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Thanks for the comments Guys I think the thread may need a little more humour Ron and had a little chuckle as I'm sure the pic could be turned to represent the edge problem I have. I too have the Texereau book and there are some interesting and relevant points and technique's in it. As you state though Ron some of it woefully out dated and it made me really laugh a bit on page 13 stating that an 8" can be fully exploited on a simple alt az mount weighing 40lb or less where as a 20" would need a more substantial mount weighing 2 tons . I'm sure there are quite a few dob owners on here that will feel mighty strong knowing that's how much their moving everytime. Soupy I hope you were using the term expert very loosely there especially seeing as I'm about as far as M31 away from that Derek I've seen the John Dobson video and to be quite frank for someone held with such esteem in the astronomy community it is quite shockingly rough Battle has recommenced with the edge so an update later guys. Back to the lap Damian

Yeah your right guys it will be nice to see a change for the better. Tomorrow will tell as I've plenty of work planned

Hiya RickI'm using the 133LPI grating as I don't have an 85. I do take measurement image's closer to radius of curvature although post the same image's just for comparison. It would be nice to see it clearer but there's an edge problem so no two ways about it I need to get rid of it through one tactic or the other. I'll do more work tomorrow and see how I go. There is a lot of glass to remove to parabolise it so was hoping that a bit of deepening would take glass off lowering the overall surface down to the edge. I think it's too far away to do that as yet so will carry on with probably a mix of what I've been doing before as it seemed to be working albeit very slowly which is no worry so long as I get there. Just patience and time and I'll get it sorted although time is drawing in Damian

A small update today as I managed an hour yesterday and today will be taken up by work. I returned to working on the edge but could see no difference in the images apart from I'd started to create a hill in the middle again. My mind wasn't fully on the task yesterday so I decided to call it a day and hoover the grass! The edge showed no improvement as can be seen Inside ROC Outside ROC I'm going to attempt more edge work tomorrow and after 2 hours test again. If the edge does not improve then I may need different tactics Damian

More work today and yesterday to the tune of 5 hours with the W type parabolising stroke It's amazing the amount of thought's that are provoked in people reading this and I had a pleasant chat over the phone earlier about the thread, progress and Ideas on what can cause edge problems. Ajohn also mentions a few things above and must have needed a stiff drink after re-reading the 13 pages again. Mind you I also think my caller probably needed one too Here are the image's Inside ROC Outside ROC Over all there is still a hill in the middle which is good and I think a slight change to the edge although the stroke gives this minimal work. More tomorrow fingers crossed and see how things progress Damian

Hi Cath. Thanks for the compliment and question. As you say the internet has lot's of info on the various testing methods that can be employed. The Ronchi is simple to set up and use I can only talk about the set up and experience I have at present. My set up consists of the following Ronchi Screen 133Lpi Camera tripod Linear translation stage LED and power source Ply frame to hold everything Small camera set on night mode A frame to hold the mirror with a sling and a wedge for adjusting the mirror focal plane (to line it up with the light source) The Ronchi screen I use actually came from the USA (http://www.ronchiscreens.com/screens.php) I bought the large screen and bought 2 the 133Lpi and 250Lpi although I only use the 133Lpi. The small screen would be o.k. I paid around £35 for both including the postage The linear translation (XY) stage is something to accurately adjust the travel in and out from the mirror. Like the one I have with the 25mm travel micrometer at the back (just an X stage) Fleabay £30 Or a micro scope XY mount about £20 Mirror frame (under Foucault knife edge test) you can see the mirror just hanging in the sling this was before the re-grind and re-polish. Lot's of problems here, It had an under polished centre, lots of finger prints around the edge as well as a hill turned down edge and just generally very poor shape You need some way of mounting these to the tripod, and then the screen and the light source to the stage for travel. In the photo I used some ply to hold the screen in front of both the LED and the camera. This is a moving light source tester as the LED travels with the screen and the camera. The LED shines through the screen and is reflected back off the mirror back through the screen to the camera lens. Light source and camera lens are as close as possible The mirror has a radius of curvature worked out roughly the depth of the curve by a spherometer so you should know this but it can be determined by getting the reflection of the led in the mirror and steadily working your way backwards till the LED image in the mirror fills the whole mirror. Looking through the camera you should be able to see the fully illuminated mirror and the lines on it. The lines alter as you move inside and outside the radius of curvature and you can use free software ROnWin20 written by John D Upton to produce matched Ronchi image's of your test mirror by entering the mirror details. The Ronchi is just a variation of the Foucault Knife edge test. It represents multiple Knife edge's as far as I believe. Phew hope that's correct, but there are plenty of much better explanations on the web. Not many talk about how to set up for some strange reason and about the moving and fixed light sources Damian

I had about an hour's more work yesterday on the mirror, but hoping for quite a bit today and see how things progress. I managed to take a few Ronchi Images of the mirror before I made a start. Inside ROC Outside ROC Time will tell if I'm going the right way. Damian

Hiya Nick, Depend's on how deep a scratch is but I very much doubt it as even though I may be taking a lot of glass off (in polishing terms) it's a small amount. Fingers crossed though as mine are very light. I have another couple of hours planned this aft so I shall take some image's later on of the progress Damian

Hiya Ron I know It sounds extreme but that's what John and also Raymond now have advised me to do. I did it quite a few time's on the larger lap to no detriment but I must admit I'm always a bit apprehensive once I've done it. I do have a couple of small scratches on the mirror which will have to stay at this point but not caused by the brushing. One will probably be under the shadow of the secondary. Damian

Hi Alan I'm glad to hear you've read the whole thread and found it interesting, if only to appreciate the incredible experience and dedication that mirror makers such as John have to create such master pieces. As an amateur I'm inexperienced and obviously feeling my way along so to say if very slowly. There is lots of input from mirror makers experienced and other wise from right around the globe here which is good and it does create quite a discussion sometimes. As with scope's there isn't one that is the same or made exactly the same, although if they were we'd all be looking through Kriege and Berry style dobs and you wouldn't have the Canopus agreed? I hope to read your first light report Alan as it's a beautiful scope and the one that inspired me to build something a little different and will do in the future. Thanks again for reading Damian P.s. I'm not saying which christmas but I think I'll get lynched if it's not done for november

I have been doing the wire brush scrub but not quite often enough by the sound's of it. Great tip and thanks for that Raymond. I will test the mirror later after a cool down and see if things have changed for the better. Can I also add that is one lovely Flickr page you have Damian

There's no rush but I thought I'd try a different tactic. As you say Nigel I will keep a close eye on the centre as I proceed and if it is going wrong I will revert to the edge. As RAC says it is a large fast mirror and the amount of glass to remove is a lot so I am trying to kill two birds with one stone by working both for now if things start to go awry I can change back. The lap has a lot of micro faceting on it, I am probably wrong but it seems to be working o.k. Damian

The pressing depends on the temperature and weight you put on. If it's warmed up too much it will just close the gaps up very quickly as will sticking lots of weight on. Smaller lap less weight (5 to 10kg). I normally soak it in luke warm water for 5-10mins so the lap is warmed through and then a half hour press. The lap is left on grease proof paper ontop of the mirror when I'm not working on it so it retains it's shape. Which reminds me it's been pressing for a good hour now

With work and chores over with yesterday and today it's back to the mirror campaign. To start with the lap facet's were looking closed up, so out with the saw and half an hour later the lap channels were cut and I was ready for a bit of warming and pressing. Not the prettiest or neatest lap in the world but it's worked up to now. This is the third time I've re-cut the facet's and it happens quite often. Today I'm going to start the parabolising process as this may bring the edge to heel. On with the W strokes Damian

Thanks Ron There's no rush as I've got this far and will get to the end. Great that you guys keep following my slow progress and offer encouragement John offers great advice and encouragement in the back ground which I've not mentioned much It's always nice to see the other opinions on what you guys think too. I may get a few strokes done today but I need to catch up on a few chores first as my cleaner has been busy fighting a turned down edge Damian ;-)

Ha ha yeah Maybe a bit more work today but I need to concentrate on other bit's today

I think there were a few others trying to corrupt me in Galloway!

Hiya Derek Everything is a good idea so no shooting will occur. I was disheartened a couple of week's ago, but spirits are quite high at present and yes things don't work out sometimes and that's where more advice and effort are required. I knew it was going to be hard from the start as John had said but it is doable. I've been thinking it may be technique but it could also be other issues that have been at the back of my thought's Raymond has made a good suggestion which confirms what John had said about large fast mirrors so after a few other bit's tomorrow I will continue. I also know your not nuts as you've not started mirror making yet ;-)

I have done about another 2 hours during the day, I think I may have a beer now as I think I deserve one

Saying it's not changed much I have a nice little hill forming in the middle just to explain why the line's aren't straight in the central area. This tallies with being opposite to the lines at the edge inside and outside ROC

I have been told this already Raymond as larger mirror's respond differently to the smaller stuff, I have tried again today to remove the offending edge using chordal strokes with the centre of the lap 2" from the edge, it has changed very little Inside ROC Outside ROC So with a view to possibly sort the edge I will have a re-read of some info then go with the W strokes. Thanks for the comments guy's Damian

No problem Derek I think it's love and hate to be actually honest ha ha

Hi Derek. What I'm looking for is the lines inside and outside the radius of curvature to be completely vertical. When the edge is turned down (as mine is) it shows as the lines bending inwards near the edge when the image is taken inside radius of curvature (ROC) and outwards outside the ROC as mine do. Here's how a perfect sphere should look inside and outside ROC And here's a TDE outside ROC As you can see it's just the very edge that's the problem at present. Once I'm to a sphere I can move on to the parabolising process although for a large mirror like this everything takes an age to alter. Judging change is very difficult so you have to stick with your chosen tactic for a while to see the difference be it right or wrong and that's why it's a back and forth testing game Hope that explains Damian

I'm now working with a 10" lap John and have been for the last few page's on the thread. More work today and concentrating more on edge work than the centre. Looking like a lovely spiral galaxy These things take time and it get's very difficult to see any movement on the image's Inside ROC Outside ROC More work to follow and I'll post an update later Damian

I managed an hour yesterday working exclusively on the edge. Not much change but I have all today planned on it so again I will concentrate on the edge and leave the middle to deal with later. Will update this evening Damian