[watsisname]

As I understand it the signal processing for optical interferometry is roughly the same, but it requires far greater precision in the alignment and stability of the instruments to work.  Atmospheric turbulence is also an important factor.

But basically yeah, jr2, we can and do combine many telescopes together to obtain far greater resolution than what a single telescope could achieve.

edit:  Slightly aside, I recall reading that there are plans in the works to use infrared interferometry to directly image the event horizon of the Milky Way's central black hole.  That would be AWESOME.

[Astronomiya]

Besides, adaptive optics has really killed the near-infrared and visual light space-based telescope.

Nope, not yet.  AO isn't good enough to beat Hubble's resolution except on bright objects even in 8-meter and above class telescopes, and the accuracy of AO is still heavily dependent on getting good seeing.  JWST would/will absolutely murder any current AO system.  The best resolution for AO is achieved in the J, H, and K NIR bands; LBT has reached upwards of 98% Strehl on a star with an R band mag of ~8 in H band, but can't crack 20% in visual wavelengths (Esposito et al., 2012), and it all gets worse as the star gets fainter.  Note that the 98% Strehl also required 0.3" seeing, which is about the best it ever gets.  With more typical 1" seeing, the 8th R band mag star resulted in a Strehl ratio of ~40% in H band.

Hubble, by contrast, gets full resolution and 99%+ Strehl all the way down to its limiting magnitude (25-30 in most bands and depending on exposure time, IIRC) in all wavelengths it can observe.

Actually, I take it back: there is one thing that Hubble has going for it in comparison to ground-based observatories, though given Hubble's relatively small size it's not clear to me it isn't illusory: it can take extremely long continuous exposures, up to 30 hours in some cases, while a ground-based telescope will have to space such a thing out over several nights.

Still, considering we've cracked the visual light adaptive optics problem (Palomar will be making the first active use of such a system at a major observatory for most of this week), even that's not an insurmountable problem.

Such an advantage isn't illusory; the Hubble Ultra Deep Field probably couldn't have been taken from a ground-based observatory.  The systematics generated by observing on multiple nights are very difficult to correct for, and you wouldn't be able to go as deep with any confidence.  You also can't just stare at it on consecutive nights, too; you have to take photometric standards several times each night to get a handle on what the atmosphere is doing.  Your resolution also isn't as good as Hubble, etc.  Hubble makes many things much, much easier because it's above the atmosphere and doesn't have to deal with changes in transparency or seeing.

Also, I've heard nothing about this revolutionary new AO system on Palomar.  You mind posting a link to some information on it, if available?

AFAIK most (all?) interferometers require physical combination of the signal, though, so large ground-based models are usually impractical.

As The E noted, this is true, but it is very easy to capture the phase and timing information on a radio signal and record it all, so the signal can just be time-matched and combined later.  This is really difficult to do in the optical, so short baselines and direct combination of the light have to be used.

[Bobboau]

/*knowledge*/

you need to post more.

...unless that was copypasta.

[Astronomiya]

Nope, no copypasta - I'm a grad student in astronomy.  I do this for a living.

Slightly aside, I recall reading that there are plans in the works to use infrared interferometry to directly image the event horizon of the Milky Way's central black hole.  That would be AWESOME.

Sub-AU resolution at over 8 kpc distance?  That would be AWESOME.  That's an angle of a few tens of microarcseconds.

[NGTM-1R]

Also, I've heard nothing about this revolutionary new AO system on Palomar.  You mind posting a link to some information on it, if available?

They just stripped down the PHARO/Palm3000 combination and rebuilt it with a newer and better system; it's been doing engineering runs lately and though I'm told it's actually commissioned, I have no idea if it's done useful science yet.

I confess I'm finding it difficult to provide a useful link since :Caltech: and their neglect of keeping information available to the public up to date. The public page on Caltech astronomy is still talking about the laser guidestar system that has not been used in quite some time. I'm not terribly surprised, the museum and visitor's gallery computer are similarly bad and make reference to instruments not active in over a year, but...all the links I can provide make reference to the older version of the system and are at least a year out of date.

[Mika]

Nope, not yet.  AO isn't good enough to beat Hubble's resolution except on bright objects even in 8-meter and above class telescopes, and the accuracy of AO is still heavily dependent on getting good seeing.  JWST would/will absolutely murder any current AO system.  The best resolution for AO is achieved in the J, H, and K NIR bands; LBT has reached upwards of 98% Strehl on a star with an R band mag of ~8 in H band, but can't crack 20% in visual wavelengths (Esposito et al., 2012), and it all gets worse as the star gets fainter.  Note that the 98% Strehl also required 0.3" seeing, which is about the best it ever gets.  With more typical 1" seeing, the 8th R band mag star resulted in a Strehl ratio of ~40% in H band.

Hubble, by contrast, gets full resolution and 99%+ Strehl all the way down to its limiting magnitude (25-30 in most bands and depending on exposure time, IIRC) in all wavelengths it can observe.

CAUTION: TECHNICAL POST FOLLOWS; READ ON YOUR OWN RISK

Why does the star brightness affect the adaptive optics result? Is this because of the longer exposure time required, thus the mirror has to deform more to compensate the changes along the path length? I suppose the Strehl ratio is easier to reach with the infrared wavelengths due to diffraction limit being larger, but I'm not sure about the seeing and it's effects in the infrared waveband. Also, I have been a little bit suspicious of what happens with the deformable mirrors with regarding the diffraction spot, it should not look like an Airy function any more if the reflective surface is not spherical.

This suggests that there are physical limits on ground based telescopes that do not allow them to achieve the equal technology level space based telescopes, ever (such a dangerous word for a Physicist, ain't it?). I wasn't aware of that AO telescopes may only achieve 20 % Strehl ratio at visual wavelengths; that's actually pretty bad. I would love to see the MTF curves of those, though. It's hard to judge between Strehl ratio and MTF, Strehl ratio being relatively stringent.

For once it would be cool to pull of the complete system design for a telescope, considering all that affects the resolution (struts, sensor pixel pitch, manufacturing errors, etc.). For example, the secondary mirror struts tend to have funny effects on the MTF, though the astronomers do not usually complain about the loss of resolution because of them. Perhaps it is impossible to do away the struts, that would likely end up with resolution problem being switch to color reproduction problems.

[watsisname]

Slightly aside, I recall reading that there are plans in the works to use infrared interferometry to directly image the event horizon of the Milky Way's central black hole.  That would be AWESOME.

Sub-AU resolution at over 8 kpc distance?  That would be AWESOME.  That's an angle of a few tens of microarcseconds.

It's an impressive angular resolution, indeed!  And I was mistaken, it would be using radio, not infrared.  There's some info on the project here.

[Astronomiya]

NGTM-1R:  Thanks, I'll ask around here and see if anyone knows anything about it.

Why does the star brightness affect the adaptive optics result? Is this because of the longer exposure time required, thus the mirror has to deform more to compensate the changes along the path length? I suppose the Strehl ratio is easier to reach with the infrared wavelengths due to diffraction limit being larger, but I'm not sure about the seeing and it's effects in the infrared waveband. Also, I have been a little bit suspicious of what happens with the deformable mirrors with regarding the diffraction spot, it should not look like an Airy function any more if the reflective surface is not spherical.

For AO to be effective, the exposure time for the guide star must be relatively fixed, as you must update as least several hundred times per second.  As you go for fainter guide stars, the diffraction pattern is less and less well sampled; for full correction, you need an extremely well sampled image, thus restricting you to very bright stars (for an 8-meter, 8th mag is insano bright).

Seeing comes in because the worse the seeing, the stronger the correction required.  If the atmosphere is giving you a 1" disk, this is obviously harder to fully shrink than a 1/2" disk.  So, because we're restricted in how long of an image we can take and be effective, worse seeing means lower Strehl, etc.  This will continue to improve as CCDs get better; the paper I linked to talks about LLL CCDs a little.  Those should be designed and prototyped in the next decade or so.  ELT class instruments like TMT and, well, the E-ELT, might be among the first to use them.

Visual wavelengths are harder to do than IR for two reasons:  first, perfect Airy disks are smaller in the optical, and second, the index of refraction of the atmosphere is higher than in the infrared, so the image intrinsically spreads out more before it even gets to the telescope.  Even so, 20% Strehl is actually a very impressive result, since open-loop (i.e., no AO) operations on large telescopes typically net you 1% Strehl at best, even on nights of excellent seeing.

To your last point, the diffraction pattern for an image is the Fourier transform of the aperture function (we're waaaaay in the far-field limit here).  Since the aperture function is still almost an evenly illuminated circle, the diffraction pattern is an Airy disk for a point source.  Furthermore, the reflection surfaces are never spherical anyway; they're usually hyperboloids, though their perimeters are always circular if possible.  Segmented mirrors like those on Keck and some other large telescopes introduce whole new problems; their PSFs are usually pretty funky and more difficult to deal with.

This suggests that there are physical limits on ground based telescopes that do not allow them to achieve the equal technology level space based telescopes, ever (such a dangerous word for a Physicist, ain't it?). I wasn't aware of that AO telescopes may only achieve 20 % Strehl ratio at visual wavelengths; that's actually pretty bad. I would love to see the MTF curves of those, though. It's hard to judge between Strehl ratio and MTF, Strehl ratio being relatively stringent.

Oh, ground based telescopes will probably catch up eventually, but they aren't quite there yet.  Pyramid wavefront sensors are, for now, the way of the future, and they'll continue to get better.  It wouldn't surprise me if the ELTs are left Hubble in their dust by the time they were done.  Since their mirrors will be ten times the diameter of Hubble's, even 20% Strehl means they beat Hubble's angular resolution.  The era of Hubble-type visible light missions is probably over.  Such IR, microwave, X-ray, and gamma-ray space telescopes are going to be launched for a while yet, though, until we can put them on the far side of the Moon or something.

For once it would be cool to pull of the complete system design for a telescope, considering all that affects the resolution (struts, sensor pixel pitch, manufacturing errors, etc.). For example, the secondary mirror struts tend to have funny effects on the MTF, though the astronomers do not usually complain about the loss of resolution because of them. Perhaps it is impossible to do away the struts, that would likely end up with resolution problem being switch to color reproduction problems.

We don't complain about the struts because their effect is small and unavoidable.  The struts cover only a tiny portion of the field, and while they do introduce diffraction effects, those usually pale in comparison to those of the atmosphere and other factors.

[NGTM-1R]

So I was actually up at the mountain most of the day and got a chance to ask one of the night assistants who run the scope about hard data on the issue of the new system. It is commissioned, it has done some science looking for extrasolar planets, but it's really more like field testing than actually out of the engineering phase. At the moment it's averaging 75-80 Strel and the lower limit of the objects it can view is magnitude 11, but those numbers are expected to improve as they work on the instrument and the people get used to working with it.

Assuming they can keep it on the telescope enough. The gestation period could be lengthy, since most of the people who want to use the 200 inch are more interested in spectroscopy and not imaging, so if the gear isn't mounted nobody's really working on/with it.

Why does the star brightness affect the adaptive optics result?

An addendum since Astronomiya didn't mention this. (It's probably so basic to someone involved they forget.)

Mirrors aren't perfect reflectors of light, they just look it. Every additional reflection costs you some relatively small amount of light, and the AO system could have quite a few mirrors. The AO system also has to suck up some of the light before it's output onto the observing instrument, so it can see what the atmosphere is actually doing and know how to deform the deformable mirror. You can easily lose half the magnitude of an object in the process of simply letting the AO system know what it needs to do.

[Astronomiya]

At the moment it's averaging 75-80 Strel and the lower limit of the objects it can view is magnitude 11, but those numbers are expected to improve as they work on the instrument and the people get used to working with it.

Did they happen to mention the band?

BTW, if you don't mind my asking, do you happen to work in the field or something related?

[NGTM-1R]

Did they happen to mention the band?

BTW, if you don't mind my asking, do you happen to work in the field or something related?

No. I didn't think to ask, it came up in the course of a generalized discussion in relation to the older PHARO system.

And I don't work in the field. I know a lot people who are on the staff up there because I'm a volunteer in the outreach program (and thus end up absorbing an awful lot of material on the subject via osmosis and the generalized fear of "oh god, what if somebody asks me"), but my actual job is pretty far from anything to do with the observatory.

[Mika]

To your last point, the diffraction pattern for an image is the Fourier transform of the aperture function (we're waaaaay in the far-field limit here).  Since the aperture function is still almost an evenly illuminated circle, the diffraction pattern is an Airy disk for a point source.  Furthermore, the reflection surfaces are never spherical anyway; they're usually hyperboloids, though their perimeters are always circular if possible.  Segmented mirrors like those on Keck and some other large telescopes introduce whole new problems; their PSFs are usually pretty funky and more difficult to deal with.

*Headdesk* Shows what I remember of Fourier optics. Yeah, you are right, neglecting aberrations, the spot is the Fourier transform of the aperture function. But because of the central obscuration and the struts, it is should not be an Airy disc (it really is not evenly illuminated), though I don't know how big is that departure in real life and whether it is possible to see the difference by naked eye. The adaptive optics mirror cannot be located on the aperture stop itself, so a local deviation in the AO mirror affects local parts of the image, thus affecting local spots (or PSF if you will). I can only imagine what sort of nightmare is the PSF of a segmented mirror telescope. The struts can be replaced in small telescopes by a glassplate or coating the middle part of a lens (Maksutov). Though the diameter of that cannot be increased much until spherical aberration starts to pop-up due to sagging effect.

But I had no idea that traditional telescopes achieve only 1 % Strehl ratio. That tells me that something else must be moving in the telescope, it is hard to believe the difference would solely be caused by seeing. Is it a combination from the vibration of object tracking mechanics and wind? I suspect that the mechanics are at least somewhat temperature compensated, but that probably plays one part too.

What it comes to reflection from the mirrors, it depends. Mid and Far IR range mirrors can be pretty reflective, the same goes for terahertz systems. Metallic mirrors in the visual and NIR range are not that good reflectors, with the possible exception of gold that works relatively well starting from NIR. I won't comment on X-ray or UV, I haven't designed devices there (yet).

[Astronomiya]

*Headdesk* Shows what I remember of Fourier optics. Yeah, you are right, neglecting aberrations, the spot is the Fourier transform of the aperture function. But because of the central obscuration and the struts, it is should not be an Airy disc (it really is not evenly illuminated), though I don't know how big is that departure in real life and whether it is possible to see the difference by naked eye. The adaptive optics mirror cannot be located on the aperture stop itself, so a local deviation in the AO mirror affects local parts of the image, thus affecting local spots (or PSF if you will). I can only imagine what sort of nightmare is the PSF of a segmented mirror telescope.

The departure from an Airy disk is quite noticeable in terms of the PSF's shape, but I don't think it necessarily has a large effect on the intensity of the central peak.  I should ask the head of the instrument lab here about it.  LBT's secondaries have triangular swingarm truss supports; see the bottom of the post for a simulated LBT PSF.  Still, I think I spoke too soon about them having a "small" effect; it clearly has to be accounted for (as a look at all the diffraction spikes in Hubble images should have immediately told me...).

I am not sure I understand your point about the aperture stop.  How would the fact that LBT has an adaptive secondary affect it, if at all?

For an example of theoretical Keck PSFs, look here.  Not exactly pretty.

But I had no idea that traditional telescopes achieve only 1 % Strehl ratio. That tells me that something else must be moving in the telescope, it is hard to believe the difference would solely be caused by seeing. Is it a combination from the vibration of object tracking mechanics and wind? I suspect that the mechanics are at least somewhat temperature compensated, but that probably plays one part too.

Well, think about it; a perfect Airy disk (yeah, yeah, it's not a perfect Airy disk, but for LBT it's close) for an 8.4 m telescope like LBT is ~1/50 of an arcsecond, but if the seeing is 1", the Strehl is going to be about 2%.  Other losses that are inherent in any design take care of the rest (I'd say it's a testament to how good we are at optical design that we can manage 98% Strehl at all, even on bright stuff).

What it comes to reflection from the mirrors, it depends. Mid and Far IR range mirrors can be pretty reflective, the same goes for terahertz systems. Metallic mirrors in the visual and NIR range are not that good reflectors, with the possible exception of gold that works relatively well starting from NIR. I won't comment on X-ray or UV, I haven't designed devices there (yet).

The aluminum coatings we use today are 90-95% reflective in the relevant bands.  Silver is 99%+ reflective in B, V, R, I, etc. bands, but crashes in the UV.  Plus it tarnishes.  Still, because silver is so awesome, there are ongoing attempts to get silver coatings that don't have to be replaced all the damn time; I think the UCSC/Lick Observatory effort is close to bearing fruit.

Most metals work acceptably in the UV until you hit their plasma frequencies; GALEX used magnesium fluoride coated aluminum for its mirror.  X-rays are just a real ***** any way you slice it; they tend to go straight through anything, so you need grazing incidence mirrors to focus them (you can see on this Chandra overview page that the reflection angles are measured in arcminutes).  X-ray mirrors tend to be made out of dense, tightly bound metals to provide the best surface for reflecting what is basically penetrating radiation. 

LBT PSF:

[Mika]

Sorry for late reply, I was on a business trip in Germany

The departure from an Airy disk is quite noticeable in terms of the PSF's shape, but I don't think it necessarily has a large effect on the intensity of the central peak.  I should ask the head of the instrument lab here about it.  LBT's secondaries have triangular swingarm truss supports; see the bottom of the post for a simulated LBT PSF.  Still, I think I spoke too soon about them having a "small" effect; it clearly has to be accounted for (as a look at all the diffraction spikes in Hubble images should have immediately told me...).

I am not sure I understand your point about the aperture stop.  How would the fact that LBT has an adaptive secondary affect it, if at all?

For an example of theoretical Keck PSFs, look here.  Not exactly pretty.

Hmm, objective lenses rarely achieve diffraction limited performance unless stopped down (at F/8 most of them do), usually the geometrical spot radius doesn't matter (much) in their case, it is mainly the concentration of energy within the central area of the spot. In the case of telescopes, I do believe that a simple spot structure might not extend a lot energy outside the middle, but in more complex cases, to attain the sufficient resolution to dim targets situated close to brighter ones, deconvolution of the spot might be necessary.

In optics we always play a sort of angle-position game - or more complexly, it should be kept in mind throughout the design process that the concept of ray has actually six dimensions, angles and position, and they often are tied to each other in a rather sneaky way. In case there is no vignetting, if something blocks the light at the aperture stop, it's effects will be visible throughout the whole field of view. Meaning that if you were to sit on the detector and looked up towards the telescope, no matter where you walked on the detector, the exit pupil will have a shadow. The same applies for an adaptive optics mirror, if it was located on the aperture stop, everything it does would be visible through out the complete detector area; hardly a local correction of the wavefront. The further the adaptive optics mirror is placed away from the aperture stop (and closer to the detector), the better it can counter local effects. It is relatively hard to explain this in words, maybe I need to show that with a graph.

Dear lord those PSFs look nasty!

I count 90 to 95 % reflectivity relatively bad performance, 90 % reflectivity means that after 3 subsequent mirrors, the contrast has gone down 30 %, and both the contrast and drawing capability affect overall resolution. While drawing capability is usually improved with more surfaces, the overall contrast has also dropped due to worse overall throughput. The corrective possibilities of adaptive optics did not become clear to me until you mentioned it could improve the Strehl ratio from 2% to 20%, that is a factor of ten improvement. Before I thought adaptive optics allowed something like 15 % improvement in the drawing capability, and I was wondering whether it was worth the cost due to possibly decreased contrast due to larger amount of mirror surfaces.

EDIT: I did not mean that the reflectivity would be a problem in UV region (well it becomes one closer to X-ray region), more often it is the scattering due to surface microroughness. UV mirrors (typically metal coatings deposited on glass substrates) have very stringent surface roughness requirements compared to VIS region.

[Astronomiya]

Hmm, objective lenses rarely achieve diffraction limited performance unless stopped down (at F/8 most of them do), usually the geometrical spot radius doesn't matter (much) in their case, it is mainly the concentration of energy within the central area of the spot. In the case of telescopes, I do believe that a simple spot structure might not extend a lot energy outside the middle, but in more complex cases, to attain the sufficient resolution to dim targets situated close to brighter ones, deconvolution of the spot might be necessary.

Yeah, deconvolution of the PSF is pretty standard in astronomy for that kind of analysis.  For segmented mirror telescopes like Keck, it is obviously necessary for a wide variety of observations, and is critical for high resolution spectra.

In optics we always play a sort of angle-position game - or more complexly, it should be kept in mind throughout the design process that the concept of ray has actually six dimensions, angles and position, and they often are tied to each other in a rather sneaky way. In case there is no vignetting, if something blocks the light at the aperture stop, it's effects will be visible throughout the whole field of view. Meaning that if you were to sit on the detector and looked up towards the telescope, no matter where you walked on the detector, the exit pupil will have a shadow. The same applies for an adaptive optics mirror, if it was located on the aperture stop, everything it does would be visible through out the complete detector area; hardly a local correction of the wavefront. The further the adaptive optics mirror is placed away from the aperture stop (and closer to the detector), the better it can counter local effects. It is relatively hard to explain this in words, maybe I need to show that with a graph.

No, I get you; whatever imperfections/corrections are present on the first couple of elements propagate on down the chain.  Since the aperture stop controls the light that actually enters the system, anything there will be visible across the field.  I don't know why I was having trouble with this concept earlier. The image is just the convolution of the object and the PSF, and anything that causes diffraction effects or blocks light will show up in the PSF.

According to the LBT website, the secondaries are purposely undersized so that their edges do set the aperture stop.  I don't know that it much matters for AO, and may actually help; you want the corrections to be applied across the entire field so that the wavefront correction isn't limited to just the guide star.  In single star systems, you end up with a small region maybe 10 or 20" in diameter that has relatively good correction, and it gets worse as you move further away from the guide star.  There are also systems (most modern ones, actually) that use multiple guide stars and even multiple deformable mirrors to do what's known as multi-conjugate AO (MCAO).  MCAO allows you to correct a larger field of view, even regions up to an arcminute or more across.

I count 90 to 95 % reflectivity relatively bad performance, 90 % reflectivity means that after 3 subsequent mirrors, the contrast has gone down 30 %, and both the contrast and drawing capability affect overall resolution. While drawing capability is usually improved with more surfaces, the overall contrast has also dropped due to worse overall throughput. The corrective possibilities of adaptive optics did not become clear to me until you mentioned it could improve the Strehl ratio from 2% to 20%, that is a factor of ten improvement. Before I thought adaptive optics allowed something like 15 % improvement in the drawing capability, and I was wondering whether it was worth the cost due to possibly decreased contrast due to larger amount of mirror surfaces.

This is why the move to adaptive secondaries was made for LBT, and I imagine the same is being done for the ELTs - fewer mirrors means fewer reflection losses.  Hopefully, silver coatings will become practical again sometime soon, since those amazing in the visible and near IR.

[Mika]

Hmm, I might need to take a look at the LBT website. Aperture stop can only be located at one place, the rest of light blocking is vignetting. I'm not sure whether LBT website describes vignetting or the placement of the aperture stop.

They plan on correcting the whole wavefront? That would imply that the seeing remains relatively constant within the atmosphere within the FOV. I had the impression that seeing would be more location dependent on the detector, but that might not be so. In this case, the AO mirror should be placed closer to the aperture stop.

However, aspherics close to aperture stop need to be produced extra carefully, less form errors are required allowed than compared to closer to the detector. Then again these aspheres would be correcting two different things.

EDIT: Try doing two different things at the same time and you don't do either of them well...

[Astronomiya]

They plan on correcting the whole wavefront? That would imply that the seeing remains relatively constant within the atmosphere within the FOV. I had the impression that seeing would be more location dependent on the detector, but that might not be so. In this case, the AO mirror should be placed closer to the aperture stop.

Atmospheric anisoplanatism is actually a serious problem; it's why the AO correction region is so limited in size.  Thus, MCAO.  Even then, making the region very large is extremely difficult, requiring many more mirrors and guide stars than we can really deal with.  But ideally, yes, we would love to correct the entire wavefront; perfect correction at optical wavelengths across the entire field would be the holy grail of AO.

However, aspherics close to aperture stop need to be produced extra carefully, less form errors are required allowed than compared to closer to the detector. Then again these aspheres would be correcting two different things.

Most astronomical optics are figured to within lambda/10 or /15 at least; LBT's primaries are maintained at about 15-30 nm rms from what I could find.