficult for me to take a photo, but it should be able to be seen on the attached photos. I have polished two sessions on the turntable and every time the fringes look a little different. I thought that it was not polished out, because I could see very small glittery flecks on checking with a laser light. That is the reflection on top of the glass. After each session of half an hour, I hose down the glass with town water, wash down with distilled water in a squeeze spray bottle and then with 70% alcohol from the spray bottle. I let this dry in the air and put it down flat. I use lens clean from my eye glasses supply and wipe it down with a very soft cloth, like velvet. I could see not difference in the polish after the last time. I tried shining the laser on the glass and wing it with a lens tissue and found that the flecks can be shifted with the tissue, they move when wiped. So I have lint after all the things I did.
Could you tell me how to get rid of the lint to see if the glass is indeed polished out. What do you use to wipe down the glass?
Did you make yourself a custom interferometer? Do you use a sodium light or diffuse the light from a laser?
Did you make yourself a custom interferometer? Do you use a sodium light or diffuse the light from a laser?
I made a newton interferometer, simple construction with a strip of red leds, the self adhesive ones. The power supply of 12 volt is from an adjustable power supply I bought for the ronchi test rig and a led from jaycar. I later will use it for an autocollimation test stand as described on Stelllafane. That is for me the simplest. I started out with diffusing a laser light, but promptly zapped it when I forgot what voltage I had the power supply on. I can make you a drawing of the interferometer if you would like.
I made a newton interferometer, simple construction with a strip of red leds, the self adhesive ones. The power supply of 12 volt is from an adjustable power supply I bought for the ronchi test rig and a led from jaycar. I later will use it for an autocollimation test stand as described on Stelllafane. That is for me the simplest. I started out with diffusing a laser light, but promptly zapped it when I forgot what voltage I had the power supply on. I can make you a drawing of the interferometer if you would like.
The box is big enough to hold a 14" disk. You can make it whatever size you want.
The top compartment has a lid that the LED strip is stuck on. The adhesive will not stick well on the mdf I used, so I put a track of superglue, the cheap stuff you buy at the cheap stores. Use a glove to spread the track of the stuff. The 5 metre strip of red LED's is cut to lengths https://www.ebay.com.au/itm/183879660663 to fit the lid.
Each strip has marked where to cut it and you need to carefully cut and scrape away the silicon cover to expose the contacts. Stick down the strip and solder copper wire from + to + and - to -. Connect you power supply to the daisy chained + and -. the LED's point downwards.
The bottom of the top compartment is plain glass 3 or 4 mm with a sheet of plain white paper over it to act as diffuser.
The testing piece on the bottom and an optical flat on the top. https://www.ebay.com.au/itm/323769219170.
The semitransparent 45deg glass is supposed to be so one can see the reflection of the test piece. The sheet of glass is to look at the test glass from the front. It did not work for me, it was too dim and did not show the fringes. I increased the number of leds and doubled it, but it did not help. When I get the time/inclination, I will remove the glass, no point having it.
Sometimes sodium light is recommended, but it is harder to get and more expensive and for my purpose the LED's work fine.
I hope that is enough for you, if not I go down to the shed tomorrow and take some photos.
Interesting construction. A hairdryer would just blast airborne dust at your mirror. You want negative clean air flow or no air movement in a clean room.
Interesting construction. A hairdryer would just blast airborne dust at your mirror. You want negative clean air flow or no air movement in a clean room.
I didn't think of taking the mirror up to the house and rinsing it off in the bathroom. It has to be the most dust free room in the house. I polished it once more yesterday and after rinsing it and drying overnight it was mostly dust free. There were a few dried marks from the water. Not many, so I could test around these. When I take it up to the bathroom, I could rinse it with distilled water after rinsing it under the tap. The polish looks good, when looking at it with the laser pointer. It has gone a little more concave, so I will do another polish and move the lap a fraction to the outside. It is flat enough for the mirror I want for the autocollimation tester, but I want to see what happens.
The box is big enough to hold a 14" disk. You can make it whatever size you want.
The top compartment has a lid that the LED strip is stuck on. The adhesive will not stick well on the mdf I used, so I put a track of superglue, the cheap stuff you buy at the cheap stores. Use a glove to spread the track of the stuff. The 5 metre strip of red LED's is cut to lengths https://www.ebay.com.au/itm/183879660663 to fit the lid.
Each strip has marked where to cut it and you need to carefully cut and scrape away the silicon cover to expose the contacts. Stick down the strip and solder copper wire from + to + and - to -. Connect you power supply to the daisy chained + and -. the LED's point downwards.
The bottom of the top compartment is plain glass 3 or 4 mm with a sheet of plain white paper over it to act as diffuser.
The testing piece on the bottom and an optical flat on the top. https://www.ebay.com.au/itm/323769219170.
The semitransparent 45deg glass is supposed to be so one can see the reflection of the test piece. The sheet of glass is to look at the test glass from the front. It did not work for me, it was too dim and did not show the fringes. I increased the number of leds and doubled it, but it did not help. When I get the time/inclination, I will remove the glass, no point having it.
Sometimes sodium light is recommended, but it is harder to get and more expensive and for my purpose the LED's work fine.
I hope that is enough for you, if not I go down to the shed tomorrow and take some photos.
did another two polish sessions and the surface is a lot better. On the outside is still a little to be polished. The flatness has also improved. So I will do one more session tomorrow.
I looked at the glass through one of my eyepieces I bought and can see what looks like dried in puddles. There are a few very faint marks on the outside of the mirror and I think they will come out at the next polish.
With the tiny glowing flecks using the laser light, I thought that they might be very tiny bubbles in the glass, even looking at them through the eyepiece I could not make out what they are. Is it possible to have tint bubbles in the glass? It is only cheap float glass that I bought to practice.
One more polish, I think and I will start the silvering process. Need to read up again, it's been a while since I bought the chemicals and the silver. Not quite clear about the priming of the glass. Anyone doing this?
Sometimes the CEO dries in small patches which are hard to remove except with lots of running water and finger rubbing. Do you have hard water? Or scratches create small circles. I have only experience with Pyrex.
My problem is I still have TDE on my small f4. The straight lines are creeeping toward the edge but I will be 100 before I finish at this rate. Perhaps I need to go back to fine grinding 600 or polish with with pads on the glass tool to get spherical. There are many conflicting techniques.
Sometimes the CEO dries in small patches which are hard to remove except with lots of running water and finger rubbing. Do you have hard water? Or scratches create small circles. I have only experience with Pyrex.
My problem is I still have TDE on my small f4. The straight lines are creeeping toward the edge but I will be 100 before I finish at this rate. Perhaps I need to go back to fine grinding 600 or polish with with pads on the glass tool to get spherical. There are many conflicting techniques.
I found that I could get rid of most of the spots by rubbing them out with the fingers. After the last session of polishing, I found that it has gone a little convex. For the purpose of the autocollimation test rig, it would be well within the limits given. So I am letting it go until I can get the stage of the test stand re-built. I don't like the way I have done the existing one. More or less copied the Stellafane one, but with the stuff I had, it did not work well. Ordered parts on ebay, but did not come. Also waiting to get some tin for priming the glass for mirroring.
I don't have any experience in fixing a turned down edge. Looked at a video https://www.youtube.com/watch?v=tMp0sI3aYEU&t=204s. It might not be applicable to you. Going back to spherical with grinding and then polishing sounds like a good solution.
"Going back to spherical with grinding and then polishing sounds like a good solution." I'd second that, from my reading you need to be spherical and polished out before parabolizing the mirror.
I am still at a very simple mirror for learning, the F4 will be a beast.
Try Mel Bartels site for how to make different pitch laps for the parabolizing of fast mirrors.
PS. I got a flash polish on my mirror enough to test. Revealed some nasty flaws, dropped back to 400 and fine tuned the F/l, then life got in the way for the last few weeks... But no matter what you do, you can always remake the surface of the mirror and go again.
I have had good luck reducing turned edges with short straight strokes mirror on top with a slightly oversized lap. Same idea as Mel Bartels. This technique should work with a standard lap, it will just take longer. You may need to work for a couple of hours to see a significant reduction. You’ll likely get some surface roughness and a central hill in the process but these are easily fixed.
A well pressed lap is essential. Make sure the lap is micro faceted and not glazed. If the lap looks shiny, get a brass brush and give the surface a light scrub. Gordon Waite shows this in one of his videos. The micro faceting and brushing improves contact significantly.
When you examine the edge, look at the image outside the ROC, it tends to show the edge defect more clearly.
