The 20 " grind has begun

[mapstar]

Ps think your biggest challenge would be getting the blank as anything over 24" is very difficult to get hold of (and 24" is hard enough)

[jamespels]

Thanks Damian, think you may be right on both counts.

I cannot see me even starting the 1m mirror until at least summer, maybe even the following one if I end up making a dob to wrap round the test mirror (might make it a 12" so it is a worthwhile upgrade from my current 10...), and possibly an obsy to wrap round the dob . Found somewhere on t'interweb that can supply 75cm blanks but I think even that was the other side of the pond.

Think this might need a few beers at PSP, and really looking forward to seeing your scope. You are right - I did mean Rich's blog and have thoroughly enjoyed reading it.

Cheers,

James.

[mapstar]

Thanks Damian, think you may be right on both counts.

I cannot see me even starting the 1m mirror until at least summer, maybe even the following one if I end up making a dob to wrap round the test mirror (might make it a 12" so it is a worthwhile upgrade from my current 10...), and possibly an obsy to wrap round the dob . Found somewhere on t'interweb that can supply 75cm blanks but I think even that was the other side of the pond.

Think this might need a few beers at PSP, and really looking forward to seeing your scope. You are right - I did mean Rich's blog and have thoroughly enjoyed reading it.

Cheers,

James.

Sorry to say my scope won't be ready for PSP but my build thread is coming on for the other.

Apologies to Rick for discussing this on your thread so back to the grinding.

I've done another hour of polishing today and still struggling getting full contact on the Lap?

The middle need's more pressing as its not making contact

On the Ronchi test its showing the centre bands bending round indicating a raised section or under polished centre which you can see by looking at the surface with naked eye.

More pressing over night and try again tomorrow but after many months of looking at hazy glass its now clear and shiny which is highly satisfying

Damian

[JRM]

Apologies to Rick for discussing this on your thread so back to the grinding.

Not to worry Damian, its all good

 after many months of looking at hazy glass its now clear and shiny which is highly satisfying

yes it is, wait till you are done, to quote the band Boston, "Oh what a feeling"

I found this pic of astigmatism on Mike Lockwoods page and am posting it here for you all to see,

would not want to find this on a glass I was working on, talk about a regrind..

I did two trips around today, I changed my tactics and want to see if it is going the right way before going to far,

the glass is on the stand and will test tomorrow, till then...

Rick M

[mapstar]

Really glad my mirror didn't look like that one on test.

Going to have another try today at polishing the centre out hopefully my lap will have full contact with mirror

[JRM]

did three trips around yesteday,tot and coc 1/3 strokes,tested lastnight and this morning, almost back to the sphere.

today I have done 4 trips around and am currently pressing and will do 4 more trips around, all tool on top and

center over center, should be back to sphere by then, will test this evening to see.

I set my glass in the stand right after rinsing it off and set my ronchi test up before calling it a session, every now

and again I will go and look at the reflection to see shape of the mirror and I am at awe as to how much the shape

of the bands will change over eight hours of cooling down, it really is important to give the glass enough time to cool

before testing, thins I now know as fact, if your pushing glass give it a try, it really is amazing, I now unerstand how

one could go inn the wrong direction just by being impatiant to wait...

Rick M

[mapstar]

Interesting Rick.

I pressed again overnight with weight and recut the channels

I have tried another 2hours polishing today working lap ontop turning regularly.I've been using longer strokes to try to polish out the centre as advised.

The mirror is getting more polished but the centre is still under polished and the Lap centre is still not contacting I've also picked up a couple of 4mm edge scratches which I'm not concerned about.

I'm pressing again tonight. The Ronchi shows the bands bending into the middle and then from about a third of the way in then there is a flat line section around the middle 10%

Gonna try again tomorrow

Damian

[JRM]

Damian,

The longer strokes should help the center, you will get it i am sure, the scratches hopefully will polish out before

long as well.

I did 3 more trips around this afternoon and tested a few minutes ago, I either have a very slight tde or my sphere

is not all the way back to the edge and this is making it look like a tde,will do a few more trip in a bit and test again

tomorrow.

I have 2 pounds of white ceo polishing compound coming, should be here tuesday,shipping costs a tad more then

the powder, works out to about 3 Canadian dollars per ounce,things sure do add up over time, oh well, gotta have

it, cheers....

Rick M

[mapstar]

I managed to get an image of the Ronchigram last night. Using a grating with 133Lpi My camera skills do leave a lot to be desired but I was quite pleased it came out o.k. 

No polishing today as work stops play! 

Damian

[Glasspusher]

Looking good Damian.  Assuming that the Ronchi is just inside COC you are under corrected. Working with the lap thru the centre should help, test after a couple of hours work to check what the result is. Looking forward to seeing your ronchigram.

John

[JRM]

I gotta try to get a photo of my ronchigram, looks good Damian.

tested today and still notice the tde, its very small, could be defraction, but to be safe I

did another 3 trip session this morning to try and push the sphere out to the edge as

this to me seems to be the issue,once the edge is back to where it should be I intend to

use a full size starllap for my next figuring session, have used starlaps before but not at

this size, should be an interesting learning curve,will cut a mask to press the pattern into

the lap, but first things first got to get the whole mirror spherical..

Rick M

[Glasspusher]

Hi Rick,

You don't have to get the mirror to be a perfect sphere, any figure showing smooth under correction with no zones or edge issues is an acceptable point to start figuring from. For what it's worth I don't figure with star laps, I use round sub-diameter laps and have not had any problems.

John

[mapstar]

Looking good Damian. Assuming that the Ronchi is just inside COC you are under corrected. Working with the lap thru the centre should help, test after a couple of hours work to check what the result is. Looking forward to seeing your ronchigram.

John

Since the seminar I have done three hours polishing through the centre and I still seem to be getting poor contact with the centre of the lap even after pressing several times over night.

It was only a rough set up so wasn't too sure it was just inside COC John I really need to set up a permanent test area.

It is still under polished in the centre which I think this Ronchigram shows and visually I can see it. Also picked up a couple of little scratches on the edge.

Sorry for the poor picture I'll try to get another one soon after some more polishing.

Damian

[JRM]

Hi John,

  

      I have use star laps when doing my 10" years ago, will give this a try once see how it goes, have

never tried sub-diameter laps but will give it a try if this does not pan out, think my edge problem is now

just diffraction but will do one last  1/2 hour session tomorrow just to be sure.

will keep you posted.

Rick M

[Astrobits]

It is not usual to need sub-diameter laps for something like a 10" mirror but it becomes much more useful, even a necessity, to use them for larger mirrors. This is because the larger mirrors are usually faster than the smaller ones and consequently have a much steeper curve which cannot be followed with a full sized lap. Don't forget that using sub-diameter laps will warm localised areas of the mirror so you must allow plenty of time for the mirror to reach equilibrium before testing, even if it is a low expansion glass. I would test the mirror and then go for a cup of tea before retesting. If the second test shows that the mirror has changed shape since the first test then wait a little while longer until there is no change in the measurements.

Star laps are a sort of "halfway house" between full sized and sub-diameter laps. The smallest lap that I have used is my thumb to attack a very narrow high zone.

Nigel

[JRM]

Okay well,have not posted in some time but if you kept track of my signature on here you would have noticed my polishing time 

change well trying to get back to a spherical surface, something I hope to never have to do again, it  took a lot longer then I thought

it would but finally  I am back to a sphere with a good edge,no scratches or sleeks and if anyone thought it was not fully polished

out before, believe me it is now, and after all the mot and tot work, short and normal strokes, my focal length has changed 1 inch to

the longer side.

So now I will decide which way to go bout getting to a parabolic curve this time, will do some more reading before the

second attempt.

round two coming soon...

Rick M

[mapstar]

Good to read a post from you again Rick and didn't know that you've had that many problem's.

I have been polishing away but I have got a few scratches one of which I can see quite easily.

Going to keep polishing as the centre is still raised

Damian

[Astrobits]

O. K. so now you have a spherical surface, the edge is precisely where you want it. You need to remove the central bulge and taper it up to the edge. No further work needs to be done on the edge itself. I would not use a full sized lap on top from here on in ( never used a full sized lap for grinding and polishing a 20" anyway, it can all be done with sub-diameter laps and it eases the load on the arms or machine ). I would use 50%-75% sized laps to start removing the central bulge. At first it will appear that a high zone is being created, just work on that high zone so that the overall shape approaches the desired parabola.

Nigel

[JRM]

no real problems Damian, just took a lot longer to get the edge back then I thought it would, I knew it

would take some work but never thought it would be that much, lesson learned.

It takes a ton of work to remove even a small T.D.E. on large mirrors best bet is to avoid turning the

edge as much as possible,

looking like a 75% lap from here on out Nigel,and a couple smaller ones as needed.

Rick M

[JRM]

Hi All,

         Still here working the glass when I get the chance, but there are not enough hours in the days lately.

Silly season at work for the last and likely the next 3-4 weeks, work so hard I am all in by the end of the day but I have been doing some work

on the mirror, I decided to star the fullsize lap and have a go at that, after some 3 hours worth of working with it C.O.C. no dicernible change

in the surface of the glass was noticed,other then it looks really really smooth the ronchi lines are still straight as can be, go figure...

here is photo of the starlap...

Yesterday I poured a 16 and a 10 inch pitch lap, and will soon groove and press them into full contact and hopefully I can get things moving again,

once thing slow down at work I shall be back in productiion, soon I hope, till then...

Rick M

[JRM]

just to update,

    Things have finally slowed down again at work so I will soon be back in full swing trying to figure the glass, I did some 50 minute

coc with the 10" lap only to find that though it has started to move the center, it is not changing very quickly, I have come to the conclusion

that the white zirconium oxide work somewhat like red rouge without the mess,very slowly,unless I get a different polishing compound figuring

may take all winter, good thing I have a couple pounds of the stuff but will be looking for quicker working ceo in the mean time.

Also I ordered my 3.5 inch secondary, it shoud be here in a week or so.

The 16" lap did not turn out right so I stuck it in the freezer for a few and stripped the pitch from it, will be remaking it again soon.

so now I hunt for ceo polishing compound,but  polish with the zirconium for now...

Rick M

[mapstar]

Good to read an update Rick and useful information about the zirconium grinding medium.

Hopefully you will acquire some ceo soon and speed things up.

I am still working away with my 17.5" dia lap on the 22" mirror coc long strokes as I still have the centre hill by the looks of the Ronchi images.

Things have slowed down for me but I'm trying for an hour every other day. I've nearly managed to sort an accurate tester where I can set reference points for comparison

One thing I've not managed to avoid is scratches 3 of which are not going to polish out but to be fair at this stage I'm not really concerned about

them.

Keep the updates coming Rick

Damian

[JRM]

Hello  All,

   ordered 1 pound of the faster cerium oxide and it should be here by tuesday, I attempted to pour the 16 inch pitch lap and messed it

up once again, I have been pouring the pitch onto the lap then pressing lap onto mirror but both times I could not even get close to

contact in the center, so tonight I made a 15 inch negative mold, to which I will make the positive of it tomorrow and saturday I will use

this mold to pour my pitch into and then set my lap on to the pitch as per the normal way of making the lap.

Damian,  some people say that using the smaller lap is more apt to cause scratches as the surface dries out more quickly, I hope to

avoid them nasty critters but only time will tell.

oh and btw, my wife ordered me a 3.5 inch secondary mirror, it arrived yesterday, more incentive to get this mirror done

so I should be pouring a 15 inch pitch lap saturday and finally get back into the groove, till then, take care all...

Rick M

[Glasspusher]

some people say that using the smaller lap is more apt to cause scratches as the surface dries out more quickly

Rick, for what its worth I use sub-diameter laps all of the time and have never made a correlation between small laps and scratches.

Good luck with your lap.

John

[Astrobits]

I'm with John ( Glasspusher ) there. I have never had scratches with sub-diameter laps. I have been particular with the grade of CeO that I use which comes as a slurry and has never been dried.

Nigel