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mapstar

The 22" mapstar mirror

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I'm sure u will crack it after ur holiday. Go have a rest from it and come back ready to go. It dose look like some improvement all the same

Mark

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I'm sure u will crack it after ur holiday.

I hope he doesn't Mark, that would be disastrous

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I hope he doesn't Mark, that would be disastrous

"There's nothing you can do now that can't be fixed.  If you were going to break the mirror you should have done that on day one."

To quote John Dobson :)

James

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Aye no cracks please! :o

The TDE looks like it is moving Damian. Great set of images.

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Might be time to make a mask and take some foucault measurements and enter them into Figure XP.  On the inside of ROC image there doesn't look like a lot of turned edge and on the outside images there is correction getting added in the outer zones. When you take some measurements you will know how turned the edge is as if it's still turned the ROC for that very outer zone will be much longer.

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I agree with RAC Damian. Foucault shadows are very revealing of the whole mirror surface. At the the centre of curvature,

the Foucault tester will really show the edge condition.

A good edge will show a dffraction ring all around the mirror edge.

Slightly brighter on the lamp source side, but a turned edge will show none  on the knife edge side at all. 

Of course the tester is not adequate for final  quantitative measuring of a mirror that size.

Ron.

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I'm by no means an expert at mirror making but between the Ronch and Foucault(with Couder mask and Figure XP) you can create a very good mirror, no other form of testing is needed if those two are used to the fullness of their abilities, fast f3 mirrors included.

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Thanks for the replies guys and the agreements in opinions as they are appreciated and valued.

I've looked at both sides of image's and it is very difficult to see any turned edge inside the centre of curvature where as outside is different.

Raymond is bang on with the correction being added to the outside as I work through the centre so make's it difficult to judge. And modest too after creating such a well crafted piece of glass so please keep up with the replies .

John has said it is still there but through centre work could help it but I may end up alternating between edge and centre work.

My main concern at present is the central hill which is still there

I'm on holiday for a week at present so a bit of a break which will do me good.

I'll get back to it when I return and thanks for looking at the image's

Damian

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As you've always said Damian, central hills are easier to correct that a TDE. Have a well earned break buddy! :)

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If you do use Faucault Damian not squaring up the tester to the mirror can cause some confusion. Vertical comes first. If the source was dead on axis the mirror would throw an image exactly back onto it. Pity a knife can't be used then. You seem to using a vertical knife 1/2 over source and below it set up so the source needed to be slightly above centre and the reflection down below it onto the knife.  The return beam can be caught on a piece of card and this sorted out. You should try and keep the separation from the source as small as you can.  I'd get them vertically aligned as well. Then comes axial alignment. You may be able to do that at least roughly with the same set up. Perhaps a more usual method is to watch to see that shadows in a mask or between pins change equally side to side across the mirror as the stage goes back and forth. Some people use an x-y stage so that at least initially until things get really close they can align it every time a separate reading is taken. That alignment is easier with a stationary source.

Actually I wonder just how much this area can give odd effects with a ronchi screen. Uncle Mel reckons that radial measurements can be taken by looking for a constant line width. So say you get one size at the centre and then move the tester it's now on the radius of the area of the mirror that has the same line width. I don't think anyone really reckons that this is a good way of measuring a mirror but when I look at the shots posted I wonder about some of the changes.  Pass really as I just knocked one up and played with it but only on a sphere and I found other methods were a lot better but in some ways tied up to a fixed source tester. Not the knife exactly on centre though but I feel that may be hard to call unless it's pretty close to a sphere and it takes an incredibly fine adjustment even then with a slit anyway, 40um slit means the image is that size too so the knife has to be put 1/2 way through it or there abouts.

As mentioned earlier - there doesn't seem to be much about on lining up a moving source tester. I maybe initially people could locate the tester on a pin at the back and swing it slightly if needed as readings are taken. 

Accuracy.  People have made mirrors of all shapes and sizes using Faucault and other checks have found them to be equally as good.  Even some famous ones later finally checked with a Hartmann test. One of the major catches as I see it is R. That needs measuring well enough to make the error insignificant. Easy to check with say fringexp. Bigger mirrors in some respects help and faster F ratio's make things worse.  Another which might be significant on some mirrors is the formulae used - it's part of a series. Another term may be needed on some. There are some ideas about on maximum separation between knife and source as well. Pass, just seen and forgotten.

John

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That all sounds properly complicated Ajohn.

I like the way Mr Dobson did it in a youtube video i watched of his 16" mirror. He stuck the mirror in his scope and pointed it at the glint of sunlight on a power line insulator and just looked at the brighter and darker bits of the out of focus image and went back to work on grinding the errors out.  :grin:

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I watch that video and its a very rough way to figure a mirror. The Foucault test is not hard or complicated. Use an led on the same knife edge as you look past and have the lot moving.

It doesn't need to be over complicated.

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A pointless comment really RAC from Nick. No doubt Mr Dob doesn't both collimating the scope before he does it so then it is simpler but still not a reliable way of checking a mirrors figure. Any form of testing needs some form of collimation. I use Mr Dob's method to check any scope I buy - high magnification so that I can see diffraction rings.

John

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The thread seems to have gone to sleep.

Damian, are you taking a break to re group, or are you working away in silence for a while.

Either way, I hope the project is still alive and well, and going well. 

It's a long journey, so rest periods won't harm anything.

Best Wishes matey.

Ron

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The thread seems to have gone to sleep.

Damian, are you taking a break to re group, or are you working away in silence for a while.

Either way, I hope the project is still alive and well, and going well. 

It's a long journey, so rest periods won't harm anything.

Best Wishes matey.

Ron

Hi Ron,

I'm still here battling away with the glass molecules this weekend after a week away last week.

Things are progressing, although not at the pace I would like.It is a long Journey but good things do take time and an update will follow soon.

Thanks for watching

Damian

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all be worth it in the end mate...has crash sorted his out yet?

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all be worth it in the end mate...has crash sorted his out yet?

Cheers Calvin. Crash is still on with his and having his own battle of a different nature.

I've not checked in with him for a couple of weeks now so I'll have to get in touch and see how he's getting on

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I see you are back on the grind again, OK polishing/figuring?

Keep it going Damian, missed all the drama!

Derek

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What an epic journey Damien, just looking at this blog make sense want to have a go, but I know my patience will get the worst of me. It's absolutely amazing and I'm so looking forward to the day when you get first light!

Sent from my iPhone using Tapatalk

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What an epic journey Damien, just looking at this blog make sense want to have a go, but I know my patience will get the worst of me. It's absolutely amazing and I'm so looking forward to the day when you get first light! Sent from my iPhone using Tapatalk

Thanks Sam, it's a fair way off being finished but steadily getting there.

It tests the best of patience and motivation as you have probably read. I have a way to go but will get there and agree first light will be something else especially in a scope built by me too

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The day is getting closer for you to feel those photons touching the back of your eye as you look through the eyepiece Damian,

all those 22 inches of light gathering beauty will be a sight to behold I am sure and will make every momment of this epic

journey worth it ... and then some my friend.

I picked my glass off the bench Saturday to test the wrist, not completely better yet, but close enough, been far to long, I

plan too pour a pitch lap this weekend and get back at it  if all goes well  :)

Cheers

Rick M

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The day is getting closer for you to feel those photons touching the back of your eye as you look through the eyepiece Damian,

all those 22 inches of light gathering beauty will be a sight to behold I am sure and will make every momment of this epic

journey worth it ... and then some my friend.

I picked my glass off the bench Saturday to test the wrist, not completely better yet, but close enough, been far to long, I

plan too pour a pitch lap this weekend and get back at it if all goes well :)

Cheers

Rick M

Hello Rick

Nice to hear your well enough to continue and fingers crossed for you that the wrist doesn't give you any more trouble.

I am getting closer although still working through the centre to get rid of the hill I have.

It may help with the slight turned edge I suspect I still have which I may have to go back to.

Will be good to see another update on your thread soon and I shall be posting one on mine sometime this week I just need to get some time on the glass again.

Damian

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Hi Damian,

I am getting withdrawal symptoms, how is the edge coming along. Any pics? No rush ;)

Hope the weeks holiday set you up again for the work ahead.

Derek

Edited by Physopto

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Hi Damian,

I am getting withdrawal symptoms, how is the edge coming along. Any pics? No rush ;)

Hope the weeks holiday set you up again for the work ahead.

Derek

Hiya Derek

I've been struggling this week for time on it and only managed 5hours of work on it last week.

I have more work planned this aft so will update with the progress images later today for definite

Damian

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