Making a 12 inch Mirror

[ponytale]

hi GP and everyone , i spent a few hours on my mirror regrind project on sunday, and i have one fine ground to 320 grit with no visible pits or scratches ,it is at f5.5 saggita and seems fairly spherical with the sharpie test , so on to polish when i have made my pitch lap later this week , i am assuming by the time i get it polished out it will be closer to F5,

The other mirror is almost at the same point but it is coming out at f5-ish also,probably my fault using the same tool so i am going to go back to a chordal stroke to deepen it then sphericalize it to f7 ish for planetary work , I am enjoying the work and getting some upper body excercise at the same time,ps i will keep you posted about the pitch laps , Tony

[Beulah]

That's brilliant, I applaud you for taking the plunge. In a way, getting started is where inertia hits the most. What OTA and mount will you use/make?

[ponytale]

hi sam, i have a diy dob that is waiting on the f5 mirror that was donated to me six months back along with an ota and focuser from someone on sgl, i have made the base etc using the same mirror before the regrind , (scratched glass and damaged silver coat) see my previous posts for pics . Tony

[Glasspusher]

Tony, if you grind or polish 50% of the time tool on top and 50% of the time mirror on top you will not significantly change the radius of curvature and hence the focal length. I am sure I have misunderstood your posting, tell me that you will not be polishing after 320 grit!!

John

[ponytale]

sorry John , I have been up since 5am and i didn't word that very well , i have 2 more grits to go( 55E 125E ) 600 and 1000 emery equivalents, but i am going to make my pitch lap bases/tools this week one 6.5" x 1.5" plywood for figureing and a cement full size lap. i hope that clears any confusion .Tony

[ponytale]

hi all, i didn't get round to making my laps last week wrong shifts at work so I spent a few hours grinding today and have one mirror at F5.5 and one at F6.9-F7 , i have 2 tiled laps and both seem spherical using the sharpie test, so tomorrow if i can i will get on to finer grits and get the f5 one ready to polish next weekend weather permitting.if the weather is bad i may make my pitch laps instead

Tony

[Glasspusher]

Tony, nice to see you are making good progress. You mention using the sharpie test for checking good contact between the tool and mirror, for those who are not familiar with it I will give a quick description. A sharpie pen, or similar, or even a pencil is used to mark a cross on each tile on the tool. The tool is then ground against the mirror with a 1/3rd centre through centre stroke, after a few minutes grinding the tool is examined to see how the pen/pencil marks have worn away. If the wear is uniform then the surfaces are in good contact and are spherical. If the marks are worn quicker at the centre or edge then contact is poor and non spherical. If this situation arises a longish spell of grinding centre thru centre with a 1/3 rd stroke will usually sort it out. Don't forget to do 50% tool on top and 50% mirror on top, this also helps maintain good contact.

John

[ponytale]

Hi John , spent 3.5 hrs grinding today i am now on 1000 grit and have a mild sheen on the glass , and at this point only 1 tiny pit mark very shallow and pinpoint in size,i am hoping it will polish out as i go as it is very shallow.

will i get to a gloss finish with more 1000grit grinding or do i go to polishing, i know its subjective , but if i shine a laser dot 1.5mm on the surface it it shows as a 1/2" diffuse light on the back of the mirror, if i wet the glass it goes through as a dot,

regards Tony

[Glasspusher]

Tony, I usually check for pits with a loupe or inverted eyepiece to examine the front surface of the mirror and find this works OK. I bright torch used to illuminate the surface helps, any pits are fairly obvious. With a 1000 surface, holding it up to the light should reveal a unifirm sheen across the surface. Perhaps someone would like to comment on using laser pointers to assess a mirror's surface, it is something I have not had a lot od experience with.

John

[FraserClarke]

I can't say I have experience of doing it myself, but I think the laser pointer test is used at the end of polishing to check the mirror is fully polished out.

The idea being that you shine a laser at a low angle on the mirror, and if the mirror is fully polished out you shouldn't see any reflection. If there are still a few small pits left, you'll see them as bright spots on the mirror surface (along with any dust particles).

[gregted]

Hey all...

Just found this thread and am loving it.

Great posts Glasspusher.

Here are some pics o a telescope I made with my son about 10 years ago. We had a ball..

The tube is a water softener tube and the telescope frame is the fibreglass housing the tube is usually mounted in.. Both cost nothing..

The spider is alluminium from an old screen door.

The focuser is different sizes of poly tube that fit perfectly inside each other and accomodate the eyepieces.

I encourage anyone to have a go at making their own telescope.

The look on my sons face when he saw the moon in detail through something he helped to build will be with me forever..

[barkis]

Just to butt in with apologies to John.

It is unlikely polishing will remove a pit left by an abrasive.

If you are a perfectionist, and it is better to be so, then revert to 320 or 400 to get rid of it. It will be quicker. I'm not sure 1000 will shift it quick style.

Make certain your grinding tool, or even you work clothing isn't harbouring any nasties from previous grits. Finger nails can be a hiding place too. I'm not suggesting you aren't washing between Grades, merely that the particles do love to hide, and jump off when you don't expect it.

Ron.

[ponytale]

Hi all , just finished 3hrs of regrinding: 1hr 320 grit (pit gone) 1.5hrs 600 ( no pits/scratches still)

1/2 hr 1000 too kna--ered to do any more today, i was up at 4/45 am for work ( bus driver) same tomorrow home at 8pm for 2 days off , so hope to do more on weekend, in meantime, on the advice of some one at the place i got my pitch from i will paint my tile tool, 1 to seal the cement back, and 2 to lock in any loose grit particles between tiles , this sounds like good advice to me, any opinions ?, regards Tony

[Glasspusher]

Tony, I assume you have stuck the tiles onto the tool with epoxy as I described in a previous post. I have never found the need to paint the tool (what with?), generally a good scrub does the job for me, having said that it might be worth a try.

Ron, thanks for your input, you have obviously made a mirror or two, I agree with your comments on cleanliness.

John

[ponytale]

hi all, yes john i used epoxy ,the suggestion was from a chap at galvoptics, he said most mirrors he sees has a small scratch or two, he said the best tool is metal which is what they use or next best glass with beveled edges ,with tiles he recomended paint (after a good scrub as john says in his tutorial), i have cleaned my tile tool thoroughly and painted it with some acrylic spray paint to try ! i will let you know how it goes. tony

[ponytale]

HI GLASSPUSHER, i have done a couple of hrs at 1000 grit today , the paint thing worked really well my tile tool washed up much quicker and i noticed a cople of things when i started grinding with the paint on the tiles , so i put it on video on my you tube channel, it became very obvious how mot and tot effect the area being ground , hope you like the videos all credit to your tutorials and advice as always regards PT.link below

YouTube - MIRROR ON TOP.MOD

[ponytale]

Hi all, now i have restarted work on making my mirror, i have put a turntable on my bbq base which helps a lot, i have made a lap and polished my first mirrror to spherical , i have tested with the focault /blade and it has a lovely straight edge all the way across,

i have some ronchi grating to use for the next test before i go any further ,

i have developed some very fine scratches,but a bit more than sleeks so i will try and polish them out after a thorough clean of everything , i suspect particles off the lap base,? so i will reseal that also, i have bought some 10mm resistored diffused led's that run on 12 volt, i have a really good 12v transformer i can use so no more flat batteries, i will try and video the test so i can post pics, has anyone else started their mirrors yet? please keep posting to keep interest in this topic going, more soon i hope

[Glasspusher]

Tony,

Nice to see that you are still making progress with your mirror, but sorry to here about the scratches/sleeks. It is useful to check the mirror's surface carefully under a bright light about 1 hour after starting to polish, this will help determine if the scrathes/sleeks are coming from fine grinding. If they are I would suggest trying to polish off 600 aluminium oxide abrasive, very fine abrasives (like 1000) can result in sleeks. The other thing to look at is pitch hardess, particularly in this cold weather. Hard pitch will cause scratches/sleeks. Do not work in cold conditions and store the mirror and lap in a warm place (not too warm!). Hope these ideas help. If woul be nice to see some ronchigrams, sounds like you had a good shere on the mirror. Like Tony I would be interested to know if anyone else has started a mirror or is thinking about it.

John

[ponytale]

hi all , this is where i am at today, reground mirror with 320 for a few minutes to clear scratches( a seen in one of the attatched pics,) polished up enough to get these ronchi test images, which clearly shows it is not polished out enough it has a hollow in centre and a zone about 1/3 rd diameter, i will need to adjust my strokes and mot/lot ratio to correct this i think, i will do some more over the next few days and test as i go , i am polishing inside the house at the moment to keep consistant pitch hardness/softness, i am desperate to avoid scratches again so i am being really careful with lap cleaning, i do have some very very minor marks at present so i am hoping they will stay minor or disapear, ??

ps, i have created photoshop file to print ronchi lines on jetprinter on acetate i used best photo quality in my printer settings , i have attached the file for anyone to use , it will print several on an a4 sheet as is , hope this is useful to someone grinding their mirror.

ronchi screens x6.psd

[Glasspusher]

Hi Tony,

Your ronchigrams look pretty good, smooth figure with no major zones or turned edge....underpolished in the middle as you know. Looks like your polishing technique is OK. Like the ronchi grating printouts, a very useful resource, thanks. I am a little concerned about the scratching this looks quite bad, can I suggest that you be very thorough with cleaning every thing up between grits to ensure no contamination. After changing grits check the surface carefully after 30 mins work to see if there are any scratches....it is important to find out at what point is the process they are occuring. Hope this helps.

John

[ponytale]

hi john , thanks for your comments , the scratches picture is before the regrind/ repolish, this time i am wire brushing the lap under running water between polishes and putting clean plastic over the turntable but under the mirror/lap each time and keeping the mirror and lap together indoors wrapped in clingfilm , you said in your last message to polish out at 600 ,would i get a satifactory finish from 600 rather than 1000 , how does that compare to the rouge for finish, PT

[Glasspusher]

Tony, I like to finish fine grinding with a good quality (ensures consistent particle size) white aluminium oxide 600. I then polish using cerium oxide (available in several forms, most are OK). You mention rouge, which is an alternative for polishing, I don't use it as it is very very messy and slow acting. When you finish with 600 aluminium oxide you may find that it takes a little longer to polish out. Aluminium oxide is softer than silicon carbide and gives a better finish, I would not recommend finishing fine grinding with 600 silicon carbide. Hope this helps.

John

[Glowingsplint]

brilliant stuff your doing, some pretty extensive DIY

[ponytale]

video link to my ronchigram of my f4.6 mirror i am grinding done a bit more polishing and have a good sphere coming on by the look of it , more polishing to go but getting closer !!!

YouTube - MOV03C

[Glasspusher]

Looking good Tony. If you post again try to show fewer bands on the image, this makes it easier to see how the mirror is progressing. Certainly there are no major errors evident at present. Well done.

John