The 20 " grind has begun

[RAC]

If the far outside zone is getting worse then the only way to fix it other than starting again is to lower the zones inside it and that can mean a small and sometimes very small lap. I like to work the mirror keeping a very close eye on the last zones reading, basically not over correcting the middle but taking the surface down evenly where needed by paying very close attention to the Figure XP tests as I go.  I don't like needing to use small laps at the end.

[Astrobits]

You must remember that a sphere viewed from the parabolic perspective is a big hill. Once you start to reduce the centre then the "edge" will appear turned down if you are then using the depressed centre as the guide and will continue to appear "turned down" until you have removed that big hill.

Nigel

[RAC]

You must remember that a sphere viewed from the parabolic perspective is a big hill. Once you start to reduce the centre then the "edge" will appear turned down if you are then using the depressed centre as the guide and will continue to appear "turned down" until you have removed that big hill.

Nigel

Yep and that's what that slider is so good for in Figure XP.

[JRM]

just to update,

   thanks for that Nigel and RAC,

    I have started doing W strokes from edge to edge and stroke to the edge only with a 10" lap, I am doing this in small

   steps (15 minute sessions) so as not to go overboard.

  Ronchi is starting to show improvement with the edge albeit slowly, will continue on this course and keep you updated.

  Thanks

  Rick M

[mapstar]

Keep up the good work Rick.

Hopefully things will head in the right direction.

Damian

[JRM]

Hi All,

        I have been steadily working the glass with 15 minute sessions and things are improving nicely using the 10" lap

with edge of lap at the edge of glass with no side movement, then to keep the cental hill in check,alternating with edge

to edge W strokes going out to the edge.

 this improved the bands moving things ever so slowly so last night I did a session with edge to edge W strokes again with a 1"

overhang and then 1" over hang strokes around the glass, not across, tested with Ronchi and things are looking better.

still have some to go but its definitly improving.

when I fing camera will post a pic .

it takes more time to get things ready and then clean after session then the seswsion itself and sometimes I am tempted to do

a half hour session but have learned slower is better, a lesson learned the hard way sometimes during this journey.

will keep you posted

Rick M

[mapstar]

Hi Rick,

Sounds like good progress you're making on the mirror.

Wise move on the slow and steady and it's painfully slow as you say with all the cleaning up.

Looking forward to seeing the Ronchi images when you post them.

Damian

[JRM]

No figuring today but here are some images, 85 lpi grating  to show you where I am so far...

These were taken with wifes Ipad, works good, the bright light is my light source,

this ipad really picks it up., I took for images to have the light shine back in different

places so you could get to see the full image.

Looks like a little more work on the edge and I need to bring down the center a tad.

but looking much better now.

as always all comments and or advice welcome

Rick M

[mapstar]

Looking very smooth Rick.

Are you still working with the figure XP along side the Ronchi tests? And have you generated a set of matched Ronchi images to compare what you are seeing at different off sets?

Damian

[JRM]

Hello Damian,

    Thanks,I have not used figure xp in awhile just been trying to correct the edge and keep the central area close.

will do another session this evening and see how things look in the morning, if better I will do both the

Ronchi comparisions and a coulder mask test to see where it stands.

Rick M

[mapstar]

I used Ronchi for Windows I think by John Upton? It is quite a good little program for generating the images at different offsets.

Main thing is it looks very smooth.

I've yet to have first light with my mirror as the weather over here has been so terrible.

Keeping my fingers crossed that it goes the right way

Damian

[JRM]

Hi All,

 just to update... I have not done much lately, one sesion on the 10th and the edge started to hook again, re-injured the wrist some what and it

makes for hard to even push the 10" lap so I have decided and I am in the process of aquiring the parts to make  a fixed post grinding / polishing

machine, was hoping to have it setup before the newyear or just after and go from there, it may not be pretty bit it shall be functionable,will definitely

keep you posted....because giving up is not an option

Rick M

[Ajohn]

Yep and that's what that slider is so good for in Figure XP.

Maybe it would be worth stressing that style of testing a bit more and using it right from the start and forgetting Ronchi all together. From my own experience knife edge testing didn't come that easily but as that is all I used on my first mirror I soon got the hang of it. My results got more consistent as time went on so in the end I did have confidence in the results.   Even so I finally checked it several other ways. I even knocked up a Ronchi screen as at the time I couldn't get my hands on one. It can be done with fine wire. Can't say I was impressed and can well understand why RAC uses it for initial work making sure he doesn't over correct. I would be inclined to ask people just how skilled they think they are to make effective use of it to that level. I'd be inclined to think it will take a lot of practice - far more than people are likely to have on their 1st mirror.

The thing I found interesting on my first is just how long I spent chasing those last few fractions of a wave. Useful really because it gave me some idea of what strokes do and how quickly.

A personal view is that I'm pretty sure people would find testing with a knife edge easier if they build in a simple method of changing the size of the source on their tester. That could just be something in front of the LED or what ever with a drilled hole in it. These old 'farts that used pin holes or slits to gain more light might not be as mad as some might think, just from thinking about what is going on when a knife edge is used especially with a mask. Tiny holes will generate strong bright diffraction rings around the holes in the mask or even around pins across the mirror but they also give sharper cut off points at the knife so what is the balance ? I'm probably wasting my time mentioning this but it is worth thinking about.

One of the problems with the mask shown earlier is that it will show up mirror sag so it's not so popular as it has been with a few due to the use of thinner and thinner mirrors.

John

-

[mapstar]

Hi Rick,

Good to have an update although I'm sorry to hear the wrist injury has reared up again to put a halt on progress.

Although frustrating there is always another option and pleased to see you are exploring other avenues with a fixed post machine.

I know they are good for polishing but can create zones if you are not careful to rotate the mirror and vary the lap facets and lap position.

We're all still here willing you to the end. Best of luck and a speedy build of the machine.

Damian

[JRM]

After 2 month I am back at it again.

I did a short 15 minute session on the turntable just to see how it performs and everything seems to be just fine.

used a 5 inch star lap on the outer edge with a 1/4 inch over hang for a couple rotations the moved in to no over hang

for a few, then moved in more until about 1 inch in from edge then back out to the edge again, finished up with a small

W stroke from the edge in 1-1/2 inches and back for 4 revolutions.

Will test later to see how the surface has changed and go from there.

Thanks

Rick M

[JRM]

Hi All,

I have been sick for awhile but worked on the edge when ever I got the chance and after a couple hours work

the edge problem is pretty much fixed and am now all but came back to the sphere, so today I did an hour of

COC strokesa with the 10 inch lap to start the parabol on its way again.

I was wondering about the turn table speed, if I should up the speed to say 6 rpm instead of the 3 rpm  it is

currently running at, would it be better at a slightly higher speed or is the 3 rpm good for figuring.

I am all but out of ceo so soon will be using the zirconium oxide I have, this stuff seems to work slower so

I am thinking turning up the speed wont hurt, it will not speed up the figuring but maybe it is better at a slightly

faster speed, maybe its best to leave as is?

any thoughts on this?

Thanks

Rick M

[Astrobits]

Apart from high speeds throwing all the polishing slurry off the edge, I cannot think of any relationship of figuring with turntable speed. 

I bet that the Chinese optics manufacturers work a lot faster than 6 rpm and have a continuous recycling of the polishing slurry that gets thrown off.

Nigel

[ChrisLX200]

I seem to recall temperature changes due to increased friction (or rather, rate of friction-induced temperature change) might be an issue? Run it faster it will warm up quicker...

ChrisH

[Astrobits]

Yes, the lap heats up noticeably under normal conditions. Extra speed will possibly need a slightly harder lap if the normal one flows too much.

Nigel

[JRM]

okay guys,

I will leave it at the 3 rpm for now.

[JRM]

Hi All,

Ronchi tests showed the curve to be moving in the right direction but with all the center over center strokes

a low zone was begining to show at the 5 inch radius, I guess I should be alternating with a bit of narrow W

strokes now and again, lesson learned, so today I did a 10 minute session of lap edge at mirrors edge

with a 2-1/2 to 3 inch strokes on turntable, then 10 minutes with edge of lap 1 inch in from the edge, stroking

edge to edge there,a 5 minutes session of the lap edge 3 inches from mirror edge again stroking to the edges

and a final 5 minutes with the lap 1 inch outside the mirrors edge with a 2 inch stroke to correct the zone issue

and blend things back nice and smooth, mirror is on test stand for testing tomorrow, will keep you posted.

Rick

[mapstar]

Looking forward to the results of the last session Rick.

[JRM]

The zone issue has been fixed, the central area is still looking flat and the surface is smooth.

will do a half hour or longer session this evening with edge of lap at edge of glass short strokes

turning the glass every 10 minutes or so on the table and see where that takes me, will keep you updated.

Rick

[JRM]

Hi All,

 been working the glass every chance I get, thing appear to be going in the right direction

for now, it sure is slow work with a sub lap, using a 10", slow but steady, will be doing a couple

sessions today.

on another note, I have ordered a dual speed focuser from scope stuff, its a gso I believe and

will do the trick for now, soon have to make the mirror cell and a jig as star testing is not to far

 down the road I hope.

Aluminum poles I can get near here as well, only thing I need to order in now is the  spider and

3.5" secondary holder, that is next on the list, but first to get close enough to start knife edge

testing again, till then it continues.

Rick

 

[JRM]

Okay,

Question

If you were doing a 20" mirror with a 10" sub lap what stroke would you use to

deepen the central 5" radius area without wrecking the outer area of the glass?

I have a hill forming again.

Thanks

Rick