mapstar

Hi all

This morning I finished the fine grinding with another hour of 600 grit alu oxide.

It now has the lap back ontop pressing but looked OK when I'd taken it off

More later

Damian

Cheers guys

I have finished today by Re cutting the lap channels and it's now sat on the mirror atop grease proof paper.

The lap will need to be pressed with just cerium on the mirror to take the full profile.

I am still in two minds what to do on the polishing front but will decide in the next couple of days.

As Ron quite rightly says the velvety surface is nice to see, I will probably give it another hour or two with the 600 aluminium oxide but at present it is scratch free so I need to be extra careful putting the tool back on.

I may get a bit of time on it tomorrow

Damian

Afternoon all,

Although quiet for a few days I have been busy in the mean time.

The mirror has had the tile tool on again and after 4 hours with 400 grit silicone carbide testing to make sure I had good contact and then a further couple of hours to make sure. Because I'm not grinding large 180 grit pits out the grind didn't take that long.

So with the 400 grind completed I moved onto two hours with the 600 Aluminium Oxide

I will probably do another couple of hours with the alu oxide to make sure.

I have also been tidying up the 18" lap ready for the move to polishing again (third time )

Damian

Mr Nichol...

Yeah

Thanks for the advice Ron.

I have read somewhere about the washing up liquid before. I wasn't sure if it was just myth but I am convinced now

The grinding today went well and threw up some interesting insights into the shape I've ended up with.

I shall post an update later tomorrow maybe. Hopefully it will be good news

Damian

If that's John's opinion then so be it. If, as John says, you have a deep zone around the central hill ( which is presumably below your final parabola ) then reverting to fine grinding is the best way to get back on course.

I find that Ronchi tests are not my cup of tea, it is difficult to tell from your last image just what the profile is. I prefer something a little more positive. Using the fringes at the edge of the knife shadow in a normal Foucault test, for example, to give an approximate depth/height of any zones. That way I can pick up any wayward area before it gets too bad. For the 20" mirrors that I have made I have never used a grinding or polishing lap bigger than 13", but then I used a machine that I had built and a spherometer that reads to 0.001mm ( direct reading of 2 waves although I could estimate to 1 wave ). The spherometer kept the grinding on target and when polishing the disappearence of the pits was closely observed and the stroke changed so as to be even over the whole surface.

Fine grinding and polishing will go much quicker now you have so much experience under your belt.

Nigel

Foucault knife edge testing is easier to look at for overall shape for my inexperienced eyes.

I will have to build myself a proper tester to supplement the ronchi I know Stellafane have an example on their site.

I have managed two hours fine grinding today with the 400 grit . I will continue tomorrow and hopefully move onto the 600 alu ox

Damian

Did John say anything about polishing out with a larger lap?

No he didn't suggest it, why do you ask have you read about it somewhere?

We discussed the 18" lap I already have and also polishing via machine to avoid the turned edge.

Damian, you aren't going backwards. You've had a bit of practice around the use of fine grits, polishing and mirror measurement. Now you are completing the basic shape and finishing the journey much better prepared than when you started out

Per Ardua ad Astra

I think you will be surprised how quickly it goes, most of my foul-up rectifications do anyway

Richard

Nicely put Richard

Quite right.I do remember the fine grinding didn't take me long last time.

As the saying goes back to the grind and hopefully by the end of the week I will be something like.

Guys

Thank you all for the kind comments and words of wisdom.

It is without doubt a difficult thing to go back wards again but the best option at present.

I have already made a start and have just moved the grinding table to the garage after cleaning it down and dug out the tile tool ready for the 400grit.

After cleaning the tile tool thoroughly I'll make a start. Hopefully it won't take long to fine grind back to a sphere.

I'll as always keep all posted on the progress

Damian

Afternoon all,

I made the three hour round trip to see John yesterday with my mirror safely tucked up in the boot.

As ever John opened the door with a welcoming grin and friendly hand shake.

After a cuppa and a chat on various bits of mirror making we moved on to testing my 22" on his auto collimation rig which is extremely sensitive to picking up surface errors.

Initially it was a play around with getting it set up after John had asked what focal length I had aimed for. This gap changed quite a bit but more on that later.

He confirmed that the turned edge had gone and the correction in the outer zone was good, uniform, and had a nice overall smoothness.

Onto the middle and after discussing technique he told me the verdict in his usual straight talking honest style,

Basically I'd not varied the lap enough whilst doing the edge work and cut a channel around the middle due to the circumference being smaller.

First option the long haul with cerium which may result in other problems along the way

Or secondly and rather reluctantly go back to fine grinding and Repolish which would save a lot of time.

I have spent a lot of time getting to where I am now but it hasn't been wasted and I've learnt a lot on the way.

Time to learn a bit more

So it's back to the 400/600 grit and then Repolish

I am still debating on whether or not to do it again myself (third time lucky) or return to John and set it up on the fixed post machine of his to save me some time and get me back to where I should be now. I have already discussed it with John who is more than willing to help.

Hey ho

Damian

Quick update to the thread today.

The mirror has been sat under wraps most of the week as I've been concentrating on other less interesting things. It has been a break and I think I needed it, but glass fever is just around the corner again and I will be hopefully back to it next week.

I have today been readying for a trip to see John, and fingers crossed, formulate a plan to get things back on track and see the mirror moving forward.

Damian

No worries Ron I may attempt a 28" one day but will have to finish the 22" first which is going to take a while ☺

Thanks for the kind words Stu and Stub Mandrel

I'm still working on the middle but it's so out of shape it will take a lot of work to pull back and I may have to work back to a sphere of sorts

Damian

I think perhaps Damian might prefer a little respite from all of us for the time being.

He is aware the job he has undertaken is a Mammoth one, and he's now at a crossroads.

His visit to see John, should result in a plan of action that will hopefully re invigorate him, and he will carry on

with a stronger purpose.

Many post have been generated in this thread, some of them with the sole intent of aiding Damian in getting to grips with the problems.

Let's face it guys, a 28" by hand is a Herculean task for anyone, let alone a first time aspirant.

We are all attempting to help by proxy, but since the task is fundamentally a hands on operation, and the analyses of the progress being made,

is the domain of the operator.

From what I have read from all the experienced contributors, their advice has been quite valid, and under normal circumstances, would have brought about

some forward progress.  That doesn't appear to be the case though.

So, with that in mind, I suggest  a respite for Damian  to allow him to regroup.

Also I would like to say, there has to be no animosity developing in the thread, because will quickly propel it downhill.

No aspersions to be cast to maintain our respect for Damian himself, who has been  a very pleasant and cooperative young man, in  his responses to all who have posted in his thread.

He is a credit to SGL his forthright attitude, and friendliness towards everyone.

I leave it open to Damian to respond of course, but If he would prefer the thread to be locked for a spell, until he has time to gather his thought, and reflect on what he gleans from John,

then that can be done, and reopened when it suits him.

This is  a suggestion only, not a threat of any kind.  For the most part, You have all built a good thread here, and it needs a successful conclusion, for Damian's sake, and for potential mirror makers too.

Ron

Afternoon all,

Thank you for the post Ron and the steadying hand on the thread., also for the compliment.

I think it would be against my best feelings to lock the thread as such. It wouldn't be what I set out to do in the first place which was to give a feel to everyone of just how much effort (in some cases) goes into creating a mirror by hand. I have to admit that I never have thought small about most projects I take on hence why I went for the size I did and why I have taken so long. 

I have had quite a journey up to now you will probably all agree, an experience that many have had, but unfortunately not documented in the public domain to quite the same extent with live discussion and progress. 

At times, as Ron fully appreciates in his post, it is quite a challenge to keep thoughts and patience together over what has become many months of staring at a piece of glass wondering just what to do next. It is slow progress and I should imagine that you feel frustration as much as I do (I know someone has commented to that end)

I didn't realise at the start just how much interest the thread would generate from those that I know and from far afield. One thing I will say is I have enjoyed it, and appreciate all who take time to post, but do agree that if you are reading this from new it is an awful lot of pages to trawl through to pick out the relevant information. So I would like to ask everyone (and I have sent out a couple of pm's to the matter) that we should keep the information relevant (with a little banter)

In the future the thread I am told can be edited to shorten it so as to condense the info which I think people would appreciate.

I will reach the end as I have never given up on a project yet and will carry on sharing my progress whether it is fruitless or moving me to first light.

After all that seriousness I do have one little bit to add about Ron's post that has enlightened me as to why I'm having problems with the 22" I'm making. It's actually a 28" now   (Ron you must get those typing fingers sorted  ) 

Thanks to all again up to now for the support on this thread and for making SGL a great forum 

Damian

It is always good to take lots of photo's for comparison and here's two Foucault type of when I started and how it stands at present

The Ronchi is a different beast altogether 

Damian

I too have had times when nothing appeared to be happening to my mirror. At those points I have decided to change something significantly and see what happens.

The last images show a central hill. I would probably make up a very small lap, about 1/3rd the dia of the hill, certainly no more than 1/2, and work entirely within the central area and see if that made a difference. I wouldn't worry about creating zones there as you can easily switch back to a larger lap once some progress has been made. Don't forget that the central area will be obscured by the flat when in use so it doesn't really matter what happens under that flat. ( Cassegrains have a rather severe central hole ). I have used laps as small as 2" on a 20" mirror and on occasions even just my thumb.

I also use a heavier tool base making mine with plaster and sealing with varnish.

As stated previously, the biggest problem with figuring large fast mirrors is that significantly different amounts of glass need to be removed from adjacent areas of the surface and that cannot be done with large tools. So a bit of practise with small laps now will give you a feel for them when you actually get to the figuring.

Here's the 200" Palomar mirror being worked with a relatively small lap:

Probably equivalent to a 2" lap on your 22" mirror.

Nigel

Hello Nigel

Thanks for the post and words of wisdom. I have been working with the 6" lap which I made up a week or so ago and have been working solely on the centre with it.

I know I did a lot of edge work and that the centre would take a lot of work to smooth down but with the glass area being smaller I didn't think it would take as long to make an impression.

My ronchi images are confusing in the sense that inside focus it just looks like a hole? 

Thanks guys for the support and suggestions

Damian

Will you be taking your laps as well as the mirror?

Richard

When you get around to making a 1 metre diameter mirror Damian, I hope you build yourself a grind and polishing

rig. It sure take a lot of the pain away .

I hope your visit to JN. is a fruitful one. I'm sure he will quide you on the right approach to fixing the last anomaly.

There's no way you can remove enough glass by polishing, sufficient to ruin it completely. Any thing you polish into it, can be undone by polishing.

If the anomaly is too bad, and needs many hours of polishing, then reverting to fine grinding would be a quicker route, and that too would not take too much glass off.

I've had to do just that myself on several occasions in the past. It takes more time, but less frustration.

Best Wishes.

Ron.

John is a busy man and to give his time to such a troublesome student is telling of his commitment to keeping amateur mirror making in this country alive.

It will get there and I've never given up on a project yet.

Chin up as they say more time will sort it

☺

hi damian, after all the work you have done and time spent and got it so close, would it cost much for john to finish it off for you, overwise you could go to far and have to junk your blank. im realy not knocking you mate you realy have tried your best and most people woulld of given up by now. you should of become a doctor, you have loads of patience

mike

Cheers Mike.

John has been helping me through out this and is prepared to share his time and experience to get the mirror to first light. It's not really a matter of cost but I guess everyone has come to that conclusion about my thread.

I'm sure things are going the right direction but just need an experienced eye to look it over and put my mind at ease.

If the time comes when I need John to put me back on track so to say then the offer is already there.

I will know soon

Damian

Very wise mate... No better man to see...

Hi all,

I have over the last few days done quite a bit more work on the mirror centre but things are not really working the way I want them to. As the days go on the images seem to be getting more confusing to read. With the amount of edge work then the centre should have had a huge hill so was going to take a lot of work to rectify and I have done quite a bit already.

I have a trip loosely planned to see John this week so the mirror is on hold until he takes a look at it and gives me his expert opinion 

Damian

Aye Damian,

Give the flat blank to Mike.

Mike will probably bring it to me and he'll hold it while I use the grinder. Got a really good diamond plate to finish it off with and a bit Brasso.

"Simples" as the Meerkat would say!

Damian,do`nt forget you can also LOOK through your creation.

Regards

Mike

Surely, once the primary is finished you will start on the flat?

Nigel

Joking aside making a flat is a whole different ball game so John tells me. I assume you have made one before Nigel I know the basic process to make one but that's about as far as my limited knowledge on the subject goes.

I think I won't disappoint anyone by saying I will bow out of that task gracefully and have someone with much more experience make it for me in a much shorter space of time.

I would like you to post about it if you have.

Damian

Can't believe how far things have moved along since I last took a look at your thread a couple of weeks ago. Again, a fascinating read whist I have my morning cuppa! You've got some fantastic support from others sharing their experience, and everyone is behind you. Keep plugging at it buddy, it's going to be amazing

Thanks Vicky and I just don't know what I'll do with my time once this is finished? Oh yeah build the scope to put it in