mapstar

It's like staring into the faces of weird Aliens.

I never made a mirror large enough to make use of the Ronchi gratings, although I did

use a home made one to get a general picture a finished 12" f6 parabola.

The Foucault/ Couder, and Everest test methods were mostly my bag.

A general overall view of the shape and smoothness, and edge diffraction were always more apparent to me.

The more modern methods seem to have relegated the old methods to the second division now though.

Whichever one uses, matter little, as long as the goal is attained.

Ron.

I use the Ronchi as I really struggle with the Foucault to take a photographic image.

Seems everything I do with it just doesn't seem to work?

I have managed another hour today which is a bit disappointing as I wanted to do more.

Back to work tomorrow but I will get some time in probably an hour or so?

Damian

How will you know when you have finished, Damian? I'm not familiar with the tests you are using

Richard

There are quite a few ways, so I understand Raymond above uses a Couder mask (like the one below) and the Foucault test in which certain areas (the cut outs) grey (null) out at certain offsets from centre of curvature.The figure XP program is used to generate the correct curve and data at what distances the areas null

I will initially use the matched Ronchi tests where you generate a series of images at different offsets (measured with a micrometer stage) from the radius of curvature. Here's something like what I will be using

 After that maybe a Ross null test, but ultimately a test on Johns auto collimation bench and his experienced eye to tell me if it needs more work.

Hope that helps a little as I'm just an amateur at this so feeling my way like everyone else 

Damian

Morning all,

Yesterday I had five half an hour sessions with the mirror and 10" lap. I've been using much less Cerium than previously and a 5KG weight atop the lap. A lot of slow and steady varied strokes and things are progressing

Here's the results in Ronchi form outside ROC as the work progressed

The central hill has begun to disappear so I know need to blend in the outer edge to the central region.

I intend some more work today and if I can I will try to get some Foucault images too showing the overall surface profile

Damian

Looks like a quarter to a third of the 'hill' gone to my eyes?

I'm already getting nervous, don't know what you must be like!

The hill has reduced a little not really sure how much in terms of measurement but it has moved.

I'm looking forward to tomorrow and seeing it change a lot more.

Damian

Afternoon fellow star gazers

Thanks again for posting Raymond with useful advice , it's interesting to see how  people go about making a mirror and there is a bewildering amount of ways to do it. I've never really looked into the figuring software side although it does produce very good mirrors as you have quite rightly shown. 

I'll have a good look at it once I've downloaded as it will be something else to check the mirror with. Final testing will be done at John's on his auto collimation rig all being well.

I have made a start today again on progressing the mirror.

Firstly I set the test equipment up again to perform both the Ronchi test and Foucault with a start image taken outside centre of curvature

To begin with I would be smoothing the overall shape as there is a hill in the middle. I worked in two half an hour sessions vary the W stroke width and length from tight wider W across the middle.

One thing I have learnt, and a tip for anyone attempting this, is that cerium in nigh on impossible to get off around the edge if it is allowed to dry and stain the glass. So I was mind full of this and kept the edge clean with a sponge soaked in filtered water.

After each session I tested and slow progress the lines are a little less bowed out (indicating a hill when measured outside COC) especially the central line which is smoother in the middle. I know the differences are very subtle to see but they are progress so I shall stick with that and progress slowly.

More work the same tomorrow, and I shall post the updates as the images progress

Onwards with pushing glass molecules 

Damian

Well done Raymond

Looks really nice and as you say smooth which is what is needed.

I will be using a 10" lap and testing regularly so as to not overshoot.

Did you slow down with your strokes towards the end as they look quite quick in your video.

Off to get sorted around then make a start tomorrow and this weekend.

Damian

Evening all,

Today I made the 3 hour round trip to pick the mirror up from John.

It was good to see him again and the enthusiasm and passion he has for mirror making is infectious

John was working on a couple of mirrors when I arrived early, and these he tested whilst I was there, interesting to see how different they could be and what corrective methods he would employ to sort them out.

We also tested my mirror which had been stood for a few days and it looks good. As in the previous test images the middle is raised although John said it wouldn't take long to sort that out. After yet more mirror making discussion I boxed the mirror up bid him farewell and headed home.

Tomorrow is a case of making ready for the mirror work by cleaning and moving the table back into the kitchen. Slow and steady with plenty of testing every half an hour to see where I'm going. 

Fingers crossed but I'll also be juggling this with the scope build as I have all the materials massed to make a start.

Damian

Mirror looking good Damian, bet you can't wait to get it back and get on with the figuring!

That is a cool video Raymond, I'm looking forward to seeing Damian's rotating table now!

Thanks for posting the video Raymond and technique as it looks to be proving very successful.

I suppose working hand wise with a spinning table makes it easier to concentrate on just the stroke width length and movement?

Moving around the table which I've been doing puts in another variable to contend with which in some cases may be a good or bad?

Looks lovely and I sincerely hope mine looks similar soon.

Damian

Interesting method and good explanation thanks.

I will obviously be hand figuring and as ajwawp it will be a challenge although one thing John has already told me is that the figure needs to be there but it's nothing without being smooth.

I've just started parabolizing my 20" f3.8 using the same method I found best with my 18" and it's working great !!!

Hopefully, I'm looking forward to picking it up to get started

Cheers Raymond.

Could well be with the colour Richard.

I'm looking forward to picking it up and carrying on.

Damian

Hi all,

I had an update from John today and after several hours on the polishing machine the mirror is polished out and has a good overall shape. No turned edge and no scratching.

It's not a perfect sphere as due to the high ambient temperatures we've had lately the lap centre has tended to sink a little creating a slightly raised middle which shouldn't be too difficult to rectify (a lot easier than TDE!!) . I will be collecting the mirror at some point this week and will discuss the next steps thoroughly with John so  that I can once again carry on my journey and keep you guys up to date with hopefully some successful progress.

Here are the images that John sent me of how the mirror stands with the ronchi test. The first two images are inside centre of curvature and the last outside

Many thanks to John for his help here and hopefully I can take this to it's conclusion

In the meantime I am going to be the proud owner of a few sheets of birch ply ready for the build  early next week.

More to follow and thanks for bearing with me guys

Damian 

It sounds like biting the bullet and letting John help you has freed up your head to get on with the rest of the scope. First light is nearer now than it was!

Good stuff, Damian - I'm looking forward to sawdust covered posts

Richard

I'll put the scope build in another thread which will be a while yet but it should be a good read.

John said he will sort it to the point where I can then continue with the figuring part which in itself is going to be very challenging.

Just have to get my plans together to make a start on the woodwork

Damian

Hi Damian,

Glad you are now getting somewhere. Hope it works, just take it as a lesson learned. If you don't mess up you don' t learn anything, I should know, it has happened many times to me on other things.

So when you build your new grinding / lapping machine for the 36" mirror........................................................

Yes I know, I can hear all the flowery words even now!

All the best,

Derek

Thanks for the encouraging words Derek.

John has a lot on his plate at present and I'm really grateful of him getting me back to where I should be.

The figuring is something else and will take a while.

On the scope front the ply is ordered so in the meantime I will March on with that as I at least know how to do those bits

Damian

Hi all,

Its been what seems like an age since I added the last update to the thread so here goes with the next installment.

After fine grinding the mirror I re-cut the channels in the lap and left it stood for a while. The warm temperatures have played havoc with the pitch with the channels closing up as it becomes more viscous. I had to remove the lap from press until the temperatures had settled back to the usual British summer.

Having discussed things with John about the re-polish I took the decision to accept his kind offer of getting me back on track by putting it on one of his machines.

So yesterday I took the trip north again after boxing up the mirror and lap to be met as usual by John at the door with his friendly grin and welcoming handshake. In the astronomical community I don't think you'll meeting a more helpful and encouraging person.

After a quick cuppa John checked the lap and mirror which were o.k. and went ahead with warming the lap to do a press with the mirror ontop looking down through it to see how the contact was. Not very good the middle due to the hot temperatures had sunk a little as I'd had the lap face up.

After several hot presses the lap was looking much better so it was onto setting up the machine to polish. A boss had to be screwed to the back of the lap to accept the centre post of the machine.  A few photos will help explain this better so here it is on the table polishing away.

After 20mins we checked the contact on the lap which was much better. I was surprised at the amount of cerium that John uses compared to me (a lot less  ) The lap was much different to hand polishing as the impregnated cerium was worn away and as some would say the lap had gone back to its natural black colour. An hour on the machine was I have to admit a lot easier than the 4 hours of hand polishing that would take me to the same point!

After an hour the mirror was removed and roughly checked as it needs a lot more polishing. There is no astigmatism but it will take a lot more to determine whether or not the fine grind was a success.

I have with a heavy heart left the mirror with John for now, as he is going to polish it back to where I should be now so I can bring it home and attempt to figure it.

All for now 

Damian

Have you a time scale for the scope?

Today's efforts have been nothing to do with the primary mirror.

I have been planning the scope build and what it will be like so gathering my thoughts.

With quite a few of the bits already made I have made a start.I have gathered together all the bits and cut the spider vanes in anticipation of assembly.

All that will be in a different thread so will be on its way later in the year.

Also I will make an update on the mirror by next week hopefully. The lap is pressing nicely in the warm temperatures

Damian

What's the depth of the mirror currently?

Hi Raymond.

I have not measured the depth of the mirror but whilst testing at Johns we worked out the the focal length is going to end up around F3.5 which is a little shorter than what I set out for. As the scope is yet to be built I have not made any calculations on the final height of the eye piece.

Damian 

Fascinating read throughout! You sir, have the patience of a saint. I think I would get about 2 hours in and then think - perhaps not...

It has been and still is a labour of love which one day will be complete and that's what keeps you going.

Damian

Looking good im always checking on your progress will be great when you finish it, keep up the posts its very interesting.

I used to spray the fine ground mirror with water after setting it up on

the test cradle, and check the radius of curvature length, and thereby /2 it's focal length.

A reasonable estimate could be gained, but also moving the knife edge back an forth to intercept the reflected light beam

could give a reasonable indication of it's spherical condition.

The illuminated mirror would darken alost instantly as the knife cut into the light source.

Not a sure fire result, but any drastic event such as astigmatism might be apparent.

It would be extremely unlikely, but not altogether impossible.

A rare occurrence after fine grinding though.

I instead went for 3 hours of polishing then test.

I can't remember where I read it but you can also shine a light source at the mirror whilst moving it side to side and the reflection should reverse around the focal point? Think I got that right but I do stand correction.

I must invest some time in making a proper foucault tester as I will then have two test methods to go on.

I will eventually call on Johns good nature and expertise to test it on his auto collimation bench to make sure it is figured correctly.

Damian

Do you have any testing piccies to show the sphere in all its glory?

Hi Nick

The mirror needs to be polished first before any testing takes place.

A sphere is the classic starting point to working the shape to a parabola. This will take quite await of time for a mirror of this size and focal length.