Future Optical Space Telescopes - i.e. Hubble 2.0 ect

[gosnold]

use spiderbot

It won't produce optical-quality surfaces, but it could be interesting for a radio telescope at high frequencies

[Burninate]

A telescope assembly is constrained by the size of the mirror, but also by structural elements of the mirror cell and the truss (which must fall outside the FOV of the mirror aperture's edges to avoid vignetting).  Hubble has a 2.4m mirror but the two cylindrical body sections have diameter 2.9m (the outer edge of the optical tube) and 4.2m (an instrument, power, & avionics bay behind it).  http://hubblesite.org/the_telescope/hand-held_hubble/the_real_thing.php#size

It's hard to generalize, but the optical tube assembly being ~20% larger than the mirror itself is probably a good starting estimate.  Outside of the optical tube, the rest of the telescope's diameter is going to depend *heavily* on the configuration choices that the designers made, and might be half the size of the OTA or twice the size.

[the_roche_lobe]

Slightly tangental because the talk is about ground based exoplanet telescope innovation, but here:



Is a very interesting recent talk about radically lightening mirror segments (second half of the talk) by using UNPOLISHED commercial float glass, slumping, and fine figure control entirely by active sensors!

I'm skeptical, but if it works it promises a revolution in space-based mirror surfaces as well.

P

[Burninate]

Slightly tangental because the talk is about ground based exoplanet telescope innovation, but here:



Is a very interesting recent talk about radically lightening mirror segments (second half of the talk) by using UNPOLISHED commercial float glass, slumping, and fine figure control entirely by active sensors!

I'm skeptical, but if it works it promises a revolution in space-based mirror surfaces as well.

P

Before watching the talk:
Slumping to form (on smaller mirrors) and fine figure control by active sensors (on larger mirrors) are existing techs.  Generally, float glass is rather wasteful for large telescopes - the mass costs scale better with commodity borosilicate blends.  There are specialty glasses with better thermal or better rigidity properties, but they're very expensive and have some compromises.

I watched Kuhn's previous talk on the Colossus, from a few years ago:


And I thought that the revolutionary bit was that they were solving for phase differences of up to 10^2 waves from one side of the mirror array to the other *in software*, albeit over a tiny FOV.  A true imaging interferometer, with software coronography, albeit over single-digit arc-seconds, without needing to penetrade the extremely difficult step of fringe-tracking optical correlators and 20nm precision alignment.  The telescope is extraordinarily specialized though - it's pretty much only good for imaging one point source in a pencil beam, and he explicitly declares that it's not even being presented as an astronomical facility, only a SETI facility.  In response to that I say "Good luck getting that funded".

Watching the new talk now.

[tea monster]

The cost of a space telescope isn't so much in the mirror glass itself. It's in logistics, software, spacecraft buss, thermal/attitude control and then hauling the whole kaboodle up into orbit. After that, you have to hire a room full of spacecraft engineers to maintain and look after the thing.

[llanitedave]

A telescope assembly is constrained by the size of the mirror, but also by structural elements of the mirror cell and the truss (which must fall outside the FOV of the mirror aperture's edges to avoid vignetting).  Hubble has a 2.4m mirror but the two cylindrical body sections have diameter 2.9m (the outer edge of the optical tube) and 4.2m (an instrument, power, & avionics bay behind it).  http://hubblesite.org/the_telescope/hand-held_hubble/the_real_thing.php#size

It's hard to generalize, but the optical tube assembly being ~20% larger than the mirror itself is probably a good starting estimate.  Outside of the optical tube, the rest of the telescope's diameter is going to depend *heavily* on the configuration choices that the designers made, and might be half the size of the OTA or twice the size.

That's not necessarily the case.  We visualize mirrors in tubes, but, especially in a space telescope, an external tube is completely unnessecary.


The Spitzer Space Telescope is a case in point:





All instrumentation and mirror mounting gear can be hidden below the mirror itself, and the primary is the only part that needs to be in one piece (assuming a monolithic mirror)  All the instruments and supporting trusses can be parts of smaller assemblies that can be handled separately.


There's no reason to cling to the mindset of futility that just assumes it can't be done.

[Jim]

That's not necessarily the case.  We visualize mirrors in tubes, but, especially in a space telescope, an external tube is completely unnessecary.

The Spitzer Space Telescope is a case in point

All instrumentation and mirror mounting gear can be hidden below the mirror itself, and the primary is the only part that needs to be in one piece (assuming a monolithic mirror)  All the instruments and supporting trusses can be parts of smaller assemblies that can be handled separately.

No, wrong again.

A.  There was a light shield around the telescope.  Most will need one or a deployable one.
b.  The telescope was very small so the secondary mirror is not a big deal.  The secondary mirror on a large telescope will be bigger than the Spitzer spacecraft itself.
c.  All the instruments and supporting trusses still have to attached to the telescope and tested before delivery to the launch site. 

testing - including acoustic, random vibe or modal survey, and thermo vac.

[Jim]

And to use your example against you, the rest of the spacecraft is larger than the telescope.

[Oli]

2. Surely you jest.  The only thing they really look at was the LSAM and that could be broken down into pieces.  There are only pie in the sky ideas like ALAST.  There is no real look at the money needed to pull it off.   Why do you think JWST costs so much?  It is still within existing fairings and can use existing planes, but still needed a new dedicated container and transporter.    Itt has to be tested in the deployed configuration and there was only one place that was large enough to test it.  They had to modify the JSC thermovac chamber and build a clean room around it.

So you think telescopes bigger than JWST won't happen because they're too expensive? What's the alternative to bigger telescopes?

[llanitedave]

And to use your example against you, the rest of the spacecraft is larger than the telescope.

That's irrelevant to what I'm talking about.  Spitzer was an infrared instrument that needed shielding, it was in low Earth orbit  as well.


An optical instrument in a high orbit does not need the shielding, and as long as you were in the mood to go searching for images, you should have found abundant examples of observatory telescopes that do not need tubes.  The largest telescopes have observatory clamshells for protection from the weather, but when in use, they are mostly exposed.  A large space telescope can get by just fine without a tube.


In fact, JWST does not have a tube either, which your attempted counterexample seems to miss.  The fact remains that an 8 meter monolithic mirror, minus maybe a small amount to accommodate the inner fairing, can be launched on SLS with an 8.5 meter fairing.  You don't need to add space for a tube.  It's a non-problem, and an artificial objection.

[hop]

And to use your example against you, the rest of the spacecraft is larger than the telescope.

That's irrelevant to what I'm talking about.

The fact that Spitzer actually uses the classic mirror in a tube design hardly seems irrelevant. Even if you are correct, the example you chose does not support your argument.

Spitzer was an infrared instrument that needed shielding, it was in low Earth orbit  as well.

Spitzer is in an earth-trailing heliocentric orbit.

JWST and Herschel are examples of space telescopes that forgo the tube, but neither contradicts Jim's point about generally needing structures larger than the primary mirror.

[Jim]

In fact, JWST does not have a tube either, which your attempted counterexample seems to miss.  The fact remains that an 8 meter monolithic mirror, minus maybe a small amount to accommodate the inner fairing, can be launched on SLS with an 8.5 meter fairing.  You don't need to add space for a tube.  It's a non-problem, and an artificial objection.

I never said it couldn't be "launched".  I said we don't have the infrastructure in the country to build and test it for flight.
It is not a non-problem, It is a very very expensive problem and it is a failure on your part to understand this simple concept.

JWST has a giant deployable sun shield (Hershel has a fixed one which still makes my point), which any SLS telescope spacecraft would need an even larger one.  Another facility which does not exist.

Also, JWST's mirrors are in a vertical plane which reduces it susceptibility to contamination.  A large mirror in the horizontal plane would need covers.

Also look at SIRTF, Herschel and JWST, the instruments and spacecraft bus are still a significant size compared to the telescope.

All the concepts here:  http://www.stsci.edu/atlast either have a tube or a large deployable sunshield.  All the concepts ignore the ground infrastructure much like JWST did and will incur large cost increases.

[llanitedave]

And to use your example against you, the rest of the spacecraft is larger than the telescope.

That's irrelevant to what I'm talking about.

The fact that Spitzer actually uses the classic mirror in a tube design hardly seems irrelevant. Even if you are correct, the example you chose does not support your argument.

Spitzer was an infrared instrument that needed shielding, it was in low Earth orbit  as well.

Spitzer is in an earth-trailing heliocentric orbit.

JWST and Herschel are examples of space telescopes that forgo the tube, but neither contradicts Jim's point about generally needing structures larger than the primary mirror.

Thanks for the correction on the orbit, but my point still stands.  The Spitzer's tube is not structural.  The supporting structure of the telescope is the mirror-mounting framework on the back, and the central support that comes up through the central perforation of the primary mirror.  There is no need for any structure outside the primary's diameter.


The tube was necessary ONLY because it is an infrared instrument and needed thermal protection.  It doesn't need it for structure, didn't need it for glare, and didn't need it to protect against launch vibrations.


An optical telescope can just as easily use a similar structural design, and can just as easily dispense with the tube.  If it still needs a sunshade, a rollout structure can be fitted to the base, one that would be far simpler than what the JWST needs.


Regardless of design details, there's nothing about an 8 meter monolithic mirror that precludes it from launching on an SLS-class rocket.


Jim, I acknowledge the lack of ground infrastructure for such instruments, and that's a valid point, but it's far from a deal-breaker.  If the SLS is going to carry larger payloads, then a larger assembly facility will have to be built anyway.  I don't envision that a single 8 meter telescope is the only package that calls for an 8.5 meter fairing.

[Jim]

If the SLS is going to carry larger payloads, then a larger assembly facility will have to be built anyway. 

That's the point, it is a deal breaker.  There is no money for such payloads much less the infrastructure upgrades.

The Cape/KSC would only need a processing facility.  The nation (meaning spacecraft  contractors and users) needs bigger manufacturing facilities like in Denver, Sunnyvale, Palo Alto, El Segundo, Renton, Redondo Beach, JPL, GSFC, etc.  And testing facilities and transportation systems to go between them.  Right now, everything is centered around 5m and still not many facilities can handle it.

tens of billions of dollars.

[Jim]

If it still needs a sunshade, a rollout structure can be fitted to the base, one that would be far simpler than what the JWST needs.

That is not true either.  It is not that simple

[Todd Martin]

Just a couple comments. 

It may be lost on some that the light collecting ability of a telescope is not linear to primary mirror diameter.  Hubble has a 2.4 meter diameter lens, which translates to 4.52 m^2 of surface area (Pi * R^2).  If we choose to replace Hubble with a 4 meter diameter lens telescope, the surface area increases to 12.57 m^2, which is a 278% increase in light gathering power.  We don't have to replace all the ground testing lab equipment to get an upgrade of over double the power of Hubble.  And this is without folding mirrors.

Secondly, Hubble has been popular in large part because of the spectacular images.  JWST simply won't provide that and public support for space astronomy will suffer.  That popularity allowed for NASA to do several servicing missions over the years.  I think a "Save the Hubble" mission could still be started today if the Scientific community would decide it was a priority.   

[Oli]

If the SLS is going to carry larger payloads, then a larger assembly facility will have to be built anyway. 

That's the point, it is a deal breaker.  There is no money for such payloads much the infrastructure upgrades.

The Cape/KSC would only need a processing facility.  The nation (meaning spacecraft  contractors and users) needs bigger manufacturing facilities like in Denver, Sunnyvale, Palo Alto, El Segundo, Renton, Redondo Beach, JPL, GSFC, etc.  And testing facilities and transportation systems to go between them.  Right now, everything is centered around 5m and still not many facilities can handle it.

tens of billions of dollars.

Off topic but how was Skylab tested? According to wiki it cost $10bn in 2010 dollars.

[RotoSequence]

If the SLS is going to carry larger payloads, then a larger assembly facility will have to be built anyway. 

That's the point, it is a deal breaker.  There is no money for such payloads much the infrastructure upgrades.

The Cape/KSC would only need a processing facility.  The nation (meaning spacecraft  contractors and users) needs bigger manufacturing facilities like in Denver, Sunnyvale, Palo Alto, El Segundo, Renton, Redondo Beach, JPL, GSFC, etc.  And testing facilities and transportation systems to go between them.  Right now, everything is centered around 5m and still not many facilities can handle it.

tens of billions of dollars.

How many different, single parts of the space telescope scale with the size of the main mirror, would require new/unique tooling, have unique transportation needs that cannot be met with existing infrastructure, and/or need to be independently tested and evaluated before being shipped to NASA?

[Jim]

How many different, single parts of the space telescope scale with the size of the main mirror, would require new/unique tooling, have unique transportation needs that cannot be met with existing infrastructure, and/or need to be independently tested and evaluated before being shipped to NASA?

The testing of a spacecraft is done at the factory and not at the launch site.  And it is done as a complete spacecraft as possible.

[Jim]

Off topic but how was Skylab tested? According to wiki it cost $10bn in 2010 dollars.

Skylab was a rocket stage and not a spacecraft.  But the ATM was treated as a science spacecraft.

Science spacecraft have closer thermal interactions between the spacecraft and instruments.