Just a little tweak of the Extremely Large Telescope(ELT)

[Exoscientist]

 The current # of mirrors on ELT is 800. Only adding 30 more brings the diameter from 39.3 to 40 meters. 
 You know there’s a considerable psychological effect of that first digit, reason why retailers like pricing their products at $39.95 rather than $40. Having the telescope size at 40 meters puts its size in good stead in relation to the 100 meter, cancelled,  OWL telescope.
 Large ground, segmented mirror telescope costs scale by collecting area, i.e., by square of aperture diameter. Increasing the size by a factor of (40/39.3)^2 would add an additional 3% to the $1.5 billion dollar cost, or $50 million. I’m sure all astronomy enthusiasts world-wide would be willing to add that extra $50 million to bring its size up to 40 meters.

  Bob Clark

 

 

Edited August 22, 2023 by Exoscientist

[darthgently]

43 minutes ago, Exoscientist said:

 The current # of mirrors on ELT is 800. Only adding 30 more brings the diameter from 39.3 to 40 meters. 
 You know there’s a considerable psychological effect of that first digit, reason why retailers like pricing their products at $39.95 rather than $40. Having the telescope size at 40 meters puts its size in good stead in relation to the 100 meter, cancelled,  OWL telescope.
 Large ground, segmented mirror telescope costs scale by collecting area, i.e., by square of aperture diameter. Increasing the size by a factor of (40/39.3)^2 would add an additional 3% to the $1.5 billion dollar cost, or $50 million. I’m sure all astronomy enthusiasts world-wide would be willing to add that extra $50 million to bring its size up to 40 meters.

  Bob Clark

 

 

They could crowdfund the difference.  If amateur astronomers are 1 in 5000 worldwide it would be $30 to $35 donation each.  If only ppl would put their money where their mouth is

[magnemoe]

I think they wanted the 40 mark but it required far more than $50M, as an 40 meter telescope would be easier to sell than an 39.3 m one. 
Probably scaling factors somehow. 
But as half build its way to late to change now. 

 

[StrandedonEarth]

1 hour ago, magnemoe said:

I think they wanted the 40 mark but it required far more than $50M, as an 40 meter telescope would be easier to sell than an 39.3 m one. 
Probably scaling factors somehow. 
But as half build its way to late to change now. 

 

Yeah, those supporting structures can be beastly…

[magnemoe]

6 hours ago, StrandedonEarth said:

Yeah, those supporting structures can be beastly…

Yes, but I guess its something else who jump the cost making diameter 70 cm larger. 
Now modern telescopes has an simple mounting. You can rotate and tilt it just like artillery. Back in Hubble's days you had to use an Equatorial mount who compensate for earth rotation along one axis. 
https://en.wikipedia.org/wiki/Equatorial_mount
That also hardly limited size.

[Hannu2]

13 hours ago, magnemoe said:

I think they wanted the 40 mark but it required far more than $50M, as an 40 meter telescope would be easier to sell than an 39.3 m one. 

 

Or maybe target was 40 m at early planning but they had to make some trade off to get some scientifically more important. I think that number psychology works better for average consumers than astronomers or professional funders.

 

13 hours ago, magnemoe said:

But as half build its way to late to change now.

This is true. They have probably ordered important parts from companies and made contracts. Modifications would cost billions now.

[kerbiloid]

Make it 40.17, make the perfectionists suffer.

7 hours ago, magnemoe said:

Now modern telescopes has an simple mounting. You can rotate and tilt it just like artillery.

Or use it as a kaleidoscope to make your own space image.

[KSK]

From Wikipedia, which does appear to include relevant citations in the article:

Early designs included a segmented primary mirror with a diameter of 42 metres (140 feet) and an area of about 1,300 m2 (14,000 sq ft), with a secondary mirror with a diameter of 5.9 m (19 ft). However, in 2011 a proposal was put forward to reduce overall size by 13% to 978 m2, with a 39.3 m (130 ft) diameter primary mirror and a 4.2 m (14 ft) diameter secondary mirror.[1] This reduced projected costs from 1.275 billion to 1.055 billion euros and should allow the telescope to be finished sooner. The smaller secondary is a particularly important change; 4.2 m (14 ft) places it within the capabilities of multiple manufacturers, and the lighter mirror unit avoids the need for high-strength materials in the secondary mirror support spider.[17]: 15 

So rather more than a $50 million saving,  along with other good reasons.  I'm with @Hannu2 on this one - I really don't think the psychology of having a 40m primary mirror was worth much of a damn in this case.

[StrandedonEarth]

4 hours ago, KSK said:

I really don't think the psychology of having a 40m primary mirror was worth much of a damn in this case.

Engineers have a way of slamming lofty ambitions into hard reality

[HebaruSan]

On 8/22/2023 at 8:35 AM, Exoscientist said:

 Large ground, segmented mirror telescope costs scale by collecting area, i.e., by square of aperture diameter.

Surprisingly, an on-site expert appears to comment directly on this question:

https://youtu.be/QqRREz0iBes?si=apLpIMl-yd5wETX3&t=1456

Quote

The cost line does not follow a linear relationship, but it goes exponential. So maybe you will add, you say, a couple of meters more, and the cost doubles.

I realize that "square of" is not the same as "linear", but it's not "exponential" either. Perhaps there are some costs in addition to simply grinding the mirrors.
(And 0.7m is not the same as "a couple of meters", but exponential is still exponential.)

[magnemoe]

13 hours ago, HebaruSan said:

Surprisingly, an on-site expert appears to comment directly on this question:

https://youtu.be/QqRREz0iBes?si=apLpIMl-yd5wETX3&t=1456

I realize that "square of" is not the same as "linear", but it's not "exponential" either. Perhaps there are some costs in addition to simply grinding the mirrors.
(And 0.7m is not the same as "a couple of meters", but exponential is still exponential.)

Price of the segments are constant or rater falling the more you make.  Building become more expensive and the support structure has the largest cost increase, probably more than square. 
If you double the area of the mirrors and controls you increase the size of the support structure behind the mirrors who then require more beefy tilt and rotate structure 

[kerbiloid]

Looks like Colosseum.
They can lease the ground floor as a stadium to fund the telescope work.
Also, it's a ready-to-use arena for postapocalyptic gladiator mortal kombat.

[magnemoe]

3 hours ago, kerbiloid said:

Looks like Colosseum.
They can lease the ground floor as a stadium to fund the telescope work.
Also, it's a ready-to-use arena for postapocalyptic gladiator mortal kombat.

I was thinking more of an barbette who its kind of is. Interesting it looks like that the housing rotates independently. 
Makes some sense as it don't need the same accuracy and they can probably use the housing as an crane. 

[Exoscientist]

 Perhaps in the video Tom Scott suggests the ELT will be the largest scope ever built because heavier scopes require heavier support equipment and at some point the mass becomes prohibitive. But it could be advancing tech could reduce the mass required. For instance this research proposes using thin Fresnel lenses to replace space mirrors at much reduced weight:

A new, thin-lensed telescope design could far surpass James Webb – goodbye mirrors, hello diffractive lenses
Published: July 12, 2023 8.39am EDT
https://theconversation.com/a-new-thin-lensed-telescope-design-could-far-surpass-james-webb-goodbye-mirrors-hello-diffractive-lenses-206055

 Then these thin lenses might work just as well for ground scopes reducing the optical element weight and therefore the support equipment weight.

  Bob Clark

[HebaruSan]

28 minutes ago, Exoscientist said:

 Perhaps in the video Tom Scott suggests the ELT will be the largest scope ever built because heavier scopes require heavier support equipment and at some point the mass becomes prohibitive.

Luckily, there's no need to guess! Three reasons were given:

1. The exponential cost line
2. Limits of construction technology (this isn't very well supported in the video; no specific example is given of something that current construction tech can't do that would need to be done for a larger telescope, aside from marvelling at how huge and powerful ELT is)
3. Someday, we'll probably figure out visible light interferometry, and at that point you'd just build several smaller telescopes instead and link them up

... but I shared that link because it featured someone working on the site discussing the actual relationship between cost and radius, not for the more speculative conclusions from the creator of the video.

Edited October 5, 2023 by HebaruSan

[monophonic]

14 hours ago, HebaruSan said:

Luckily, there's no need to guess! Three reasons were given:

1. The exponential cost line
2. Limits of construction technology (this isn't very well supported in the video; no specific example is given of something that current construction tech can't do that would need to be done for a larger telescope, aside from marvelling at how huge and powerful ELT is)
3. Someday, we'll probably figure out visible light interferometry, and at that point you'd just build several smaller telescopes instead and link them up

... but I shared that link because it featured someone working on the site discussing the actual relationship between cost and radius, not for the more speculative conclusions from the creator of the video.

The "some day" of point number 3 is well into the past. ELT even belongs to the same organisation that operates the VLTI and building the former was started over 10 years after the latter saw first light. Therefore we can safely conclude that interferometry is not a panacea that makes singular telescopes unviable and point 3 is thus invalid.

https://www.eso.org/public/teles-instr/paranal-observatory/vlt/vlti/

Point 1 will always be true, but it may simply result in exponentially increasing time between new largest telescopes getting built.

[magnemoe]

36 minutes ago, monophonic said:

The "some day" of point number 3 is well into the past. ELT even belongs to the same organisation that operates the VLTI and building the former was started over 10 years after the latter saw first light. Therefore we can safely conclude that interferometry is not a panacea that makes singular telescopes unviable and point 3 is thus invalid.

https://www.eso.org/public/teles-instr/paranal-observatory/vlt/vlti/

Point 1 will always be true, but it may simply result in exponentially increasing time between new largest telescopes getting built.

Agree, Scott talked about doing it like radio telescopes, I however doubt this is even theoretical possible with technology we can predict. Problem is that reading the chip with the image data takes time. Worse you tend to observer over longer periods of time to collect more photons before reading. 
I agree that expanding on the VLTI technology for longer baselines would be pretty feasible. 

 

[NFUN]

https://arxiv.org/pdf/1107.2939.pdf

[HebaruSan]

10 hours ago, monophonic said:

The "some day" of point number 3 is well into the past.

Indeed, I mis-stated point 3 (the creator even tours the VLTI earlier in the video); the hoped-for development was the ability to do visible light interferometry with computers instead of physically with big pipes of light.

[monophonic]

On 10/6/2023 at 7:02 PM, HebaruSan said:

Indeed, I mis-stated point 3 (the creator even tours the VLTI earlier in the video); the hoped-for development was the ability to do visible light interferometry with computers instead of physically with big pipes of light.

Oh... ok... that one's gonna be far away. Considering the sampling rates required to retain phase information alone the data rates are just insane. Nyquist frequencies for visible light go into the petahertz. A digitizing device needs to run at a some multiple of that, and we have taken two decades to get from 3GHz to 6GHz. 6-7 levels of magnitude to go. Not to mention we don't have sensors that can sample combined waveforms at those wavelengths either. Do we even have any idea how such a sensor could work?

[darthgently]

On 10/6/2023 at 12:02 PM, HebaruSan said:

Indeed, I mis-stated point 3 (the creator even tours the VLTI earlier in the video); the hoped-for development was the ability to do visible light interferometry with computers instead of physically with big pipes of light.

That extremely hypothetical computer would make a great gaming machine.  Sign me up