800mm - the mirror

[Michele Scotti]

Hi everybody - I'm humbled that some of you are following the thread on the 800mm telescope. I thought it was worth starting a sub-topic specifically related to the mirror making.

So as the works on the mount got halted by the lockdown we had some time to virtually meet-up and discuss the optics for this project.

 

The onset was pretty straightforward with an 'if we are doing this we are making the mirror'. And that sorts out the make vs buy, I suppose.

As of now, the only two things decided so far are the diameter -at 800mm- and the f/3.3 - of course we can accommodate some variance.

 

Some aspects of the making are pretty unchartered territories for our club so I'd like to seek some good advice from anybody in terms of direct experience or rather point us at some resources/threads.

Back-ground: to make it short, our senior member had three 500mm f/5 done years ago. He did of course faster optics up to f/3 among many other mirrors - I actually never asked him how many, uh! He has always worked with full tool and has no experience with slumped glass. Btw we do have some experience in slumping glass but -weirdly enough- not in machining.

 

During our initial discussions we boiled the scenarios down to 3:

• float glass, 35/40mm thickness considering 15mm of sagitta. This is a thermal challenge with its big outward mass, it's going to make the machining more of a challenge and the stabilization time longer
• float glass, 25mm (which seems more of a commercially available), slumped. Trading the thermal challenge with the slumping
• borosilicate, 25mm, slumped. Tbh this is just a better version of the previous point at a cost that is not prohibitive

 

What are the sources of glass in Europe for thickness over the bog standard 25mm? 

 

Disclaimer: this is surely an ambitious endeavor and by no means we are underestimating that. Not only the bigger diameter is a step-up; the fast optics is a challenge too. 

 

To start with we have some questions about slumped glass. It looks like an attractive, modern approach to mirrors that exceed a given diameter. I think I saw already some threads specifically about the slumping itself - that's golden.

However it's the grinding/finishing/parabolizing that is puzzling us. Are there specific techniques or is it the same as the flat back glass?

Also, how do you support the mirror? Would a support that replicates the telescope mirror cage be appropriate - a 27-point in our case? Or is it a matter to build a concave support that holds the back of the mirror? How accurate/solid should that support be?

Any experience out there??

 

I reckon that's enough as a start - thanks everybody in advance for your contribution.

 

Stay safe! Michele

[Chriske]

That's a challenge indeed 800mm f/3.3...😳

The largest I've made so far is 20" f5.6.( by hand). Two identical disks to make a bino. Almost polished out. On hold because I've promised...   right now there's again a 20" f/4 on it's way(for a friend), but this time it will be made using a M-o-M. Most of the time I use these M-o-M's fixed post.
All these mirrors will be supported 27-point (PLOP)

My problem is : there are only 24 hours in one day...😬

[Peter Drew]

I have no experience with making large thin mirrors but I do have a 750mm F4.1 professionally made mirror which I have built into a traditional style truss tube Dobsonian.  The mirror is plate glass, flat backed, 35mm thick at the edge and with a 75mm perforation.  The mirror cell is a composite of 6mm aluminium discs separated by a radial arrangement of square tubing, the edges are closed off with a peripheral band of aluminium.  The mirror is supported by a 75mm central spigot with a retaining disc to stop the mirror riding forwards.  There is no conventional mirror support system, interspaced between the mirror and the backplate is a disc of heavy duty bubble pack type material providing 750 points of support.  The inventor of "PLOP" would have a fit, but it works.  Initially there was clearance between the central spigot and the perforation in the mirror, this was to allow an edge supporting strap to take the weight of the mirror.  This actually introduced astigmatism at low angles due to the mirror "potato chipping", it was cured by wrapping tape round the spigot until it was a good fit in the mirror perforation and then dispensing with the strap.

Seeing conditions and mirror cooling are the main influences on its performance.     🙂 

[Astrobits]

I'd scrub option 2. If you are going to slump it then low expansion will be a safer bet than float. Low expansion can be cooled a bit more quickly than float. With the development of toughened and laminated glass the thickest float glass now commonly produced is 19mm although 25mm is listed by some Chinese suppliers. Really thick stuff can still be had when aquaria replace their very big glass tanks with acrylic and occasionally when old ships are broken up.  Unfortunately most of these sources have already gone, so these are not very easy to come by but there might be some available from someone who just grabbed some when the going was good.

You will need to experiment with the support during grinding/ polishing as I am sure that you will get some astigmatism at first. I tried supporting a 40mm thick 500mm dia mirror on bubble wrap and got triangular astigmatism because I didn't rotate the mirror adequately ( I was in a hurry and bubble wrap has a triangular pattern of bubbles).

As for testing I think that you will need to make a tester specifically for this mirror. There is plenty of literature on test equipment.

Nigel

 

[Michele Scotti]

14 hours ago, Chriske said:

That's a challenge indeed 800mm f/3.3...😳

The largest I've made so far is 20" f5.6.( by hand). Two identical disks to make a bino. Almost polished out. On hold because I've promised...   right now there's again a 20" f/4 on it's way(for a friend), but this time it will be made using a M-o-M. Most of the time I use these M-o-M's fixed post.
All these mirrors will be supported 27-point (PLOP)

My problem is : there are only 24 hours in one day...😬

We like challenges, don't we?

The mirror making machine is something I'm curious about - how does yours look like? Can you recall a thread were various kind of machines are discussed? That topic is something I just know I need to dive in sooner rather than later.. 

Edited May 26, 2020 by Michele Scotti

[Michele Scotti]

13 hours ago, Peter Drew said:

I have no experience with making large thin mirrors but I do have a 750mm F4.1 professionally made mirror which I have built into a traditional style truss tube Dobsonian.  The mirror is plate glass, flat backed, 35mm thick at the edge and with a 75mm perforation.  The mirror cell is a composite of 6mm aluminium discs separated by a radial arrangement of square tubing, the edges are closed off with a peripheral band of aluminium.  The mirror is supported by a 75mm central spigot with a retaining disc to stop the mirror riding forwards.  There is no conventional mirror support system, interspaced between the mirror and the backplate is a disc of heavy duty bubble pack type material providing 750 points of support.  The inventor of "PLOP" would have a fit, but it works.  Initially there was clearance between the central spigot and the perforation in the mirror, this was to allow an edge supporting strap to take the weight of the mirror.  This actually introduced astigmatism at low angles due to the mirror "potato chipping", it was cured by wrapping tape round the spigot until it was a good fit in the mirror perforation and then dispensing with the strap.

Seeing conditions and mirror cooling are the main influences on its performance.     🙂 

Peter, thanks for sharing the details of your optics - it sounds like a monster of a telescope! Are you able to do some tracking or is it just visual?

35mm thickness is really the minium I would dare to go - ending up with 20ish mm in the center.

4 hours ago, Astrobits said:

I'd scrub option 2. If you are going to slump it then low expansion will be a safer bet than float. Low expansion can be cooled a bit more quickly than float. With the development of toughened and laminated glass the thickest float glass now commonly produced is 19mm although 25mm is listed by some Chinese suppliers. Really thick stuff can still be had when aquaria replace their very big glass tanks with acrylic and occasionally when old ships are broken up.  Unfortunately most of these sources have already gone, so these are not very easy to come by but there might be some available from someone who just grabbed some when the going was good.

You will need to experiment with the support during grinding/ polishing as I am sure that you will get some astigmatism at first. I tried supporting a 40mm thick 500mm dia mirror on bubble wrap and got triangular astigmatism because I didn't rotate the mirror adequately ( I was in a hurry and bubble wrap has a triangular pattern of bubbles).

As for testing I think that you will need to make a tester specifically for this mirror. There is plenty of literature on test equipment.

Nigel

 

I agree - if we have to slump  then Borosilicate would be better and at a marginally higher cost. Btw I've just enquired today Galvoptics in Basildon - they have borosilicate 25mm...they need to check the inventory for thicker material..

On testing I have all the bits to pull together a Bath - however I reckon our mirror maker memeber is going to prefer his Ronchi test.

[Chriske]

A M-o-M (Mirror-o-matic) is a very good concept. The mirror rotates and the 75% diameter tool swings back and forth. There's lots of information on the net about the technique 'how-to'. I have two of these machines that can easily handle 20" mirrors. I'm busy right now grinding a 20" f/4. Carbo is fed by a peristaltic pump. To avoid contamination every grid has it's own separate system. These machine's have a autonomy of about 1.5 hours. When the containers, filled with a mix of carbo/water(stir constantly), are nearly empty the machine halts automatically.  The courser grit is fed by hand. There was no way to feed carbo 80 through that peristaltic pump.

In fact one of these machines is Marc's. Side by side we use to grind our own mirrors in my workshop. We had lots of fun building them and grinding mirrors with it. Almost every year we tried to make a new scope, every time a different scope. Target and goal was open door at our local observatory. Very often a weird scope that has been built only once by an amateur. This time we were busy building a Stevick-Paul. A few month's back Marc past away at age 57, and I never touched our machines again from that day on.
Sorry to go of topic...😧

At about the same time we made our MoM's there was that guy Gordon Waite building his mirror mirror grinding machine. He uses a fixed post grinding machine. iow the tool is stationary.
He has a set of movies on YouTube. There's even a set of movies on very fast mirrors.
If we would do it all over, we probably would build a 'Waite version'. Simple to build. But these MoM's can easily grind in 'fixed post' modus too.
 

[Michele Scotti]

On 27/05/2020 at 07:59, Chriske said:

A M-o-M (Mirror-o-matic) is a very good concept. The mirror rotates and the 75% diameter tool swings back and forth. There's lots of information on the net about the technique 'how-to'. I have two of these machines that can easily handle 20" mirrors. I'm busy right now grinding a 20" f/4. Carbo is fed by a peristaltic pump. To avoid contamination every grid has it's own separate system. These machine's have a autonomy of about 1.5 hours. When the containers, filled with a mix of carbo/water(stir constantly), are nearly empty the machine halts automatically.  The courser grit is fed by hand. There was no way to feed carbo 80 through that peristaltic pump.

In fact one of these machines is Marc's. Side by side we use to grind our own mirrors in my workshop. We had lots of fun building them and grinding mirrors with it. Almost every year we tried to make a new scope, every time a different scope. Target and goal was open door at our local observatory. Very often a weird scope that has been built only once by an amateur. This time we were busy building a Stevick-Paul. A few month's back Marc past away at age 57, and I never touched our machines again from that day on.
Sorry to go of topic...😧

At about the same time we made our MoM's there was that guy Gordon Waite building his mirror mirror grinding machine. He uses a fixed post grinding machine. iow the tool is stationary.
He has a set of movies on YouTube. There's even a set of movies on very fast mirrors.
If we would do it all over, we probably would build a 'Waite version'. Simple to build. But these MoM's can easily grind in 'fixed post' modus too.
 

Thanks for openly sharing! You separate frit pump system seems very clever.

I'm very familiar with Gordon's videos - among my favorites! I like the concept of fixed post as it simplifies the constrcution and/or make it more robust. However I do not get what is the disadvantage of that

[Glasspusher]

As a mirror maker with many years experience I wonder if I might share some thoughts on your ambitious project. When making a big mirror the first thing to consider is how do I test it? If you can’t test it don’t make it. Why go through all of that work in the hope that testing will be achievable?

When testing mirrors the difficulty increases with larger diameter and shorter focal ratio. So a 300mm F5 mirror can be figured in a few hours or less by an experienced optician. An 800mm F3.3 is an entirely different proposition.  All forms of testing require a degree of skill which is acquired over years of experience. What level of accuracy are you hoping to achieve? It is unlikely that you will end up with a high resolution planetary telescope; a low to medium power deep sky telescope is a more likely outcome. Thinking about testing you might consider:

The Foucault test. The mirror could be tested at the centre of curvature with a zonal mask but this test becomes less reliable with increasing diameter and speed of the mirror. Reading the shadows is a subjective business and requires years of experience to get it mastered.

http://www.jeffbaldwin.org/figure.htm

The Ronchi test. Again this could be used. The test is basically qualitative but Mel Bartels amongst others, has developed a semi-quantitative version. The observed Ronchi patterns are compared to computer generated images of an ideal Ronchi pattern. The mirror is worked until the observed Ronchi pattern matches the computer generated pattern. This is done for a number of positions both inside and outside of the centre of curvature of the mirror.

https://www.atm-workshop.com/ronchi-test.html

https://www.bbastrodesigns.com/ronchi.html

Star testing. The best method of testing a mirror is on the stars. You need a tube assembly to pop the mirror in between figuring spells. Mirror could be figured using the matching Ronchi test to get is as near as possible, final figuring being done using the star test.

https://www.bbastrodesigns.com/JoyOfMirrorMaking/StarTesting.html

Other ‘less common’ tests include:

Double Pass Autocollimation test (DPAC). An excellent null test used extensively by professional optical shops the world over.  You need a high quality optical flat, usually the same diameter as the test optics. A smaller flat might be used with care. Optical flats are very expensive items.

Waineo test. This uses a spherical mirror to perform the null; the mirror can be smaller than the test optic. Again additional optics are needed on this occasion a spherical mirror of good quality. The test goes by several alternative names.

https://www.bbastrodesigns.com/waineo.html

Interferometry. The Bath interferometer has become popular with amateur telescope makers in recent years. Unfortunately it is not suited to large fast mirrors.

http://rohr.aiax.de/Using a Bath - EN.pdf

 

Optical testing: https://www.telescope-optics.net/testing_optical_quality.htm

The Secondary mirror. A large flat secondary mirror is a very expensive item and not one which is easily made. Most telescope makers prefer to buy a secondary mirror which will be a very expensive item.

I have experimented with both fused and slumped mirrors over the years. You would need access to a large (and expensive) kiln for a project such as this. There is a steep learning curve which can be very expensive due to the cost of electricity and breakage of glass. Annealing takes much longer than anticipated which means running the kiln for a numbers of days. Expensive! I think a monolithic blank would be better, as to the thickness, the thicker the better unless you have had experience in controlling astigmatism in thin mirrors. The problem with astigmatism is that in its more subtle forms it is easily missed until you come to examine a star image with the telescope.

Sorry that much of this sounds a little negative but undertaking a project like this is no trivial task, many have tried, a few have succeeded.

Good Luck.

John

[davidc135]

I'd contact Stathis Firstlight in Germany regarding Borofloat glass. His web page says he can supply  discs of low expansion glass in thicknesses from 5mm to 57mm with order times up to 2 months for the big stuff. Up to 20 ins in stock but larger blanks need to be ordered.  David

PS I should add that Stathis seems a person very much after your own heart and I should think will be very keen to help your project.

Edited May 31, 2020 by davidc135

[Glasspusher]

Just few further thoughts regarding the mirror blank. Slumping is very much in it’s infancy as far as amateur telescope makers are concerned. If you choose the slumping route you will have to consider:

The cost of a kiln and how to use it effectively.

How to make a suitable mould. If the mould is not accurate enough you might have to do extra grinding to get the radius required. This will result in loss of thickness which is not helpful if you are starting with what is already thin glass.

Supporting the mirror during grinding/polishing, if not done properly you will end up with astigmatism. These comments are based partly on the experience of slumping and grinding/polishing a 20 inch x 25mm thick mirror.

With a pre-generated monolithic blank you could cast a dental stone tool using tiles or steel nuts and begin work straight away. If you start with a flat blank and grind in the curve with a sub-diameter tool you will loose some blank thickness, this should not happen with a pre-generated blank. Such a blank can be purchased here…

http://www.reginato.it/blanks.htm

Alternatively, Thompson telescopes have a 760mm F3 blank listed on their website…

https://www.thomsontelescopes.com/store.html

Hope this helps.

John

[Michele Scotti]

5 hours ago, davidc135 said:

I'd contact Stathis Firstlight in Germany regarding Borofloat glass. His web page says he can supply  discs of low expansion glass in thicknesses from 5mm to 57mm with order times up to 2 months for the big stuff. Up to 20 ins in stock but larger blanks need to be ordered.  David

PS I should add that Stathis seems a person very much after your own heart and I should think will be very keen to help your project.

I had a quotation from Stathis some couple of months ago - it seems that he isn't able to provide an 800mm borofloat disk more than 25mm thick, 

Yeah, Stathis is top bloke  

[Michele Scotti]

On 30/05/2020 at 18:58, Glasspusher said:

As a mirror maker with many years experience I wonder if I might share some thoughts on your ambitious project. When making a big mirror the first thing to consider is how do I test it? If you can’t test it don’t make it. Why go through all of that work in the hope that testing will be achievable?

When testing mirrors the difficulty increases with larger diameter and shorter focal ratio. So a 300mm F5 mirror can be figured in a few hours or less by an experienced optician. An 800mm F3.3 is an entirely different proposition.  All forms of testing require a degree of skill which is acquired over years of experience. What level of accuracy are you hoping to achieve? It is unlikely that you will end up with a high resolution planetary telescope; a low to medium power deep sky telescope is a more likely outcome. Thinking about testing you might consider:

The Foucault test. The mirror could be tested at the centre of curvature with a zonal mask but this test becomes less reliable with increasing diameter and speed of the mirror. Reading the shadows is a subjective business and requires years of experience to get it mastered.

http://www.jeffbaldwin.org/figure.htm

The Ronchi test. Again this could be used. The test is basically qualitative but Mel Bartels amongst others, has developed a semi-quantitative version. The observed Ronchi patterns are compared to computer generated images of an ideal Ronchi pattern. The mirror is worked until the observed Ronchi pattern matches the computer generated pattern. This is done for a number of positions both inside and outside of the centre of curvature of the mirror.

https://www.atm-workshop.com/ronchi-test.html

https://www.bbastrodesigns.com/ronchi.html

Star testing. The best method of testing a mirror is on the stars. You need a tube assembly to pop the mirror in between figuring spells. Mirror could be figured using the matching Ronchi test to get is as near as possible, final figuring being done using the star test.

https://www.bbastrodesigns.com/JoyOfMirrorMaking/StarTesting.html

Other ‘less common’ tests include:

Double Pass Autocollimation test (DPAC). An excellent null test used extensively by professional optical shops the world over.  You need a high quality optical flat, usually the same diameter as the test optics. A smaller flat might be used with care. Optical flats are very expensive items.

Waineo test. This uses a spherical mirror to perform the null; the mirror can be smaller than the test optic. Again additional optics are needed on this occasion a spherical mirror of good quality. The test goes by several alternative names.

https://www.bbastrodesigns.com/waineo.html

Interferometry. The Bath interferometer has become popular with amateur telescope makers in recent years. Unfortunately it is not suited to large fast mirrors.

http://rohr.aiax.de/Using a Bath - EN.pdf

 

Optical testing: https://www.telescope-optics.net/testing_optical_quality.htm

The Secondary mirror. A large flat secondary mirror is a very expensive item and not one which is easily made. Most telescope makers prefer to buy a secondary mirror which will be a very expensive item.

I have experimented with both fused and slumped mirrors over the years. You would need access to a large (and expensive) kiln for a project such as this. There is a steep learning curve which can be very expensive due to the cost of electricity and breakage of glass. Annealing takes much longer than anticipated which means running the kiln for a numbers of days. Expensive! I think a monolithic blank would be better, as to the thickness, the thicker the better unless you have had experience in controlling astigmatism in thin mirrors. The problem with astigmatism is that in its more subtle forms it is easily missed until you come to examine a star image with the telescope.

Sorry that much of this sounds a little negative but undertaking a project like this is no trivial task, many have tried, a few have succeeded.

Good Luck.

John

Hi John, thanks for taking the time to write - by no means I take this as negative. It's rather a realistic approach.

As we speak I'm going through the resources you shared - same new, some already known.

To start with I agree on the testing - I'm not the 'optician' of the group but I pushed some glass in the past and at least I appreciate where your concern is coming from. 

May I ask you why you reckon Bath is not suitable for a big+fast mirror? I recall some discussions in the Bath Interpehromertry group but I not sure it was conclusive.

About slumping: so far our group has slumped up to 400mm, thin mirrors - it was quite some time ago and it was dotted with quite some shattered glass....
We are investigating where to go 'nearby' to get an oven big enough - and available for such things. I'd really like to avoid building one from scratch though.

I agree on menisci (aww, that looks and sounds terrible!) haven't proven yet to be a fully viable way - it's nonetheless pretty attractive because for instance in our case the alternative is a more standard but hefty 4k blank.

Btw, ...Thompson....isn't he the guy that built some 800mm in Liverpool years ago? That group had a radical approach to using the Bath as far as I remember.

Lastly, thanking you again for pitching in, have you wrote anything about that slumped 20"/25mm? That would be a golden experience.

Regards,
Michele

[Glasspusher]

Yes that was Dave Thompson who was part of the group that built the 30 inch scope.

The problem in using a Bath interferometer is that the resulting interferogram will have crowded fringes due to the size and speed of the mirror making analysis difficult. You will need to obtain good clean high resolution igrams with such a fast mirror.

It has been a few years since I did kiln work. I made the mould from kiln fire bricks glued together with kiln cement as shown in the pictures. I sanded in the curve using a glass disk with the approximately the same curve as required.  A circular self adhesive sanding disk was stuck onto the glass which was about 50% the diameter of the mould. The process is just like grinding a mirror and was continued until the correct curve depth was reached as measured with a spherometer. I have attached some images, first one is of the finished mould. Next three are the finished 20inch diameter by 25mm thick meniscus mirror. The forth one is the mirror stress test, even after a second annealing run irregular stress still present. With kiln work it is essential to do all heating and cooling at the slowest rate particularly when annealing. The final image was my attempt at fusing a 20 inch mirror using 15mm thick plate glass. In the end the cost of running the kiln was getting silly. I reverted to the tried and tested monolithic blank. Hope some of this helps.

John

 

[Chriske]

Did exactly the same 35(or so) years ago.
In my case I glued the 'sandwich' together with silicone.
Resulting in very weird star patterns.
That scope was nothing more then a 'light bucket'.

Using very low magn. it was doable, but high magnification revealed the true problem. Six fat spikes were visible, and no, not spider diffraction. I had glued  6 thick glass-blocks between two 19mm thick sheets of glass. Strangely enough after a few our the image got better.

One of my earlier experiments...😏

[Glasspusher]

Gluing is definitely not the way to go. Fusing and careful annealing will produce a usable result. The Hubble sandwich mirrors are of this type and many are happy with the performance. See also the work of Normand Fullum: http://www.optiquesfullum.com/optical-mirrors/

 

John

[Chriske]

16 minutes ago, Glasspusher said:

Gluing is definitely not the way to go.

I know  ...😟

[Michele Scotti]

On 04/06/2020 at 14:17, Glasspusher said:

Yes that was Dave Thompson who was part of the group that built the 30 inch scope.

The problem in using a Bath interferometer is that the resulting interferogram will have crowded fringes due to the size and speed of the mirror making analysis difficult. You will need to obtain good clean high resolution igrams with such a fast mirror.

It has been a few years since I did kiln work. I made the mould from kiln fire bricks glued together with kiln cement as shown in the pictures. I sanded in the curve using a glass disk with the approximately the same curve as required.  A circular self adhesive sanding disk was stuck onto the glass which was about 50% the diameter of the mould. The process is just like grinding a mirror and was continued until the correct curve depth was reached as measured with a spherometer. I have attached some images, first one is of the finished mould. Next three are the finished 20inch diameter by 25mm thick meniscus mirror. The forth one is the mirror stress test, even after a second annealing run irregular stress still present. With kiln work it is essential to do all heating and cooling at the slowest rate particularly when annealing. The final image was my attempt at fusing a 20 inch mirror using 15mm thick plate glass. In the end the cost of running the kiln was getting silly. I reverted to the tried and tested monolithic blank. Hope some of this helps.

John

Fire bricks sound like an excellent idea - did it last all runs in the kiln? We used some dental plaster - worked fine but was expensive although I know there are suitable alternatives now.

So were you able to complete the mirror and test it? Did you use float glass or borosilicate?

The last experiment it is indeed what Fullum and Hubble Optics are doing - very interestinf - you weren't happy with the quality?

 

Again thanks for sharing experience and pics - it's really appreciated. Annd thanks for your patience in getting back to all my questions...

[Glasspusher]

The fire brick mould worked well and survived several firings. The 500mm mirror is plate glass; with thin mirrors like this it is not so essential to use borosilicate glass as the glass cools quite quickly being relatively thin. The mirror was finished and tested but suffered from astigmatism probably due to poor support during grinding/polishing. I should probably go back and have another go at it. The reason that I did not continued with kiln work was that the cost of running the kiln was becoming a little excessive. It was apparent that it would not represent a significant saving, if any, over a monolithic blank.  The literature available on making slumped mirrors, as you are finding, is limited compared to that on monolithic mirrors which are more ‘tried and tested’ so I decided to stay with the  monolithic route.

John

[Michele Scotti]

8 hours ago, Glasspusher said:

The fire brick mould worked well and survived several firings. The 500mm mirror is plate glass; with thin mirrors like this it is not so essential to use borosilicate glass as the glass cools quite quickly being relatively thin. The mirror was finished and tested but suffered from astigmatism probably due to poor support during grinding/polishing. I should probably go back and have another go at it. The reason that I did not continued with kiln work was that the cost of running the kiln was becoming a little excessive. It was apparent that it would not represent a significant saving, if any, over a monolithic blank.  The literature available on making slumped mirrors, as you are finding, is limited compared to that on monolithic mirrors which are more ‘tried and tested’ so I decided to stay with the  monolithic route.

John

John, I'd agree that the volume/surface ratio on big thin menisci provides an opportunity to stick to float glass - I just wonder about using borosilicate. It costs more - for a 800mm, 25mm it'si around £800. vs float that I suppose would be £250(?). Borosilicate should behave better during figuring as well making it a bit easier (??). Possibly the slumping temeprature is higher - 800degC vs 650degC (???).

You see - a lot of question marks....

On the support I had a lot of thought s for quite long time - my idea would be to provide the same support as if it would be on a flat-back. What I'm trying to say is that I'm thinking about a solid support that is shaped as the back of the meniscus -like carved-out wood- and a layer of compressible material - the same that you would put under the mirror of a M-o-M . Again I'd replicate the 120deg holder on the side.

Stacking some (several) boards like well seasoneds pallets would be quite inexpensive. The a curved jig/rail to guide a router to carve-out the 'pit'. I woun't be quick but I feel it's inevitable. IF the meniscus back face is irregular - whcih is usually the case- I suppose one can use the meniscus itself to mould a layer of any material (plaster/sawdust+glue/etc) to have a cozy rest. I reckon I'd need to sketch it out to make sense of it.

Oh btw, I've read that Mel Bartels uses towels as a support....

 

"I should probably go back and have another go at it"     ----->   Oh yeah!  

[Glasspusher]

As you say the advantage of using borosilicate glass would be reduced time between figuring sessions due to the better thermal properties of the borosilicate glass.

Regarding supporting the convex back during working, I ground the convex back to get a smooth regular figure I then cast a support against the convex back to ensure the support matched as well as possible. Despite this and regular rotation relative to the support  I still got astigmatism. I don't think a wooden support would be stable enough for a mirror of this size. The stability and uniformity of wood is  a concern. The slightest deviation from a regular support can result in astigmatism.  I haven't asked as yet, but what sort of surface accuracy are you hoping to achieve?

John

[Chriske]

We use float and never wait to measure mirrors on the optical bench.
How..? We always rinse with water at ambient temp and dry immediately afterward..

[Michele Scotti]

On 08/06/2020 at 14:49, Glasspusher said:

Regarding supporting the convex back during working, I ground the convex back to get a smooth regular figure I then cast a support against the convex back to ensure the support matched as well as possible. Despite this and regular rotation relative to the support  I still got astigmatism. I don't think a wooden support would be stable enough for a mirror of this size. The stability and uniformity of wood is  a concern. The slightest deviation from a regular support can result in astigmatism.  I haven't asked as yet, but what sort of surface accuracy are you hoping to achieve?

What I'd look fo ris a support that mimick a flat turning table of a M-o-M. In that case you have a decently flat surface (sometimes is made of wood) and a layer of "softer" material to compensate for small irregulatiries. With the difference and complication that here I'd have a curved back. I woulnd't rotate if I can make it match properly or alternatively I'd properly rough-ground the back - again, isn't this what it is done for the back of a  standard flat-black glass?

The accuracu targegt is a tricky question. Lambda/4 at least - anything les than that and it would be more of a light buck than anything

[Chriske]

On 08/06/2020 at 15:49, Glasspusher said:

 The slightest deviation from a regular support can result in astigmatism. 

yep.

To overcome this I use bubble-wrap or a soft carpet under the mirror(flat back) while working on my M-o-M.

[Chriske]

14 hours ago, Michele Scotti said:

The accuracu targegt is a tricky question. Lambda/4 at least - anything les than that and it would be more of a light buck than anything

You need to make or buy a secondary far beyond that quality if you stop at 1/4, or you indeed will end up with a light bucket.

I'd go for at least 1/8.