First mirror grind advice

[Chris]

Hi all, every year or so I get thoughts of grinding my own mirror, however, this time round I've took things a tiny step further and made enquiries about plate glass blanks at my local glass merchants. The thickest glass they have on site is just 6mm, but they said they can order 19mm plate, so I've asked for a quote on a couple of 8.5" blanks with beveled edges. 

I would be grinding a mirror more for the experience and achievement than anything, but having said this, I would obviously wan't to use any successful mirror as part of a telescope build in order to enjoy it. Basically I know it's not a money saving exercise with any mirror below 12", but realise I'm more likely to be successful with a small and slow mirror.

First question - would 19mm plate glass be thick enough for a 213mm 8.5" mirror? I've toyed with doing a 6"f/10 through to a 10" f/6, and feel something like a 8.5" f/7 would be a good compromise. The mirror would form the basis of a Dob, so a 8.5" f/7 would give a focal length of 1500mm which I feel would give a good eyepiece position close to zenith. 

I'm very much open to discussion on mirror size and f/ratio or anything else that might be handy. Trying to tame thoughts of a big mirror as my first grind - money is tight, time can be reasonably tight with 3 young kids, and my skill set at grinding mirrors is currently zero.

 

[Alien 13]

I cant see how anything below 20 inches or perhaps even larger is cost effective for DIY once you factor in your time and all the kit you have to buy or make to test it but its always worth having a go.

Alan 

[Moonshane]

Don't know much about grinding mirrors but I tink the point is the achievement rather than any cost saving. Surely DIY time is free? No need to factor that in unless it takes you away from paid work.

[barkis]

Start off small. A 6" f10 is going to give you a good Idea of what is  required of you to 
Make your own. It won't be a difficult task, and a 19mm blank would suffice for such a mirror.
You will learn a lot, and you won't need to parabolise a mirror of that sze and focal length, you just have to work it to a nice smooth spherical
figure. You will need to make a test rig called the foucault tester, named after the Frenchman who devised the test.
However, the testing of a sphere is simple, you just need it to go dark all over when the knife edge of the tester cuts
into the reflected beam of divergent light source of the tester.
Is it easy, No, but it is not hard either, just requires fortitude and stickability, the will to succeed.
A number of members have made some excellent and massive mirrors, those are feats of daring indeed, I salute all of them. Brilliant, not just the mirror, but the 
 mounts too.  You start with small, and work your way up, I think that is the best route, but entirely up to you. If you make mistakes, it's no big deal with  Small disc.

I have NE. Howards How to Make a Telescope Book, it served me well in the past.It's a bit threadbare now, but
still perfectly readable.  I'd be happy to loan it to you for as long as you need it. Just let me know.
There is another Book by Jean Texereux which a bit deeper on the subject. I sold mine some time ago,
I wish I hadn't, but I believe it is still in print.
 

 

 

[Chris]

It must be difficult to put a price on the pride you must feel having successfully ground, polished and figured to reasonable Strehl your very own mirror, really what ever size  However, I do understand those that jump in to doing a huge mirror as it's a way of making a huge Dob more afforedable, and I very much take my hat off to them....very brave!

[Chris]

35 minutes ago, barkis said:

Start off small. A 6" f10 is going to give you a good Idea of what is  required of you to 
Make your own. It won't be a difficult task, and a 19mm blank would suffice for such a mirror.
You will learn a lot, and you won't need to parabolise a mirror of that sze and focal length, you just have to work it to a nice smooth spherical
figure. You will need to make a test rig called the foucault tester, named after the Frenchman who devised the test.
However, the testing of a sphere is simple, you just need it to go dark all over when the knife edge of the tester cuts
into the reflected beam of divergent light source of the tester.
Is it easy, No, but it is not hard either, just requires fortitude and stickability, the will to succeed.
A number of members have made some excellent and massive mirrors, those are feats of daring indeed, I salute all of them. Brilliant, not just the mirror, but the 
 mounts too.  You start with small, and work your way up, I think that is the best route, but entirely up to you. If you make mistakes, it's no big deal with  Small disc.

I have NE. Howards How to Make a Telescope Book, it served me well in the past.It's a bit threadbare now, but
still perfectly readable.  I'd be happy to loan it to you for as long as you need it. Just let me know.
There is another Book by Jean Texereux which a bit deeper on the subject. I sold mine some time ago,
I wish I hadn't, but I believe it is still in print.
 

 

 

Thanks, we really do need a double like button on SGL     I certainly won't dismiss a 6" f/10 at this stage, it would also make for an interesting instrument, I'm very fond of the 6" f/8 Dob my wife bought me for Christmas so just thought it may be too similar to that and wont teach me anything about figuring the parabola, as like you say it wouldn't need to be parabolic. It may be a good first step, I'm just trying to think how much harder a 210mm f/7 would be? 

I too solute those that have undertaken massive mirrors, I've just finished reading @Rustysplit thread circa 2013 where he took on a 14" mirror as his first grind! Super impressed! And didn't @RAC start out with a very fast 18", that turned out to have a very high Strehl......beyond amazing really!  

Thanks for the overview of the foucault test, I must admit I've glanced at info on both the Foucault and Ronchi testing and thought maybe I'll just tackle that hurdle when I get to it You've definitely put me at ease about the Foucault

Very kind of you to offer me a loan of your book, that's much appreciated  I may take you up on that at some point if I can't find the book I found a couple of years ago online in PDF format. Other than old SGL threads I've just been looking at Youtube vids so far.

 

 

[Hicks]

Not sure how much help this will be, but if you've not seen it, there's this youtube video by John Dobson which you may find interesting and hopefully some of the tips will be of use

[Chris]

The man him self! I must admit I did know about this one, coincidentally I linked this vid to someone the other day who was asking about grinding 16" for his first mirror. Much appreciated all the same   Reminds me I must watch it again. Last time I found it re assuring that even the man himself got the temp of his pitch lap wrong once or twice during his work shops. He didn't half work quick though! 

Cheers!

[Chris]

Other than asking if a 19mm blank is ok for a 210mm f/7 mirror? any advice about grit sequence? 

Everyone seems to start at 80 grit, but what would be a good sequence for a 6-8" slow mirror after that please. I think you can get some grits from Fleabay but not sure about pitch lap? Galvoptics pitch is a bit on the hard side and low on amount according to Rustysplit's 14" grind thread. I think he had to temper it with linseed oil or something?

Also not sure about quantities of each grit size needed for a 6-8" mirror? I think Fleaby sell them in 100g bags.

[M106]

1 hour ago, Hicks said:

Not sure how much help this will be, but if you've not seen it, there's this youtube video by John Dobson which you may find interesting and hopefully some of the tips will be of use

was supposed to get to sleep early lol great watch

esp loved his smashed up van hehe

[barkis]

10 hours ago, Chris Lock said:

Other than asking if a 19mm blank is ok for a 210mm f/7 mirror? any advice about grit sequence? 

Everyone seems to start at 80 grit, but what would be a good sequence for a 6-8" slow mirror after that please. I think you can get some grits from Fleabay but not sure about pitch lap? Galvoptics pitch is a bit on the hard side and low on amount according to Rustysplit's 14" grind thread. I think he had to temper it with linseed oil or something?

Also not sure about quantities of each grit size needed for a 6-8" mirror? I think Fleaby sell them in 100g bags.

There are companies that sell mirror grinding kits, Galvoptics in Basildon do them, but if you obtain your own blanks, then
It's only the Carbo grits, Alox fine grind grades, Cerium Oxide and Optical pitch to find. They will probably sell those items separately.
I bought my first Mirror Kit from Charles Frank in Glasgow, way back when Methuselah was a lad. It had two superb  8.25" Pyrex Blanks, and enough
Grinding materials to make a 8" f6 mirror. I did need much more Cerium Oxide polishing agent though, I made so many mistakes  trying to Figure it, I kept running 
out of it. Mistakes are how you learn though, and anyone who hasn't made mistakes in life, haven't made anything.
Shortly after that episode, I found the Sinden Optical Company.  David Sinden was Grubb Parson's top Optician before he branched out on his own, first at Byker in Newcastle, then later to Ryton near Gateshead in Tyneside.  John Nichol was also a very good friend of Davids, and you would be hard pressed to find a better producer of large Objectives than John Nichol.
 Dave Sinden passed away some time ago, which came as a big shock to me, although I knew he had been unwell. Such a fine man, and good friend he is sorely missed.
There will be no shortage of help and advice should you need any, Faulksy, Mapstar, Moonshane, and others have much experience in the art of mirror making,
so you are in good hands.  I'm sure you'll do well.

 

[Chris]

38 minutes ago, barkis said:

There are companies that sell mirror grinding kits, Galvoptics in Basildon do them, but if you obtain your own blanks, then
It's only the Carbo grits, Alox fine grind grades, Cerium Oxide and Optical pitch to find. They will probably sell those items separately.
I bought my first Mirror Kit from Charles Frank in Glasgow, way back when Methuselah was a lad. It had two superb  8.25" Pyrex Blanks, and enough
Grinding materials to make a 8" f6 mirror. I did need much more Cerium Oxide polishing agent though, I made so many mistakes  trying to Figure it, I kept running 
out of it. Mistakes are how you learn though, and anyone who hasn't made mistakes in life, haven't made anything.
Shortly after that episode, I found the Sinden Optical Company.  David Sinden was Grubb Parson's top Optician before he branched out on his own, first at Byker in Newcastle, then later to Ryton near Gateshead in Tyneside.  John Nichol was also a very good friend of Davids, and you would be hard pressed to find a better producer of large Objectives than John Nichol.
 Dave Sinden passed away some time ago, which came as a big shock to me, although I knew he had been unwell. Such a fine man, and good friend he is sorely missed.
There will be no shortage of help and advice should you need any, Faulksy, Mapstar, Moonshane, and others have much experience in the art of mirror making,
so you are in good hands.  I'm sure you'll do well.

 

Thanks, I may get one or two things from Galvoptics if I can't find them anywhere else. They do seem quite expensive i.e. £155 for a 210mm 40mm blank, although I'm sure it's of the highest quality plus low expansion.

They do a 6" kit still which is a very reasonable £80. comes with two 25mm blanks, so one as the tool and one as the mirror. Some grits and polish, but lacks 80 grit and pitch so it's not a complete kit by the looks of it. 

I may still give them a call and discuss my plans, they may be able to supply plain plate glass. I'm sure low expansion comes into play with large mirrors but I think I can get away with plate for a little one. Plus it will grind faster and easier from my reading so far

You're a fountain of experience barkis, and I'm sorry to hear about your friend. I lot of great people have been lost recently to add to this  Makes you realise you should get things ticked off the old bucket list, and making a mirror is one on the list for me. Without wishing to be morbid, you never know when your number is up!

I'm looking forward to adding to the pool of info on mirror grinding, I'll do a thread when it starts outlining my process, and hopefully it may inspire others

 

 

[Astrobits]

I have made many 8" and 10" mirrors ( 8" f/6 and 10" f/5 ) from 19mm thick glass and all have produced good images.  So no problem there.

I purchased most of my blanks from J.B.Treasure in Liverpool. This was about 15-20 years ago so don't know if they are still in business and I purchased them 10 at a time so don't know if they would do one or two. You can try Galvoptics/Image Optics in Basildon and VCSM in Walthamstow ( tel: 0208 520 5353) for mirror making supplies. Beacon Hill also did mirror making supplies, again, don't know if they still do.

I do not know how this idea of spherical mirrors being acceptable has surfaced. 4" f/9 and 6" f/12 can be left spherical but any bigger and shorter mirrors WILL need to be parabolised to get an acceptable image. The difference between spherical and parabolic surfaces on a 6" f/8 mirror is 0.6 wavelengths of yellow-green light. That will give a wavefront error of 1.2 wavelengths. Some years ago Peter Ceravolo made three 6" f/8 telescopes with 1/2, 1/4 and 1/10 wavefront errors. Even the inexperienced observers picked out the one with 1/2 wave error but only the experienced observers could discern the difference between 1/4 and 1/10 wavefront error mirrors. For bigger and  faster mirrors the difference between spherical and parabolic surfaces, and hence wavefront error, is even greater. You can get an idea of the image quality by simply defocussing an existing telescope by a small amount. The difference in focus points between the centre and edge will be about half the sagitta of the mirror which is given by the formula S=r^2/2R ;  r being the radius of the mirror ( eg 75mm for a 150mm mirror) and R the radius of curvature ( = twice the focal length ). A 6" f/8 mirror has a sagitta of 1.2mm so defocus by about 0.6mm on a telescope of those dimensions and assess the image quality. For an 8" f/7 you will need to defocus by about 1.8mm. I am sure that you don't even need to do the exercise to know the loss of detail that these defocus amounts would produce.

Nigel

[Astrobits]

For grit supplies you could try:

ww.ukge.com/en-gb/Lapidary/Grits,-Media,-and-Polish__c-p-0-0-14-243.aspx

You will need 80 grit as that is needed to remove the bulk of the glass to get your curve.

For pitch I always used the pitch from roofing suppliers softened with genuine turps. Turps substitute and mineral spirits can have components that are insoluble in pitch and have to be boiled off before the pitch can be used. As this is dangerous ( nasty and flammable fumes ) I have never tried to use anything but genuine turps, always turning off the heat before adding the turps to the molten pitch. When I lived in South Africa the local mirror making group did not use pitch at all. They used resin softened with engine oil so getting the genuine optical stuff is not an absolute necessity.

Nigel

 

[barkis]

44 minutes ago, Chris Lock said:

Thanks, I may get one or two things from Galvoptics if I can't find them anywhere else. They do seem quite expensive i.e. £155 for a 210mm 40mm blank, although I'm sure it's of the highest quality plus low expansion.

They do a 6" kit still which is a very reasonable £80. comes with two 25mm blanks, so one as the tool and one as the mirror. Some grits and polish, but lacks 80 grit and pitch so it's not a complete kit by the looks of it. 

I may still give them a call and discuss my plans, they may be able to supply plain plate glass. I'm sure low expansion comes into play with large mirrors but I think I can get away with plate for a little one. Plus it will grind faster and easier from my reading so far

You're a fountain of experience barkis, and I'm sorry to hear about your friend. I lot of great people have been lost recently to add to this  Makes you realise you should get things ticked off the old bucket list, and making a mirror is one on the list for me. Without wishing to be morbid, you never know when your number is up!

I'm looking forward to adding to the pool of info on mirror grinding, I'll do a thread when it starts outlining my process, and hopefully it may inspire others

 

 

Damian (Mapstar)  had a thread running when he was working his large disc, and it was both  popular and Interesting. Many members chipped in with their own tips and suggestions,
and Damian was very receptive to the advice and assistance offered.  He did a fantastic job there.
We'll look forward to your adventure Chris.
 

[Chris]

47 minutes ago, Astrobits said:

I have made many 8" and 10" mirrors ( 8" f/6 and 10" f/5 ) from 19mm thick glass and all have produced good images.  So no problem there.

I purchased most of my blanks from J.B.Treasure in Liverpool. This was about 15-20 years ago so don't know if they are still in business and I purchased them 10 at a time so don't know if they would do one or two. You can try Galvoptics/Image Optics in Basildon and VCSM in Walthamstow ( tel: 0208 520 5353) for mirror making supplies. Beacon Hill also did mirror making supplies, again, don't know if they still do.

I do not know how this idea of spherical mirrors being acceptable has surfaced. 4" f/9 and 6" f/12 can be left spherical but any bigger and shorter mirrors WILL need to be parabolised to get an acceptable image. The difference between spherical and parabolic surfaces on a 6" f/8 mirror is 0.6 wavelengths of yellow-green light. That will give a wavefront error of 1.2 wavelengths. Some years ago Peter Ceravolo made three 6" f/8 telescopes with 1/2, 1/4 and 1/10 wavefront errors. Even the inexperienced observers picked out the one with 1/2 wave error but only the experienced observers could discern the difference between 1/4 and 1/10 wavefront error mirrors. For bigger and  faster mirrors the difference between spherical and parabolic surfaces, and hence wavefront error, is even greater. You can get an idea of the image quality by simply defocussing an existing telescope by a small amount. The difference in focus points between the centre and edge will be about half the sagitta of the mirror which is given by the formula S=r^2/2R ;  r being the radius of the mirror ( eg 75mm for a 150mm mirror) and R the radius of curvature ( = twice the focal length ). A 6" f/8 mirror has a sagitta of 1.2mm so defocus by about 0.6mm on a telescope of those dimensions and assess the image quality. For an 8" f/7 you will need to defocus by about 1.8mm. I am sure that you don't even need to do the exercise to know the loss of detail that these defocus amounts would produce.

Nigel

Hi Nigel  Really great to hear 19mm will be fine for the kind of size mirror I'm planning. I've not yet heard back from my local glass merchant, but if they do get back to me and the price is good I think I'll go that way. I'll probably talk to Galvoptics also and ask about the possibility of plate glass blanks just incase they can sort me out with what I need.

Ok, I'll take on board that any practical f/ratio/focal length scope will need to be parabolic. As much as I like the sound of a 6" f/12, I'd probably need to take oxygen upto the eyepiece/summit

Your defocus formula really gets the point across, I think I non parabolised 8" f7 would be pretty blurry by the sounds of it ! 

 

[Chris]

20 minutes ago, barkis said:

Damian (Mapstar)  had a thread running when he was working his large disc, and it was both  popular and Interesting. Many members chipped in with their own tips and suggestions,
and Damian was very receptive to the advice and assistance offered.  He did a fantastic job there.
We'll look forward to your adventure Chris.
 

Thank you very much, looking forward to it  Also looking forward to reading Mapstar's thread, I'll dig it out, I hadn't spotted that one! 

[Chris]

43 minutes ago, Astrobits said:

For grit supplies you could try:

ww.ukge.com/en-gb/Lapidary/Grits,-Media,-and-Polish__c-p-0-0-14-243.aspx

You will need 80 grit as that is needed to remove the bulk of the glass to get your curve.

For pitch I always used the pitch from roofing suppliers softened with genuine turps. Turps substitute and mineral spirits can have components that are insoluble in pitch and have to be boiled off before the pitch can be used. As this is dangerous ( nasty and flammable fumes ) I have never tried to use anything but genuine turps, always turning off the heat before adding the turps to the molten pitch. When I lived in South Africa the local mirror making group did not use pitch at all. They used resin softened with engine oil so getting the genuine optical stuff is not an absolute necessity.

Nigel

 

Pitch from roofing suppliers, I would have never known, I'll check that out, thanks  If so I'll be sure to use the genuine Turps and have the flames off when adding, especially with several small children in my house!

I guess I've got a bit of time before this stage, I'll be knocking together a tile tool first.

And before that.....would you use a barbel weight or another matching glass blank to hog out?

Which do you think is best for a beginner?

Ooh and which stroke for hogging out, I've seen people do it a number of ways on Youtube etc, so not sure which stroke to use?

 

[SilverAstro]

ooops, dupe, deleted !

[SilverAstro]

45 minutes ago, Chris Lock said:

 a 6" f/12, I'd probably need to take oxygen upto the eyepiece/summit

That is when you start into the realm of the schiefspiegler and TCT, see elswhere in this parish

Looks like Beacon Hill are still 'doing' - good to see, (grit and things in the price list page)

http://beaconhilltelescopes.org.uk/  

for grit sequence, strokes and all else ! go :-

https://stellafane.org/tm/atm/index.html

https://stellafane.org/misc/links.html

but beware, one can spend so much time following and reading those links you will not have time to grind !! (dont ask )

Good luck.

 

[Chris]

3 minutes ago, SilverAstro said:

That is when you start into the realm of the schiefspiegler and TCT, see elswhere in this parish

Looks like Beacon Hill are still 'doing' - good to see, (grit and things in the price list page)

http://beaconhilltelescopes.org.uk/  

for grit sequence, strokes and all else ! go :-

https://stellafane.org/tm/atm/index.html

https://stellafane.org/misc/links.html

but beware, one can spend so much time following and reading those links you will not have time to grind !! (dont ask )

Good luck.

 

I'll be honest, I don't get the reference lol but I'd forgotten about Beacon Hill. I once bought a completed mirror from them circa 2004, glad they are still on the scene! I'll check them out

lol I think I know what you mean about reading verses doing, I suffer from bad memory which doesn't help lol

The sooner I get my hands on the blanks, grits, and pitch....the sooner I can start

Cheers for the links!

[SilverAstro]

4 minutes ago, Chris Lock said:

I'll be honest, I don't get the reference lol

Sorry, yes I was a bit cryptic ! It is the Yolo topic in the Astro lounge,, hang on ,,

here : https://stargazerslounge.com/topic/285317-yolo-on-ebay/

but dont go there - you have enough to do with grinding lol   !

Yes Beacon Hill through thick&thin good to see,

[Astrobits]

If you can get some scrap 10-12mm thick plate and cut/break it into approx 1" squarish bits then you will be able to hog out and fine grind with them as a tile tool. If thinner glass is used then you will have to make two( or more ) tiled tools, the following one made against the partial curve made by the previous one. Stroke approx 1/3 dia centre over centre near the end but off centre to get things going quickly. If you use two glass blanks beware that they can lock together if the curves on the mirror and tool don't match which will occur if you do too extreme strokes. The tile tool will not lock, it has that advantage.

Making pitch laps needs to be done outside for safety, and the smell will v.quickly get the wrath of SWMBO if you try it in the kitchen.

Nigel

[Astrobits]

Just a quick point on the glass tile tools, the edges will be very sharp so no fingers hanging over the edge of the mirror when pushing/pulling it.

Nigel

[Chris]

17 hours ago, Astrobits said:

If you can get some scrap 10-12mm thick plate and cut/break it into approx 1" squarish bits then you will be able to hog out and fine grind with them as a tile tool. If thinner glass is used then you will have to make two( or more ) tiled tools, the following one made against the partial curve made by the previous one. Stroke approx 1/3 dia centre over centre near the end but off centre to get things going quickly. If you use two glass blanks beware that they can lock together if the curves on the mirror and tool don't match which will occur if you do too extreme strokes. The tile tool will not lock, it has that advantage.

Making pitch laps needs to be done outside for safety, and the smell will v.quickly get the wrath of SWMBO if you try it in the kitchen.

Nigel

17 hours ago, Astrobits said:

Just a quick point on the glass tile tools, the edges will be very sharp so no fingers hanging over the edge of the mirror when pushing/pulling it.

Nigel

Hi Nigel, Thanks for the heads up regarding heating the pitch out doors! I need to tread carefully when it comes to SWMBO, I doubt she would be amused if I stunk the house out cooking pitch and Turps, net alone blowing the kitchen up! Maybe a camping stove in the garden would be a safer bet

I've not heard about breaking up thick glass to make a tile tool, I can see the logic if it means using one tool throughout grinding, although I'm a bit concerned about the safety aspect of it in a family environment  I can see the logic, but I don't think I can go down this route, Nigel.  

Whilst on the above subject, before work I spoke to a guy at Galvoptics about my plans and He's going to call me back with a quote on a 210mm 25mm plate glass blank. He was a friendly talkative kind of guy and said "you're not going to use a tile tool are you!?". Apparently they get hundreds of mirrors to coat through their doors, and they noticed that many of the ones produced by a tile tool exhibit scratches. I asked why this would be the case? he said because it's very difficult to remove all the previous grit size from a tile tool before proceeding with a finer grit. He said this is the case even if you scrub the tool. he recommended using a slightly sub diameter glass blank tool about 19mm thick, and said I should use it to both hog out and fine grind, followed by laying the pitch on it for polishing. Based on this info I asked for a quote on a glass tool also, but wondered what you thought of this? I must admit I like the sound of no sharp glass shards which could cut fingers.