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Old 09-07-2021, 01:12 AM
bgilbert (Barry gilbert)
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Diffraction spikes in Newt's

G'day all.

Why do we observe 6 diffraction spikes in a Newtonian with 3 support vanes to support the secondary mirror, and only 4 spikes if we add another vane, making 4 vanes ? Assume all vanes are equally spaced.

Barry
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Old 09-07-2021, 01:31 AM
JA
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Hi
Quote:
Originally Posted by bgilbert View Post
G'day all.

Why do we observe 6 diffraction spikes in a Newtonian with 3 support vanes to support the secondary mirror, and only 4 spikes if we add another vane, making 4 vanes ? Assume all vanes are equally spaced.

Barry
Hi Barry,
I believe a clue to the answer is to be found in looking at / induction from the diffraction spike pattern of 1 and 2 vane spiders and comparing with what is observed in 3 and 4 vane spiders..... And really questioning what one is seeing with the 4 vane spider....is it 4 spikes or 2 sets of four spikes superimposed on top of one another?

What you are really observing is diffraction of light around the edges of the vane, this creates an interference pattern / diffraction pattern which is perpendicular to the edge/s so that ...

No vanes
will simply have a round (airy disk) diffraction pattern around a star

1 vane
will have round (airy disk) pattern around the star & a spike either side of the star like a line passing through it, which may appear as a dotted/dashed line due to interference or possibly not so dashed depending on how well the scope is collimated and focused

2 vanes - in line
will have round (airy disk) pattern around the star & a spike either side of the star like a line passing through it, which may appear as a dotted/dashed line due to interference and slightly more pronounced than the 1 vane pattern

2 vanes - at 90degrees to each other
This is the kicker in terms of understanding....
will have round (airy disk) pattern around the star & a 4 sided spike evenly distributed around the star. All else being equal it will appear to be a slightly more pronounced pattern than the 2 vane spider with vanes at 90 degrees, because there are more edges (of vanes) in the way of the light and hence more diffraction patterns superimposed.

3 vanes - at 120degrees to each other
will have round (airy disk) pattern around the star & a 6 sided spike evenly distributed around the star. Each of the vanes will create a diffraction pattern (line) perpendicular to it. Looking at the OTA end-on we have 3 vanes: one at say 0 degrees (TOP), another at 120 degrees and another at 240 degrees around its circumference. For simplicity say we have a star at the dead centre of the OTA, then
taking Vane 1 (at 0degrees on the OTA circumference) it creates a diffraction pattern (dashed line) perpendicular to it which passes through the star and therefore goes towards 90 degrees on one side and 270 degrees on the other side of THE IMAGE.)
taking Vane 2 (at 120degrees on the OTA circumference) it creates a diffraction pattern (dashed line) perpendicular to it which passes through the star and therefore goes towards 30 degrees on one side and 210 degrees on the other side of THE IMAGE.)
taking Vane 3 (at 240degrees on the OTA circumference) it creates a diffraction pattern (dashed line) perpendicular to it which passes through the star and therefore goes towards 150 degrees on one side and 330 degrees on the other side of THE IMAGE.)
This results in a 6 pointed star shaped diffraction spike pattern. With spikes at 30, 90, 150, 210, 270 and 330 degrees being seen IN THE IMAGE relative to the position of Vane 1 ON THE OTA, measuring the angle clockwise.


4 vanes - at 90degrees to each other
will have round (airy disk) pattern around the star & a 4 sided spike evenly distributed around the star. It will appear to be a more pronounced pattern than the 2 vane spider, because there are more edges (of vanes) in the way of the light and hence more diffraction patterns superimposed. The reasoning behind there being 4 spikes has to do with the diffraction pattern of any one vane being at perpendicular to it and culminates in 4 diffraction spikes only at 0, 90, 180 and 270 degrees in a similar reasoning to that used in considering the 3 vane spider, detailed above, except that the patterns can be thought of as being superimposed on one another and hence only showing up at 4 locations.

5 vanes (evenly distributed @72 degrees)
10 pointed spike pattern


and so on..... OR NOT .....(kidding)

A drawing may have been easier, but it needed some explanation (at least to me)

Hopefully no typos or worse.

Best
JA

Last edited by JA; 09-07-2021 at 01:49 PM. Reason: Added reasoning
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Old 09-07-2021, 05:37 AM
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mura_gadi (Steve)
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Hello,

If you look at some of the less professional shots around on a F5 or faster newt, you'll see 8 spikes from a 4 vane spider. A smaller set, 45degrees off set from the large spikes as well. I say the less professional ones as the spikes tend to show up a little better in them, or look for very bright stars with long exposures.



Steve
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Old 09-07-2021, 07:15 AM
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Think double slit experiment.
Light acts as a particle and a wave and as a wave creats interference pattens.
Alex
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Old 09-07-2021, 07:37 AM
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The answer is complicated and a lot of mathematics is involved.

In free space photons behave like a stream of bullets but when they strike an obstacle photons behave like sine waves and oscillations occur creating harmonics.

Apparently Leonardo Da Vinci in 1490 first observed this diffraction effect of shadows from a straight edge cast onto a floor.

This article here has a good explanation of this phenomena, but a lot of maths is involved and its over my head.

http://serge.bertorello.free.fr/opti...c/diffrac.html
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Old 09-07-2021, 05:21 PM
bgilbert (Barry gilbert)
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There are some pretty good answer in your posts, but Iíll throw my two bobís worth in. Firstly there are no photons, bullets or particles of light! Maxwellian waves represent light perfectly. The mere act of diffraction and interference proves that. The point spread function (PSF) in the case of a Newt displays Airy rings plus spikes, the PSF represents the two dimensional Fourier transform of the aperture, The spikes are the transform of the blades, the rings represent the transform of the edges of the primary mirror plus the transform of the edges of the secondary mirror and its holder etc. If the edges of your primary mirror were softened (not so sharp) you could eliminate the Airy rings. This is very difficult in your Newt but relatively simple for RF satellite dishes and antenna arrays. Modifying edge sharpness is referred to a Apodization, this can improve planetary and lunar detail The Fourier transform of a blade usually looks similar to a rotation of the blade by 90 degrees. In other words a vertical blade will put a horizontal spike on the PSF. The spikes can be reduced or decorrelated by curving the blades in various ways. This distributes the apparent energy of the spikes into faint noise which is considered less distracting by some but enjoyed by others! Although apodizing optical elements (lenses and mirrors) is difficult it is useful for tailoring the point spread function for such things as finding exoplanets. By designing a deep null in the PSF close to the main beam helps see close in objects to bright stars or finding doubles. Just to finish off the Fourier transform of a Gaussian aperture is a Gaussian PSF. This can currently be achieved in RF antennas but difficult and expensive for optics. The main beamwidth of an aperture is inversely proportional to the aperture diameter, so very large apertures are required to improve resolution. Large baseline interferometry allows RF antennas to have very fine resolution, but is extremely difficult for current optical technology.

Barry
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Old 09-07-2021, 06:13 PM
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Barry
These waves are they similar to an ocean wave where it would appear the water responds to energy passing through a medium, the water...with out a medium what is a wave? Can a wave exist without a medium?
Alex
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Old 09-07-2021, 06:17 PM
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The spikes on the 4 vane spider overlap whereas on the 3 vane they do not. You may notice the 4 vane spices are brighter.
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Old 09-07-2021, 07:22 PM
bgilbert (Barry gilbert)
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G'day Alex.
It's a good point you make, it troubled Newton to such an extent that he apologized to future generations for leaving them with the awful legacy of action at a distance without a medium. I prefer to struggle with Newtons action at a distance rather than the absurd paradox's that result from Einsteins little "wave packet". How do you explain G. I. Taylor's 1909 experiment with photons. His experiment claims to have taken 3 months to expose the film because the photons were so well spaced or feeble as to guarantee only one photon passed the needle at time. A magnetic field seems to act at a distance without a medium, so does gravity, do you suggest we need gravity particles or magnetic particles? The impedance of free space is 377 Ohms, why? It is somewhat of a mystery that light particles don't help explain. The fact that the repulsive force of Coulomb charges prevent you from falling through the floor is not helped by particles of light! Or any other particles. Where does the energy come from to support these force type particles? Do gravity particles push or pull? All the identified massive particles are 99.999% empty space, the concept of solid matter is anthropomorphic. It seems to indicate we have fields, mass and inertia and we don't understand any of it, but we can measure, observe and manipulate it to some extent.

Just my observations?
Barry
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Old 09-07-2021, 10:38 PM
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Hi Barry
Of course I dont have all the answers nor profess that the ones I have are correct but my simple approach is that all things have a particle to represent them...and so yes, gravity, fields (including magnetic) even energy are made from particles ...just very very little ones..perhaps a primative approach but that is the way I can imagine things...or a "billiard ball" universe...but there is no one holding a cue or triangle nor any edge for the table which has always been there.

Alex
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Old 10-07-2021, 07:12 AM
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Quote:
Originally Posted by bgilbert View Post
G'day all.

Why do we observe 6 diffraction spikes in a Newtonian with 3 support vanes to support the secondary mirror, and only 4 spikes if we add another vane, making 4 vanes ? Assume all vanes are equally spaced.

Barry
In a reflector with two mirrors the easiest way to vizualise this is to think of an extra spike extending opposite each spider vane. The resulting number is always 2xnumber of spider vanes but what you see will differ if they are overlapping or not.

So zero spider vanes = zero spikes (SCT)
1 spider vane = 2 spikes (maybe a hyperstar cable over the corrector)
2 spider vanes = 2 overlapping spikes if in line, 4 spikes otherwise.
3 spider vanes = 4 spikes (2 overlapping) if 2 are in line and one at 90 degrees, 6 otherwise.
4 spider vanes = 4 spikes (2 overlapping if properly squared) otherwise 8 spikes.

And so on. Clear as mud?
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Old 10-07-2021, 11:35 AM
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Quote:
Originally Posted by multiweb View Post
"3 spider vanes = 4 spikes (2 overlapping) if 2 are in line and one at 90 degrees, 6 otherwise."
I doubt there are too many reflectors out there with 3 vane spiders that have two vanes inline and one at 90 deg. A significant majority would be 3 vanes at 120 deg. apart producing 6 spikes. What you wrote is correct but perhaps "3 vanes = 6 spikes otherwise 4 spikes if 2 vanes are in line and one vane at 90 deg".

I'm sure the OP appreciated the various replies explaining all the reasons behind diffraction spikes but I got the impression that he was simply asking why 6 spikes on 3 vane spider and 4 spikes on 4 vanes spider.
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Old 10-07-2021, 04:16 PM
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Why ask the leading question if you think you know the answer ?

Quote:
Originally Posted by bgilbert View Post
G'day Alex.
It's a good point you make, it troubled Newton to such an extent that he apologized to future generations for leaving them with the awful legacy of action at a distance without a medium. I prefer to struggle with Newtons action at a distance rather than the absurd paradox's that result from Einsteins little "wave packet". How do you explain G. I. Taylor's 1909 experiment with photons. His experiment claims to have taken 3 months to expose the film because the photons were so well spaced or feeble as to guarantee only one photon passed the needle at time. A magnetic field seems to act at a distance without a medium, so does gravity, do you suggest we need gravity particles or magnetic particles? The impedance of free space is 377 Ohms, why? It is somewhat of a mystery that light particles don't help explain. The fact that the repulsive force of Coulomb charges prevent you from falling through the floor is not helped by particles of light! Or any other particles. Where does the energy come from to support these force type particles? Do gravity particles push or pull? All the identified massive particles are 99.999% empty space, the concept of solid matter is anthropomorphic. It seems to indicate we have fields, mass and inertia and we don't understand any of it, but we can measure, observe and manipulate it to some extent.

Just my observations?
Barry
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Old 10-07-2021, 06:15 PM
bgilbert (Barry gilbert)
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Sunfish
You seem agitated? Can you be more explicit what is troubling you? I'm showing by demonstration that Maxwell is capable of solving many of the paradox's of QM, optics and physics. Perhaps you could explain the subtleties of diffraction, refraction, interference and the PSF with "photon" theory? At least bring me up to speed on forum etiquette.

Barry
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Old 10-07-2021, 06:48 PM
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Quote:
Originally Posted by astro744 View Post
I doubt there are too many reflectors out there with 3 vane spiders that have two vanes inline and one at 90 deg. A significant majority would be 3 vanes at 120 deg. apart producing 6 spikes. What you wrote is correct but perhaps "3 vanes = 6 spikes otherwise 4 spikes if 2 vanes are in line and one vane at 90 deg".

I'm sure the OP appreciated the various replies explaining all the reasons behind diffraction spikes but I got the impression that he was simply asking why 6 spikes on 3 vane spider and 4 spikes on 4 vanes spider.
I was just showing that the basic formula works for all cases starting from zero to four. Not describing any particular type of scope.
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Old 10-07-2021, 07:56 PM
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I am not an experimental physicist and cannot refer you to any current peer reviewed papers in high impact factor journals and so my reply would be pointless.

Quote:
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Sunfish
You seem agitated? Can you be more explicit what is troubling you? I'm showing by demonstration that Maxwell is capable of solving many of the paradox's of QM, optics and physics. Perhaps you could explain the subtleties of diffraction, refraction, interference and the PSF with "photon" theory? At least bring me up to speed on forum etiquette.

Barry
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