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Why does it take longer to make bigger mirrors

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#1 brebisson

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Posted 19 February 2019 - 08:12 AM

Hello,

 

It might look like a stupid question at first glance.

After all, a bigger "thing" requires more work than a smaller thing, isn't it?

 

But, on a mirror, the work, (grinding action) is done by the tool. And the tools grow in size with the mirror.

So, if a 6" tool is able to remove 1mm of glass of a 6" mirror in 2h... Should not a 12" tool be also able to remove 1mm of glass of a 12" mirror?

 

Same goes for polishing. This is a surface thing... and the surface of the tool grows with the size of the tool....

 

So, why is it that the general wisdom is that a larger mirror is more work than a small one?

 

Cyrille



#2 pyrasanth

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Posted 19 February 2019 - 08:26 AM

I don't think the problem is the initial grinding but the final configuring and polishing. Remember the surface area increases massively as the aperture increases and that is a shed load more problems to overcome in the final polishing & configuring. I've never made a mirror so this is more an educated guess but I can quickly work out that the initial grinding would be the least of any problem.



#3 Cames

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Posted 19 February 2019 - 08:42 AM

One factor that occurs to me is that the volume of glass that must be removed increases as the cube of the mirror diameter whereas the grit surface increases roughly as the square of the diameter.

-----------

C


Edited by Cames, 19 February 2019 - 08:54 AM.

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#4 TOMDEY

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Posted 19 February 2019 - 10:45 AM

Good observations and good points... all !

 

Where I worked (B&L, Kodak, ITT, Harris, Excelis, Navy... others) we made mirrors from rather small to several meters. The biggest difficulty (and therefore time, cost) driver, as a function of SIZE... is the wavelength-normalized accuracy needed on the optical surface of the mirror.

 

My explanation goes like this >>>

 

Consider a nice, ten-inch, smooth, 1/10-wave paraboloidal Primary Mirror. Use it to view the heavens, and everything is perfection... as good as the laws of physics permit. It is, functionally, a ~perfect~ mirror.

 

Now, just take that exact same mirror, and grow it ten times bigger. It is now a sorely-deficient, hundred-inch, rough, 1-wave paraboloidalish Primary Mirror. Use it to view the heavens, and everything is blurry, aberrated, hairy... as bad as the terribly-flawed Hubble mirror produced at its disastrous first-light.

 

So, in that sense, a mirror that is ten times as big has to be ten times as accurate... just to perform as well as the smaller one. Think of a good 100-incher as comprising one hundred ten inchers, all connected together, and needing to deliver simultaneously great wavefront, plus tip-tilted, pistoned and radiused to match  one common focal point. That's asking a LOT. Add to those considerations, the amplified difficulties of mounting, aligning, pointing... the whole task is extremely non-linear. More like cubic, quartic, quintic?! All of this because the wavelength of light does not scale up with aperture! So, our perfection yardstick... remains fixed!   Tom


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#5 stargazer193857

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Posted 19 February 2019 - 01:07 PM

To make a 16", 4x as much glass is hogged out using traditional methods, and fine grinding is 4x the surface area.

Using the same amount of force, which already is high, requires a subdiameter tool that must be controlled better, or using a lighter weight tool. Lighter weight might remove less even proportionally.

Big mirrors also get heavier, which can be an issue when rinsing the grit.

#6 stargazer193857

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Posted 19 February 2019 - 01:14 PM

Hello,

It might look like a stupid question at first glance.
After all, a bigger "thing" requires more work than a smaller thing, isn't it?

But, on a mirror, the work, (grinding action) is done by the tool. And the tools grow in size with the mirror.
So, if a 6" tool is able to remove 1mm of glass of a 6" mirror in 2h... Should not a 12" tool be also able to remove 1mm of glass of a 12" mirror?

Same goes for polishing. This is a surface thing... and the surface of the tool grows with the size of the tool....

So, why is it that the general wisdom is that a larger mirror is more work than a small one?

Cyrille


8x as much power is needed. You could run 8x as many small mirrors with that power. It is much easier to rinse the smaller mirrors. And the smaller mirrors would have half as much friction hear per square inch.

#7 tommm

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Posted 19 February 2019 - 01:18 PM

More material must be removed from a larger mirror so more energy input is required to do that extra work.  The removal rate depends on tool pressure, force per unit area. A larger tool then requires more force for a given pressure.  You can increase the removal rate by adding more weight to the tool, increasing pressure, and you can use larger grit size to increase the rate of work during rough grinding, but that will tend to leave a rougher surface with deeper pits which require more finer grinding work to remove, so grinding will necessarily take somewhat longer.

 

But that is still a minor part of the time, most is spent figuring.  As TOMDEY said, you need to hold the same tolerance to deviation from the ideal curve over a larger area. So for one thing you generally do not use a full size tool to figure a large mirror because you tend to mess up the curve at some radii (zones) while improving it at others unless the mirror is high f number.  Most larger mirrors made these days are low f number, < f/4, so people use sub-diameter tools.  Correcting a large mirror is then not just a simple scale up using the same technique of correcting a small mirror. You generally have to use more varied strokes and localized pressure to remove more material in targeted areas. By large here I mean on the order of 16" to 30" that an amateur might attempt.  The larger mirrors used in professional mirrors like TOMDEY was involved with are another story with the many other issues he mentioned.

 

One rough analogy might be in making a cut at a specific angle in wood, say to fit 8 pieces together to form an octagonal ring.  The narrower the width of the boards used, the less maximum gap you will have along a joint for a given error in cutting the angle,say 22.2 deg rather than 22.5 deg.  Using much wider boards with the same angular error will result in a much larger gap, so you have to take more pains to get that angle cut "just right" to have a well-fit joint. You will likely have to fiddle with making repeated tiny adjustments to the miter arm on the table saw and using scrap wood to test to get the angle just right.



#8 Pierre Lemay

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Posted 19 February 2019 - 02:26 PM

Having made a couple of mirrors, I've noticed the following when hand working with full size grinding and polishing tools:

  • Rough grinding from a flat surface to a given radius of curvature (say, f/5) takes longer as the diameter increases
  • Fine grinding takes a similar amount of time whatever the diameter
  • Polishing takes a similar amount of time whatever the diameter
  • Parabolizing takes a similar amount of time but is more dependant on:
    • Focal ratio (faster = longer time to complete)
    • Thickness of glass (cool down time before test is longer for thicker mirrors)
    • Use of sub diameter tools takes more time

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#9 Mattimac

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Posted 19 February 2019 - 10:32 PM

One factor that occurs to me is that the volume of glass that must be removed increases as the cube of the mirror diameter whereas the grit surface increases roughly as the square of the diameter.

-----------

C

The night is totally cloudy here so I thought I'd calculate how much glass is actually removed for 6" f/8 and a 12" f/8 parabolas:

 

6" removes about 0.66 cubic inches; the 12" removes about 5.3 cubic inches.  Ratio 1:8, as Cames indicated already.

 

PS - my wife asked what I was doing!

 

Cheers...


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#10 stargazer193857

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Posted 20 February 2019 - 12:20 AM

The night is totally cloudy here so I thought I'd calculate how much glass is actually removed for 6" f/8 and a 12" f/8 parabolas:

6" removes about 0.66 cubic inches; the 12" removes about 5.3 cubic inches. Ratio 1:8, as Cames indicated already.

PS - my wife asked what I was doing!

Cheers...

The edges will likely decrease during hogging.

...

I just checked the numbers based on perfect curve removal. I got the same. 0.66.

Edited by stargazer193857, 20 February 2019 - 12:23 AM.

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#11 stargazer193857

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Posted 20 February 2019 - 02:15 PM

<p>

Having made a couple of mirrors, I've noticed the following when hand working with full size grinding and polishing tools:

  • Rough grinding from a flat surface to a given radius of curvature (say, f/5) takes longer as the diameter increases
  • Fine grinding takes a similar amount of time whatever the diameter
  • Polishing takes a similar amount of time whatever the diameter
  • Parabolizing takes a similar amount of time but is more dependant on:
  • Focal ratio (faster = longer time to complete)
  • Thickness of glass (cool down time before test is longer for thicker mirrors)
  • Use of sub diameter tools takes more time

Thank you for the info. I think Danny said required fine grind time is proportional to diameter, which is still less than proportional to area. But it also depends on technique used.



...

The first respondant also made an interesting point: a proportionally bigger mirror may have proportionally bigger grit and need an extra session to get as fine.

#12 TOMDEY

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Posted 20 February 2019 - 09:38 PM

For the record... I've seen Full Diameter pitch laps in action on reasonably fast 2.5 meter mirrors... great for "smoothing runs" to blend out the zones, otherwise so common on big mirrors. My opinion is that we should do that on our little amateur mirrors, too!  Always have one full-sized lap handy, then you can use those sub-diam ones with a lot more confidence that smoothing the mirror and chasing out any astig will go nicely, using the big lap.

 

I know we're discussing grinding here, but that's the easy part!   Tom


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#13 gregj888

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Posted 22 February 2019 - 11:18 AM

Doing exactly that now with my 20"... 

 

 

For the record... I've seen Full Diameter pitch laps in action on reasonably fast 2.5 meter mirrors... great for "smoothing runs" to blend out the zones, otherwise so common on big mirrors. My opinion is that we should do that on our little amateur mirrors, too!  Always have one full-sized lap handy, then you can use those sub-diam ones with a lot more confidence that smoothing the mirror and chasing out any astig will go nicely, using the big lap.

 

I know we're discussing grinding here, but that's the easy part!   Tom


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#14 stargazer193857

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Posted 22 February 2019 - 11:34 PM

For the record... I've seen Full Diameter pitch laps in action on reasonably fast 2.5 meter mirrors... great for "smoothing runs" to blend out the zones, otherwise so common on big mirrors. My opinion is that we should do that on our little amateur mirrors, too! Always have one full-sized lap handy, then you can use those sub-diam ones with a lot more confidence that smoothing the mirror and chasing out any astig will go nicely, using the big lap.

I know we're discussing grinding here, but that's the easy part! Tom

That's been my plan all along.
And no need to go after just the center on a thin mirror.

#15 Pinbout

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Posted 23 February 2019 - 05:45 PM

 

My opinion is that we should do that on our little amateur mirrors, too!

cause there's a little atm'ing mirror maker in all of us... lol.gif



#16 TOMDEY

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Posted 23 February 2019 - 08:24 PM

I just realized something... If a mirror is a light year across, it would have to take at least six months to make it, and probably a lot longer than that, for innumerable practical reasons.

 

Then pointing it around... maybe a string telescope?    Tom



#17 stargazer193857

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Posted 23 February 2019 - 09:56 PM

I just realized something... If a mirror is a light year across, it would have to take at least six months to make it, and probably a lot longer than that, for innumerable practical reasons.

Then pointing it around... maybe a string telescope? Tom


They should also make is a fast f#, not a slow one, or they would have to wait longer for each result.

#18 stargazer193857

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Posted 23 February 2019 - 09:59 PM

Eventually, with 1/4" slumped spherical mirrors made parabolic with pressure, laser controlled pressure could read the atmosphere and clean up the seeing for that scope. Expensive, but Bartels' successor will do that.

#19 TOMDEY

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Posted 24 February 2019 - 02:48 AM

Eventually, with 1/4" slumped spherical mirrors made parabolic with pressure, laser controlled pressure could read the atmosphere and clean up the seeing for that scope. Expensive, but Bartels' successor will do that.

Yep, few minor technical details, and we'll all be enjoying that. Pearl-divers are already on the waiting list!    Tom

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#20 mashirts

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Posted 24 February 2019 - 08:15 AM

1950 computers filled a large room. Now they are in your back pocket and much faster.

#21 brebisson

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Posted 24 February 2019 - 10:34 AM

1950 computers filled a large room. Now they are in your back pocket and much faster.

Hello,

 

Are you saying that now a light year diameter telescope would be a light year accros, but that in 70 years they will only be 100m accross?

 

Cyrille


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#22 mashirts

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Posted 24 February 2019 - 11:29 AM

Not referring to size of mirror. More referring to inevitability of technical progress and how quickly it can happen.

#23 mashirts

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Posted 24 February 2019 - 11:42 AM

Currently the world has 7 billion cell phones. If there was a demand for 7 billion 20 inch Newtonian telescope mirrors with 1/10 wave front accuracy I suspect we'd already have the the technology to produce those relatively inexpensively by now. I believe inevitably we will. More an issue of how long you hold your breath for it.

#24 mashirts

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Posted 24 February 2019 - 12:04 PM

But Tom mentioning adaptive optics for amateur use is a stretch is correct. But who knows what the future holds.


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