The 22" mapstar mirror

[mapstar]

More work today and yesterday to the tune of 5 hours with the W type parabolising stroke

It's amazing the amount of thought's that are provoked in people reading this and I had a pleasant chat over the phone earlier about the thread, progress and Ideas on what can cause edge problems. Ajohn also mentions a few things above and must have needed a stiff drink after re-reading the 13 pages again. Mind you I also think my caller probably needed one too 

Here are the image's 

Inside ROC

Outside ROC

Over all there is still a hill in the middle which is good and I think a slight change to the edge although the stroke gives this minimal work. More tomorrow fingers crossed and see how things progress 

Damian

[Physopto]

Ay a gin and tonic,

Well maybe more than one. Looking good Damian.

[Soupy]

Looks like another slight improvement Damian. Keep it up buddy it's amazing work and will produce an amazing mirror

[gooseholla]

I'm no expert, but I can definitely tell those lines are straighter than a few days ago! It's amazing the amount of work that goes into making a mirror. I'm glad I paid someone to do it though.

John

[Mark62]

It's really coming along. They do look straighter let's hope ur over the worst and things carry going well

Sent from my iPhone using Tapatalk

[mapstar]

A small update today as I managed an hour yesterday and today will be taken up by work.

I returned to working on the edge but could see no difference in the images apart from I'd started to create a hill in the middle again. My mind wasn't fully on the task yesterday so I decided to call it a day and hoover the grass!

The edge showed no improvement as can be seen 

Inside ROC

Outside ROC

I'm going to attempt more edge work tomorrow and after 2 hours test again. If the edge does not improve then I may need different tactics 

Damian

[Soupy]

Like you said last night Damian, parabolising it might work, and I know you have already tried this but you never know.

[Astrobits]

The fact that you are generating a hill in the centre indicates that you are removing glass from the outer areas. You are going in the right direction.

Keep it up.

Nigel

[JRM]

I'm going to attempt more edge work tomorrow and after 2 hours test again. If the edge does not improve then I may need different tactics 

Damian

Damian,

What are you using to test, the 133lpi ronchi?, if so can you do a couple shots with  a lesser one say 85lpi?

it should show less difrraction and give a clear picture of the edge,just a though... unless of course you are

using the 85.

Rick

[mapstar]

Damian,

What are you using to test, the 133lpi ronchi?, if so can you do a couple shots with a lesser one say 85lpi?

it should show less difrraction and give a clear picture of the edge,just a though... unless of course you are

using the 85.

Rick

Hiya Rick

I'm using the 133LPI grating as I don't have an 85. I do take measurement image's closer to radius of curvature although post the same image's just for comparison.

It would be nice to see it clearer but there's an edge problem so no two ways about it I need to get rid of it through one tactic or the other.

I'll do more work tomorrow and see how I go. There is a lot of glass to remove to parabolise it so was hoping that a bit of deepening would take glass off lowering the overall surface down to the edge.

I think it's too far away to do that as yet so will carry on with probably a mix of what I've been doing before as it seemed to be working albeit very slowly which is no worry so long as I get there.

Just patience and time and I'll get it sorted although time is drawing in

Damian

[Soupy]

Damian the only time constraints are your own. It will be ready when it is ready. Its your mirror and scope not something you're doing for someone else so take your time. There are lots of people here who are willing you on and know you will get there!

[Mark62]

Damian the only time constraints are your own. It will be ready when it is ready. Its your mirror and scope not something you're doing for someone else so take your time. There are lots of people here who are willing you on and know you will get there!

Could not have put it any better

Sent from my iPhone using Tapatalk

[mapstar]

Yeah your right guys it will be nice to see a change for the better.

Tomorrow will tell as I've plenty of work planned

[Ajohn]

Texereau shows a demo of a ronchi using 5 lp/mm but only 4 lines across the mirror using a 40um slit. From the photo diffraction doesn't look to be much of a problem, it's a 125mm diameter with a 2m rad and a turned down edge. The faults are pretty evident including surface finish. This area is a bit of a pass  for me. I couldn't get my hands on one easily on my 1st mirror so knocked one up using fine wire but wasn't impressed as his method actually measures the problem.

Damian could use the same method. The knife would be down below the led but that doesn't matter. Just fasten the knife blade 1/2 over the LED, find something shorter to form the other half of the slit and adjust it by eye with the led on - just look for a narrow parallel slit. Put that some way in from the ROC of the mirror and fringes will be seen. These need to be straight too but the advantage is that the first fringe will be 3/4 of a wave from the knife edge. He also uses this to set the slit width. A very fine slit will cover half of the mirror with fringes and this will also give strong diffraction effects. If the slit is widened so that only 2 or 3 are seen the test can still be accurate and the diffraction effects will be minimised. You hope. Adjusting it isn't as bad as people might think.

The radius of curvature can be measured roughly with a torch or led etc. Wet or oiled when only fine ground. Wave the light around at the side of your eye and look for the reflection back from the mirror. Probably best to start too close. As this moved back the direction the reflection moves  will reverse as it goes past the centre of the ROC. Hone in on the position so that the mirror looks to be full of light and slight movements of the light makes no difference.

Lining a tester up is fairly easy but if it's truly slitless it would be best to put something thin centrally over the led with a small hole or rough slit etc in it.  The mirror will reflect it back and the position can be found with piece of card. Move things around / tilt the mirror until it's where it should be. A slit and knife edge are handy again here - focus a loupe onto the knife edge and adjust to bring the return image sharply in focus with the knife edge about 1/2 way across it. An eyepiece used backwards should make a decent loupe.  25mm will give about 10x. With source stationary set up there is no need have the knife cut into the beam as the knife can be moved independently.

Lining up towards the mirror is a bit more tricky but the return image should remain half masked by the knife through a significant amount of movement. There is a need to worry about this  when shadow style testing is used during figuring. Some people add an x traverse to keep the shadow generation even when source and knife move together and only worry about getting it exact when things are nearly finished. I've wondered about using a camera macro slide/rail for this and making the knob bigger to get fine adjustment. It's not so much of a problem with stationary source as the knife edge is usually swung into the beam via mounting it on an L shaped part. Probably all difficult to understand but his rig is shown here

http://astro-foren.de/index.php/Thread/4323-Foucault-Test-Einrichtung-nach-Texereau/

a better shot of it here

http://www.iucaa.ernet.in/~scipop/Literature/hbt1/fig17000.jpg

The basic idea is that the sliding part runs on 2 V's formed by metal plates running on a piece of bar or tube supported by a screw that runs on something slippery. (I just used a bit of metal rather than glass) That screw is used to move the knife edge but the same set up can be used for source and knife moving together. The table usually needs a bit of weight adding to keep it stable and another one can be mounted on top of it to give x-y movement if needed.

I'd guess source and knife moving together is the most precise method but the fixed source is probably easier to set up and allows the amount the knife cuts into the beam to be easily adjusted. Trouble is it tends to finish up with a wide separation between the source and knife and that needs keeping down to a minimum really. I have seen figures given based on % rad of the mirror but I'm sometimes dubious about info on the web, i've seen a photo of a pro set up used to test mirrors up to and above 60in dia. Slit and source move together and I would estimate were about 6mm apart. It was mounted on a lathe top slide giving X-Y adjustment and modified to read to 0.0001in.

John

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[Soupy]

John I hope others can understand all this and I am sure it makes sense but you have totally lost me!

[Ajohn]

There are toooooooooo many video's on youtube about mirror testing and making the bits for that and even the mirror. I think there are a few purely on the Foucault test too.  They should aid understanding. There are some good ones. There should also be examples of fixed source and the other type of tester.

 I can't write a book on here. Texereau wrote 2 books. How To Make A Telescope and the 2nd edition of the same book which as far as this thread goes doesn't add anything useful. The book is fairly old now but explains Foucault testing and mirror making better than any other source I am aware of.  There was a copy of the 1st edition on the internet archive. Like many famous books it's been bought up by some one "William Bell" I think and so now any ebook copies are removed. It mentions Ronchi and some other tests but doesn't really rate it as an accurate test - as some one on here has already mentioned perhaps the best option for making a very accurate mirror is Ronchi followed by Foucault. There are other tests as well that use additional optics but like most things there are catches. Perhaps the easiest way of getting to grips with Foucault testing is to do it once the basic idea is firmly in the head. It sounds a lot worse than it is  in practice.

This page and any links off it might help but I don't know of any web source detailing fringes ahead of the knife edge

http://www.telescope-optics.net/foucault_test.htm

I've come across another twist on Foucault but it's past my bed time so will add it later.

John

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Edited March 29, 2015 by Ajohn

[barkis]

I sold my Texereau book. All I have now are NE Howards Handbook for Telescope Making,

and Albert Ingalls  Volumes 1, 2, and 3. Old, but crammed with useful stuff. 

Modern methods have long overtaken them, but one can still home produce good optics from them.

Ron.

PS.

I'm sure Damian won't need to attempt the following  .

A little humerous drawing from Ingalls book 1.

[Soupy]

Ron and John thanks for trying to help us non-glasspushers to understand a very intricate process but I think I'll leave the mirror grinding and figuring details to experts such as yourselves, Damian, and the other very knowledgeable glasspushers on here I'll just enjoy the thread all your posts and the progress it shows Damian is making, and I am sure when Damian finishes the mirror I'll get to have a look through it (him being one of my best mates an all that)

[Physopto]

Hi again Damian,

Well if you haven't see it go look it is a real eye opener.

Telescope Building with John Dobson (1:28:30)  in SGL.

You were annoyed at a small chip on the edge of your mirror, well NUTS is all I can say!

He proves you don't even need a sledge hammer.  Forget all this rubbish about Ronchi and Foucault tests. Ordinary tap water will do. Mind you, you will probably need to move to America for the sun shine to do it.  Hummmm maybe not such a daft idea!

Regards,

Derek

[JRM]

Hi again Damian,

Well if you haven't see it go look it is a real eye opener.

Telescope Building with John Dobson (1:28:30)  in SGL.

  will have to go and spend an hour and a half watching this then

Thanks for the info Derek.

Rick M

[mapstar]

I sold my Texereau book. All I have now are NE Howards Handbook for Telescope Making,

and Albert Ingalls  Volumes 1, 2, and 3. Old, but crammed with useful stuff. 

Modern methods have long overtaken them, but one can still home produce good optics from them.

Ron.

PS.

I'm sure Damian won't need to attempt the following  .

A little humerous drawing from Ingalls book 1.

Pit Pic.jpg

Thanks for the comments Guys 

I think the thread may need a little more humour Ron and had a little chuckle as I'm sure the pic could be turned to represent the edge problem I have.

I too have the Texereau book and there are some interesting and relevant points and technique's in it. As you state though Ron some of it woefully out dated and it made me really laugh a bit on page 13 stating that an 8" can be fully exploited on a simple alt az mount weighing 40lb or less where as a 20" would need a more substantial mount weighing 2 tons . I'm sure there are quite a few dob owners on here that will feel mighty strong knowing that's how much their moving everytime. 

Soupy I hope you were using the term expert very loosely there especially seeing as I'm about as far as M31 away from that 

Derek I've seen the John Dobson video and to be quite frank for someone held with such esteem in the astronomy community it is quite shockingly rough 

Battle has recommenced with the edge so an update later guys. 

Back to the lap

Damian

[mapstar]

Ron and John thanks for trying to help us non-glasspushers to understand a very intricate process but I think I'll leave the mirror grinding and figuring details to experts such as yourselves, Damian, and the other very knowledgeable glasspushers on here I'll just enjoy the thread all your posts and the progress it shows Damian is making, and I am sure when Damian finishes the mirror I'll get to have a look through it (him being one of my best mates an all that)

Wondered what all those beers and clouds we've shared were all about 

[Physopto]

Rough ain't quite the word I would have used  but fun to watch!

[Soupy]

Wondered what all those beers and clouds we've shared were all about 

We've been sharing beers and clouds since well before the monster mirror... I might be shallow but not that shallow

[JRM]

Battle has recommenced with the edge so an update later guys. 

Back to the lap

Damian

Battle on Damian, thinking it will be atleast june before I can return to it, really sucks but I am reading everything you post,

I just watch the John Dobson video, a bit rough yes but bold and to the point, no sense beating around the bush I always

say but heck I would never want to carry a tubed 20" around, thank goodness for the truss tube design.

Rick