electronic telescope primary mirror

Hi all,

Would it be possible to substitute a 2d CCD array for the 3d parabola mirror in a telescope if each CCD pixel only grabbed light with a phase delay proportional to its "virtual" position on a parabolic mirror?

__ __ ''-_ __-'' '-__ _--' virtual primary mirror ''-_ _--'' ''' --------------------------------- CCD array

(created by AACircuit v1.28.4 beta 13/12/04

So if a pixel in the CCD array was 1meter away vertically from the virtual mirror, you would delay sampling the pixel about 3 nanoseconds (approximate time it takes light to travel 1meter) compared to the pixel at the root of the parabola, and you would sample the pixels at a rate proportional to the frequency of the light you are receiving. I think the light would also have to be polarized so that it is only travelling vertically to the CCD. Also the CCD array would have to be custom in that each pixel would have to be able to be triggered seperately and very quickly.

I have always wanted to be able to build a telescope using just a planar sheet of material and avoid having to have a precision mirror so this is just a wild idea about that! :)

cheers, Jamie

This would work if the CCD pixels responded to the actual waveform of the light - giving an AC signal out at however many Teraherz light goes at. Unfortunately they don't - they integrate and give an output simply proportional to intensity, so phase information is lost. A nice idea, but a non-starter, I'm afraid.

d Pearce Consulting

In theory something like this may be possible, but in practice no.

If you could record the waveform of the light at each point and maintain the phase information it would be possible, in theory, to process the data to produce an image. However, in order to avoid grating lobes, you would need a receive element spacing less than the wavelength of light, i.e. < 500nm. A 1m square array would therefore need >400,0000,000,000 pixels, each sampling at thousands of THz. Note that each image pixel will be the sum, with appropriate phase, of all the receive elements, not just the data from a single receive element. It quickly becomes obvious that even if you could build the array (which you can't) there is no way you could process that amount of data.

With longer wavelengths and lower frequencies it is possible. For example phased array radars use a similar idea to produce an electronically steered beam.

Gareth.

In addition to what was said : 1 in^2 of aCCD is much more expensive that 1 in^2 of a mirror.

Rene

Gareth, marketing just called; they need that CCD array by Friday, at a retail cost of less than $10 per array...

"Jamie Morken" wrote

Close, but no cigar: Look up 'long baseline interferometer' and 'phased array antenna' w/ google.

I suspect that That Planar Sheet of Material would still have to be Figured to the same Degree of Precision as the Primary Mirror it is replacing, so there is no simplification . eg Flat to a fraction of a wave-length of Light ! Very much harder than making a precision mirror !!

Did you know that the Hubble mirror could have been QC inspected and tested with only a Pin-hole Light source and a Knife-edge. Idea was overruled as too primitive, like building the Panama Canel with a pick and shovel.!

Yukio YANO

An old amateur telescope maker !

I don't think you understand how a lense works. You would get the same image with a parabolic array of CCDs as you would with a flat array. With a flat array you need a lense to image onto the array. With a parabolic CCD array you would need a different kind of lense to image onto the array. You can image onto a CCD using a pinhole lense. The pinhole breaks up the image into little pieces putting a different piece of the image onto different pixels of the CCD by the relationship of the angle of the object to the pin hole and the CCD. Just imagine looking through a pinhole and by moving your head around. As you move your head you glimpse a little different piece of the scene on the other side of the pin hole. If you move your head around real fast the little piece will combine and you will see the whole scene. With a mirror or lense the entire scene ends up on every spot of the lense. What happens to create an image is that the light is bent in such a way as to create an interference pattern that generates the image. With a device as you describe you would only get a blur. No interference pattern; no image.

Paul C

do a google search for Focault testing !

I came across some web pages on the Hubble Telescope Mirror tests

I made my own mirror as a teenager fifty years ago

This is what happens when the Paper Pushers over-rule the Grunts that do the actual work !! PP insisted on Gilding the lily by adding a couple of extra lenses in an "autocollamator setup" and got it wrong somewhere along the line ! A simple Focault Test setup would have tipped them off that something was amiss. But hey this a Electronics Design Newsgroup, but then again this is how I got interested in telescopes , electronics, CCD's, Electron Microscopes, and Video.

Yukio YANO

But light is quantized into photons. One photon hits one and only one detector, no matter the size of the detectors.

There is a single-photon-sensitive high-speed detector: a microchannel plate multiplier followed by a position-sensitive charge detector, like a 2-d delay-line detector. That will log the time of hit and x-y location of photons hitting a surface, up to maybe 1kx1k resolution, at rates around 10M/second. But it won't image unless fronted with a lens or something. A photon hit on a surface doesn't tell you which direction the photon came from.

John

There was plenty of evidence that something was wrong, but they ignored it and stuck with the results of one defective test fixture. The Hubble mirror was so bad that any competant hobbyist-level mirror grinder would have caught it.

Read "The Hubble Wars" by Eric Chaisson. After weeks of trying to focus the scope in space, a big meeting was called, and they figured out what must have happened. One of the prime optical designers excused himself, stepped into the hallway, and vomited.

John

Rich, Telescopes can be tested against a point source. In earlier times one used aluminum foil with a pinhole in front of a lamp, nowadays, I'd rather use a laserdiode without optics in 500m distance. The 10um in square are pretty close to a point soutce. Through the telescope one then sees the bessel rings of the fourier transformed point source if the telescope is right. Otherwise one sees the distortions in interferometry style.

Rene

Photons are photons, and all the rules apply regardless of wavelength. One photon can only excite one atom, so it can hit only one place. The reason a microwave phased-array detector works is because it detects huge sloshes of many, many fairly coherent photons. The energy in a single microwave photon is undetectable. A 1 GHz microwave photon packs about 7e-25 joules.

For light, the equivalent would be for a fast pulse of lots of photons to hit the planar detector all at once, like from a pulsed point source. Then you could analyze the timing of the hits at each pixel and determine the direction. That is technically feasible. But you can't image random photon hits using just a planar detector, because one photon can only hit one detector.

They can be neglected because you can't detect a microwave photon! So all you can do is work with huge bunches of them, and they average out to appear like classical, non-quantum waves.

John

.....because they've sold 200 already!

Ken

I had not considered quantum effects, but microwaves are EM radiation and phased array antennas work. Obviously each light photon will have a much higher energy than a microwave photon due to the higher frequency, but I can't see why the fundamental theory would be any different.

To form an image you would need a sample rate high enough record the actual EM wave and be able to preserve the phase. If you have that data then you should be able to do the math to simulate the lens or mirror. This can be done with microwaves, so I can't see why it would not be theoretically possible with higher frequency EM waves such as light.

I suppose not, but if you have a sufficiently large number of photons such that the light appears as a continuous wave I think it should still work. A phased array antenna can tell which direction a radio signal comes from, though obviously not from a single photon.

One key difference between light and microwaves is that for microwaves the energy of a photon is very small compared to thermal noise (hf

CCD will be inchoherent detection. The system could be theoretically possible for single spevtral line sources. if the ccd elements were replaced with mixer diodes with response up to light em waves. The diodes would be illuminated with coherent local oscillator light and the mixed down em result detected in a narroe band magnitude/phase detector.

t tIn message , Jamie Morken writes