barkis

Certainly not me lad, I don't offend easily anyway. I've taken time off myself for a little while, trying to come to terms with a very persistent cough I've inherited from a recent chest infection. It has wearied me somewhat, and still no end in sight. Maybe it's waiting until your mirror is figured and ready for the Vacuum :grin:. (Now that is meant to be a lighthearted jest ). I sincerely hope you are on the last lap. (Whoops!, a pun now too.) . Good Luck, you deserve some. Ron.

Some very sound advice in your post John. I hope when Damian surfaces again, he has some positive news. Goodness knows, he deserves it. Ron.

What size is the polisher you are using, and how are you working it on the mirror, When you said 2" and 6", is the centre of the lap traversing that variation in vertical strokes across the centre of the mirror. or are they chordal?

Forgive this question Damian, it might seem a little impudent. Have you looked at the edge to make sure that it is polished all the way? Just a thought that crossed my mind. I'm sure it will be, just being nosey is all. Ron.

Jewelers Rouge is slow acting as a polishing agent. The big drawback with it, is the mess due to staining it makes. A mutual friend of John Nichol and I, the late David Sinden, amused me with his tales of rouge. He would relate that, standing in a corner of his Optical Shop, carefully opening a container to extract a small measure which he would then place in a squeezie bottle to make up a mixture. On turning around, he would be confronted by an area behind him covered in Red staining. He was exaggerating of course, merely trying to emphasise how badly the stuff stains anything it contacts.. David was good at telling jokes. Much better than I can even write them. Ron.

Please be very careful with the small lap, they can change an area in double quick time. Goes without say my friend, but keep a wary eye on what is happening by very frequent tests after allowing cooling periods. The Video AJohn suppled is a great guide to working the edge with the small tool, but I still urge caution. A 22" disc is far from an 8" one, when it comes to remedial work. Sorry if I seem to be Clucking like a mother hen, but I dread the thought of you having to return to fine grinding. I'm sure it won't happen though. That beefed up table will make a difference too. Good job that.

Hi. John. That video is very informative, and that TDE correction method is indeed a great way to correct it. Short centre over centre with a hard lap. An Oblate centre is the usual penalty for such an action, as the gentleman in the demonstration explained. That fortunately, is easily corrected, again adequately explained. Unfortunately, the mirror being worked in the Demo, looks like a 150mm dia disc to me, worked with a full size lap. Damian's disc is 22", and a full sized lap is going to be a beast to work on a mirror of those dimensions. As much as I personally dislike sub diameter corrective laps, they are necessary on Damians project. Many instances of TDE are a legacy from the fine grinding process, and in severe cases, reverting to fine grinding is the only sensible way to reverse it. Damian is persevering in correcting the anomaly using his sub diameter tool, and it does appear to be responding. It's a condition mirror makers dread, and either one gives up, and decide to mask it off, or prepare himself for the difficult job of Eradicatiing it, which is Damian's goal I'm sure. I think Damian is very pleased with all the attention his project has attracted, and all of us watching, and sometimes adding our owns tips and suggestions to the mix, maybe getting a little tiresome for him at times. He is probably too much of a gentleman to say as much, and in fact I might be wrong in suggesting such a thing. However, at such a critical time in his progress, he might wish to enter the last phase of the job with less comment from us, and just let him alone for a while. He knows what is required, and if he needs qualified help and advice, he has John Nichol to turn to, and in better hands he couldn't be. I hope no one is offended by what I've said, and indeed, if Damian himself doesn't agree, he is perfectly at liberty to say so, and I won't mind one bit. Ron. Edit] My apologies. The mirror in the Vid. is a 200mm Diameter Disc.

The reason is usually because the central areas of the lap tends to collapse due to the constant friction. There is no respite in the middle. The softer pitch unsupported on the mirror due to the sagging, causes the lap to plough into the edge zones, resulting in turning the edge over, and can get pretty bad if not recognised soon enough The mirror also turns Oblate, because the central areas are not being worked by the lap, due to the same sagging condition. Hard pitched laps are an insurance against TDE, but can result sleeks on the mirror. Not as bad as scratches though, and can be polished out. Lots of pressing is also mandatory to maintain good contact all over. Ron.

Everything that feels good, is good, so stay in that mindset Damian, it's worth hours of work. Ron.

Onward and Upward!, as they say. Not that I know who they are . Ron.

Cheers mate, I thought it was an indoor environment. My other thought was, if you're married, then you have a very understanding wife. Mind wouldn't let me near the kitchen. It was bad enough I transported pitch chippings into the house on my person. Trimming the channels after periods of polishing with a trim knife, the little bits flew all over the place, in my hair usually, where some little bit would drop off during the night, and embed itself into the pillowcase. Then the Flak flew I can tell you . The garage can be a cold environment in which to work, but a hard cold lap was a plus in preventing a turned edge, but contact conformity was also difficult, so pressings were frequent through necessity. One had to guard against sleeks too. Good Luck in your forthcoming sessions, I think the mirror is about ready to do your bidding . Ron.

Just where are you doing the work Damian? You seem to be in the house somewhere. The Kitchen maybe? Either there, or you have a very posh Garage/Workshop . Apologies if you've explained this elsewhere in your thread. Ron.

For Sure matey .

Good luck on the run in Damian. I'll be back to join in the celebrations, and I may send you a Pack of Jaffa Cakes, SGL's usual celebratory snack . Best of Luck Matey. Ron.

You're certainly approaching the tape Damian. Carry on the excellent work, and I'll recede from the workplace and let you get to Nirvana. I'll be back to join in the celebrations. I'll send you a pack of Jaffa Cakes, SGL's usual celebratory snack . Best of luck on the Final Approach Matey. Ron.

I don't think King George III would miss the £4,000 he granted for the telescope John. Cheap for the 40 foot monster I would say. Consider the discoveries made with it. I don't think there would be much for William and his Sister Caroline, (Who helped him) to pocket after paying for the materials. It was the largest scope in the World for 50 years. Believed to have helped in the discovery of Saturn's sixth and seventh Moons, Enceladus, and Mimas. Anyway, I think we have hogged Damians thread too much as it is. He must be getting fed up with interruptions :icon_mrgreen:. Ron.

Try to imagine William Herschel figuring that 49" Speculum metal mirror of his, way back when Damian. I think he used a star as his light source, and lugged the mirror in and out of the huge truss to make corrections to the surface as required. Some Job Heh? Carry on yor good work sir, I wish I wasn't so far away from Barnsley, I'd love to pop in to watch progress, and maybe even get a few wets in. You've got me yearning to get back on the grind again you bad lad . Ron.

I would love a big Dob, a 36" f3.5 will do nicely thank you. Housed in a weatherproof Obs, an a simple Alt Az Swing through Fork Mount to gain access to the Zenith. Not too high to climb to the focuser, and no awkward eyepiece positions to annoy me. Just wait for the Cosmos to roll by me nightly through the years, and pour the starlight down to the big collecting eye, and back up to mine. Nice thought. Sorry Damian, I digress in your thread. Ron.

I sold my Texereau book. All I have now are NE Howards Handbook for Telescope Making, and Albert Ingalls Volumes 1, 2, and 3. Old, but crammed with useful stuff. Modern methods have long overtaken them, but one can still home produce good optics from them. Ron. PS. I'm sure Damian won't need to attempt the following . A little humerous drawing from Ingalls book 1.

Eeeek!! Wire Brush?? Tell me you are kidding please . Imagine the myriad scratches a piece of wire could do stuck in the pitch. Ron.

This post isn't to try and offer any advice Damian. I just want to make the point about the incredible attention you are giving this project. Not just your almost running commentary on the work, but to highlight the extreme patience you posses, both in applying yourself to the task in hand, and your willingness to pop on SGL every day to reply to everyone's posts, without an Iota of impatience reflected in your replies. I know each one of us is willing you success, and it will happen. The only equation left is being resolved by you, and that is.. Patience x Time + Thought = Eureka!!! We'll all share in your elation when that moment arrives. Keep pushing those glass molecules up the hill to the edge . Ron.

I always used micro faceting on my laps, whether it was a 12" lap, or a 6". The theory that it is the edges of the lap squares that facilitated the polishing action, means that multi. facets theoretically would speed up the polishing, and also the figuring process. Of course we know that speed is not the factor that is important. Good contact, and at a speed that prevents micro ripple is important. Keep the Faith Damian, you've already got the winning post in sight, you just have to give the mirror a quiet talking to . Ron.

Damian, I'm sure you were under no illusions about the task you embarked upon with this large objective, and John Nichol would certainly have highlighted the difficulties you would encounter. Frustration will always be your companion when you are attempting the figuring process, and when it gets too bad, you should take a break. A 22" f3.7 is a very difficult adversary for anyone to tackle. All of us following your progress are pretty much mesmerised by what you've achieved so far, and certainly willing you passed the winning post. You have the discipline to complete the job, but don't be averse to leaving it to one side for a spell, and get back to it when you feel like it. Have a read of Mel Bartel's Parabolising pages might inspire , they are quite interesting. Best Wishes. Ron. http://www.bbastrodesigns.com/JoyOfMirrorMaking/Parabolizing.html

Have you been using 1/3rd centre over centre strokes Damian? 5 minute periods with cold pressing regularly? You can shorten to 1/4 strokes if it is reluctant to cooperate. I assume your lap is fairly hard, which is good? That is a good way to fix TDE, but you might inherit an Oblate surface in the centre, which is not unusual. That is easy to remedy though. Not trying to teach Grannie to suck eggs matey, and you are winning the battle with that huge lump of glass. You're a braver man the I . Ron.

You'll get there alright, you have achieved enough already to show you have the will to get past the winning post. I look forward to that day . Ron.