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Cardboard Optical Interferometer

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#1 Ken Simmons

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Posted 27 September 2021 - 01:27 PM

Hi there, I was wondering if anyone might have plans for making a Interferometer similar to the one in the book "Double and Multiple Stars and How to Observe Them" by James Mullaney. The photo is on page 13 of the book. I have attached a crop version of the photo. 

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  • Optical Interferometer.png

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#2 steveastrouk

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Posted 27 September 2021 - 04:45 PM

The book sounds interesting. I've ordered a copy. Thanks !



#3 Charl

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Posted 27 September 2021 - 06:58 PM

See Sky & Telescope March 1997:

 

1997 3 91 Telescope Making Cardboard Double-Star Interferometer "Maurer, Andreas"


Edited by Charl, 27 September 2021 - 07:03 PM.


#4 MKV

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Posted 27 September 2021 - 10:12 PM

The simplest thing to do even in a workshop using a telescope collimamator.  A classic stellar Michaelson IF. Cut a cardboard mask with two holes, as shown. Two aperture holes create two star images. Bring them together by refocusing and see interference fringes.

 

Mladen

 

star interferometry.jpg


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#5 Ken Simmons

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Posted 28 September 2021 - 10:48 AM

Thanks for the reply guy's. Steveastrouk, some other books I have are "Observing and Measuring Visual Double Stars" 2nd. Edition by R.W Argyle (Editor), "The Cambridge Double Star Atlas" 2nd. by Bruce MacEvoy and Wil Tirion, "Starlight: An Intro. to Stellar Physics" by Keith Robinson and "The Life and Death of Stars" by Kenneth R. Lang. I have others but these are my favorites. The books on the stars have little or no math.

Charl, thanks for the info. I have the Sky and Telescope DVD collection. Will lookup this article and see if I can find others. MKV, this would be the easiest one to make. I have a 8" Meade SCT, I wonder if the holes have to be a certain diameter. ?. Is there a formula for this. ? You guys take care.



#6 MKV

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Posted 28 September 2021 - 11:26 AM

MKV, this would be the easiest one to make. I have a 8" Meade SCT, I wonder if the holes have to be a certain diameter. ?. Is there a formula for this. ? You guys take care.

Good question. l think it's not crucial. You're just superimposing two separate wavefronts which leads to interference. This is the basis for any interferometer.  In the  Twyman-Green IF, an offshoot of the Michaelson. you get two focused images (one reference, the other test) and to get interference they have to be precisely superimposed. As shown below

 

T-G alignment issue.jpg


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#7 Ken Simmons

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Posted 29 September 2021 - 10:59 AM

Ok, thanks MKV.



#8 Itz marcus

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Posted 01 October 2021 - 01:58 PM

Hi,

How does that mask work? I see that you put pictures with ronchi lines. Is that a pic through a ronchi slide? Is this useful in testing telescope quality and if yes, is this a more sensitive than a ronchi screen straight without the mask? Is it as sensitive as a double pass test?

Clear Skies

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#9 BGRE

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Posted 01 October 2021 - 06:03 PM

Nothing to do with either testing a telescope or Ronchi's grating interferometer.

In extended form with the distance between the apertures of several meters Michelson used it to measure the angular diameters of several large stars including Betelgeuse.

 

The telescope optics are merely a convenient means of combining the light from the pair of apertures. .



#10 davidc135

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Posted 02 October 2021 - 03:25 AM

Interesting thread. What does the interferometer enable someone with, say, an 8'' sct to do? On for instance the double double.

 

David


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#11 Gleb1964

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Posted 03 October 2021 - 04:49 AM

The ability to discern the two components of a binary star system defined by ‘Rayleigh Criterion’ is 1.22 λ/D [rad], while the maximum transmit frequency is λ/D before cut-off. The resolution in interferometer mode defined by λ/2b[rad] and can be as twice as high for the same aperture.

 

The carrier frequency in a star image is λ/b and in case of double star the minimal modulation can be detected if two patterns shifted by the half of pattern wave.

 

Screenshot_20211003-114119.png


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#12 BGRE

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Posted 03 October 2021 - 05:02 AM

The ability to discern the two components of a binary star system defined by ‘Rayleigh Criterion’ is 1.22 λ/D [rad], while the maximum transmit frequency is λ/D before cut-off. The resolution in interferometer mode defined by λ/2b[rad] and can be as twice as high for the same aperture.

 

The carrier frequency in a star image is λ/b and in case of double star the minimal modulation can be detected if two patterns shifted by the half of pattern wave.

 

attachicon.gifScreenshot_20211003-114119.png

If the two components of the double star aren't of equal brightness then the fringe modulation is non zero when the incoherent interference patterns produced by the 2 stars are 180 degrees out of phase. The light from each of the 2 stars is incoherent with the light from the other star and each star produces a separate dual "slit" interference pattern. Since the sources are mutually incoherent the intensities of the individual interference patterns add. If the spectral bandwidth is too large few fringes are seen. The angular separation can be measured by adjusting the "slit" separation to minimise the fringe visibility of the superimposed interferograms.


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#13 davidc135

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Posted 03 October 2021 - 01:10 PM

Thanks for the explanations.  David



#14 MKV

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Posted 03 October 2021 - 09:31 PM

In general, the larger the separation (d) of the the two apertures, the better the resolution of the interference pattern. 

220px-Michelson_stellar_interferometer.s

source: Wikipedia (https://en.wikipedia..._interferometer)

 

Large observatory telescopes have such devices with d spanning several meters.

 

There's no reason why a similar device (with adjustable d) couldn't be placed atop of an 8-inch (0.2 meter) Cassegrain, spanning much more than 0.2 meters.

 

c-8 stellar interferometer.jpg

 

Mladen



#15 BGRE

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Posted 03 October 2021 - 10:05 PM

When the "slit" separation is sufficient to measure the angular diameter of a star the effect of limb darkening of the stellar disk has to be taken into account when attempting to measure the angular diameter of a star. The observed interferogram is actually an incoherent superposition of the mutually incoherent interferograms produced by each radiating atom on the stellar disk. Limb darkening reduces the relative amplitude of the interferograms produced by points closer to the limb of the stellar disk.



#16 Gleb1964

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Posted 04 October 2021 - 06:35 AM

In general, the larger the separation (d) of the the two apertures, the better the resolution of the interference pattern. 

220px-Michelson_stellar_interferometer.s

source: Wikipedia (https://en.wikipedia..._interferometer)

 

Large observatory telescopes have such devices with d spanning several meters.

 

There's no reason why a similar device (with adjustable d) couldn't be placed atop of an 8-inch (0.2 meter) Cassegrain, spanning much more than 0.2 meters.

 

attachicon.gifc-8 stellar interferometer.jpg

 

Mladen

Mladen,

 

that would complicate things a lot.

The simplest way to try interferometer is to use two holes within telescope aperture. It works with white light and within entire field.

If you use construction, like on picture, that complicates all things a lot. The increased separation between interferometer apertures, the consequences of that is, that you need use ether monochrome light and ether you getting minuscules field where interference may take place. With very large separation you need limit both spectral range and field. That is the cost of expanding interferometer apertures beyond the telescope aperture.

Think, stellar interferometer is getting two tilted focal planes of constant phase, crossing each other somewhere inside or outside of field of view of telescope. With white light there can be just a few microns of coherence length, so both arms needs to be properly aligned at identical optical path length to the portion of wavelength. That would give you a line in the field where phase is identical for two arms. Variating phase delay between arms would move equal phase line within the field or beyond (most probably). White light interference can be observed only very close to the line of equal phase. Adding narrowband filter the region of interference can be extended in proportion of ∿λ/∆λ.    

 

Gleb



#17 MKV

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Posted 04 October 2021 - 07:24 AM

Mladen,

 

that would complicate things a lot.

The simplest way to try interferometer is to use two holes within telescope aperture. It works with white light and within entire field.

If you use construction, like on picture, that complicates all things a lot...

Oy, Gleb it's used in the same fashion in observatories to increase resolution. The link I provided shows it clearly. A 100-inch (2.5 meter) telescope using a 6-meter stellar IF. Translated into amateur size 8-inch to a 19  inch stellar IF.  If the starlight is good enough for the big one it's probably good enough for the small one. :o)

 

Hooker_interferometer.jpg

Source: Wikipedia. Same link as is #14



#18 Gleb1964

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Posted 04 October 2021 - 09:26 AM

220px-Michelson_stellar_interferometer.s

I think, something is missed in that simplified layout. That should be something to help adjust equal optical path within a few microns tolerance. For example, parallel glass, that can tweak path length by tilting it. Otherwise, that construction will never achieve interference.
Last time you had recommended the right thing:

It can be done using something like a C8 lens cap. Just drill 2 holes in the cap same distance apart from the center.
post-177548-0-36954900-1580394608.png



#19 MKV

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Posted 04 October 2021 - 09:48 AM

Last time you had recommended the right thing:

Well, I recommended it again on this thread in #4

 

https://www.cloudyni...ter/?p=11392421

 

Nonetheless, it is clear that with sufficient controls you should be able to get more fringes, as it is  a function of separation of the two apertures.



#20 gr5org

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Posted 04 October 2021 - 12:12 PM

@MKV - I love your diagram showing the interference fringes.  But how do I determine the diameter of a star (even very roughly)?  Do I count the fringes or something?  Or will the diameter of the "circle of fringes" match the diameter of the star?



#21 MKV

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Posted 04 October 2021 - 02:37 PM

@MKV - I love your diagram showing the interference fringes.  But how do I determine the diameter of a star (even very roughly)?  Do I count the fringes or something?  Or will the diameter of the "circle of fringes" match the diameter of the star?

Thanks. I'm afraid I don't know the answer to that. I now for a fact that there were other threads on this topic in the past on CN. I'll defer to our theoretical gurus -- BGRE and Gleb. I made a 2-aperture mask just out of curiosity to see fringes of a single star-like signal. I never tried to observe a double star of close magnitudes. If I were to do it, I would point the telescope to Castor in Gemini. The "star" resolves into two "stars" of close  magnitudes (1.9 and 3.0), but with higher power, each "star" is further resolved into another double! Well, there are actually six stars in the whole Cast system, so I think it would make a perfect test object to learn how to use a stellar interferometer and to find out, empirically, its limitations.   

 

Cheers!

Mladen



#22 BGRE

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Posted 04 October 2021 - 03:53 PM

The fringe contrast is measured as the separation between the apertures is varied and a curve fit is used to determine the stellar diameter. For a crude measurement determine the aperture separation for which the fringe contrast vanishes. The added complication with Michelsons setup is ensuring equal optical path lengths from each aperture to the focus.

Its even possible to interferometrically combine the light from a pair of separate telescopes if a suitable differential optical path length compensator is used. In effect a pair of variable delay coupled optical delay lines is used to equalise the OPD,

 

Michelson was only able to measure the angular diameter of stars like alpha Orionis which have a large angular diameter (0.047 arsec for alpha Orionis). Most stars (apart from the sun) have a much smaller angular diameter requiring a much larger separation between apertures than Michelson used to measure their angular diameters.

 

The same technique could be used to measure some solar system objects with a relatively small angular diameter (~1 arcsec or less).if they are sufficiently bright. 


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