Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_imaging_challenge_banner_30_second_exp_2.thumb.jpg.7719b6f2fbecda044d407d8aba503777.jpg

Lockie

The 8.75" f/6.7 Mirror Grind

Recommended Posts

1 minute ago, barkis said:

You're welcome to borrow my Handbook for 

Cheers for the offer Ron, no worries though, I'll have the one I've ordered in a few weeks. Still got one more grade of fine grit to go in the mean time, plus I'd better look at putting some kind of test rig together too. I'm sure there's some info online for this.    

Share this post


Link to post
Share on other sites
5 minutes ago, Lockie said:

Cheers for the offer Ron, no worries though, I'll have the one I've ordered in a few weeks. Still got one more grade of fine grit to go in the mean time, plus I'd better look at putting some kind of test rig together too. I'm sure there's some info online for this.    

Sorry for the short message. I must inadvertently pressed enter long before I'd finished. H
Full edition below. :happy11:

  • Like 1

Share this post


Link to post
Share on other sites

Hi everyone, I prepped my tool for the final Alu 600 grind before the polishing stage, but after having a real good look at the mirror magnified under an upturned 25mm Plossl, I'm a bit concerned that not all the pits have gone. I don't want to start the Alu 600 if I should be going back a grit size? With this being my first mirror I'm having a bit of trouble deciding what pits are created by the grit size I'm currently using and therefore what is acceptable? 

I've therefore uploaded a video of me sweeping the surface of the mirror with the eyepiece, so as to give a better idea to the extent of the pits and there size.

Could any of you experienced mirror makers kindly cast your thoughts on my pitted mirror and advise as to whether I should be moving forward to Alu 600 or backwards a step or two?

I don't expect perfection for my first mirror, but having said this, I don't want it too look too terrible once silvered.

  

Share this post


Link to post
Share on other sites

The scale of the "pits" suggests tiny bubbles or very early rough grinding sizes.

Going back one stage is unlikely to satisfy your desire for near perfection.

One learns to place the mirror or tool down, very carefully, without pressure, at the beginning of each wet.

When you roll the glass gently around this gives the glass a chance to crush the 'boulders' before they can do serious damage.

Great care is also needed not to contaminate later grinding and smoothing powders with coarse 'gravel.'

Fingernails short and wells scrubbed between grades? Don't open a pot or bag until you really need it.

Don't let coarse gravel go on a higher shelf where it can rain down, etc.

 

Pits will have zero visible effect on your image. Do you really want to go back several stages?

The correct answer is no. :thumbsup:

 

And, finally, always set embedded videos NOT to show further video suggestions. 

There are YT boxes to tick. You can go back and edit this afterwards.

  • Like 1

Share this post


Link to post
Share on other sites

400# is a tenacious, but a very good grit for tackling small pits.
Unless you have craters left over from bubble breakthroughs it should clean up
small pits for you. 
Persevere with it, it will get you where you need to be.
Post above by Rusted is very good advice too.

  • Like 1

Share this post


Link to post
Share on other sites

There are not many of them and they are small. Most, possibly all, can be removed with the 600 grit. Identify the exact location of a selection, 3 or 4, and check them after a couple of 600 wets. Then decide if they are likely to go with further 600 wets. If not then go back to 400 for a few wets. It is quite possible that they are caused by the largest particles in the 400 grit anyway so the 400 will remove some and produce some more.

Nigel

  • Like 2

Share this post


Link to post
Share on other sites
3 hours ago, Rusted said:

The scale of the "pits" suggests tiny bubbles or very early rough grinding sizes.

Going back one stage is unlikely to satisfy your desire for near perfection.

One learns to place the mirror or tool down, very carefully, without pressure, at the beginning of each wet.

When you roll the glass gently around this gives the glass a chance to crush the 'boulders' before they can do serious damage.

Great care is also needed not to contaminate later grinding and smoothing powders with coarse 'gravel.'

Fingernails short and wells scrubbed between grades? Don't open a pot or bag until you really need it.

Don't let coarse gravel go on a higher shelf where it can rain down, etc.

 

Pits will have zero visible effect on your image. Do you really want to go back several stages?

The correct answer is no. :thumbsup:

 

And, finally, always set embedded videos NOT to show further video suggestions. 

There are YT boxes to tick. You can go back and edit this afterwards.

Thanks for your reply, Rusted.

I'm not sure if they're pits from very early rough grinding? My logic for saying this is that I used the method of selecting the largest pit I could find, then grinding until gone. I think I may have over ground with the early grits because the pits seemed to move around. It's only in hindsight I saw I may have been creating the pits with the grit I was using at the time. Maybe I wasn't as careful as I should have been at the start of wets as you say, and maybe I didn't bed in the wets gently enough before they caused damage? I've been careful to not cross contaminate grits. Considering this along with what you have said, I think it's either pits from one of the later grits or very small bubbles in the glass. 

I'm leaning towards carrying on as you say, as I could go back several grit sizes and still end up with these pits if it does turn out to be glass bubbles.

I know it won't effect the image noticeably, but I did wonder if they would be visible to the eye once the mirror is silvered? Having said this I can't see them unless I magnify the mirror considerably so maybe it will be ok.

 

Share this post


Link to post
Share on other sites
3 hours ago, barkis said:

400# is a tenacious, but a very good grit for tackling small pits.
Unless you have craters left over from bubble breakthroughs it should clean up
small pits for you. 
Persevere with it, it will get you where you need to be.
Post above by Rusted is very good advice too.

Thanks Ron. I'm leaning towards taking Rusted's advice and just carrying on, but it's good to know that if I do go back, I don't absolutely need to go right back!

Share this post


Link to post
Share on other sites
11 minutes ago, Astrobits said:

There are not many of them and they are small. Most, possibly all, can be removed with the 600 grit. Identify the exact location of a selection, 3 or 4, and check them after a couple of 600 wets. Then decide if they are likely to go with further 600 wets. If not then go back to 400 for a few wets. It is quite possible that they are caused by the largest particles in the 400 grit anyway so the 400 will remove some and produce some more.

Nigel

Hi Nigel, thanks. If the 400 grit could possibly remove the pits and create more in a different place, what do you think about Rusted's idea of carrying on? Do you think these pits are likely to look bad once the mirror is silvered? 

I don't mind too much going back to 600 grit, I'd need to order some more grits. I guess what I'm asking is, if you were in my shoes and making progress as slowly as I am, do you think the gain would be worth going back two or three grit sizes?  

Share this post


Link to post
Share on other sites

My suggestion was based on the large scale of the pits compared with the smooth [almost invisible texture] of the overall finish.

If they are bubbles then the entire surface of the glass has to be lowered to remove them.

Perhaps you could clean the mirror well and then rear illuminate it while examining the pits/bubbles with a more powerful lens? 

Bubbles should be identifiable as reasonably round or spherical. Pits more obviously jagged.

If they really are bubbles then you may end up with a thinner mirror and still have them breaking through.

If you decide they are pits then by all means continue smoothing.

Wetting the surface will  give you a better sense of depth if you rock the mirror blank over the light.

A smear of Glycerine will make the surface more transparent and it won't quickly evaporate.

Are there more bubbles below the surface which threaten to surface? Let this be a guide.

Aluminising will certainly not hide them. It will make them even more obvious by contrast.

But only if you stare through the inevitable dust which collects on every optical surface.

They will still not be visible in the final image.

EDIT: All that said: You will get a better sphere if you continue smoothing MOT, TOT, MOT, etc.

Only you can decide how best to proceed. It will not be the end of the world whichever route you take.

Edited by Rusted

Share this post


Link to post
Share on other sites

Your last post says going back to 600 grit??

I thought from a previous post that you were only ready to start the 600 grit.

I would go on to the next grit size and see what happens, checking a selection of the larger pits as you go.

Nigel

Share this post


Link to post
Share on other sites
On 27/02/2018 at 06:38, Rusted said:

My suggestion was based on the large scale of the pits compared with the smooth [almost invisible texture] of the overall finish.

If they are bubbles then the entire surface of the glass has to be lowered to remove them.

Perhaps you could clean the mirror well and then rear illuminate it while examining the pits/bubbles with a more powerful lens? 

Bubbles should be identifiable as reasonably round or spherical. Pits more obviously jagged.

Thanks, after thinking about your post I've managed to capture these shots of the largest defects I can find across the entire mirror using a USB microscope. Then I snapped a shot of a tape measure for reference:

The largest pit looks to be around 0.5mm and the smallest around 0.1mm to my eyes.

Most look jagged so likely pits, but one or two may be bubbles as they are rounder.

Image8.jpg

Image9.jpg

Image10.jpg

Image11.jpg

Image12.jpg

Image13.jpg

Image14.jpg

Image15.jpg

Image16.jpg

Image17.jpg

Image19.jpg

Edited by Lockie

Share this post


Link to post
Share on other sites
On 27/02/2018 at 15:36, Astrobits said:

Your last post says going back to 600 grit??

I thought from a previous post that you were only ready to start the 600 grit.

I would go on to the next grit size and see what happens, checking a selection of the larger pits as you go.

Nigel

Sorry Nigel, I miss read your post and thought you were saying to go back to 400-600 Carbo. Yes I'm ready to start 600 Alu Oxide.

I've managed to get some clearer pics of the pits with a scale in the above post, if after seeing the above pics your advice still stands, then I'd be very happy to move onto 600 Alu and see what happens. 

I think the USB microscope may well be a handy tool for checking on these pits! :) 

Share this post


Link to post
Share on other sites

As you are using a pyrex blank ( just had to look back to the start of this thread for that info) some of those will be bubbles that have broken through. If you try to eliminate them you will almost certainly break through into others. That is the problem with pyrex.

I would ignore these defects ( as they will not significantly affect the performance of the finished mirror ) and continue with the next stage.

Nigel

  • Like 1

Share this post


Link to post
Share on other sites
1 minute ago, Astrobits said:

As you are using a pyrex blank ( just had to look back to the start of this thread for that info) some of those will be bubbles that have broken through. If you try to eliminate them you will almost certainly break through into others. That is the problem with pyrex.

I would ignore these defects ( as they will not significantly affect the performance of the finished mirror ) and continue with the next stage.

Nigel

A done deal! Thanks Nigel, onwards and upwards :icon_salut:

Share this post


Link to post
Share on other sites

Roughly 15-20 wets with the Alu oxide 600 done today, plus another little vid to add to the mirror grinding library on Youtube:

 

  • Like 3

Share this post


Link to post
Share on other sites
1 minute ago, Daniel-K said:

wicked project matey 

Cheers Danny, it's getting there....slowly :) 

 

Share this post


Link to post
Share on other sites

I think this means the grinding stage may be complete!? I've re-examined the pits and I'm pleased to say they have improved with the Alu 600, probably 50% better. The surface is looking smoother too, you can see large text quite easily through the mirror when it's dry, and it's very nearly as clear as glass when wet.

Looking for a bit of confirmation, does the mirror look like it's ready for polishing?

Dry:

IMG_20180309_135202.jpg

 

Wet:

 

 

IMG_20180309_140754.jpg

Edited by Lockie
  • Like 3

Share this post


Link to post
Share on other sites

It's go for polishing. :hello2:

I would always do about 5 minutes of polishing and then clean up and  check the polish pattern appearing on the mirror. This just checks the lap contact and stroke pattern is good or bad. If good, the mirror will show polish all over, probably a bit more in the center than the edge with MOT, while if it is bad then there will be a big difference and you can do something the correct it like pressing for longer , changing the stroke or switching to TOT. Keep checking with only short polishing spells until you are happy that the mirror is polishing evenly all over.

If I had to leave the polishing for a long time between sessions I would leave the lap in contact with the mirror and put them in a stout plastic bag or box with extra water to prevent drying out. That way the lap was ready pressed to restart polishing immediately.

Nigel

  • Like 1
  • Thanks 1

Share this post


Link to post
Share on other sites

Woohoo! :hello2: Thanks for all the help so far Nigel, it's much appreciated! I can't quite believe it's took me 13 months to get this far, but I'm really pleased with how it's looking now, so well worth being meticulous and spending ages prepping each grit size. I can't see a single scratch now, and the pits are surprisingly improved with a couple of hours using Alu oxide 600.

I'll check the mirror regularly as you say, and keep the lap wet and pressed. Do you think it's worth making up a test apparatus first so I don't have a delay between polishing and figuring? If so, would you recommend the Foucault test or the grating one?  

 

  • Like 3

Share this post


Link to post
Share on other sites

Looking great Chris,
how you have the patience to do this is beyond me.
I would have been climbing the walls a long time ago!

Nice Video too.  

  • Thanks 1

Share this post


Link to post
Share on other sites
10 minutes ago, Alan White said:

Looking great Chris,
how you have the patience to do this is beyond me.
I would have been climbing the walls a long time ago!

Nice Video too.  

Haha thanks Alan, much appreciated :) My family comes first a lot of the time, but I honestly enjoy this kind of thing when I get the chance to do it. My kids sometimes make me climb the walls though :grin:

  • Haha 1

Share this post


Link to post
Share on other sites

It's probably a bit late now, but I'd humbly suggest you are far more generous with the grinding/smoothing powder than I ever was.

Probably by a factor of three to four or even much more. The pros I watched, with their decades of experience, used very little abrasive indeed.

The more powder you use, the quicker it turns to mud as it grinds away at itself instead of rolling and crushing the glass surfaces.

It is the micro-fracturing of the glass surfaces by crushing which wears it away. It's nothing like using sandpaper!

Smaller quantities of grinding/smoothing powder sound and feel much sharper, louder, metallic and cut faster, but with a much shorter wet.

Mud "floats" the mirror or tool away from the opposite surface and may increase or reduce stiction.

Making it harder to control the blank and follow the desired stroke or even to tell when the wet has finished.

I used individual salt or mustard shakers and suspended the medium in water with a smidgen of washing up liquid.

I'd shake the little bottle thoroughly each time and then distributed the watery liquid evenly before gently lowering the mirror or tool to crush any 'boulders.'

Learn to never carry implements or containers directly over the working area. Sweep them in an arc around the work or place them to one side rather beyond.

Wash your hands and forearms carefully between changes of grit size. Scrub your nails thoroughly too.

Like I said: It's too late now but your mirror blank looks fine.

 

Have you tried the dry glass pencil test for matching spherical surfaces?

You thoroughly clean and dry both blanks. Wipe clean with bare DRY hands to remove any fluff from drying tissue.

Then mark pencil lines across both, fine ground surfaces form edge across the center to the far edge.

Now VERY GENTLY lower the mirror onto the tool and very gently rub one surface across the other in short strokes.

Avoid any moisture AT ALL or they may stick hydraulically together! DO NOT LEAVE THEM LYING IN CONTACT!

Now gently lift off and check the pencil lines for being evenly rubbed away.

Because the pencil lines are so thin they are a good check for even contact.

Which shows closely matching radius and sphericity between two fine ground blanks.

  • Thanks 1

Share this post


Link to post
Share on other sites

When you make up the test apparatus is up to you. Personally, I don't test until I am satisfied that all the pits have been polished out. The best way to check for pits is to examine the surface with a 10x loupe ( 25mm eyepiece ). I used a light box with a sheet of black paper/card on top from which holes had been cut. The mirror is put face up on this. Examine the area just into the dark edge and the pits stand out as bright specks.

The first test that I do when starting figuring is to examine a pinhole image with the 10x loupe in and out of focus. This is a very quick check for astigmatism. Thereafter I use a knife edge for the normal Foucault set up. Moving the knife edge either side of focus gives the same info to me as a single edge of a Ronchi grating hence I have never owned or used a Ronchi grating. When I am getting nearer to the parabola I include a null test ( Dall Null ) which I find is much easier than using Foucault or Ronchi tests. There is info on using a the basic Foucault on the internet to finish mirrors.

Nigel

  • Thanks 1

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.