Jump to content

sgl_imaging_challenge_2021_3.thumb.jpg.30e9b298c34c80517e8b443ce153fce3.jpg

First time mirror grinding - 6" F4.5ish


DuncanC
 Share

Recommended Posts

Hi,

After about 15years of thinking about it, I finally got started on my first mirror grind today. I bought a used mirror grinding kit from AstroBuySell which includes a 6" 25mm plate glass blank, matching glass tool, pitch, selection of grits etc. The kit appears to be a 30 year kit from Beacon Hill Telescopes. The first owner had taken copious notes while rough grinding out to ~F11 (0.9mm), then switching to 120 grit and smoothing. Theres a couple of small chips on the back of the blank and tool, and a tiny chip on the edge of the mirror. They are not mentioned on the notes so suspect they may have happened during storage.

I realise I'm aiming for a fast mirror for a beginner. This is really driven by the fact that I want to use this mirror in a telescope optimised for astrophotography at prime focus and like the 700mm focal length. Whilst a faster mirror like this might be a stretch I'm going to try and make things as easy as possible for my self during the figuring stage by making the best tester I possibly can and a sub-diameter lap if needed. Being 6" I'm prepared to take lots of attempts to figure if needed as returning to a sphere should be quicer than larger mirrors.

First task was to bevel the edge of the back some more as this was a little small. This was a lot quicer than expected using the grinding stone. It's around 3mm. Is it better to have this as a 45 deg chamfer, or a smooth radius? I'm assuming the former to avoid too much light scatter?

The little chip has been reduced by the increased chamfer and I also blended it a bt using a wet diamond file to avoid further damage.

After that was done I started increasing the sagitta from the current 0.9mm up to my target of ~2.1mm. After just over an hour of the chordial stroke at 40%, reducing to 25% I got the sagitta upto about 1.6mm. At the moment I'm measuring using feeler gauges and a straight edge. Maybe I'll make a spherometer at some point. Chip much less noticable.

IMG_20210507_172244696.jpg

IMG_20210507_172309585.jpg

IMG_20210508_190350581.jpg

  • Like 7
Link to comment
Share on other sites

Good luck! Have often wondered how much effort and how long this would take, so would love to hear how you get on.

Gordon

Link to comment
Share on other sites

That takes me back to my early years trying to follow my 1st edition of Texereau's 'How to make a telescope'.  Saw the light in the end a bought the mirrors (although it was fun trying) 😊

Link to comment
Share on other sites

Thaks for the encouragement guys. To answer the questions I'm working in the workshop I built a few years ago. You need somewhere you can make a little messy I guess during grinding. It's not ideal though as I've done lots of GRP work in there recently so lots of dust around. I think I'll do my polishing outside if I can to avoid contamination.

I did another hour or so mirror on top with 80 grit using a reducing chordial stroke down to a normal 1/3 centre over centre stroke. This got me up to around 2mm where I decided to check the focal length. Luckily the sun was shining earier so I was able to measure the focal length and it's about 700mm, basically where I wanted. I decided to call it a day with the 80 grit and do a good clean up.

After lunch I added a bevel to the tool (didn't know this was needed), and increased the bevel a little on the mirror. I switched to the 120 grit to clean up the pits left by the 80 grit. I started with a quick sharpie test to see if the shape of the curve was okay by seeing it it grind out evenly. I think it was okay, but it only took like ten strokes before it had gone and seemed to be even, so not worried. Will check again at the next grit. The fine grinding continued with a mix of tool on top and mirror on top to avoid changing the focal length too much. I did a couple of hours like this and seem to be pretty much done with the 120grit. Unfortunately the first owner of the grit used up most of the 120 grit (the notes say 2 tsp per wet!) so I've run out. The images below show the surface with the worse imperfections I could find. Am I okay to move to 220 or do I need to order some 120 grit? Thoughts? Ignore the out of focus dust on the old 25mm eyepiece I used for the photos.

IMG_20210509_182938786_HDR.jpg

IMG_20210509_183055834.jpg

  • Like 1
Link to comment
Share on other sites

I measured my focal length earlier using the wet mirror in the sun. It had gone up a bit from where it was, upto around 750mm, with a reduction in sagitta of about 0.15-0.2mm during my grinding with 120 grit. I must have had the tool on top too long during grinding. I shall grind with mirror on top from now on to increase the sagitta back to where it was.

Did about another 1 3/4 hr this evening with the 220 grit and happy to have gotten rid of those pits. Surface is pretty uniform now and I've used all the 220 grit I had in the it. Not going to worry and move to the 320 grit.

Measuring the sagitta using feeler guages after the 220 grit it appears to have gone back to ~2mm, which should put the focal length to ~680mm/F4.6 ish (taking the bevel into account).

Increased the bevel again on the tool and mirror. Should be the last time now.

Link to comment
Share on other sites

Gld to see a fellow ATM'er. I just completed 400 grit on my 6" f/8 visual only scope (I am otherwise an astrophotographer). I've also procured an 8" blank which was half finished to 800 grade by <God knows who>. I have a 4.5" f/10 ready to polish 😛

Its going to be fun.

I hope you have procured optical grade Cerium Oxide 2-3 microns?

  • Like 2
Link to comment
Share on other sites

Hi, Thanks for the interest. I can see this becoming quite addictive! It seems nobody ever finishes just one mirror. You either don't complete it, or if you do finish it you go on to make several more! Quite rewarding I have to say. Just wish the sun would come out today so I could check the focal length again.

I have a tub in the second hand kit I bought labelled "Cerox" - but no idea if it's still any good or what grade. Will buy some more if needed - luckily there's a couple of suppliers here in the UK. From what I've read in India getting blanks and grit isn't too bad, but pitch is difficult to come by.

Link to comment
Share on other sites

Posted (edited)
4 hours ago, DuncanC said:

Hi, Thanks for the interest. I can see this becoming quite addictive! It seems nobody ever finishes just one mirror. You either don't complete it, or if you do finish it you go on to make several more! Quite rewarding I have to say. Just wish the sun would come out today so I could check the focal length again.

I have a tub in the second hand kit I bought labelled "Cerox" - but no idea if it's still any good or what grade. Will buy some more if needed - luckily there's a couple of suppliers here in the UK. From what I've read in India getting blanks and grit isn't too bad, but pitch is difficult to come by.

If the Cerox is offwhite colour, almost like but smoother than Flour or Skimmed Milk powder, then you are near optical grade. The others are off pink colour. 

You could think of automating the grinding/polishing by making a simple fixed post machine too. I've recently built one, but haven't tested it yet.

Edited by astrosathya
  • Like 1
Link to comment
Share on other sites

A grinding machine would be nice, but it seems you can do a better job by hand due to the random nature, and it would take longer to build than grind my mirror. I have to admit to looking at a few for future mirrors if I decide to make another one. I wouldn't want to do much more tha an 8-10" by hand! I think the cerox is the right colour, but haven't tried the consistnacy as I dont want to open it before I have too.

Measured the focal length in the sun earlier. Appears to be as close to 680mm as I can measure, so sagitta was correct as far as I can roughly measure.

Did a good hour this evening with the 320grit ali oxide. It looked to be pretty much ground out after half an hour but I decided to do the regulation hour anyway to be sure. Image below.  Ive been lucky for stray grit so far, but there's been the odd bit floating around I've managed to spot before grinding. Hopefully be able to do the finder grinding and polishing outside to reduce the chances of contamination.

IMG_20210511_224525680.jpg

Link to comment
Share on other sites

I did a good hour on each of the 225, 175, 125 and 95 (22.5 -9.5 micron?)  optical powders. Probably didn't need a full hour, but no problem to continue for a bit longer to make sure it was ground out. Was very lucky (I think due to the glass tool and keeping everything clean) that I didn't get any scratches during fine grinding.

With fine grinding done I'm moving on to polishing. i spent the day making a pitch lap for polishing the mirror. The 30yr old black pitch from Beacon Hill was very hard and failed the thumbnail test. I bought some proper turps from HobbyCraft and added a third of a cap full into the pitch. Made a massive difference and was now probably a bit on the soft side. I raised the heat on the pitch a little and let the volatiles boil off till it was a bit harder. Pouring the lap seemed to go okay and I think I have contact. Hopefully get some polishing done this week. Expecting 6+ hours of polishing prior to figuring.

  • Like 1
Link to comment
Share on other sites

Watch the polishing pattern carefully. With the mirror on top you will see a difference between the centre and edge after as little as 5 minutes.

I work both mirror and tool on top to even out the polish. If I left the mirror on top until the mirror was fully polished there is a danger of going too deep in the centre and having a hyperbolic figure which is difficult to polish back to parabolic. A hyperbolic figure can occur very easily with an f/8 but your f/4.5 will take more effort and time to go hyperbolic.

Nigel

  • Like 1
Link to comment
Share on other sites

Hi Nigel, Thank you for your post. I will keep an eye out. From what I've read polishing is quite hard work so taking a break occasionally to see how the polish is developing won't be an issue! It appears that 50/50 Mirror on top / lap on top is the way to go? Will try to monitor it.

Link to comment
Share on other sites

It is no more difficult than grinding, if anything it is easier not having to stop so often to replenish the abrasive material. However it takes longer than you first expect. Initially, a polish appears quite quickly but to remove the last traces of pits seems to take much longer as you will also be pressing the lap to the mirror often to maintain the contact and you can then go have a cuppa or whatever🍻.  The one drawback that I found with the pitch lap on top is that the polishing slurry doesn't stay in the lap grooves  but falls out and is pushed off the edge of the mirror which means that you tend to use more slurry when lap on top.

Nigel

  • Like 2
Link to comment
Share on other sites

A mirror grinding and polishing machine is a serious time waster.
You can put a lot more effort into manual grinding and polishing.
Plus you get the randomness of strokes and rotation to smooth things out.
A machine has to be deliberately varied to avoid zones.
I spent years using home made machines.
 But would have been more productive without.
You can manually make any mirror you can lift.
Though the drag of polishing a big mirror can be quite hard work.

An achromatic lens is far more than just four sides.
The glass cost is considerably higher than a mirror making kit.
The flint glass is fragile and prone to edge chipping.
You have to closely control four radii and minimize wedge. 
A spherometer with a decent dial [digital] gauge helps.
Ideally you need a larger optical flat for testing.
Plus a monochromatic light source.
A six inch is far easier to make than anything smaller.
The strokes are more easily managed working manually.
You need curved lens holders and extra tools and laps.

Fortunately there is plenty of help available online these days.

 

  • Like 1
Link to comment
Share on other sites

Thank you for the interest guys. Agree that anything smaller than a 6" would be a struggle. My hands are struggling with a 6" as it is. 8" would be easier! Was thinking it would be a nice job to help tone the old muscles without a machine, but not sure they agree!

After a busy week where I didn't get anything done I did an hour and quarter earlier MOT. I'm doing 1/3 centre over centre with slow speed and lots of pressure. 6-10 strokes, rotate the mirror ~10-30deg, repeat maybe 3-4 times, then move to the right a step. Once Ive gone round the corner of the table I rotate the lap 30 deg or so. Does that sound about right?

Photo of the lap before polishing below. I cold pressed for 45mins with a net from a pack of oranges and a counterweight prior to polishing.

before.jpg.efe5543feff173948dc6ced4138a639d.jpg

After an 1.25hrs the lap looked like this. I had a few chunks of pitch come off round the edge. I think contact is okay, but time to open up the channles maybe?

after.thumb.jpg.de563b49996437c9c7c933e0672d2f10.jpg

Given how much my hands hurt I'm wondering if I'm using too much pressure which is why the channels have closed up so quickly. That or the lap is a bit soft.

Just as you predicted Nigel, the mirror almost looks polished out to the naked eye bar a slight duller area in the outside 3-5mm. I fully intend to polish for at least 6hrs before testing though. Apart from anything else I need to build a tester!

 

Link to comment
Share on other sites

The lap channels need to be opened up and the lap needs the net pressed again before proceeding.

Do you have a stand that can hold the mirror vertical for testing? I can tell you how to measure focal length accurately using a stand, torch and some water. We did that for years in a mirror making class I taught at North Carolina State University (USA).

Keep up the good work!

- Phyllis

  • Like 1
Link to comment
Share on other sites

As Phyllis says-- re-cut and press.  You don't need to press very hard, if at all. Also, you don't need to use long strokes. In grinding that is used to deepen the centre of the mirror but you don't want to do that now until figuring. Thus the shorter the stroke the better in polishing. I worked my mirrors on a pedestal/barrel and rotated the mirror and moved a step round every stroke in an almost continuous movement.

I was told, many years ago, that the polishing action occurs at the edged of the blocks not on the faces. I don't know if that is true but without those channels the polishing slurry will not get to the centre of the mirror. Using a net produces more edges so that might be the reason for that technique.

I always checked the whole surface of my mirrors with a 10x loupe and placing the mirror on to a light box ( easily made- does not need to be evenly illuminated ) with a black card which had holes cut out. This gave an obliquely illuminated mirror surface with a black background when viewed near the edges of the cut-outs.

As you have a reasonable polish I would suggest that you check the mirror now using a simple pinhole and 10x loupe at the centre of radius. You will get a fairly accurate measure of the FL and by looking at the discs of light inside and outside of focus you can check for any astigmatism.

Nigel

  • Like 1
Link to comment
Share on other sites

Sorry for the delayed replies again. Over the weekend I opened up the channels and re-pressed the lap with the net in place. I did another 1.5hrs MOT, taking me to three hours. I then opened the channels again with the soldering iron and pressed again.

The mirror looks really polished out. I checked with the eyepiece and could see much but there were a few odd isolated pits. I was holding the mirror up to the window, and it was easier to see where my fingers blocked the sunlight, illuminating the surface obliquely as you say Nigel. When grinding I supported the mirror on a couple of blocks of 3x2 and slid a garage inspection lamp underneath which worked quite well as a simple light box, might do that again.

I did do a laser pointer test - I thought I was polished out. I really wasnt! I still have a spot on the front surface of the mirror that is slightly stronger toward the edge as expected. The mirror doesn't look frosted at all however. I will complete the planned 6hrs polishing and check again. Not going to worry too much though- I suspect parabolising may take a few attempts so plenty more time to get it's totally polished!

Another hour completed this evening taking me to 4 hours. This time Lap On Top.  I will do the rest like this to make it roughly 50/50. My lap is 152mm diamter vs 155mm for the mirror, so I have to move the cleats each time I swap over.

I will probably build a tester sometime this week before the polishing is finished. Intending to build a mirror support similar to the one in the BBC video, but with a strap to support it. My tester will be based on my smaller milling machine so I can take advantage of the x-y table. I'll use a magnetic dial indicator stand as a variable height prop for the head. I'll probably 3d print a couple of heads, one with an LED and stanley knife blade (slitless Foucault) and the other with some form of Ronchi set up. I had better get busy!

  • Like 1
Link to comment
Share on other sites

Now upto 6 hrs polishing 50/50 Mirror On Top / Lap On Top. Polishing is nearly completed. I was seeing resisdue on the glass with the laser pointer not a lack of polish. Things improrved a lot with a clean with an IPA lens wipe. It's only the outside 3-5mm that is unpolished now. Will do another couple of hours, but not panic too much as I'm expecting to have a lot of porabolising attempts which will polih further. Couple of images below.

 

Centre

IMG_20210529_220950644.thumb.jpg.579f4f593ec271cb39382b1d365545cb.jpg

Edge

IMG_20210529_220958722.thumb.jpg.937fca62d925948fe50b085a2a7b7272.jpg

 

Link to comment
Share on other sites

What if you hit a nice parabola while the edge is still incompletely polished?

To parabolise a 6" f4.5 from a sphere you need to remove approx 2 microns of glass from the centre and none from the edge.

Finish polishing before you even attempt to parabolise.

Nigel

Link to comment
Share on other sites

I totally agree with your thinking Nigel, in fact I've seen similar written elsewhere. It's not my intention to test earlier, more that I will polish until I'm convinced I'm finished, but am likely to have a good safety margin.

Another 2 1/2hrs completed, taking me up to ~8.5 hrs. Laser completely invisible on the front surface nearly all the way to the edge. I'd say it's just visible around 3-4mm  from the edge and there's a slightly hazy ring of about 1mm at the far edge. I'll do another 2-3hr's and see where I am.

In between polishing I built a mirror stand over the weekend. There's a single foot at the back for adjusting the angle. I made it a little oversized, big enough for a 12" if needed. Mirror support by the sling at the front and stopped from falling forward by a plastic head wing bolt.

 

IMG_20210531_165043115.thumb.jpg.c2db79d433c23cc7d14df7b8d4e961f5.jpg

  • Like 1
Link to comment
Share on other sites

After 10.5hrs of polishing I decided to to call it done. Laser pointer test seems good as long as the mirror is cleaned with a lens wipe first.

Did my first tests of the mirror - I found it very hard to align the mirror and tester to get decent results. I need to sort a better Ronchi screen as it's currently homemade from 0.1mm fishing line.

Initial tests below.

Inside ROC

IMG_20210606_115652.thumb.jpg.8820b2edb7fd240abb70a88fc8963fbd.jpg

Roughly ROC

IMG_20210606_115714.thumb.jpg.438df43d5debc9b1b490a17324103e5a.jpg

Outside ROC

IMG_20210606_115730.thumb.jpg.1b525b7add9d671967d2055fa0cb502b.jpg

 

Looks like I have okay polish, but slight hill in the centre (under corrected sphere) and a slight turned down edge. I've recut the facets in the lap and cold pressing it. Plan to do 30mins with 1/4 Centre over Centre strokes and see where that gets me.

  • Like 1
Link to comment
Share on other sites

Results after 30mins 14 CoC MOT and an hour or so cooling. Please ignore the finger prints (since cleaned).

Inside RoC. Not much change but maybe slighty reduction in the centre hill. Bit worried I might be increasing the TDE.

IMG_20210606_180448.thumb.jpg.4bee5b6e65dd5b1ae07980a05c073353.jpg

Foucault Test. Noticed a lot rougher surface since the last 30mins of short strokes. This is just inside RoC. Central hill smaller but more pointy... Odd.

IMG_20210606_180211.jpg.41addc0b8231e0a47e39327b55a715d1.jpg

Outside RoC. Bumpy.

IMG_20210606_180320.jpg.eca131662767e9eb8b14c88347df5607.jpg

Lap is pressing now. I might do a little bit more 1/4 CoC MOT later. Not sure if I should try to fix the TDE or the central hill as I'm oblate at the moment.

  • Like 1
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.