The 20 " grind has begun

[mapstar]

Hiya Rick,

I've not updated my thread but will be doing soon so may be a good few bits to read there.

On the kink be careful not to deepen the centre too much as I have witnessed with the guidance of John just how long it takes to pull back with edge work.

I've found it difficult to judge the Ronchi images and they really can lead you astray. They are good for overall smoothness which is actually the ultimate goal not as though I'm anything like an expert as I've not managed to complete the mirror myself.

It is a difficult thing to do and as I watched John he would work the centre and then onto the 70-75% zone to gently ease the parabola out to the edge without creating zones that would take massive amounts to pull back in.

Without expensive test gear I would look into the way Raymond (RAC) has tested his mirrors and worked with foucault and couder mask to complement the Ronchi test. Ultimately the ronchi will show if the correction is smooth if done with the matched Ronchi test images.

Damian

[JRM]

Hi All,

 Thinking the edge is high I have drawn a 10" circle on the back of the lap and started doing parabolizing strokes side to side with more work on the edge then through center and the circle as my guide to stroke length and width, I did 50 minutes worth last night and a 20 minute session tonight and thing appear to be improving, will have to test again in the morning but according to ronchi my images with 50, 85 and 133 LPI gratings are looking much better and much closer to where they should be, things may be looking up, will keep you posted...

Rick M

[mapstar]

Sounds like it's moving tho the right direction Rick. Watch out for those outer zones forming as they take some smoothing out.

Hope things are still coming along. I have to make an update to my thread and build.

Damian

[JRM]

Damian and All,

      I did a test with a couder mask to see where everything stood figure wise and now know what the hook on the outer edge is.

My edge zone ona 7 zone mask is focusing about 200 thousandth of an inch shorter then the center of the glass...hmmm.

two remedies I can think of are work the inner section of the glass to shorten the radius or work the outer zone to increase the radius,so

I will likely make a small lap and work the outer area of glass and hope not to turn the edge down.

still in the thought process but looking like the option most likely to take.

Rick M

[Astrobits]

A spherical mirror has a longer focus from the centre zones than the edge. There are two ways to parabolize: lengthen the focus of the edge zones or, more normally, shorten the focus of the centre zones.

It strikes me that you have been in the process of shortening the centre zones but are not there yet. I would stick with work in the centre. Use the edge zone as the base reference and adjust the rest of the mirror to that datum.

Nigel

[RAC]

A spherical mirror has a longer focus from the centre zones than the edge. There are two ways to parabolize: lengthen the focus of the edge zones or, more normally, shorten the focus of the centre zones.

It strikes me that you have been in the process of shortening the centre zones but are not there yet. I would stick with work in the centre. Use the edge zone as the base reference and adjust the rest of the mirror to that datum.

Nigel

Totally agree, working the edge to a longer roc will turn bad.

[JRM]

Nigel

         I would have thought working the edge zone would be quicker then deepening the inner zone,I have read where one has used

a ten inch lap to work the zones progressively inward which would make sense to a point. ( will post lik as soon as I find it again)

mind you I am indulging in a few beers as the ball game has ended, way to go blue jays,and my post are usual;ly a mess when I have

a few in me  but hey,. back to mirror talk.,I am working a 3/4 diameter stared lap,( mnd you I can make a 10" easily enough) what stroke

would you recommend?

CoC sounds like the way to go with a tad of a swing say one inch off center maybe more to keep things smooth but what length,

should I be running longer strokes, this is where I get lost so to speak.

you help is always appreciated.
Rick M

[JRM]

okay, here is where we stand...

    stripped of the 15" lap and made a new 10" yesterday.

will start to polish tonight with short central W strokes to deepen the centerl area of glass until I am 3/4 of the way to

where it needs to be in reference to the edge zone, once there will work to push  parabol out to meet the edge.

thats my thinking anyway, and please correct me if my logic is wrong.

Rick M

[RAC]

Have you got a Figure XP image to show us what it currently looks like?

You idea sounds ok but don't spend too long on the center or in any one spot for too long. It's too easy to dig too larger hole or make zones. I would work the center then a bit further out the out more then test.

[JRM]

Have been doing some work and today did a test, here are my latest readings...

The edge is no longer short compared to center, but I get confussed with zone 4 & 6, my

thinking is zone 4 is a depression and 6 is a high zone,correct me if I am wrong please.

thinking if 6 is high I should tackle it with a small lap first.

all advice appreciated

Rick M

[RAC]

So this is what you have if you numbers are correct.

I'm not sure if you use the slider but this is how I would be attacking the mirror. You can't add glass into the ditch at zone 5 in the image above so we must deepen the inside 70% to match the correction outside the 70% mark. Don't run the tool too far over the edge as we don't want any longer ROC out in that last zone. I "personally" would be using a small 3in tool over zone 3 for a bit to take the corner off it and a larger 8-10in tool to smooth out and drop the middle with W's over only the middle out to about zone 4 1/2 but not out to the exact same spot at the end of each push and then a bit over the whole mirror to help keep things smooth. Retest often and work were required. But remember this is only what I would do, some one else may have a better idea.

[mapstar]

Hi Rick

Looks like there's a fair way to go and although I'm not going to offer any advice I'm still here watching and willing you on to the end.

Hopefully you will get your head around the figure XP program and with the help of Raymond above you'll be able to get it near with that.

If your going to use a smaller tool be careful to vary it as you will rough the surface up.

I'm sure there will be some more advice along soon

Damian

[Astrobits]

The only thing I can add to Raymond's post is that I would make several smaller laps ( e.g. 3-4", 5-6", 8" ) as the 10" could be too big for the very steep profile of the final parabola and the 3" might be a bit small for anything other than narrow high zones.

Nigel

[RAC]

And never push down on the lap particularly a small lap. Only enough to keep the lap flat on the glass or you will really rough things up.

[JRM]

okay thanks guys, will make some smaller laps and give it a go.

Rick M

[JRM]

guys,

   thinking the numbers did not match the Ronchi image I was seeing I did some reading and it appears I was doing it wrong.  guess I

was suppose to measure one set of zones first then turm mask 90 degrees to get the other set, pays to read instructions.

so now I have some new numbers

sorry about the mixup, live and learn I say, so this tells me if I am reading it right that zone 3, 4 and 5 need more

work  which is what I have been thinking all along.

Can someone confirm this for me please?

Here is the surface error

btw

I have the radius set at 200 even though my focus was 205 as a sphere, just because I am shooting for the 200 ROC

Thanks

Rick M

[RAC]

Are you doing it like this?

[RAC]

You need to set the ROC at what the mirror is not what you want it to be. Null the middle zone out then measure from the knife edge to the middle of the mirror. That is your ROC to enter into Figure XP.

[mapstar]

Interesting Video Raymond which I've not seen before. I'd love to know what he's imaging it through as it works superb. thanks for posting it.

Damian

[RAC]

I think its just a video camera. My canon 70D with a cheap 70-300 lens works just as well. Some lenses just don't work and some do. I only take measuments by eye though.

[JRM]

this is my mask, if you have a link that gives them all in same orientation please post it, but this 

does work fine once used correctly.

mind you the next one I make will mask the central 10% of glass as well.

but the numbers posted are the correct ones now.

thanks

Rick M

[RAC]

It'll be a lot faster to get the readings and less room for error if you make a mask like the one in the video. Just type couder mask calculator into google.

[mapstar]

Watching with interest Rick

Damian

[JRM]

It'll be a lot faster to get the readings and less room for error if you make a mask like the one in the video. Just type couder mask calculator into google.

I agree completely RAC and will work on it for sure, Thanks

Rick M

[JRM]

what can I say, 

     the zone out near the edge just keeps getting worse so I gave up on round two and I am currently on the way back to a sphere in an attempt to

finally correct the zonal problem, figuring round three will begin once this situation is corrected.

Rick M