Why no Phase Contrast Telescopes?

[Quarker]

TL;DR Summary

There are phase contrast microscopes, why aren’t there phase contrast telescopes for visual use on the night sky?

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[Ibix]

Look at where the specimen goes in a phase contrast microscope. How would you put a star there?

 

[Quarker]

Look at where the specimen goes in a phase contrast microscope. How would you put a star there?

It seems like it should be possible, with a properly designed eyepiece, for a telescope to create two images, one normal, and one phased. The star can stay where it is.

 

[Ibix]

Possibly - what design did you have in mind? And what would be the point?

 

[Quarker]

Maybe by adding an additional lens set to an eyepiece, or tweaking the design of current eyepieces, a second light cone can be projected onto the eye. If this second light cone can be optically phased with the primary image, it may be possible to constructively interfere the two images, making dim objects appear brighter, without the use of additional aperture.

 

[sophiecentaur]

it may be possible to constructively interfere the two images,

Interference patterns can only work when there is good phase coherence in the light from the sources. In a microscope the path differences are both short and stable to much less than a wavelength so you can obtain good cancellation by interference. That allows phase contrast imaging to work. Over a long path that includes the atmosphere, the path variations can be many wavelengths. I have always understood that the 'twinkling' of stars is due to random phase variations along the different paths of light entering the eye.
How would you arrange for half of the paths through the scope be constantly out of phase with the other half, to cause cancellation?

 

[Quarker]

The light that reaches the eyepiece has already traveled through the atmosphere, and the primary lens. It would be the job of the eyepiece alone to create a second light cone. With only a slightly longer path through the eyepiece, it should be possible for the second light cone to maintain coherence. And to clarify, the two images should be in phase, not out of phase. The object is to increase the amplitude of the light waves from a dim astronomical source, hopefully making it easier to see.

 

[Drakkith]

The object is to increase the amplitude of the light waves from a dim astronomical source, hopefully making it easier to see.

This is not possible. The amount of light captured by the telescope, usually governed by the clear area of the primary mirror or lens, is the ultimate deciding factor on how bright something can appear. All you can do is focus the light down to a smaller, brighter image or a larger, dimmer image. Photons can't be produced by constructive interference, they have to come from somewhere. If they don't enter the telescope, you can't have them.

 

[andrew s 1905]

This is not possible. The amount of light captured by the telescope, usually governed by the clear area of the primary mirror or lens, is the ultimate deciding factor on how bright something can appear. All you can do is focus the light down to a smaller, brighter image or a larger, dimmer image. Photons can't be produced by constructive interference, they have to come from somewhere. If they don't enter the telescope, you can't have them.

The image of a star, is the diffraction pattern formed by the telescope of what you see is the result of constructive interference. Yes you can't add more photons but you can concentrate them more effectively. Focus is a key mechanism but other techniques are used such as adaptive optics to remove or limit blurring due to the atmosphere. Also speckle interferometry and apodizing masks have been used in special cases.

Regards Andrew

 

[sophiecentaur]

The image of a star, is the diffraction pattern formed by the telescope of what you see is the result of constructive interference. Yes you can't add more photons but you can concentrate them more effectively. Focus is a key mechanism but other techniques are used such as adaptive optics to remove or limit blurring due to the atmosphere. Also speckle interferometry and apodizing masks have been used in special cases.

Regards Andrew

Interferometry and multiple mirrors are used very successfully for large apertures but would all that be classed as "phase contrast"?

 

[andrew s 1905]

Interferometry and multiple mirrors are used very successfully for large apertures but would all that be classed as "phase contrast"?

No, not as I understand it, I had not intended to imply that. Regards Andrew

 

[sophiecentaur]

Phase contrast technique:
I always understood that phase contrast imaging is used for thin objects that are almost transparent and a direct path through the object is added to a reference, side path to produce an interference pattern to increase contrast and which can reveal the structure that would be hard to see by direct observation. The transparent structures we would see in space would have to be millions (and more) of wavelengths thick (at the longest of radio waves that we observe) so what sort of interference pattern would be see? Also, these nebulae are visible by reflected / scattered light A microscopic object will be perhaps tens of microns thick - or tens of optical wavelengths. A very different kettle of fish.

But the OP is actually a reasonable question for a beginner, bearing in mind all the other similarities between micro and tele scopes. It's all in the detail.

 

[Quarker]

Phase contrast technique:
I always understood that phase contrast imaging is used for thin objects that are almost transparent and a direct path through the object is added to a reference, side path to produce an interference pattern to increase contrast and which can reveal the structure that would be hard to see by direct observation. The transparent structures we would see in space would have to be millions (and more) of wavelengths thick (at the longest of radio waves that we observe) so what sort of interference pattern would be see? Also, these nebulae are visible by reflected / scattered light A microscopic object will be perhaps tens of microns thick - or tens of optical wavelengths. A very different kettle of fish.

But the OP is actually a reasonable question for a beginner, bearing in mind all the other similarities between micro and tele scopes. It's all in the detail.

A phase contrast microscope is designed to highlight the three dimensional structure of small objects. A phase contrast telescope won’t resolve the three dimensional structure of galaxies, since the image is flat. But by projecting a phased image over a visual image, the dim galaxy that requires averted vision to see even in large telescopes might be visible using direct vision in relatively small telescopes.

 

[Quarker]

This is not possible. The amount of light captured by the telescope, usually governed by the clear area of the primary mirror or lens, is the ultimate deciding factor on how bright something can appear. All you can do is focus the light down to a smaller, brighter image or a larger, dimmer image. Photons can't be produced by constructive interference, they have to come from somewhere. If they don't enter the telescope, you can't have them.

All photons would come from the primary mirror or lens. Any photons used to create a phased image would be subtracted from that. There may be some minimal threshold of light that the eye needs to recognize the phased image, and some objects may simply be too dim, or so bright that the phased image is overwhelmed, but that could only be determined by trial and error.

 

[sophiecentaur]

A phase contrast microscope is designed to highlight the three dimensional structure of small objects.

My only experience was with transparent biological tissue (by transmission) I guess the same thing could be done by reflection but the coherence length of the light source could be a problem - almost the same as for holography.
Anything like that requires a small range of relative path differences - hence my problem with astronomy.

All photons would come from the primary mirror or lens. Any photons used to create a phased image would be subtracted from that.

Is there really any point introducing photons into this discussion. Phase contrast is a totally wave phenomenon. The only time photons are relevant would be when the light is detected. Wherever did you come across the idea of 'subtracted photons'?

 

[sophiecentaur]

But by projecting a phased image

What's one of them?

 

[Quarker]

The main problem with a phase contrast telescope comes from the lack of a phase focus mechanism. If an additional phased image is projected onto the eye by a second light cone, the eye will ignore it in favor of the visual image. The phased image will be seen as mere extraneous light. But if the observer’s eye can be held within the depth of focus above the eyepiece, the phased image will lie within the optical lines of the visual image, and the eye will hopefully combine the two into one bright image.
Keeping the eye properly positioned may prove to be the most difficult part of the process. The depth of focus of most telescopes is very narrow. Slower telescopes may be necessary. Achromatic telescopes may not work because the residual light may drown out the phased image, but possibly not for very dim objects. Apochromatic refractors will probably work best. The secondary mirror in Newtonian telescopes may cause issues with center of field images.
Because the phased light cone will probably be narrow, only a small, circular area within the eyepiece field of view will be resolved.
And this is my idea for a phase contrast telescope. If someone has a question about any specific function, I will try to answer. But I’m not going to discuss any theoretical aspects. Either it works, or it doesn’t.

 

[Drakkith]

All photons would come from the primary mirror or lens. Any photons used to create a phased image would be subtracted from that. There may be some minimal threshold of light that the eye needs to recognize the phased image, and some objects may simply be too dim, or so bright that the phased image is overwhelmed, but that could only be determined by trial and error.

So you take an already very, very dim image, and you subtract light from it to make another image? That would make the phase contrast image even dimmer than the original. Okay.

And that assumes that it's possible to make such a phase contrast image. What are you interfering the target's light with after it enters the telescope?

As I said before, if you're aiming to increase the amplitude of the light from the target (which makes it brighter) then that's just not going to happen. Note that there's a difference between increasing the brightness of the image and increasing the contrast. You can't do the former, that's limited by the telescope's aperture and focal ratio. So the question is whether you can do the latter, and I am skeptical.

The image of a star, is the diffraction pattern formed by the telescope of what you see is the result of constructive interference. Yes you can't add more photons but you can concentrate them more effectively. Focus is a key mechanism but other techniques are used such as adaptive optics to remove or limit blurring due to the atmosphere. Also speckle interferometry and apodizing masks have been used in special cases.

You could certainly improve focus, but that doesn't really have any bearing on what I said in my post you quoted. Maximum brightness of the image is still determined by how much light comes in the aperture, and nothing you do will increase this brightness. Bad focus, atmospheric blurring, and other effects simply reduce the contrast and detail.

The phased image will be seen as mere extraneous light. But if the observer’s eye can be held within the depth of focus above the eyepiece, the phased image will lie within the optical lines of the visual image, and the eye will hopefully combine the two into one bright image.

What's making the image bright? Even if your technique works, you've taken X amount of light, divided it into two pieces, and then recombined it into X amount of light. No brightness change.

 

[sophiecentaur]

If an additional phased image

The phased image

the phased image

drown out the phased image

What do you mean by a phased image? How would you form it? What would be the appropriate optics for it? Somehow you would split the field and introduce a phase shift / delay?

 

[Ibix]

I still don't understand what OP is hoping to achieve. Phase contrast microscopy is useful for looking at transparent objects, where it highlights tiny variations in optical path distance across the field, converting them into lighter and darker regions. The phase shift we're interested in is induced by the object - the eyepiece is just designed to combine straight through and phase structured light.

What transparent object are you planning to look at? Usually in astronomy you are interested in the light source, not the stuff between you and the light source. In microscopy, vice versa.

 

[sophiecentaur]

Yes - that process is not possible in a massive object because of the path lengths involved and the coherence length of the light.

Incidentally, there are many structures in space that are transparent and are not primary light sources; Nebulae (the most photogenic objects to see, up there), with varying thicknesses, so varying phase shifts. Just like squidgy animal tissue. The only difference is the size and COHERENCE involved.

Edit. There is a corresponding effect to phase contrast microscopy and that is Thin Film Interference. The clue is in the word "Thin". Thick films (like nebulae - or even thick glass) will not exhibit such interference effects unless light with very long coherence length is used . For nebulae, you be needing a monochromatic source (laser) that has been running for millions (and more) of years. The size matters.

 

[andrew s 1905]

You could certainly improve focus, but that doesn't really have any bearing on what I said in my post you quoted. Maximum brightness of the image is still determined by how much light comes in the aperture, and nothing you do will increase this brightness. Bad focus, atmospheric blurring, and other effects simply reduce the contrast and detail.

Indeed, I explicitly agreed with your point that the maximum "brightness" is determined by how much you energy you capture.

I was just adding some additional points on how you can maximise the observed brightness which is what the OP was talking about. Real world observing, visual or imaging, is much more than just aperture and focus.

Sorry if you feel I quoted you inappropriately but I was trying to build on your comments.

Regards Andrew

 

[Quarker]

What do you mean by a phased image? How would you form it? What would be the appropriate optics for it? Somehow you would split the field and introduce a phase shift / delay?

Correct. Good telescope eyepieces are quite complicated because they are correcting for multiple aberrations. Some of the light entering the center of the eyepiece from the edge of the field is actually scattered away from the eye. It may be possible to slightly alter the design of the eyepiece to send this light into the eye. A good optician, which I am not, may be able to figure out a way to phase that light, with the light that forms the visual image. An adjustable eyepiece that holds the eye in place, but can be adjusted to raise the phased light up to the eye, would be even better. Theoretically, the image in the eyepiece would blink in and out of view as the phased image moves between constructive and destructive interference.

 

[Quarker]

What's making the image bright? Even if your technique works, you've taken X amount of light, divided it into two pieces, and then recombined it into X amount of light. No brightness change.

Okay, bright might be an overstatement. But taking a nearly invisible object like a distant galaxy, and making it clearly visible, would be something many amateur astronomers would love to be able to do.

 

[Quarker]

Indeed, I explicitly agreed with your point that the maximum "brightness" is determined by how much you energy you capture.

I was just adding some additional points on how you can maximise the observed brightness which is what the OP was talking about. Real world observing, visual or imaging, is much more than just aperture and focus.

Sorry if you feel I quoted you inappropriately but I was trying to build on your comments.

Regards Andrew

Obviously, bright is not the adjective I should have used. Images from phase contrast microscopes look gray and colorless. Phased galaxies may look the same. Which is still better than barely visible.

 

[Quarker]

I still don't understand what OP is hoping to achieve. Phase contrast microscopy is useful for looking at transparent objects, where it highlights tiny variations in optical path distance across the field, converting them into lighter and darker regions. The phase shift we're interested in is induced by the object - the eyepiece is just designed to combine straight through and phase structured light.

What transparent object are you planning to look at? Usually in astronomy you are interested in the light source, not the stuff between you and the light source. In microscopy, vice versa.

Ask an amateur astronomer how much they would spend for an eyepiece that would allow them to directly view galaxies.

 

[sophiecentaur]

A good optician, which I am not, may be able to figure out a way to phase that light, with the light that forms the visual image.

You have now put the onus on a fictional "optician" to perform a function which you haven't specified.

But perhaps you are confusing the function of a photographic lens / telescope in getting improved astronomical images and the function of a PC microscope which is for revealing different densities and thicknesses of thin objects.
There are a number of ways to enhance the sharpness of an image by shaping the thickness a lens (which is putting an extra and subtle phase slope across it). The effect of this sort of thing is to make images look sharper but ti distort edges and it's basically the same sort of process as can be done by image processing. This sort of treatment of an astronomical target can produce the illusion of detail and introduce features that are not actually there.
If you want a near equivalent of a PC microscope system it is in Schlieren photography which will reveal areas of different density (temperature, perhaps) in a gas. Collimated light is passed through, say convection currents in air, an image of the source will be formed by a long focus concave mirror. A sharp edge is used to block off half of the light passing through and the resulting asymmetry produces changes in amplitude where there were originally differences in phase. But this system can only produce a useful image for relatively small differences in path length and 'clean' gas. Astronomical objects are too big for the same effect to be seen; any interference pattern will be formless and extremely fine. Schlieren images have been obtained of supersonic shock waves from planes passing in front of the Sun ( a good, bright source).

 

[Drakkith]

Okay, bright might be an overstatement. But taking a nearly invisible object like a distant galaxy, and making it clearly visible, would be something many amateur astronomers would love to be able to do.

Why is it clearly visible? For that to happen it would have to be brighter, which isn't possible. Remember that the background is already very nearly pure black, so the lack of contrast is almost entirely due to the lack of light. The only way to increase the contrast of an object is to either make the light from the object brighter or to reduce the background light so that the object stands out more from the background. PCM does both compared to normal light microscopy. We normally can't do either in astronomical imaging except by building larger telescopes to gather more light. Reducing the background light is limited to very specific targets that are much dimmer than a nearby bright source, like exoplanet imaging.

 

[sophiecentaur]

so the lack of contrast is almost entirely due to the lack of light.

Absolutely. The back light in a microscope is as bright as you want (limited by frying the specimen). The light from a nebula / galaxy is just not quite enough without an 'improvement'. In the end, that improvement will have to involve gathering more light with a bigger aperture.

 

[andrew s 1905]

@Drakkith is right for galaxies and the like visual astronomers use large "light bucket " telescopes and seek out dark sky locations to get the required contrast between target and sky.

For the rest they switch to electronics to help using image intensifies, integrating video cameras or go the whole hog and switch to imaging and data processing.

For star like, point sources, the old trick of increasing magnification works as it darkens the, typically less than perfect, skies while having less impact on point sources hence increasing contrast.

Regards Andrew

 

[sophiecentaur]

visual astronomers use large "light bucket " telescopes

Same goes for AP'ers. Twice the area of the aperture will mean half the exposure time and, wallet permitting, that a good way to go. Resolving power can be increased by interferometry (with more scopes) but interferometry will probably affect noise performance.

 

[andrew s 1905]

Same goes for AP'ers. Twice the area of the aperture will mean half the exposure time and, wallet permitting, that a good way to go. Resolving power can be increased by interferometry (with more scopes) but interferometry will probably affect noise performance.

Yes, however, given the general poor seeking in the UK and Europe most AP'ers use small aperture (60 - 100mm) short focal length (< 1m) well corrected refractors for ap. I think in contrast in the US larger RC reflectors are prefered.

Regards Andrew

 

[Ibix]

Ask an amateur astronomer how much they would spend for an eyepiece that would allow them to directly view galaxies.

But why do you think an eyepiece will do this? In phase contrast microscopy you use an eyepiece to select bits of light from a source that have gone straight through your sample and bits of light that have had their relative phase altered by the sample and interfere them. A galaxy is a large incoherent light source, not a layer of varying refractive index with a light source behind it, so there won't be any information coded in the phase.

 

[sophiecentaur]

@Quarker The title of the thread asks "Why no . . . ". Do you imagine that the fact that there are none is because the telescope trade is trying to be awkward and withheld a moneymaking article from us? There are none because it's actually a nonsense concept. You have not described what you actually mean by the term and you have no idea how you would design one.

I don't understand why you are continuing to argue that there 'should be' a PCT. If you can't provide a reference to one then there is no argument.

I have wasted enough time on this.

 

[chemisttree]

Consider the Sun?