The 20 " grind has begun

[Astrobits]

Ground backs can make it difficult to see the pits.

For my mirror making I made a light box and covered the glass top with a sheet of black paper which had holes in it. The holes were about 1" ( 25mm ) square. The mirror is put face up on the light box and the surface examined with a 10x loupe. The pits can then be seen best around the boundary of the holes where you have angled lighting against a black background. It is easy to focus on the surface as it is always covered with small bits of dust which can be moved with a slight rub of a finger. The ones that don't move are the pits.

Wetting the back is more effective if you use something that matches the refractive index of the glass suchas an oil. Water is not a very good match for glass refractive index. If oil seems a bit messy then Glycerol can be used.

Nigel

[JRM]

just to update.. I did a half hour polishing mirror on top on thursday, and today after pressing for 45 minutes I did another half hour mirror on top

then pressed again for half an hour with a mesh like material and did one more half hour mirror on top before flipping everything over and polished 

with 10 lbs weight on top of pitch lap for 1 more hour,contact with lap and glass is great, I honestly stunk like an dirty wet dog so to speak when I was

done but oh my how well the pitch works when you do longer sessions.

took photo of pressing with 36 lbs on top the glass..

and this one..

Nigel, nice concept with the light box, I found on the web where a gent was using a flashlight shining it up through the bottom with a upside down eyepieces as a magnifier as well.

just looking at the surface naked eye i can see little tiny bubbles (and I mean tiny) inside the glass that I never seen before, so things are getting polished for sure, hope to do a couple more hours tomorrow, till then...

Rick M

[Astrobits]

Yes, 25mm eyepiece = 10x loupe.   I find that an eyepiece is less user-freindly than a loupe but a is still useable substitute. You can, of course, use other focal length eyepieces for different magnifications, 12.5mm eyepiece = 20x mag.

As long as there is light shining through the glass at a shallow angle and you view against a dark background it doesn't matter how you do it. With small mirrors hust holding them up to a window or light does the trick but holding a 20" bit of glass up with one hand and holding a loupe in the other to examine the whole surface is a bit trickier

As I was doing more than one mirror I made the light box as it was the easiest solution. It also allowed me to make pencil marks on the side of the mirror so I could keep track of any particular features and see how they change with the grinding/polishing.

Nigel

[JRM]

I spent yesterday going over the surface with light shining from underneith the glass and a 20mm eyepiece upside down, its really amazing what you can see

through the upside down eyepiece, at first I thought I was looking at pits but my they were big for pits at this stage, then I though no, they are pits on the bottom

of the glass having been ground flat... but what I come to figure it to be is tiny bubbles in the glass, can not see it being from the back of the glass and they are

to big to be pits, will pick up a stronger magnifying lens and give it another look over but willk polish more just the same.

I injured my wrist at work this morning but did manage to get a half hour in M.O.T. after pressing wish mess in between glass and lap for about an hour total with

36lbs weight on top. the lap is working great, the micro fauciting is an added bonus,helps with pressing and makes the lap work even better. 

hope to get a polishing session in each night this week time permitting, will keep you posted...

Rick M

[Astrobits]

That's the one negative to "PYREX". It seems to always have lots of bubbles in it and they often break through the surface at the most inconvenient times. As long as the open bubbles are few and small they won't have a significant effect on the optical performance of the finished mirror. It does bug the mirror maker though, having crafted a surface accurate to fractions of a wavelength of light, there is this +/- 0.5mm hole.

You can always go back to grinding to remove them of course

Hope yours stay below the surface.

Nigel

[mapstar]

I've spotted a few in mine near the surface after the 600 grit grind and I'm keeping my fingers crossed not to go through any Nigel.

[JRM]

Nigel,  I do not yet have any that came through the surface, though some look mighty close I doubt any will during polishing, fingers crossed just the same. 

its really hard to focus on the surface,actually I can not do so unless I place a piece of thread or find a speck of dirt on it, and yet its like the bottom of the

glass is the top.

my honest opinion is the polishing is done,but still I will do more, as much as I hate the thought of a scratch happening, more is always better to a point.

today I pressed t.o.t. for 30 minutes with 36lbs weight, then did 45 minutes of polishing  all t.o.t. and c.o.c.

My pinkish ceo  is runnig low (and to think I though I had lots) but still have enough for a few more hours so polish tell then atleast.

I have more of the other stuff for figuring and even some red rouge so think I am good but may need more, only time will tell, will try to do a 1/2 hour session

or more each night this week and go from there... till then it looks like a great night for the 12" dob must go get it set up, later all..

Rick M

[JRM]

just to update...

today I did a  30 minute press with 36 lbs on top of glass then polished m.o.t for 45 minutes, channels will have to be touched up

before I do my next session other then that everything is moving along fine..

Rick M

[JRM]

 update for yesterday Aug 14th,

I noticed a sleek or very light scratch on the glass about 3 inches long when checking the surface, darn thing, thinking it happened when I

was done for the day and lifting the glass off the lap, so I redid the channels on the lap, pressed for 40 minutes t.o.t. with 36 lbs weight then

polished for 30 minutes t.o.t.

now for today,

I checked the glass and no scratch or sleeks to be seen, thank goodness, now I have but two choices, continue to polish even though i

think the polish is done and risk a scratch or call it and move on to test and figure.

Even though I am all but sure it is done I intend to do a couple more hours, hate the thoughts of a scatch but the contrast killing unfinished

polish would be far worse then any scratch left on the surface in my opinion.

Rick M

[mapstar]

I bet its looking really nice now.

I have yet to cast my lap and start polishing but may do it tomorrow. I still have the first of three scope's to finish for a friend so this take's priority.

[JRM]

Today after pressing for 50 minutes with the weights, I did another 45 minutes m.o.t. and c.o.c. strokes.after which I cleaned the surface of the glass

and let it dry, then inspected the whole surface from every angle with glancing light and a magnifying glass and can honestly say the polishing is done.

tomorrow I shall build that testing stand that was suppose to be built long ago and begin the testing to see where things stand with the optical

quality and what may need to be done before figuring begins.

yes Damian,it is looking real good, here is a photo of the polished surface...

keep us posted on your progress Damian, looking forward to the updates.

In a few days I should have an initial surface test report, till then...

Rick M

[crashtestdummy]

Nice work fella.I was advised to polish for an hour and then test just to get a general idea rather than polish fully before the initial test.I'm hoping to get some work done this year as I've stalled a bit with a new job and a week's training course away last week.resigned to maybe not finishing mine this year

[Peter Drew]

Same stars next year.  

[Rustysplit]

Looking really nice Rik. Keep us posted on the testing.

[mapstar]

That's a great looking piece of work Rick. Can't wait till mine is looking similar.

I have started building the rocker box today and at 770mm Sq its a big thing! so hopefully have it glueing up by the end of tomorrow, one day I'll make a start on mine but it may be round sumerian style. I love the round look but it may work out a little large thats why I went for a square one for my friends scope.

I will eventually do a build thread but thats when its all finished.

Looking forward to the test report and well done up to now.

Damian

[Sfarndell]

Well done Rick - excellent progress!

If you can, do a Ronchi test initially just to check the general shape until you start seeing some parabolisation. It's much quicker and easier than doing a foucalt test, but at the cost of quantitative accuracy. Stellaphane has a link to print a 2"x2" 133 lpi ronchigram, which I printed at high resolution on an old Overhead projector film and it works really well!

Looking forward to your test report.

Scott

[JRM]

okay,

  tried to test the other day but my tester was not working properly, would not slide smoothly on the sliders, sitting for 10

years took it course I guess so I did a revamp to the tester,it was built as per the directions on the stellafane site, worked

great last time around, go figure...

I removed the tensioner spring, the guides and the slide beam, all of these I replaced by putting a one foot long threaded

1/4 by 20 rod locked in place on both ends, one end of course has the turning knob attached, as for my knife edge carriage

I secured two T nuts, one on each end and the 1/4" rod threads through these, so there are no,slides or guides and no need

for the spring, the whole carriage moves back and forth on the threaded rod itself,the real nice thing about this is that it elimanates

the slack that was there when goinging from one direction to the other, works real nice and smooth too.

My r.o.c. is 205 inches or F 5.125, the surfaces is showing some zones, alomst like when your star testing and out of focus, you see

the rings, best way I can discribe it, maybe 5 rings in total, no turned edge to be seen so far, the zones appear to be only minor

so I will press the lap tomorrow and do a half hour with extra thin polish mix to smooth everything out, then test again,other then that

the surface is good so once I smooth it out more it will be good to go figuring, till then..

Rick M

[mapstar]

Any chance you can post some pictures of the tester Rick as I've made one myself but not sure if It's correct?

Will you be casting another figuring lap soon then?

[JRM]

Rich, it will be there waiting, you will get her done, I hope to get through with the figure this year but if not, next year is fine, no rush here but would

like to finish by next summer if possible.

Alan and Scott, thanks and will do..

Damian, thanks also, sounds like you have your hands full.

I may try figuring with the full size lap I have but may make a sub diameter lap, still contemplating

that very idea,cross that bridge soon enough I guess...

 my knife edge tester is rough, nothing fancy, t nuts, hot glue and plywood, shes not pretty but only has the one job to do,and the prinicple is sound

i believe...once you are done laughing look it over, it is very sturdy, just rough, will save my wood working skills for the telescope build.

here is an overview.... see where the slide bar was located before on the surface...

a view of the threaded rod assemble, you can see where I removed the V block slides from under the knife edge assembly...

a front view...

and a view from the dial...

The two screws in the knife edge tower are for the ronchi screen, the two small holes are for the L.E.D. light which I have hooked to a

12 volt transformer (power cord for a video game machine or something like that) and I still need to put a marking pin over the dial indicator.

It does work really well, thats all that matters to me.

today I pressed for 2 hours t.o.t. and 36 lbs weight for first 1-1/2 hours,10 lbs for last 1/2 to insure a good mating of lap and glass,

moving the lap around every 1/2 hour and spraying water to keep it all moist, then polished for 1/2 hour t.o.t. with a slight W stroke, tomorrow I will check to see if it smoothed the surface, oh and I diluted my mix 20 to 1 to help with any roughness.

Rick M

[JRM]

I did a test to see if the patterns have gone, they are less but still there slightly,so I check my channels on the lap and

come to find that when last I channelled I went the wrong way with center most groove,it was almost dead on center in

one direction,  this is more then likely the cause, so I fixed the channels and will press for a bit then polish some more,

test again tonight, will let you know how it goes..

Rick M

[JRM]

after testing last night the zones are  still there but getting better, so today I brushed off the surface of the lap,marked the

direction of by channels on the back of the lap both ways, this to insure I do not push the lap parallel with the channels,

thinking this may have been an issue as well after reading up on it last night, my mix may also have been to thin for

smoothing everything out as well so I mixed at 10 to 1 for this round of polishing, after a good hour plus cold pressing I did

a one hour session alternating between c.o.c. and a narrow W stroke, will test later to see what changes are, till then...

Rick M

[Astrobits]

Provided you are doing random strokes ( as you should ) then it doesn't matter where the channels in the lap are ( either positional across the lap or relative to your strokes ). Trying to avoid certain patterns will almost certainly have you getting into another pattern with zones as the result. A truly random arrangement does not have a certain pattern removed as you will then have a pattern of missing components

Nigel

[mapstar]

Thanks for posting the pic's of the tester Rick very interesting.

I may need to re make mine? I didn't think the tester needed that much movement in out?

I am eager to get started on the polishing.

[JRM]

Nigel,

 Everything has been random, be it stroke length, step around, rotation of lap and glass, have been very careful of that,

after last nights testing I am starting to think its from the back or bottom of the glass, ever hear of something like this, my

reason for trying a session without polishing on the grooves was because if nothing else it would have an effect on the 

whole situation one way or another, but nothing has changed, its still the same.

I notice the effect with Ronchi testing the most, but the lines are straight as can be, no visual blur or fuzz to the lines at

all,under foucault it is even harder to notice but the blink out at roc is even, sphere looks great.

your thoughts on this would be appreciated.

Damian, you don't need a whole lot of swing, just makes it easier in my opinion to get setup.

Rick M

[Astrobits]

If the simple Foucault shows an essentially good sphere then it's go for Parabolizing. It is amazing how small subtle zones can be when seen with these tests. You have many wavelength of glass to remove during parabolizing (a 20" f/5 needs 8 waves of glass from the centre tapering to 0 at the edge ) and fractions of a wave of glass now will make no difference to the final outcome.

James Muirden's book "Beginners Guide to Astronomical Telescope Making" has an example of just how sensitive the Foucault test can be. Take a mirror of 3000mm roc which has a 20mm wide zone. To null this zone requires the knife edge ( fixed pinhole position ) to be moved towards the mirror by 10mm.

I am sure that you would agree that any such zone would be VERY prominent and cast horror in the mirrormake's mind. Yet it is only 1/20 wavelength of yellow light deep!!!

Try to imaging how the subtle zones you are now seeing would appear superimposed on the Black/White donut you will have when parabolized. Chances are you would not be able to find them.

Happy Parabolizing.

Nigel