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Quarker
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- There are phase contrast microscopes, why aren’t there phase contrast telescopes for visual use on the night sky?
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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.Look at where the specimen goes in a phase contrast microscope. How would you put a star there?
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.it may be possible to constructively interfere the two images,
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 object is to increase the amplitude of the light waves from a dim astronomical source, hopefully making it easier to see.
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.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.
Interferometry and multiple mirrors are used very successfully for large apertures but would all that be classed as "phase contrast"?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
No, not as I understand it, I had not intended to imply that. Regards AndrewInterferometry and multiple mirrors are used very successfully for large apertures but would all that be classed as "phase contrast"?
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.
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.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.
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.A phase contrast microscope is designed to highlight the three dimensional structure of small objects.
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'?All photons would come from the primary mirror or lens. Any photons used to create a phased image would be subtracted from that.
What's one of them?But by projecting a phased image
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.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.
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 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.
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.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.
If an additional phased image
The phased image
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?drown out the phased image
Indeed, I explicitly agreed with your point that the maximum "brightness" is determined by how much you energy you capture.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.
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.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?
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.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.
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.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
Ask an amateur astronomer how much they would spend for an eyepiece that would allow them to directly view galaxies.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.
You have now put the onus on a fictional "optician" to perform a function which you haven't specified.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.
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.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.
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.so the lack of contrast is almost entirely due to the lack of light.
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.visual astronomers use large "light bucket " telescopes
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.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.
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.Ask an amateur astronomer how much they would spend for an eyepiece that would allow them to directly view galaxies.