The 22" mapstar mirror

[mapstar]

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

Thanks John and we'd make you more than welcome.

[Soupy]

Indeed John and you can put it in a line with midday of our even smaller scopes

[Soupy]

Indeed John and you can put it in a line with midday of our even smaller scopes

I meant many... that's auto correct for you!

[sammak127]

Good luck Damian, an intreaguing journey it has been to follow.. What a feeling it will be when it is finally done.

[saac]

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

Damien,

Thanks for that - I appreciate your time in answering.  I started my own mirror project about 2 years ago, modest 12 inch.  I got to the stage where I was ready to start polishing - started reading a bit on the testing using ronchi and foucault but I ended up getting distracted on another project (interferometer to use in school physics class).  I tested a couple of commercially made mirrors which tended to produce interferograms with straight fringes  - I had no idea really how to interpret them though beyond what the software was saying.    Looking through your posts has encouraged me to pick up on where I left off.   I've got  a lot of reading to do ahead of me,  thanks again and good luck with mirror.

Jim

[mapstar]

Damien,

Thanks for that - I appreciate your time in answering.  I started my own mirror project about 2 years ago, modest 12 inch.  I got to the stage where I was ready to start polishing - started reading a bit on the testing using ronchi and foucault but I ended up getting distracted on another project (interferometer to use in school physics class).  I tested a couple of commercially made mirrors which tended to produce interferograms with straight fringes  - I had no idea really how to interpret them though beyond what the software was saying.    Looking through your posts has encouraged me to pick up on where I left off.   I've got  a lot of reading to do ahead of me,  thanks again and good luck with mirror.

Jim

No problem Jim and really glad you have decided to go back to it   Maybe you will post about it here as there are lot's who will encourage and motivate you. No mirror is modest if your making it yourself either and I hope you will see it to the end.

It's been a fair while since I updated the thread (nearly 3 weeks) and although quiet I've not been resting on my laurels. A further 35 hours work in total have ensued since to try to turn the edge around.

Here are the shots as I progressed trying to remove the turned down edge.

The middle is a complete mess but can be sorted later. Not sure how much more edge work there is but it looking better. I shall post a series of images how it is now moving through from inside to outside 

Damian

Edited April 28, 2015 by mapstar

[mapstar]

Here's the series of images as I move from inside centre of curvature to outside 

The central hill is quite large (think olympus mons) and you'll probably get the picture of how much time it will take to smooth it off finally

Damian

[gooseholla]

Man, that's some 60s Dr Who intro going on there! So is the edge sorted now?

John

 

[Soupy]

The edge is def on the move Damian. As for the middle... Well that just looks weird, but you already told me that and I know you can get that to where it needs to be, so well done on the progress, the end is getting closer.

[mapstar]

Man, that's some 60s Dr Who intro going on there! So is the edge sorted now?

John

Yeah I suppose you could say that John

The edge need's more work as the lines still turn at the very ends so I will continue

I am hoping to have the edge sorted before I go away for a week in early may I can then come home and start the figuring process.

Damian

[Mark62]

You have been really busy Damian. It's nice to see how it's coming along and it's looking really good. Good to see you have not lost ur motivation. Keep up the good work

Mark

[Stu]

Well done Damian!

Must be such a relief to see it moving in the right direction finally.

I've been silently cheering you on from the sidelines!!

Keep at it :-)

Stu

[Uplooker]

Well done Damian, looking good. I found it possible to see the difference in the photos by using the "Page Up" & "Page Down" buttons to scroll through the images quickly, you can really see the difference in them.

Really good news that your mojo is still intact and progress is being made. The time away in May will really help things as you will come back invigorated.

[mapstar]

Well done Damian!

Must be such a relief to see it moving in the right direction finally.

I've been silently cheering you on from the sidelines!!

Keep at it :-)

Stu

Thanks Stu

Good to know I've not bored everyone stupid yet with the barrage of Ronchigrams but the work continues and yes it is a big relief that it's finally heading in the right direction.

[Stu]

Thanks Stu

Good to know I've not bored everyone stupid yet with the barrage of Ronchigrams but the work continues and yes it is a big relief that it's finally heading in the right direction.

Not at all. I think I'm finally beginning to understand them now!! I was confused above what a 'TDE' looked like in the Ronchi despite your explanations. With the improvements I can really see what I'm looking at now

[mapstar]

Well done Damian, looking good. I found it possible to see the difference in the photos by using the "Page Up" & "Page Down" buttons to scroll through the images quickly, you can really see the difference in them.

Really good news that your mojo is still intact and progress is being made. The time away in May will really help things as you will come back invigorated.

Cheers Ian

As Stu says it's difficult to see and explain changes and that's why I posted the series of image's above which give a good indication. I didn't want to keep posting what looked like the same image's until there was A significant change.

The lines move in a very subtle way and probably not how you would expect them to?

The main problem from the point of the person doing the mirror (one this size) is that it's about 3 hours work before any discernable difference can be picked up, so if it's the wrong direction there's a lot of time wasted.

I'm getting there and progress is glacial, but the week away from it will be good as you point out.

Edited April 29, 2015 by mapstar

[dark star]

I really admire your staying power!

I have nearly finished a 14 inch mirror which has taken around a year. The truss dobsonian mount for it is taking a long time to finish and will probably take another few months to get right. I am about to start grinding a 20 inch mirror (I aim to start this weekend having made a very heavy duty grinding table).

I hope to show the same determination as you!

By the way, I have found it impossible so far to take a photograph of a Ronchi or Foucault test that shows anything useful at all, but will try again at some point.

David

[mapstar]

I really admire your staying power!

I have nearly finished a 14 inch mirror which has taken around a year. The truss dobsonian mount for it is taking a long time to finish and will probably take another few months to get right. I am about to start grinding a 20 inch mirror (I aim to start this weekend having made a very heavy duty grinding table).

I hope to show the same determination as you!

By the way, I have found it impossible so far to take a photograph of a Ronchi or Foucault test that shows anything useful at all, but will try again at some point.

David

Hi David and many thanks for the kind words

If you're about to finish a 14" then you definitely don't lack staying power or determination in my eye's and taking on a 20" afterwards galvanizes that thought.

Anything worth doing take's time so there is no need to rush.

Personally I have found mirror making up to now much more challenging and demanding than the scope building side.

I have had many a moment where my willpower has been severely tested, and without understanding support I wouldn't be here now as probably many others would agree with.

On the testing front I use a simple Samsung L100 instant camera to take my image's.

Settings are Scene and night mode and because I test in my house with floor boards it's difficult to get an image with the line down the middle. Everytime I press the shutter the camera moves! Would be better with a remote cable but there's no facility for that.

I usually set the camera with a two second timer and actually move one leg of the tripod whilst it's timing out! Sounds quite heath Robinson but it does work.

Is the table a powered one or will you be hand grinding?

Damian

[Ajohn]

If your thinking the powered grinding machine way Damian for this or other projects there is a lot of information on this mans youtube video's on all sorts of things.

https://www.youtube.com/user/GordonWaite/videos

He uses a fixed post machine - no stroke arm. Much simpler to make. Using them is a mix of hand and pure action of the machine depending on what's being done. The bearings for mine arrived today - working in the house and not much space so a sensible way to go.

Out of interest it looks like dental paster is dental stone in the UK and easy to get. I think another goes by the name of hydrastone but I can't find it.

John

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[RLWP]

Damian,

I don't have anything practical to add, I would like to count myself as one of your admirers for your tenacity and willingness to share what you are doing. Thank you, it's very inspirational

I think I might have a go, perhaps on a 1" diameter mirror - or is that too big a project?

Richard

[RLWP]

Out of interest it looks like dental paster is dental stone in the UK and easy to get. I think another goes by the name of hydrastone but I can't find it.

John

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Hydrostone is a modelling plaster: http://plaster.com/HYDROSTONE.html

Richard

[RAC]

I would like to see what would happen to the edge if you were to fix the center by doing long straight stokes from one side to the other with a 1/2 inch or so offset from the center and an overhang of NO more that an inch either side over the edge. You may need to work just the center with no offset for some time to get the bulk of the hill down.

To me you must be making this turned edge as fast as you are trying to fix it.

[Ajohn]

I've been wondering that for some time too RAC and suggested periods fixing the centre which should be short earlier, also not working with the tool edge past centre when trying to fix the edge plus not much overhang when doing that. This will still leave a hump in the middle but not an extreme one which could turn out to be difficult to fix.

However from comments I shouldn't post in this thread so over and out. It's just difficult to resist as it seems to me that Damian has spent more that sufficient time to finish the entire mirror even accounting for figuring for the first time.

John

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[mapstar]

I would like to see what would happen to the edge if you were to fix the center by doing long straight stokes from one side to the other with a 1/2 inch or so offset from the center and an overhang of NO more that an inch either side over the edge. You may need to work just the center with no offset for some time to get the bulk of the hill down.

To me you must be making this turned edge as fast as you are trying to fix it.

Thanks for the comment Raymond

I started tidying up the centre earlier this week. I've been thinking about returning to the same lines as your previous post which was to get nearly there and then start to parabolize just to work the edge gently. My thoughts on going over the edge with such a hump in the middle with long strokes may add to the edge problem so until the middle come's down to something like I am just going to concentrate on that.

As someone else said I may find myself going from edge to middle several times to sort the edge.

Damian

[dark star]

I am in the process of planning a fixed post grinding/ polishing machine. First I am going to try to make a powered turntable and see how I go from there. I have one pulley so far and need another one or two, I am probably going to use the motor from my drill press. I need to calculate the sizes of the pulleys to get the right rpm. This will all take a while especially the construction!

 But in any case I first need to grind both sides of the mirror flat, as it has ridges, and this will take a lot of work and quite a long time. I will be doing that by hand.

David