The 20 " grind has begun

[JRM]

Rick, for what its worth I use sub-diameter laps all of the time and have never made a correlation between small laps and scratches.

Good luck with your lap.

John

Thats great news the John, was something I read on the web somewhere, was a main worry of mine about using

sub diameter laps, thanks for clearing that up, I feel better already..

as I was typing this my cerium oxide was delivered to the door so I should be back in business real soon.

Cheers All.

Rick M

[JRM]

Hello All,

finally back at it, spent over a week pressing and channelling but have the lap  conforming nicely with the glass, so I know have a 10" and a 15" lap,

donw 1-1/2 hours figuring over the last two days,two sessions of 15 minutes with the 15" then 30 minutes with the 10" c.o.c. with the occasional narrow

W one trip around to try to keep things smooth, this cerium is working much better, curve is starting to take shape already, will test tonight and hit it early

in the morning that way I can test and hit it again in the evening for another session but already thinking my next session will be with the 15" lap only to

try and keep everything smooth and avoid the surface roughness before it gets out of hand, shall keep you posted...

Rick M

[mapstar]

Thanks for the update Rick. Things sound to be going good with the figuring will you be posting the Ronchi Image's?

Mine unfortunately has taken a turn for the worse, as you can see from my profile picture the Foucault test showed the profile to have a large hill and steep edge.

To sort it out I was advised to do long centre through centre strokes with lots of weight on to flatten the figure.

Unfortunately this resulted in lots of scratching not sure if it was cold lap or contamination?

Result is that I am currently back to 400grit carbide to remove the scratches and hopefully flatten the mirror off.

Damian

[JRM]

will see if I can get a ronchi image this week,my biggest problem with this project is time,never enough im the day, have the

15" lap pressing now for a session later. half an hour here and half an hour there adds up in the end but it sure goes slow,

would be nice to have a week to do nothing but work the glass, it would be done then...I think....

It is all a learning curve Damian, we learn as we go and hope not to forget it all the next time around, good to hear you are still

at it, keep it going, we will get there eventually and it should feel real nice by then..

Rick M

[JRM]

since last report I did a total of 45 minutes with the 15" lap COC strokes with different stroke lenghts of one to three inch over hangs only

to find the central area for the most part flattened out again, maybe should have used shorter strokes?

surface smoothed up real nice though.

since then I have scalloped the edge of the 10" lap and worked exclusively with it of a total of 2-1/4 hours, using coc and narrow w strokes

with stokes of zero up to 2 inches of overhang to keep things smooth,the curve is looking nice though not yet deep enough and there is no

surface roughness to be seen but I have a depression in the center I wish to correct before it gets to far along,these small laps are new to me

so I am curious as to which stroke to use with the 10" lap, I am thinking shorter coc or very slight w strokes of maybe 5" total movement, thinking

with the w it would move to correction out more smoothly over a larger area, then back to longer strokes again after that, sound right?

I can attempt the strokes and learn from it but ask the question to save any unneeded and unguided work, Thanks...

Rick M

[Astrobits]

If you feel that you are overdoing the centre then avoid coc strokes as they will work the centre most! The centre of a sub-diameter tool should, on average, be centred on the peak of the zone you are trying to lower, and always oscillate left and right - the W stroke otherwise you get some tool edge high/low zones. So, W strokes off centre for a while.

Nigel

[JRM]

many thanks Nigel...

just to update,I did 45 minute worth yesterday with the 10" lap with some improvement but roughed up the surface so tonight I pressed the 15" lap

and did 45 minutes more,will test tomorrow for smoothness and correction and decide the path to follow next...

with everything that has been going on in this life lately, there is no more time to be a wasting, it is time as they say, no more slacking it, the push is on,

will test every morning and do a figuring session every evening, the time has come to get er done...

Rick M

[mapstar]

Hi Rick

The home stretch seem's to be in sight by the sounds of it with a plan like that.

Hope to see some test image's soon I've found taking them invaluable in monitoring how things progress or don't as the case may be?

After grinding back with 400grit then 600grit I have just done another 6 hours of polishing with the table repositioned in the house for warmth

Initial inspection after 3 hours is the lap need's more pressing as the centre is barely polished, the edge has some polish but there is a ring about 5" in that has polished much better.

Another press overnight and another 3 hours polishing has the edge more polished but still little contact with centre.

Pressing again but I'm out of CeO which should hopefully be here Friday so I can make a restart and try to get it fully polished out?

The warmer surroundings are making the polishing a lot better thankfully and hoping by early next week to get some test image's again and see what shape I have

Damian

[JRM]

test this morning and things are going in the right direction, surface is smoothing out real nice as well.

did anther 35 minutes tonight coc with differing lengths of no overhang to an inch or so in an attempt

to prevent zoning, will test in morning again, my session consist of 10 -15 minutes slow steady strokes,

press with 25 pounds for 5 minutes and do another session, then press again.

I have tried to take images Damian,but it never turns out with good results, will have to make a stand for

the cam, will definitely work on it.

sorry to hear about the return to fine grind, its a learning curve for sure but you will persevere through to

completion I am sure, and what a beauty it will be

Rick M

[JRM]

after a few more hours of work the center is near perfect, the glass is free of surface roughness and the curves in the Ronchi test are looking real good, I have a small zone about 1-1/2 inches in from the edge that I am currently working and have just finished a 25 minute session of c.o.c strokes with the 15 inch lap with different lenghts from 1 to  2-1/2 inch overhang. will test in morning, time to make a measure stick to see how far these lines really are in comparision with the Ronchi calculater..

Rick M

[mapstar]

Hi Rick,

Thanks for the update things sound to be moving well. I have done roughly 16 hours polishing again but still having problem's.

I've been pressing with lots of weight and heating the lap but I am struggling with polishing the centre out.

Slow steady Coc strokes which have been really long but just as a test to see the shape I did a Ronchi I still see the same sort of image's as last time which is quite frustrating. The centre is still hazy which I expected.

I am beginning to wonder if my problem's lie elsewhere?

My lap seem's to be really thin on the edge now so its looking like I will have to make a new lap and the table on which I've been grinding/polishing on has a slight cup to it which could be a problem. I use a piece of firm foam mat to polish on.

I'm sure I will sort things out but It's just one of the trough's of mirror making I seem to be stuck in at present.

I'm hoping to write a thread myself once its all done and put in the trials and tribulations of it all.(must be hope as the word if hasn't crept Io there)

I will add the test image's I have as I've found these few and far between on the web and I think it's as you state the difficulty in getting the camera to perform well on them.

Damian

[JRM]

 sounds like you really need  to remake the lap Damian, the thinner the pitch the harder it becomes, I would think you need the pitch to flow good with the focal length

you are working, just might be why center is not getting contact if pitch is to thin to flow on the outer area of the lap, you must have to press often when polishing as well

to keep center in contact, just my thoughts... and yes, you will work it out I am sure...

 I made my strokes too long on my last session and now have straight bands in the center half of glass, live and learn, just finished a session of coc short and shorter strokes,

with the 15" lap,will test this evening and go from there, and to think I thought I was close...

Rick M

[JRM]

have since returned to using the 10" lap to help with central area and did two sessions of 4 trips around of coc starting with short 2" total length strokes followed by

two more of 6 inch total length, then 2 more out to the edge, then yesterday one session of 6 trips around with coc strokes edge to edge, tested this afternoon during

lunch break and took the following image....

from what I see the center still needs work as well as the area about 2" in from the edge which is looking better then the last test..

any opinions on this ronchi image are welcome.

Thanks

Rick M

[mapstar]

Great to see an image Rick. I'm sure someone will be along soon to cast an experienced eye over it and give a bit of helpful advice

Damian

[Astrobits]

Your assessment appears correct. I would work on the edge first and get that right before attacking the centre.

Nigel

[JRM]

Here is a series of images from + 84 ouside to -14 inside.

20" mirror 205 roc,50 lpi grating.

+ 84                                                                                                          +74
  

+64                                                                                                           +54

  

+44                                                                                                           +34

  

+24                                                                                                           +14

  

+04                                                                                                           -04

[JRM]

Here is a series of images from + 84 ouside to -14 inside.

20" mirror 205 roc,50 lpi grating,all images are in 1/10 inch increments

+84                                                                                                          +74

 

+64                                                                                                          +54

 

+44                                                                                                          +34

 

+24                                                                                                          +14

  

+04                                                                                                          -04

 

-14

sorry about last post, no edit, wanted them all on same page for easier viewing

so you can see just where I am with this mirror.

I hurt my wrist at work so will take a few days off the mirror to recoup, give me time to decide which

way to go about this,strokes and such that is.

Rick M

                                            

[mapstar]

Thanks for posting the Images Rick. I'm very inexperienced in reading the images but think I can see the middle needing more work. Just wondering why image +54 doesn't look symmetrical is it just a case of how the Ronchi was lined up on the mirror? 

I'm just on with making a new lap as the old one was too thin and I wasn't happy with it anyway so decided to start again. Hopefully I will be able to post some test images myself once I have the middle polished out? 

I can see me needing a lot of advice on the test results 

Damian

[Ajohn]

I have a similar problem to you Damain on what has turned out to be a long term project. Mixed designing it and making the mirror. The blank was hogged out for me and despite having done it before it hasn't finished up spherical after fine grinding. Not saying you have that problem but if you do it's best to go back to grinding again. I can tell I have a problem with simple lamp test when the mirror is wet. The image point and the distance at which the movement changes is too woolly.

The usual way to persuade a basically spherical mirror to polish evenly is to alternate tool and mirror on top. I feel I would still work that way with a floppy thin mirror. Tool on top is an excellent way of getting a good clean edge and that is also probably the best way to finish off a thin mirror.  Either way use say a 1/3 overhang max but vary it and maybe a slight W or V. You'll find mirror on top tends to polish up the centre more especially if off centre but again for a sphere not extreme. The other thing is to avoid getting into a rhythm.  Vary the steps  around and how much what ever is on top is rotated etc. It's the random aspect that makes the sphere automatically. If it's a thin mirror on top make sure you only apply pressure in the centre - a weight maybe.  Once the worst if over I'd also try say 1/2 hr polish and then a much longer press but I'm and old fashioned disciple of Texereau other than mirror on bottom for finishing.  He does describe a good way of testing for a polished sphere but it needs an adjustable slit.

John

-

[mapstar]

I have a similar problem to you Damain on what has turned out to be a long term project. Mixed designing it and making the mirror. The blank was hogged out for me and despite having done it before it hasn't finished up spherical after fine grinding. Not saying you have that problem but if you do it's best to go back to grinding again. I can tell I have a problem with simple lamp test when the mirror is wet. The image point and the distance at which the movement changes is too woolly.

The usual way to persuade a basically spherical mirror to polish evenly is to alternate tool and mirror on top. I feel I would still work that way with a floppy thin mirror. Tool on top is an excellent way of getting a good clean edge and that is also probably the best way to finish off a thin mirror.  Either way use say a 1/3 overhang max but vary it and maybe a slight W or V. You'll find mirror on top tends to polish up the centre more especially if off centre but again for a sphere not extreme. The other thing is to avoid getting into a rhythm.  Vary the steps  around and how much what ever is on top is rotated etc. It's the random aspect that makes the sphere automatically. If it's a thin mirror on top make sure you only apply pressure in the centre - a weight maybe.  Once the worst if over I'd also try say 1/2 hr polish and then a much longer press but I'm and old fashioned disciple of Texereau other than mirror on bottom for finishing.  He does describe a good way of testing for a polished sphere but it needs an adjustable slit.

John

-

Hiya John,

I went back to fine grinding from the image in my Avatar, as I'd tried polishing with lots of weight on the lap and long through centre strokes. Unfortunately I was trying to do this in the garage where the temperature was below 10deg and the pitch was too hard. Result was a badly scratched surface so a return to fine grinding was the only option. It did seem to grind much better though. Being random is something I am good at    

I'm working a 22" mirror so mirror on top isn't really an option or practical as advised by a very experienced mirror maker (Texereau 12" and below` mirror either way) .  I have learnt valuable lessons with casting the lap twice before and have a better plan to do try again this time. I have read a few bits out of the Texereau book on and there are some good bits in it, but its also quite old fashioned in some respects. Many things like materials and their quality move on and become easier to work with. There is some really interesting literature in the book and one I would advise anyone thinking of creating a mirror to take a look at it and take heed of its teachings.

Experience and help count volumes, and having a wealth of it to fall back on is a real bonus. Sadly in this country at least it's by all accounts becoming few and far between  . 

Are you fully polished out? and have you tried foucault or Ronchi tests? 

Apologies for hijacking the thread a little Rick

Damian

[Ajohn]

I'll be brief. It's small 220mm but < F3 for a cassegrain. I'm not sure I will ever finish it. I've completed the design including a corrector for it. That took a while. If I do any more work on it it will be via a machine. I have collected a few bits and pieces for that but too busy with other things.

I made a ronchi screen but wasn't impressed. I used something with the same basic design as Texereau's but with micrometer spindles to drive it and adapted to to do x-y type knife edge testing as well which I found slightly easier in some ways when things got close. As a final check I knocked up a Dall null tester.

 With your problem I wouldn't care about what experienced mirror makers thought, just find out for myself. Many use machines in any case. If I did that I would probably see what laps about 4/5 the diameter of the mirror did. Might be an idea doing it by hand as well mirror on the bottom. 5/6 is supposed to maintain the radius with suitable strokes.

Not all that much interest in dobsonians now. A 10in put me off a bit some time ago. I made a hexagonal tube from thin ply with glued butt joints -  worked well except I found it wasn't entirely square to the mirror a little too late.

John

-

[Ajohn]

I may as well elaborate on the x-y knife testing for if some one wants to measure rather accurately. It's mentioned in one of the ATM books.

It uses a mask but treats them as separate small mirrors and a double edged knife its used - a wire in other words. Each of the small mirror produces it's own cone of light, It's a little like the pure Hartman test. With that the path of each beam used to be recorded on film. Some might be doing it on dslr's now.  Each beam produces a line in the final image. Take 2 shots from different distances and the profile of the mirror can be calculated.

With the wire test the wire is traversed across the path of the beams and viewed with an eyepiece so it just has to be moved back along the axis by a set distance., I used a simple 10x loop. When the wire is exactly centred in one of the beams diffraction patterns form on either side of it so the position is adjusted until these are symmetrical.  The original article reckoned that a 1in diameter hole in a mask is accurate to 1/100 wave in relationship to a sphere so they test rather well this way even on fast mirrors. I can't remember what the accuracy claims were for the test as a whole but it really does need a delicate touch on a real micrometer spindle to get the diffraction pattern to be dead symmetrical. The maths to convert to normal Foucault readings were in the article.

As I couldn't get the type of lens Dall suggested I used one out of a 5x eye loop and knocked one up an open one on a piece of square bar illuminated with a pin hole over a LED. The diffraction colour effects were spectacular especially in the mirror itself but I could see that the mirror was smooth and that the distances were about right.  I used perspex to hold the lens which is probably explains the colour  effects. I've since read about an professional optician using an achromatic in a similar way but more as a smoothness test to spot ridges etc on very fast mirrors that might even need some of the figuring grinding in. With this one the lens is moved to null the mirror or at least get a gradually changing shadow on it.

Both tests need decent slits.  Texereau describes a good way of making them but various blades can also be polished.

John

-

[JRM]

just to update...

   My wrist has healed, 9 months since last I pushed glass but have finally started back, using a 15 inch star lap to help avoid

surface roughness, my sessions will be short, around 15 minutes but I hope to be at it every day if possible starting next

week.

I have done three sessions this week  with some 20 trips around so far, I am new to this sub diameter laps so every session

has been a test to see how the glass responds,first session was with lap 3" off center with 1/2" over hang strokes, second session

was with lap 2-1/2" off cener with no over hang stoke length and tonights session was with lap 5" off center with 2-1/2" over hang

for strokes.

I will not be giving daily updates, things will not be changing that quick, I will try to update on a weekly basis or as events require,

but I am still up for any questions or tips you all may have to offer.

Thanks

Rick M

[mapstar]

Great to see an update Rick and glad you've managed to do some more on the glass.

I hadn't realised it had been so many months since you last touched it? Time flies when you have as many problems as what I've had.

We all look forward to your progress and like me watch those lines carefully as they can quickly run wild!

Damian

[JRM]

I guess I can't add, it was some 8 months Damian, but I have done some 2 more hours of polishing with the 15" lap, keeping off center about 2 to 2-1/2" with a stroke that had a 1" overhang for the first few sessions.

I have a kink almost exactly like the one in the photo you have on here Damian,to much CoC before I guess with no W  its been there since I started polishing again,so I am trying different strokes to see what does what for changes but my sessions were to short at 15 minutes,so my last two

sessions were 30 and 45 minutes, with the first session lap flush to outer edge and stroke with 2" overhang  and the session tonight with lap starting flush to the edge with parabolizing  W strokes  from the edge to almost CoC and an overhang of 3" on each stroke. 

Its a learning process to determine what this sub lap will do with what stroke so I take it one session at a time till I figure this stuff out, lets see tomorrow just what todays session has done, I am in a learn by each session mode for now.

Rick M