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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

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

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.

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.

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

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

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

Just read Daniel K is selling up! Shocked to say the least

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

Don't worry folk's "give up" is not in my Vocabulary but thank's for everyone's support throughout the thread

Hi Nick I did it just as an experiment the other day to see what I'm dealing with as I know sometime's telescope makers do this as a final option, I've even read of the mirror being turned upside down on a piece of plat glass and the edge flatted off then painted to eleviate the edge, seems a bit extreme and not something I'll be considering The limit I suppose will be when I get fed up or think it is futile but I am stubborn so it's not an option

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

To be honest it's not a very good spectator event as once you've seen the strokes and a few laps around the table the excitement is over! I suppose it's interesting to watch the progress speeded up and their are many video's to watch that show this. Most of them relate to powdered table's and spinning mirrors as the old laborious technique of hand grinding has all but disappeared (as is shown in my thread and a few others on here) due to the vast amount of work and time required to do it.

Thanks Gerry for the encouraging comment. Much appreciated Damian

AJohn As Ron quite rightly put Thanks

Hi John Not a silly question at all and quite a good one The amount of glass removed is measured in thousandths of a millimetre or fractions of the wavelength of visible light. You will often see measurement's like a 1/4 wave or 1/8th wave quoted this means that the reflected light can be up to that much out of phase with the other light reflected thus the image is not quite as sharp. There's a lot more to it than that as you have to get that light to focus to a single point (if your lucky) which is where shaping the mirror into a parabola (called figuring) has to be done. This can be done to a matched Ronchi image using your intended mirror parameters and a program to generate the image's you should see. Of course this is how the amateur will do it there are many other tests that are used to improve on this which are often called null test's. A mirror will become unusable when it get's too thin and thus not capable of holding the shape (figure) you have created on it so the image's will not form to a sharp image referred to as Astigmatism. I think I'm safe that it would probably take 200years at this rate for me to get to that stage although I think it may take me that much time to get rid of the turned edge! Damian

Onwards with a little update as I have been a bit absent of late so apologies to all. I have managed a further 8 hours work on the mirror moving further to the edge. This seems to have amplified the edge problem , rather than making it better, so I have now moved back towards the centre with the occasional W strokes to smooth everything down and hopefully improve things again only time will tell so I have a couple of days to see how things progress and I should be able to update with a few images Damian

Good to see you too Rich and have a catch up on everything. I'm sure you'll sort your mirror problems out quickly now you have the motivation back to continue. Finding the time is difficult but hopefully the plan you have will make short work getting it back on track and ready for the next season.

More work this afternoon folks once I've finished work and done a few chores. I have Rich visiting this aft at some point and will be good to see him A long chat with him about each others mirror making and how to fit the new flag stone table top to his existing table frame. By the looks of it he has a similar edge problem to me so will probably be viewing my notes with interest Will be nice to see some progress this week and fingers crossed I'll be able to grace you akk with some image's of a smoothly spherical mirror. So the plan is to smooth the centre a little more, long press and then work the edge again.

Nice one Derek

Hi Derek, Yes things from the state's sometimes are not quite what we expect. No work on the mirror today I'm afraid, as I was readying a second table top mod like mine for Rich (Crashtestdummy blog) to take home with him to hampshire. Edge is the same grass is cut though Damian

Now there's a long term project

Not quite Mark but I'll sure celebrate when I do

Thanks I hope it won't be too long a wait

Cheers for the post Alan Very ambitious I know but yes it is my first mirror so the process is even longer as I'm feeling my way along, although with a good guide it's a bit easier. I may get some more work on it this aft but I have a few bits to make up for Rich and his mirror blog (crashtestdummy) if you also want to check that out as he's doing the same. Damian