The 22" mapstar mirror

[crashtestdummy]

Yes I will update my blog in the next week.i still have a slight middle problem but not as bad as before but I'm back to the tile tool again doing short w strokes as I have my edge turned down.was good to see you again fella and glad of the new flag to grind on.hopefully retest my mirror end of next week to see if the edge has improved

[mapstar]

Yes I will update my blog in the next week.i still have a slight middle problem but not as bad as before but I'm back to the tile tool again doing short w strokes as I have my edge turned down.was good to see you again fella and glad of the new flag to grind on.hopefully retest my mirror end of next week to see if the edge has improved

Good to see you too Rich and have a catch up on everything. I'm sure you'll sort your mirror problems out quickly now you have the motivation back to continue.

Finding the time is difficult but hopefully the plan you have will make short work getting it back on track and ready for the next season. 

[mapstar]

Onwards with a little update as I have been a bit absent of late so apologies to all.

I have managed a further 8 hours work on the mirror moving further to the edge. This seems to have amplified the edge problem  , rather than making it better, so I have now moved back towards the centre with the occasional W strokes to smooth everything down and hopefully improve things again only time will tell so I have a couple of days to see how things progress and I should be able to update with a few images

Damian 

[Soupy]

Can't believe you are going backwards Damian... Did I get the post right?

Eh up Rich... Hope all is well

Edited April 14, 2015 by Soupy

[gooseholla]

This may seem a silly question but I don't really know much about glass and mirror making. Is there ever a point where a piece of glass becomes unusable as a mirror, or do you never take off enough material for that to happen? What effects does it have having to take further bits out of the centre or the edges? Presumably the focal length/ratio is changed? I guess what I am asking is: is it a process that can go on forever on the same bit of glass and achieve the desired result you set out to achieve?

John

 

[Soupy]

I think the polishing is only removing tiny amounts of glass at a time John... And it is on the scale of a few atoms I believe, so it can be polished quite a bit before the glass will get noticeably thinner, or unusable. Once Damian has it all to the right spheroidial shape he will then figure it to a parabolic shape. I think I am right

[mapstar]

This may seem a silly question but I don't really know much about glass and mirror making. Is there ever a point where a piece of glass becomes unusable as a mirror, or do you never take off enough material for that to happen? What effects does it have having to take further bits out of the centre or the edges? Presumably the focal length/ratio is changed? I guess what I am asking is: is it a process that can go on forever on the same bit of glass and achieve the desired result you set out to achieve?

John

Hi John 

Not a silly question at all and quite a good one 

The amount of glass removed is measured in thousandths of a millimetre or fractions of the wavelength of visible light. You will often see measurement's like a 1/4 wave or 1/8th wave quoted this means that the reflected light can be up to that much out of phase with the other light reflected thus the image is not quite as sharp. There's a lot more to it than that as you have to get that light to focus to a single point (if your lucky) which is where shaping the mirror into a parabola (called figuring) has to be done. This can be done to a matched Ronchi image using your intended mirror parameters and a program to generate the image's you should see.

Of course this is how the amateur will do it there are many other tests that are used to improve on this which are often called null test's. 

A mirror will become unusable when it get's too thin and thus not capable of holding the shape (figure) you have created on it so the image's will not form to a sharp image referred to as Astigmatism.

I think I'm safe that it would probably take 200years at this rate for me to get to that stage although I think it may take me that much time to get rid of the turned edge!

Damian

[Soupy]

Excellent description Damian. Couldn't have said it better myself

[Ajohn]

I have a book by a Scottish  optician that did work for nasa and a number of universities in the USA and some here I  believe. He discusses making rather large by our standards very fast mirrors. His comment on figuring is that he works out the depth of the glass that has to be removed. If more than 0.001in he grinds rather than polishes it away.  Sounds like a good route to apoplexy to me but how else could very fast mirrors be made. Using conventional grinding and polishing gear he grinds and polishes the entire mirror  to some radius. Then fits a smaller tool and grinds and polishes the centre to another rad and then blends the whole thing in. In terms of very high precision mirrors he reckons F4 is around the limit. I'd guess his decision on the 0.001 would also depend on where it is and how much glass has to be removed.

The fact that things change though the whole process is interesting. I've abandoned a design I came up with because I couldn't hope to make some of the optics in it. I came up with another recently that uses mirrors and bought lenses that aren't too expensive. Looked great until I toleranced the parts I would have to make. Rad of the main mirror to better the 25mms and a lot less than that on the second mirror. It's a slightly modified version of a corrected Dall Kirkam that's about on the web - sort of Tak Mewlon with the corrector built in rather than having to be bought separately.  Great but months to make the bits and if a rad goes out of spec ......................... So I've decided on an F5 10in Newtonian plus coma corrector.  The fact that it might turn out not to be exactly F5 doesn't matter.

I don't want to hijack the thread but a question that might relate to anyone reading this thread. Does anyone know of places where I can buy cerium that hasn't been shovelled into a plastic bag by some one who also sells grits from a shed etc? I have just ordered the mirror blank of ebay - pre milled for F5 -   must mean great minds think  alike. Just wish it was a bit thicker. Also a ronchi screen off the same source mostly out of curiosity. Next will be some bits of brass for a tester.

John

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[mapstar]

AJohn

Let us try to keep this thread on topic folks.

Anyone with their own projects running, make a thread of your own and don't post in this one.

I'm sure Damian would prefer continuity, with no interruptions.

Ron.

As Ron quite rightly put 

Thanks

[jetstream]

Damian, keep up the good work on the mirror. The thread is very informative- thanks.

[Ajohn]

AJohn

As Ron quite rightly put 

Thanks

Thanks for your help Damian. Most of the post did relate to the question about glass removal earlier.

John

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[mapstar]

Damian, keep up the good work on the mirror. The thread is very informative- thanks.

Thanks Gerry for the encouraging comment. Much appreciated

Damian

Edited April 14, 2015 by mapstar

[gooseholla]

Thanks for that. Always interesting to learn how things are done. Maybe one day i'll get to see someone grinding one in person!

John

 

[mapstar]

Thanks for that. Always interesting to learn how things are done. Maybe one day i'll get to see someone grinding one in person!

John

To be honest it's not a very good spectator event as once you've seen the strokes and a few laps around the table the excitement is over!

I suppose it's interesting to watch the progress speeded up and their are many video's to watch that show this.

Most of them relate to powdered table's and spinning mirrors as the old laborious technique of hand grinding has all but disappeared (as is shown in my thread and a few others on here) due to the vast amount of work and time required to do it.

[Soupy]

I could always set you up a time lapse Damian...

[saac]

mapstar  - I've been admiring your Ronchi images over the past few pages and the progress you've been making.  This is going to be a daft question but here goes - how do you know when to stop?  Are you looking for the interference lines to run parallel to each other with equal  gap between each successive fringe?  

[nicks90]

Damian,

at what point do you mask off the outer 1/2" of the mirror to 'fix' you TDE? Have you set an maximum limit on man hours, or is that not an option?

[mapstar]

mapstar  - I've been admiring your Ronchi images over the past few pages and the progress you've been making.  This is going to be a daft question but here goes - how do you know when to stop?  Are you looking for the interference lines to run parallel to each other with equal  gap between each successive fringe?  

Thank you for the compliment, progress is regrettably slow and picking out any difference in the image's is quite difficult.

No such thing as a daft question by the way.

At present my first goal will be to remove the turned edge as this is the starting point to which the rest of the surface has to take it's reference point from to create a parabola. The classic way of creating the desired shape is to work the surface to a perfect sphere which is shown in the Ronchigram as straight lines top to bottom right to the edge in the test image's, whether this is inside or outside the radius of curvature (sometimes called the centre of curvature of focus point) Then to move onto making the parabolic curve using a set of matched image's generated by a computer program using the details of the mirror (diameter, focal length) then final test's which I should come onto later.

The lines are not interference lines such as those used in an interferometer, but the principle behind the Ronchi test is the same as the Foucault Knife edge test, the Ronchi screen or grating presenting many knife edge's to the reflected light. The foucault test is very accurate, but hard to quantify. Both are good at showing the surface smoothness and overall shape, a description of both the Foucault and Ronchi test can be found here:-

http://www.atm-workshop.com/foucault.html

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

Damian

[mapstar]

Damian,

at what point do you mask off the outer 1/2" of the mirror to 'fix' you TDE? Have you set an maximum limit on man hours, or is that not an option?

Hi Nick

I did it just as an experiment the other day to see what I'm dealing with as I know sometime's telescope makers do this as a final option, I've even read of the mirror being turned upside down on a piece of plat glass and the edge flatted off then painted to eleviate the edge, seems a bit extreme   and not something I'll be considering

The limit I suppose will be when I get fed up or think it is futile but I am stubborn so it's not an option 

[Cath]

Don't give up Damian.

You'll probably not do this again, at least not without thinking about transferring a lot of the required time into making something that can do most of the work without you doing it.

[barkis]

You could polish a mirror for years before it got appreciably thinner to any degree.

Actually, the School is still out on whether the glass molecules get removed at all,  

or just merely pushed  around.

Damian won't quit, I think it's quite obvious the man has grit and determination. If anyone sees a Pun there,

it wasn't an intended one.  He could take a break, which shouldn't shock anyone. After all, many makers of mirrors of

this size would have used a grinding and polishing machine, It's damned hard work pushing glass around.

We talk of microns, and could assume  'Oh, it can't be that difficult then'. WRONG.

It's the getting them in the correct place that brings the difficulty, and the larger the mirror,

the more difficult it becomes.  It's largely why Newtonian, and compound  telescopes are so expensive.

There are cases of potential mirror makers not even getting started, but it's rarer to find many who 

don't finish what they start. It may seem a glamorous thing, but not a walk in the park either,

and this one could be described as Herculean. He's a braver man than me, even if I was 50 years younger .

Ron.

[mapstar]

Don't give up Damian.

You'll probably not do this again, at least not without thinking about transferring a lot of the required time into making something that can do most of the work without you doing it.

Don't worry folk's "give up" is not in my Vocabulary but thank's for everyone's support throughout the thread 

[barkis]

"What's with the Negative Waves"

To quote (Vernon L Pinkley), Donald Sutherland in the Movie 'The  Dirty Dozen'.

Damian needs  inspiration from us, and hopefully we can give it.

[gooseholla]

Don't worry folk's "give up" is not in my Vocabulary but thank's for everyone's support throughout the thread 

It used to be the only thing in mine, but somehow I managed to build an 18" scope without giving up! Power of support!

Can't wait to see it finished in all its glory one day. Maybe one day I'll bring my tiny scope up to Yorkshire