360mm First time mirror grind.

[8kids]

Gary, not sure I agree with Ajohn about the noise. The fine grits, 180 and finer, are pretty quiet for sure, but the 80/120 grit I used for hogging out the initial curve made a reasonable amount of noise. The sound reminded me of a power hacksaw, the reciprocating type. Not enough to upset the neighbours unless you are doing it at unsocial hours. As to the Wife, only you know the answer to that

If you do a quick search on youtube for mirror grinding, you will get an idea of the noise level for rough grinding.

Thanks for the info, If I grind the mirror at the same time shes nagging me she won't hear it. ...I'm only joking......she'll probably still hear it. .

Looking at the youtube vids it's not exactly a quite process so I'll grind mine at my father inlaws workshop. All I have to do is decide on a size. I have a 12" dob at the moment so do I go larger or smaller? Is one harder than the other to grind?

Keep up the good work and I'll be following your thread with interest.

Gary.

[Ajohn]

I would say this one is a bit quieter than it will be in practice. Imagine the sound boosted until you could hear the spray being used at the right level.

Some one that doesn't have some water to swill off spent grit.

Bigger mirror same F ratio more glass to be removed. For the same size long focus slow F ratio mirrors sometimes have frighteningly little glass needing to be removed to figure it. Fast F ratio's more glass to remove during figuring. This touches on why F6 or there abouts mirrors were very popular. Testing gets more difficult with faster F ratio's and can be a problem at rather slow F ratios too.

Say some one decided that they would make an F5 scope. Easier maybe than a F4.5, probably harder than an F6.or 7. If its 250mm mirror the scope is 1.25mtr long. If it's a 500mm it's 2.5mtr long. Longer the scope is the more rigid it needs to be. Same for reductions in F ratio too if resolution is needed. Texereau points out that few people will be able to get the full resolution a good 10in mirror is capable of. It's true but fuzzies are extended objects so light capture comes into it as well. As the scope size goes up the view that fits into a 2in eyepiece gets less.

It's all swings and round abouts really and there isn't such a thing as a perfect scope.

1in thick blanks are the biggest problem is some ways. Texereau's idea of a blank is so thick it wont be a problem what ever you do with it even when grinding let alone in the telescope. I was glad to hear that Alan knew about Plop. He's likely to need it even if just to see which rather expensive mirror cell he will buy. Maybe he is going to make his own. The more expensive mirror blanks are thicker. Might give some problems grinding etc but don't need so much care in the telescope.

John

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[Ajohn]

LOL Just thought - a lot lot quieter than the washing machine, food processor, a dog barking and many kids.

John

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[Rustysplit]

After a couple more hours this weekend, the fine grinding is finished. My sequence was about an hour on each grit of 180/400/303. I was quite surprised by how little of each grit it actually took to polish out the previous grit's pits. They may be fine, but they seem to cut very well. I think it was helped by having very good contact between the tile tool and blank. Almost all of the tiles were in contact.

For comparison, here is the appearance of the pits after 400 grit..

...and following 303 grit...

I have managed to get all the pits out, except one right near the centre of the mirror. This is within the shadow of the secondary, so I am not gonna sweat over it. Here is the blank as it stands now..

Next job is making the pitch lap.....

[Glasspusher]

That looks good Alan. I always like to check for any scratches at this stage, if we get a sunny day (!) take the mirror outside and catch the sun's reflection in it. No danger here as the mirror is not yet polished. Examine the surface VERY carefully for scratches. If you see any, do some grinding, with deep scratches you need to go back to a courser abrasive. It is best to eliminate all scratches (as far as possible) before moving on to polishing. If it is not sunny, then use an inverted long focal length eyepiece and a light source to illuminate the surface. Remember to check the whole surface.

John

[Rustysplit]

I take your point John. I have gone over the surface with an inverted 25mm ep and a white led torch. I think it is clear from all scratches, but I will check it again. No point finding them after polishing for a couple of hours

May I ask where would you suggest buying a ronchi grating from? I have a quick buzz around the sites and can't find any except the Orion optics Ronchi ep at about 60 odd quid

[Glasspusher]

Alan, I have used http://www.ronchiscreens.com/ on a couple of occasions and found them to be helpful.

Hope this helps.

John

[fwm891]

I've found a strong light (torch) from the back surface of the ground disc to show scratches up as straight or swirly lines depending on the stroke that generated them. It will also give an idea of how far you need to go back when comparing the scratch with the surrounding ground surface.

[Rustysplit]

Francis, that is a very good tip. I tried it earlier and whilst it confirmed no scratches, it did show up a couple of pits that I had not spotted. They seem to show up much better when illuminated from behind. Thanks for the suggestion.

[Ajohn]

You can print your own Alan see some way down here

http://stellafane.or...t/tester-2.html

I have an idea that there is some software about to print them too. Pay for / free don't know.

If you are going to make a tester their isn't any need for one really. Texereau again. If the knife is in the right position fringes can be seen and the straightness indicates roundness in much the same way as a ronchi screen does. The positioning for that isn't critical. He also uses the same thing to set a sensible slit width. Fiddly to do even with his slit but could also be done with the 2 scraper blades etc type combined knife and slit arrangements. Curiously Tex.... is the only person that even bothers to mention this and it does have some bearing on how easy the test is to do.

When you make a tester ebay number 200908090072 might help if you go that way. With M6 Imm pitch thread you can measure to about 1/100mm with that and just have to remember to count turns - or create a scale for that too. Probably best use all thread as the pitch is generally accurate. Don't know if the knob fits M6 or 1/4 so it may need padding up some how. It might tell you on the Maplin web site. The thread will probably be loose if it's M6 anyway. Just try to keep the knob central on it. Maybe epoxy putty could be useful. Lot's of £ shops sell it.

If it doesn't polish up evenly best go back to grinding. I think I would have bedded the tile tool in with the coarse grit. One way to check that you have a sphere is to mark the mirror with pencil lines all over the place - a bit of grinding with short strokes should remove every trace of them very evenly at the state you are at. This can only really be done with a more or less full sized tool. If you haven't got full contact it's best to do some more grinding. Quickest route the way you have worked will probably be back to the coarse stuff again. You can use the pencil test on that too. The problem with the finer grits is that they remove so little material so correcting can take for ever.

John

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[Rustysplit]

Aaaaggghhh!

Just tried to pour the pitch lap, and there is nowhere near enough pitch. The edges have barely any coating and it cooled so quick due to it's lack of depth that it is very uneven. I tried wafting a blow torch over it to even out, but a waste of effort.

So, the question is, can I pour over fresh pitch onto this set layer? If not, do I have to make a new tool and start again?

I shall get onto Galvoptics in the morning about some more.

[Rustysplit]

I have just had a look on youtube at Gordon Waite removing and re-using old pitch. He puts the tool in the freezer for a few hours, then it just chisels off easily, ready to be remelted. Should I do this so that it can mix together with the new pitch? It seems the best plan to me at the moment. Start afresh on the old base rather than try and fudge it.

[upahill]

I cant offer any advice, but recently read a good section about pitch laps in Allyn J Thompsons book, some of it might be a little outdated, but if nothing else it can be good bedtime reading.

http://ubuntuone.com/5xj7SSWjrnE3j8PK8ujnhm

That link is for the PDF version of the book, which I believe is now public access / out of copyright. I found it much easier reading than Texerau too.

[fwm891]

Francis, that is a very good tip. I tried it earlier and whilst it confirmed no scratches, it did show up a couple of pits that I had not spotted. They seem to show up much better when illuminated from behind. Thanks for the suggestion.

Pits unless they are much bigger than the surrounding surface are a fact of life unfortuneately. You will get grains that clump together as they roll around and they exert more local pressure and dig a small hole. Once you get onto the softer working aluminium oxides you wont get half the pits forming, those that do will work out quite easily (I tend to apply AO's as a slurry with just a touch of washing up liquid, this soaking in dilute washing up liquid seems to break down the clumps. The finer silicon carbides can also be applied in that way for the same reason.

Good luck with reforming the lap

[Ajohn]

Ronchi screens can be bought from the UK at

http://www.awrtech.c...htm#RONCHI_TEXT

Gordon Waite seems to use hard pitch laps otherwise he would "temper" it each time he used it. It gets harder each time it's melted. Texereau did the same thing initially adding a little true turps when it was used again. Later he switched to castor oil. Waite is using a machine so some techniques are different.

Hope it passed the pencil test.

Texereau reckons it's best to figure bigger faster mirror as close as possible with a full sized lap and switch to the smaller lap when it stops being effective. Probably as it will be easier for beginners - less chance of digging holes. Some will no doubt go straight for it. He does show the most suitable stroke for doing it that way.

John

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[Glasspusher]

Yes Alan, you can pour pitch on top of your partially made lap. Make sure it is clean and dry before pouring.

Good Luck,

John

[Ajohn]

Some UK people go through the fine aluminium oxide grades that are usually available and then a tube of tooth paste. It's reckoned to save time polishing by a useful amount. I noticed that there is some white aluminium oxide on ebay. I intend to ask about the grade as I suspect it's for toothpaste.

I use a saw to cut channels in the lap. Just cut either side and dig it out. Only mentioned in Howards book as far as I'm aware. Works for me. Texereau's cast strips and cut squares from that and stick them on it is how rather large professional laps were made. I suspect he suggests that as it saves getting the tool hot which in his case is glass so there is some risk of it cracking. I find the mirror has to be hot anyway so that the pitch carries on spreading after it's covered with what ever polish is used and the mirror is rubbed all over it. Howard also mentions finally pressing fine plastic mesh into the lap - helps it deform to shape during polishing and also speeds up the process. Mesh with a 1 to 1 1/2mm spacing is about right. I've tried pressing very coarse mesh - drain board mats - into the pitch to form the channels but it didn't work out so out came the saw..

John

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[Ajohn]

My Foucault tester has been in a loft room that used to have a bit of a leak for some years. Whole roof replaced but this is what it's done.

No sign of the slit yet, some one has tidied up. The light source is in the die cast box on a pole. I tilted that to align the slit with the knife as it saved making an extra bit for the knife. Most of the rest was made from aluminium scrap off cuts and cleaned up with a file. A bench drill helps as it easy to keep holes etc square. The whole thing can be made of wood etc really except the V's that run on the bars. Some use wood for that too but they are always rather long and are best kept short to keep down friction. I suspect thin ply or even upvc, perspex and all sort of other things would be better. These are just fastened on the end of the table. Mine runs on a steel bar but tube would do. Some use copper water pipe.

Later I added the cross feed part to try the caustic test. It's a lot more precise. Details can be found on the web. They often mention eyepieces but I used a 5 or 10x eye cup type loupe. I found that it was possible to measure shadows in a mask to around 0.001in or better but only after a lot of practice just using Foucault. I think that aspect is the biggest problem on a 1st mirror.

Problems with it. The standard micrometer spindle is adequate for measurement but the low diameter makes it touchy to use. The screw that tilts the table to cause the knife to enter the beam is incredibly touchy when used on a good sphere and doesn't help much in normal use. It really aught to have a disc on the end say 30mm dia or more. I can't use it to grey out a sphere as it is so I use Texereau's fringes.

I'm just going to clean it up and make a slitless type tester (these do have a slit) to sit on the table. The blade that forms the knife and one side of the slit will be fixed in place - the other blade that forms the other side of the slit will be clamped some how so I can adjust it.

The extra bar that the micrometer spindle works against is down to the size of the aluminium I could find and the shortness of a 0-1in micrometer spindle. Normally this would work against a small piece of metal fastened to the table - screw or bold head or anything else that is to hand. Ideally it needs to be square to the face of the screw that's used for the measurements. - epoxy, same but putty so it sets square.

The weight added to the table is 1/2 in x 1/4 lead strip epoxied to it's underside. Some added weight is needed to make these slides work well however they are made. Maybe a 2 or 3oz ledger etc from a fishing tackle shop, steel bar or what ever can be found. It's over towards the screw that tilts the table to keep that firmly down. There should be a tension spring pulling the table back against the micrometer spindle. I have some now but did use elastic bands. I found a bag of springs at a car boot. It needs to pull back by more than the weight of all of the moving bits and tend to pull down rather than up otherwise the table will wobble up and down.

This is an interesting area. Some one on one of the atm groups in despair commented that people are prepared to put hours and hours into making a mirror but wont spend time making something to test it with. I would say if some one is going to make a tester make a decent one or push a slitless type around over paper or just a knife on a block of wood and mark positions with a sharp pencil. Or add a guide and do the same.

John

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[Rustysplit]

Thanks for the awr link John. It seems Gordon does temper his pitch with oil, castor or linseed, can't remember which now. I think he says he favours slightly softer pitch. I will be glad just to get the damn thing made

Glasspusher John, I drove up to Galvoptics today and picked up 2 fresh lots of pitch. I placed the tool in the freezer overnight and chipped off the first lot this morning. I can pour the whole thing afresh now without worrying about lumps. I can reuse the old stuff for the figuring lap I guess.

I have some ali channel for pressing in the facets. I can always clean them out a bit with the handsaw if needed.

[Ajohn]

Alan out of interest more has been written about the Ronchi test than any other. Often followed by the comment that it isn't a very good test for anything really. I knocked one up out of curiosity using fine wire. It wasn't worth the effort. Best you search the web and try and find a good description of how it works and what you should see in and out of focus.

Still it's often mentioned so it must be good.

Better really to build a tester and make sure you have a fine control of it. That way you will get some experience of using it before trying to use it to take measurements. The problem with a sphere is simple really. Say the source is a slit or what ever 0.040mm wide. The image on the knife edge will need to be about 1/2 way through it and the size is the same. The distance the image is along the axis of the mirror varies but it's also tight. Get the knife there and you will see any errors easily. Move the knife slightly along the axis and the position of the errors will change. It can be done with a knife on a block of wood too with a separate light source or any similar tester. Once you can do this your effectively using the tester the same way as it's used to figure the mirror. You can also try correcting any errors, the 1/2 sized lap might come in handy and with luck a smaller one if you happen to severely mess up the edge or centre - the small one will probably make other errors that are much worse and often as with figuring it's sometimes best to go back to the big lap and back to a sphere and start again.

You might also find that the source you are using doesn't really work out. Better then than later.

Texereau's fringes are really meant to check that the mirror is spherical right up to the edge. It gets round the bright diffraction ring that forms on the rim of the mirror making errors hard to see. They are straight were ever the knife is though if its a sphere and the knife itself forms something to compare them with. If it's a slitless type tester and you make it so that the slit and light can be removed on a small panel it's easy to hold it up and set it to very fine limits. Probably down to 0.01mm or even less. At that sort of width there will be loads of fringes. Not a good idea as the diffraction effects on holes in masks or around pins will be more intense than they need be.

Not mentioned on the web so it can't be correct can it.

John

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[Rustysplit]

Blimey!!! That is quite frantic work. I think I have made an ok pitch lap. I will need to open up the channels with the saw tomorrow, but as it stands, I have good contact over around 80%. It is pressing now with greaseproof paper between blank and tool. Will see how it settles out.

Just as well the wife is away at the moment. I can almost hear the comments as she sees me melting pitch on the cooker and putting failed tools in the freezer!!

I think I have read and understood more about the ronchi test, so will probably go with that. Depends on how long it will take to get grating. It may be quicker and cheaper to make the knife edge tester and read up a bit more.

[Rustysplit]

After about 1.5hrs polishing, here is the progress so far...

Along way to go, but coming along OK. I had read that the lap will bed in during use as it warms and forms better contact. This is certainly true, whilst it feels good when you start the polishing session, about 20 mins or so in, I have found that it starts to feel better. I have managed to pick up a few very fine scratches . I think it may be from the jagged edges where the pitch splintered when opening up the channels. Also my pitch seems very hard to my inexperienced eyes. It takes a good deal of pressure to make a small mark with the thumbnail test.

[Glasspusher]

Alan, did you go for a full sized tool in the end? The mirror looks to be polishing out in the middle faster than the edge, have you been working mirror on top? If so I would work with the lap on top for a while, it will help the edge to catch up to the middle.

John

[Peter Drew]

Re testing, it could be instructive to set up a professionally finished mirror to give you some idea of what appearance you are looking for. I presume a parabola will look pretty much the same even if the mirror is of a different diameter.

[Rustysplit]

I did go for the full size lap John,yes. I have also been working TOT all the time, due to the thin blank. Could it be that I have been using fairly conservative 1/3 strokes, to try and avoid TDE?