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The 22" mapstar mirror


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Hi Derek,

In really simple terms you should be able to see perfectly straight lines across the full face of the mirror. The dark lines show where the beams of light have cancelled each other out. Others, including Damian will give a more definitive answer.

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Hi Damian,

What are we expecting to see exactly when it looks right to you? I'm completely in the dark so to speak with that.

Looks like you will be able to arm wrestle a bit when finished :p

Derek

Hi Derek.

What I'm looking for is the lines inside and outside the radius of curvature to be completely vertical.

When the edge is turned down (as mine is) it shows as the lines bending inwards near the edge when the image is taken inside radius of curvature (ROC) and outwards outside the ROC as mine do.

Here's how a perfect sphere should look inside and outside ROC

post-28847-0-51610800-1426954292.jpg 

And here's a TDE outside ROC

post-28847-0-46520500-1426954742.jpg

As you can see it's just the very edge that's the problem at present. Once I'm to a sphere I can move on to the parabolising process although for a large mirror like this everything takes an age to alter. Judging change is very difficult so you have to stick with your chosen tactic for a while to see the difference be it right or wrong and that's why it's a back and forth testing game

Hope that explains

Damian 

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Hi Derek,

In really simple terms you should be able to see perfectly straight lines across the full face of the mirror. The dark lines show where the beams of light have cancelled each other out. Others, including Damian will give a more definitive answer.

Sent from my Fone

Hi Ian,

Nice to hear from you. I sort of guessed a bit about it, I'm familiar with constructive and destructive light effects, but not with this test. So that was very helpful, thanks. How are you doing? hope all is well and will see you again shortly I hope.

Look after  yourself.

Best regards,

 Derek

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Hi Derek.

What I'm looking for is the lines inside and outside the radius of curvature to be completely vertical.

When the edge is turned down (as mine is) it shows as the lines bending inwards near the edge when the image is taken inside radius of curvature (ROC) and outwards outside the ROC as mine do.

Here's how a perfect sphere should look inside and outside ROC

attachicon.gifimage002.jpg

And here's a TDE outside ROC

attachicon.gifimage004.jpg

As you can see it's just the very edge that's the problem at present. Once I'm to a sphere I can move on to the parabolising process although for a large mirror like this everything takes an age to alter. Judging change is very difficult so you have to stick with your chosen tactic for a while to see the difference be it right or wrong and that's why it's a back and forth testing game

Hope that explains

Damian 

Hi Damian,

Many thanks,

 Looks like you are getting there. I wasn't sure, only a bit of an idea about what you were looking for. Between you and Ian I now get it. It is definitely a labour of love. As I said., again thanks, the pics  help a lot, "a pic is worth a thousand etc etc".

I'll keep watching.

Regards,

 Derek

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The dark lines show where the beams of light have cancelled each other out.

Ian, you are confusing the Ronchi test with an Interference test. There is no cancelling of light due to interference in the Ronchi test which is essentially a variation of the Foucault knife edge test. The bands seen in the test are giving information about the local slope on the surface of the glass, as is the case with the knife edge test. A spherical mirror shows straight bands with the Ronchi test and greys out evenly (nulls) with a knife edge test. Unfortunately the shape of the finished mirror is not spherical, it is paraboloidal.

John

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I would try some w strokes across the whole mirror with no more than an inch over hang using that same lap. Don't worry if it deepens the mirror more than a sphere because you have alot of glass to remove anyway. When I had about the same amount of turned edge this is what removed it the fastest for me and it also it also gets some of the next phase of the job done also so it could save you hours and hit two birds with one stone.

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I think the confusion has arisen because an interferogram can look similar to a ronchigram in that both have a series of bands running vertically across the mirror. A spherical mirror will show straight bands in either test. For a paraboloidal mirror things get a little more complicated. I hope this helps make things a little clearer.

John

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I would try some w strokes across the whole mirror with no more than an inch over hang using that same lap. Don't worry if it deepens the mirror more than a sphere because you have alot of glass to remove anyway. When I had about the same amount of turned edge this is what removed it the fastest for me and it also it also gets some of the next phase of the job done also so it could save you hours and hit two birds with one stone.

          

Good call RAC

John

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Regarding the ronchi test. One of the most confusing things is that a turned edge looks just like too much correction. Your brain would tell you that a turned edge would kink the lines the other way not the same way.

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I would try some w strokes across the whole mirror with no more than an inch over hang using that same lap. Don't worry if it deepens the mirror more than a sphere because you have alot of glass to remove anyway. When I had about the same amount of turned edge this is what removed it the fastest for me and it also it also gets some of the next phase of the job done also so it could save you hours and hit two birds with one stone.

I have been told this already Raymond as larger mirror's respond differently to the smaller stuff, I have tried again today to remove the offending edge using chordal strokes with the centre of the lap 2" from the edge, it has changed very little

Inside ROC

post-28847-0-42694600-1426964881_thumb.j

Outside ROC

post-28847-0-02309700-1426964909_thumb.j

So with a view to possibly sort the edge I will have a re-read of some info then go with the W strokes. Thanks for the comments guy's

Damian

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Saying it's not changed much I have a nice little hill forming in the middle just to explain why the line's aren't straight in the central area. This tallies with being opposite to the lines at the edge inside and outside ROC

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Well hi all,

Thanks for you input.

 Just spent a productive half hour or so on this website.

http://www.atm-workshop.com/ronchi-test.html

Very informative. So now I understand what you are doing a bit better. It is a test based on reflected light/shadows through  a grating from a pinhole or a grating parallel to the one being viewed through. Nice idea. The Foucault test is a bit different. Not sure as yet what the knife edge does. I'll figure it out.

I did see the effects of diffraction ( producing a double image in the Ronchi test). My original optical and atomic physics was more to do with lasers, so this is all new to me. It's nice to learn something new. What a really great hobby this is! :p  

Derek

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Hi Damian,

You may think me nuts but here goes, please don't shoot me for this. Your pictures are great showing the mirror. Also the Ronchi test results are very interesting to see.

But !!!

How about a short video of you actually figuring/grinding the mirror. It is not for me, although that would be informative. It is just a thought that as you seem to be getting a bit disheartened or fed up with things not panning out. Maybe John could see a possible problem with the method you are using, I.e. Stroke shape. As I say don't shoot me, it took me some time to understand what a "W" stroke was. It is just a possibility as time has passed, that you have slipped into a slight methodical error. (if that is the correct word). I have done the same in other endeavours in the past without realising it. It may, may just save you pulling out some hair.

I'm receding so I wouldn't want that for me ;)

Derek

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Hiya Derek

Everything is a good idea so no shooting will occur.

I was disheartened a couple of week's ago, but spirits are quite high at present and yes things don't work out sometimes and that's where more advice and effort are required.

I knew it was going to be hard from the start as John had said but it is doable.

I've been thinking it may be technique but it could also be other issues that have been at the back of my thought's

Raymond has made a good suggestion which confirms what John had said about large fast mirrors so after a few other bit's tomorrow I will continue.

I also know your not nuts as you've not started mirror making yet ;-)

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Damian,keep up the good work it will be well worth the effort you are putting in,just think you are going to have a b****y big  light bucket when your finished and the views you will get at Galloway will be stunning,when do the tickets to view o on sale? :Envy:  :Envy:  :Envy:

Hope your enjoying your beer,or have you been corupted by Derek :Envy:  and gone for a whisky?

Regards

Mike

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Well now there's a thought!

Been at a loss for something to annoy the wife for a while. Maybe a mirror on the newly finished dinning room oak floor. Better still in the middle of the living room. That should put a stop to Coroation Street.;)

I'm sure she would be right behind me on that. Probably with an iron poker!

Once I start, all ...you know what I mean.., breaks loose.

Last good idea ( instigated by wife, hasten to add) to keep me occupied was astronomy, now several shekels poorer.

Me, very happy, wife Err well.

Derek

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