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Cheers Paul

John mentioned the reflection test with fine misted mirror but I never tried it.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

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.

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

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

Thanks again for commenting 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

Hiya RaymondNo 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.

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

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

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

Not sure Richard maybe if John requests it

I can't think I'd ever attempt something that big but a grinding rig would be the first acquisition.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 ☺

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

Agreed Calvin a true wizard in mirror terms

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

I'd better make a good job of this mirror then to follow such finesse.