Why no Phase Contrast Telescopes?

[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?

 

[sophiecentaur]

Consider the Sun?

I’ve considered it. Now what?

 

[Quarker]

Consider the Sun?

I don’t think phase contrast will work on the sun. A filter would be required. The phased image would be completely blocked out.

 

[sophiecentaur]

I don’t think phase contrast will work on the sun. A filter would be required. The phased image would be completely blocked out.

How could phase contrast be applied to an object the size of the sun? Could you provide a diagram?

 

[Quarker]

How could phase contrast be applied to an object the size of the sun? Could you provide a diagram?

It would work like any other telescope of similar size and f ratio. The only difference would be the additional phased image provided by the eyepiece. As I said earlier, there are aberrations inherent in the design of eyepieces that could be phased with only minor adjustments. The problem is, no such eyepiece is being made.

 

[Drakkith]

I don’t think phase contrast will work on the sun. A filter would be required. The phased image would be completely blocked out.

Why would it not work for the Sun, but work for a nebula or other deep sky object?

 

[sophiecentaur]

It would work like any other telescope of similar size and f ratio. The only difference would be the additional phased image provided by the eyepiece. As I said earlier, there are aberrations inherent in the design of eyepieces that could be phased with only minor adjustments. The problem is, no such eyepiece is being made.

I’m probably missing something but could you actually indulge me with a diagram showing how the “phasing” takes place and what the result would be on a solar image [Edit: or any astronomical image]?
I can appreciate that lenses can be designed to produce ‘sharpening’ at the expense of added diffraction effects and resolution. But is that Phase Contrast?
I have already made the point that the evidence is a present lack of such an attractive Astro tool. Amateur astronomers are all desperate to spend their money and the industry would have produced thousands of PC eyepieces if there were even cat’s chance of having possible models available.

 

[Quarker]

Why would it not work for the Sun, but work for a nebula or other deep sky object?

The solar filter that would be required to view the sun only allows a small percent of light to reach the eye. The phased image will probably not be very bright to begin with. The solar filter will dim the phased image to the point where it can’t be seen.

 

[Quarker]

I’m probably missing something but could you actually indulge me with a diagram showing how the “phasing” takes place and what the result would be on a solar image [Edit: or any astronomical image]?
I can appreciate that lenses can be designed to produce ‘sharpening’ at the expense of added diffraction effects and resolution. But is that Phase Contrast?
I have already made the point that the evidence is a present lack of such an attractive Astro tool. Amateur astronomers are all desperate to spend their money and the industry would have produced thousands of PC eyepieces if there were even cat’s chance of having possible models available.

If anyone ever tried to design a phase contrast telescope by adding a phased image to the visual image, they would have had a problem combining the two images. The eye will simply ignore the phased image in favor of the visual image. I think this problem can be solved by using a slow telescope with a wide depth of focus. If the eye can be held within the depth of focus above the eyepiece, the visual and phased images will align perfectly with one another. When the visual image is focused, this will hopefully force the eye to detect the phased image as well. Basic physics says that the amplitude of the visual image should double at that point. What that means visually is still an open question. But if dim objects like galaxies can suddenly become visible using a simple property of light, it seems worth it to me.

 

[Quarker]

I’m probably missing something but could you actually indulge me with a diagram showing how the “phasing” takes place and what the result would be on a solar image [Edit: or any astronomical image]?
I can appreciate that lenses can be designed to produce ‘sharpening’ at the expense of added diffraction effects and resolution. But is that Phase Contrast?
I have already made the point that the evidence is a present lack of such an attractive Astro tool. Amateur astronomers are all desperate to spend their money and the industry would have produced thousands of PC eyepieces if there were even cat’s chance of having possible models available.

I wish I could provide a detailed ray diagram showing exactly what I’m talking about when I refer to depth of focus, but they seem to have disappeared from the internet. All I can find are simplistic sketches of undergrad homework assignments.

 

[Ibix]

Is it worth me pointing out, again, that a phase contrast microscope let's you look at transparent objects on a background made bright by a light source you control, and that's almost exactly the opposite of what astronomical targets are?

All that @Quarker is doing is repeating that some system that they haven't described and apparently can't draw "ought to" make galaxies brighter by diverting scarce photons to do... something. I recommend this thread be closed if there isn't a ray diagram in OP's next post, because until we see such a thing we appear to be talking about a fantasy, not an actual device or even a proposal for one.

 

[sophiecentaur]

The phased image will probably not be very bright to begin with.

I wish I could provide a detailed ray diagram showing exactly what I’m talking about when I refer to depth of focus

Yet again, you haven't actually described what you mean by "phased image" or even the basics of how your eyepiece could work. I don't need a 'detailed' diagram- just something that tells us what you have in mind.
Also, why has it not already been made available? Do you really not understand the difference between an object that's just 1mm thick and one that's 100Light Years thick and how that affects the phase shifts of light passing through different parts?
Please consider that you could just possibly be totally wrong about this. Do you know the basic principles of interference?

 

[Quarker]

I simply want to make dim images more visible. That seems self explanatory, given the stargazing flag at the top of the post. But maybe not. Amateur astronomers know it as going from averted to direct vision. They look off-axis at galaxies, but directly at bright objects like planets and the moon. I want to use constructive interference to increase the amplitude of the light coming from a dim source like a face-on galaxy. This should, theoretically, make the image easier to see.

As to the source of the constructive interference, I’ve been as specific as I can be for a non-optician. If I could create the first phase contrast eyepiece, I would. Because if it works, it will be groundbreaking.

 

[sophiecentaur]

I simply want to make dim images more visible.

Of course, and so do we all but you can't just hope your idea will work without good cause.

As to the source of the constructive interference, I’ve been as specific as I can be for a non-optician.

You have to realize that every part of the normal image, formed by a lens is, in fact due to constructive interference of light taking all the alternative paths from object to image. In the case of an astronomical telescope, you want as much light to land in the appropriate place. That's the best you can do.

In the case of a microscope, you have as much light as you want (from the lamp) and you can do interference tricks to detect changes in thickness of the (transparent) object by the phase along the different path lengths. This reduces the peaks of transmission and enhances the differences in path length. In the case of an astronomical object, there is no common source of light across it - whether it's the Sun or a nebula.

I’ve been as specific as I can be for a non-optician.

As a non-optician you should realize your limitations and treat the subject seriously. "If' doesn't make devices work; knowledge sometimes does. Give the subject some respect.

 

[Drakkith]

I want to use constructive interference to increase the amplitude of the light coming from a dim source like a face-on galaxy. This should, theoretically, make the image easier to see.

I've already explained that this isn't possible. If you want to make it brighter you either get a bigger aperture or you reduce the focal ratio. The latter of which only works because you're making the image smaller, cramming the same light into a smaller area.

Increasing contrast is a separate but related topic that has few more options. Generally this boils down to reducing background (foreground?) light from nearby light sources via filters or observing from a location far away from cities and towns. Getting a larger aperture telescope also helps.

Remember that when you look through the eyepiece you are already viewing ALL of the available light that has entered the telescope, minus a small amount lost to reflections off the lens surfaces and other such effects. Phase contrast can't create light (or amplitude) from nothing.

 

[jim mcnamara]

Let's close off this discussion.
The current understanding is that brightness is directly related to the number of light particles hitting the eye. This means that the path to a brighter view requires gathering more light particles when objects we want to see are dim. Otherwise why spend money to build gigantic mirrors like the Keck observatory or James Webb.