mapstar

I have done a further half an hour today, I've mainly been working on the centre out to about 60% and trying to blend the hill into the outer zone for a smoother curve. Here are the images Inside ROC Outside ROC I will agree with AJohn that the foucault test is very sensitive to alignment along the optical axis. This I'm going to have to work on probably by adding some jacking screws on the test stand to tilt the mirror so that the plane of the mirror is vertical Hopefully more work tomorrow Damian

I was doing something similar last time I took some foucault pics. I would be interested to see your testing set up Raymond if you could post a pic And the way you managed to take your pics Thanks

Hi Ron I use the Ronchi as I really struggle with the Foucault to take a photographic image. Seems everything I do with it just doesn't seem to work? I have managed another hour today which is a bit disappointing as I wanted to do more. Back to work tomorrow but I will get some time in probably an hour or so? Damian

There are quite a few ways, so I understand Raymond above uses a Couder mask (like the one below) and the Foucault test in which certain areas (the cut outs) grey (null) out at certain offsets from centre of curvature.The figure XP program is used to generate the correct curve and data at what distances the areas null I will initially use the matched Ronchi tests where you generate a series of images at different offsets (measured with a micrometer stage) from the radius of curvature. Here's something like what I will be using After that maybe a Ross null test, but ultimately a test on Johns auto collimation bench and his experienced eye to tell me if it needs more work. Hope that helps a little as I'm just an amateur at this so feeling my way like everyone else Damian

Morning all, Yesterday I had five half an hour sessions with the mirror and 10" lap. I've been using much less Cerium than previously and a 5KG weight atop the lap. A lot of slow and steady varied strokes and things are progressing Here's the results in Ronchi form outside ROC as the work progressed The central hill has begun to disappear so I know need to blend in the outer edge to the central region. I intend some more work today and if I can I will try to get some Foucault images too showing the overall surface profile Damian

Yes nerves are jangling so slow and steady is the way.The hill has reduced a little not really sure how much in terms of measurement but it has moved. I'm looking forward to tomorrow and seeing it change a lot more. Damian

Afternoon fellow star gazers Thanks again for posting Raymond with useful advice , it's interesting to see how people go about making a mirror and there is a bewildering amount of ways to do it. I've never really looked into the figuring software side although it does produce very good mirrors as you have quite rightly shown. I'll have a good look at it once I've downloaded as it will be something else to check the mirror with. Final testing will be done at John's on his auto collimation rig all being well. I have made a start today again on progressing the mirror. Firstly I set the test equipment up again to perform both the Ronchi test and Foucault with a start image taken outside centre of curvature To begin with I would be smoothing the overall shape as there is a hill in the middle. I worked in two half an hour sessions vary the W stroke width and length from tight wider W across the middle. One thing I have learnt, and a tip for anyone attempting this, is that cerium in nigh on impossible to get off around the edge if it is allowed to dry and stain the glass. So I was mind full of this and kept the edge clean with a sponge soaked in filtered water. After each session I tested and slow progress the lines are a little less bowed out (indicating a hill when measured outside COC) especially the central line which is smoother in the middle. I know the differences are very subtle to see but they are progress so I shall stick with that and progress slowly. More work the same tomorrow, and I shall post the updates as the images progress Onwards with pushing glass molecules Damian

Well done Raymond Looks really nice and as you say smooth which is what is needed. I will be using a 10" lap and testing regularly so as to not overshoot. Did you slow down with your strokes towards the end as they look quite quick in your video. Off to get sorted around then make a start tomorrow and this weekend. Damian

Evening all, Today I made the 3 hour round trip to pick the mirror up from John. It was good to see him again and the enthusiasm and passion he has for mirror making is infectious John was working on a couple of mirrors when I arrived early, and these he tested whilst I was there, interesting to see how different they could be and what corrective methods he would employ to sort them out. We also tested my mirror which had been stood for a few days and it looks good. As in the previous test images the middle is raised although John said it wouldn't take long to sort that out. After yet more mirror making discussion I boxed the mirror up bid him farewell and headed home. Tomorrow is a case of making ready for the mirror work by cleaning and moving the table back into the kitchen. Slow and steady with plenty of testing every half an hour to see where I'm going. Fingers crossed but I'll also be juggling this with the scope build as I have all the materials massed to make a start. Damian

If I attempt another mirror! I will make one, but for this one the manual table will have to do.

Thanks for posting the video Raymond and technique as it looks to be proving very successful. I suppose working hand wise with a spinning table makes it easier to concentrate on just the stroke width length and movement? Moving around the table which I've been doing puts in another variable to contend with which in some cases may be a good or bad? Looks lovely and I sincerely hope mine looks similar soon. Damian

Interesting method and good explanation thanks. I will obviously be hand figuring and as ajwawp it will be a challenge although one thing John has already told me is that the figure needs to be there but it's nothing without being smooth.

Is that on your machine? If so can you explain how your going about it?

Hopefully, I'm looking forward to picking it up to get started

Cheers Raymond.

Could well be with the colour Richard. I'm looking forward to picking it up and carrying on. Damian

Hi all, I had an update from John today and after several hours on the polishing machine the mirror is polished out and has a good overall shape. No turned edge and no scratching. It's not a perfect sphere as due to the high ambient temperatures we've had lately the lap centre has tended to sink a little creating a slightly raised middle which shouldn't be too difficult to rectify (a lot easier than TDE!!) . I will be collecting the mirror at some point this week and will discuss the next steps thoroughly with John so that I can once again carry on my journey and keep you guys up to date with hopefully some successful progress. Here are the images that John sent me of how the mirror stands with the ronchi test. The first two images are inside centre of curvature and the last outside Many thanks to John for his help here and hopefully I can take this to it's conclusion In the meantime I am going to be the proud owner of a few sheets of birch ply ready for the build early next week. More to follow and thanks for bearing with me guys Damian

Thanks Richard I'll put the scope build in another thread which will be a while yet but it should be a good read. John said he will sort it to the point where I can then continue with the figuring part which in itself is going to be very challenging. Just have to get my plans together to make a start on the woodwork Damian

Thanks for the encouraging words Derek. John has a lot on his plate at present and I'm really grateful of him getting me back to where I should be. The figuring is something else and will take a while. On the scope front the ply is ordered so in the meantime I will March on with that as I at least know how to do those bits Damian

Hi all, Its been what seems like an age since I added the last update to the thread so here goes with the next installment. After fine grinding the mirror I re-cut the channels in the lap and left it stood for a while. The warm temperatures have played havoc with the pitch with the channels closing up as it becomes more viscous. I had to remove the lap from press until the temperatures had settled back to the usual British summer. Having discussed things with John about the re-polish I took the decision to accept his kind offer of getting me back on track by putting it on one of his machines. So yesterday I took the trip north again after boxing up the mirror and lap to be met as usual by John at the door with his friendly grin and welcoming handshake. In the astronomical community I don't think you'll meeting a more helpful and encouraging person. After a quick cuppa John checked the lap and mirror which were o.k. and went ahead with warming the lap to do a press with the mirror ontop looking down through it to see how the contact was. Not very good the middle due to the hot temperatures had sunk a little as I'd had the lap face up. After several hot presses the lap was looking much better so it was onto setting up the machine to polish. A boss had to be screwed to the back of the lap to accept the centre post of the machine. A few photos will help explain this better so here it is on the table polishing away. After 20mins we checked the contact on the lap which was much better. I was surprised at the amount of cerium that John uses compared to me (a lot less ) The lap was much different to hand polishing as the impregnated cerium was worn away and as some would say the lap had gone back to its natural black colour. An hour on the machine was I have to admit a lot easier than the 4 hours of hand polishing that would take me to the same point! After an hour the mirror was removed and roughly checked as it needs a lot more polishing. There is no astigmatism but it will take a lot more to determine whether or not the fine grind was a success. I have with a heavy heart left the mirror with John for now, as he is going to polish it back to where I should be now so I can bring it home and attempt to figure it. All for now Damian

I'll be hoping to be using it in September Calv but these things have a habit of a drifting timescale

Today's efforts have been nothing to do with the primary mirror. I have been planning the scope build and what it will be like so gathering my thoughts. With quite a few of the bits already made I have made a start.I have gathered together all the bits and cut the spider vanes in anticipation of assembly. All that will be in a different thread so will be on its way later in the year. Also I will make an update on the mirror by next week hopefully. The lap is pressing nicely in the warm temperatures Damian

Hi Raymond. I have not measured the depth of the mirror but whilst testing at Johns we worked out the the focal length is going to end up around F3.5 which is a little shorter than what I set out for. As the scope is yet to be built I have not made any calculations on the final height of the eye piece. Damian

Hi and thanks for reading the thread and for the compliment.It has been and still is a labour of love which one day will be complete and that's what keeps you going. Damian

Cheers Paul