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Equipment Discussions >> Reflectors

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Jon Isaacs
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Re: Diffraction spikes and secondary vanes new [Re: GlennLeDrew]
      #5366296 - 08/13/12 11:50 AM

Quote:

If spike intensity scales with obstructor width, then one would naively expect the sub-micron width of a roof prism roof line to produce a spike of very substantial faintness. Yet to me it's about as bad as having, oh, a 1mm thick obstructor, which is more than 1,000 times thicker. I'll have to do some controlled experiments...




Based on my experience using correct image diagonals at high magnifications, there is more going on with a roof prism than just a diffraction effect. Maybe a highest quality, perfectly aligned Amici prism would be different but they really mess double stars in an 80mm F/11 Achromat.

JOn


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Starman1
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Re: Diffraction spikes and secondary vanes new [Re: Jon Isaacs]
      #5366323 - 08/13/12 12:07 PM

A scratch on a lens in the focal plane of the eyepiece is easily visible because it is in focus.
The middle line in a roof prism in a scope is also very close to the focal plane.
Spider vanes in a secondary holder are nowhere near the focal plane so they are not in focus.
The line down the center of the field of view of an erecting prism is one reason they are not recommended for astronomical use.


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Thomas Karpf
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Re: Diffraction spikes and secondary vanes new [Re: Starman1]
      #5366375 - 08/13/12 12:37 PM

If any bright light source can create a diffraction disk (if visible to the primary or secondary mirror) and/or spike (if visible to the spider), then even a light source seen through the struts of the scope could create them.

If one has a top-of-the-line scope with regards to optics, flocking, and dark surfaces (as Don does), I wonder if it's worth using a shroud even in very dark locations...


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Starman1
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Re: Diffraction spikes and secondary vanes new [Re: Thomas Karpf]
      #5366552 - 08/13/12 02:06 PM

Quote:

If any bright light source can create a diffraction disk (if visible to the primary or secondary mirror) and/or spike (if visible to the spider), then even a light source seen through the struts of the scope could create them.

If one has a top-of-the-line scope with regards to optics, flocking, and dark surfaces (as Don does), I wonder if it's worth using a shroud even in very dark locations...



Light suppression extends to:
--all interior surfaces and nuts and bolts in a scope
--the inside of the focuser and end of focuser drawtube
--the reflective ends of the eyepieces themselves
--light suppression from off angle sources such as: around the primary mirror up through the tube to the secondary mirror; light through the poles to the primary mirror; light into the bottom of the focuser and eyepiece from over the top of the UTA on the side opposite the focuser.
--reflective edges from secondary mirrors

Essentially, anything that could, at low angles, reflect any stray light into the field of view. Even light reflected at angles not directly in the field of view could cause reflections off surfaces that do reflect in the field of view.
Rob Teeter goes almost to extremes to suppress stray light, Carl Zambuto's mirrors do likewise in terms of his smoothness of polish, and TeleVue eyepieces suppress stray light better than any other brand I've used (however, I do darken the bottoms of the barrels and the filter threads "just in case"). And I use a black tarp on the ground under the scope to darken reflections from that source.

Light suppression is an art unto itself. The biggest difference I ever saw in a scope was completely flocking and baffling an SCT to the Nth degree. The difference in contrast was startling--almost on the level of adding a broadband filter to the scope.

And that was under dark skies. Certainly, light suppression is important under brighter skies, but it is equally important under dark skies, if, for no other reason than that the observer has traveled so far to get to them and getting every last bit of contrast out of the scope is so desirable.

One of the ironic features of good light suppression, however, is that it makes diffraction spikes MORE visible by improving contrast.



Edited by Starman1 (08/13/12 02:08 PM)


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Nils Olof Carlin
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Re: Diffraction spikes and secondary vanes new [Re: GlennLeDrew]
      #5366740 - 08/13/12 03:47 PM

Quote:

one would naively expect the sub-micron width of a roof prism roof line to produce a spike of very substantial faintness.




I think submicron is underestimating some, but who knows? Don gives the clue:

Quote:

The middle line in a roof prism in a scope is also very close to the focal plane.




Here, the wavefront may be a few mm in diameter, so even if the roof line is a hundredth of a mm, the relative width (and the spikes generated) could be comparable to a mm wide vane across a few hundred mm aperture.


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wh48gs
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Re: Diffraction spikes and secondary vanes new [Re: Starman1]
      #5366915 - 08/13/12 05:13 PM Attachment (23 downloads)

Don,

Quote:

Pascal's site (linked in my earlier post), at the bottom of the page pretty much agrees with what I see.
And I agree the thinner the vane, the less noticeable the spike.
The "waves" in the intensity that occur closer together in the Suiter graph with increasing obstruction can be seen in the spikes in the illustrations with thick vanes.





Those bright sections of the spike are not subsidiary maximas from the graph. All of them belong to that one central maxima, and the discontinuity is a result of interference with the main pattern, onto which the vane pattern is effectively sumperimposed. Site illustration shows vane width of about 7% of the aperture diameter, which means that its 1st minima-to-minima length is about 14 times the Airy disc diameter.

Discontinuity seen on the simulation shown in Suiter's book is also resulting from superimposing vanes' central maxima onto the ring structure.

Here's simulation by OSLO, for a 4-vane spider 4% aperture width (obstruction neglected). In this case, the central maxima is 25 times the Airy disc diameter long, and only about half of it shows (simulation is not representative of the visual image, but is useful to show pattern structure).

Vla


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Starman1
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Re: Diffraction spikes and secondary vanes new [Re: wh48gs]
      #5366919 - 08/13/12 05:16 PM

Thanks for the clarification.
Does that mean that when I see a spike from a bright star extending over a degree away from the star that the entire length of that spike is within the first maximum away from the star?


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mark cowan
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Re: Diffraction spikes and secondary vanes new [Re: wh48gs]
      #5366990 - 08/13/12 05:46 PM

Quote:

More importantly, there is no spikes to speak of - or is there?




There shouldn't be... But on the road to finishing this dob project I've taken several turns, twice shortening the design f/ratio (now at f/3 ) and haven't got the prototype finished yet! Two other people have built that spider in the meantime. However I'll be putting it in a test scope with an experimental 14.7"x7mm f/3 meniscus mirror (<4 lbs) once I get the mirror sorted. So I'll find out.

Thanks for the nice comments, BTW. The design owes something to native american petroglyphs that I've seen ("she who watches").

Best,
Mark


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mark cowan
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Re: Diffraction spikes and secondary vanes new [Re: Starman1]
      #5366994 - 08/13/12 05:49 PM

Quote:

This would seem to be yet another advantage of the wire spider design, though I'm not certain what would happen off axis if there were pairs of wires going from the secondary holder to each attachment point. Would you then get doubled diffraction spikes?




I'd have to calculate it, but for aligned pairs of wires that are accurately superimposed (not too hard to achieve) the effective width does increase offaxis, just like a wide vane.

Best,
Mark


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wh48gs
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Re: Diffraction spikes and secondary vanes new [Re: Starman1]
      #5367030 - 08/13/12 06:18 PM

Quote:

Does that mean that when I see a spike from a bright star extending over a degree away from the star that the entire length of that spike is within the first maximum away from the star?




Does that imply a 2-degree visual spike? It is easy to figure out, since the entire central maksima is approximately D/W times the Airy disc diameter, where D is the aperture diameter and W the vane width. It is never seen in its entirety, though, since the portion close to the minima is of extremely low intensity. Of course, a sufficiently bright point-like object would ultimately show second and/or third maxima of the vane pattern, but such objects are not a part of the usual sky inventory.


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Starman1
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Re: Diffraction spikes and secondary vanes new [Re: wh48gs]
      #5367163 - 08/13/12 07:28 PM

Quote:

Quote:

Does that mean that when I see a spike from a bright star extending over a degree away from the star that the entire length of that spike is within the first maximum away from the star?




Does that imply a 2-degree visual spike? It is easy to figure out, since the entire central maximum is approximately D/W times the Airy disc diameter, where D is the aperture diameter and W the vane width. It is never seen in its entirety, though, since the portion close to the minimum is of extremely low intensity. Of course, a sufficiently bright point-like object would ultimately show second and/or third maxima of the vane pattern, but such objects are not a part of the usual sky inventory.




Oy vey!
That means that as the spider vanes get thinner, the length of the spike gets longer, though the minimum after the first maximum near the star should be invisible.

On a 317.5mm primary with 0.5mm blades, that would make the spike length 635X the width of the Airy disc.

That would be substantially less than the visible length of the spike I see.

The Airy disc on the 12.5" f/5 scope is .00665mm (2.43932 x lambda in mm x focal ratio) , so the length of the spike should be 4.23mm.

At the image scale of the 12.5" f/5 scope, .0361 degrees per millimeter, a 2 degree spike (and it's longer than that, but I'm not sure by how much) would cover 55.4mm, or over 13X the width of the first maximum.

Ergo, I am seeing well out into further maxima well beyond the width of the first maximum if your formula D/W x Airy Disc holds up.

The spike length (as you would expect) shortens as the magnitude of the star goes down. By the time I get to 3rd magnitude, the spikes fit entirely inside a field 22mm wide.
And, by 5th magnitude, inside a 10mm field.

So tell me, is there something wrong with my math, or does that mean that many more maxima are visible in the star spikes than just the first one near the star?


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wh48gs
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Re: Diffraction spikes and secondary vanes new [Re: Starman1]
      #5367276 - 08/13/12 08:36 PM

Don,

Your math is correct, and the relation is what the theory says (if the sinc function is the right one, the spike length would be somewhat less, D/1.22W times the Airy disc). But if you see that far from the star (assuming zero magnitude), that is at least the 12th subsidiary maxima, which is according to Suiter's graph - which should apply in that respect regardless of the degree of light coherence - some 1,600 times fainter than the central spike maxima. Since the maxima itself is, for this vane area (~160sq.mm) about 500 times fainter than mirror maxima (about 1/10 of the first bright ring in aberration-free aperture, or about 3rd bright ring level, assuming intensity proportional to the area), that makes it some 800,000 times - or 5.9 logarithmic units - fainter than the mirror maxima. For zero magnitude star, this translates into 14.8 magnitude down for that last maxima.

That would be comparable to the brightness of the 38th bright ring in monochromatic light. Since the starlight is not monochromatic, the ring structure is suppressed quite a bit, and one probably can't see more than 10 rings, or so, under any circumstances. But if you really se that far down, you should be able to see nearly ten rings with zero magnitude stars (with the end ring still significantly brighter than the spike end).

Also, the spike should be much (several magnitudes) fainter in its outer portion. You should clearly see the maximas separated (in about a dozen sections), since the minimas are of much lower intensity. Does this fit what you see?

Vla


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wzmek
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Re: Diffraction spikes and secondary vanes new [Re: wh48gs]
      #5367549 - 08/13/12 11:19 PM

Vla,

I agree with the thrust of your posting on 8/10 at 8:49 PM. I want to comment and elaborate on the following snip from that post:

Quote:

Actual vane is much more likely acts as an elongated (inverse) aperture emitting incoherent light, with the first minima lengtwise given by 1.22Lambda/W and width-wise by 1.22Lambda/H. Coheremce factor is important in determining the spike intensity: with coherent light, the complex amplitude is summed up in the pupil and squared at the focus, so doubling the flux increases peak intensity fourfold. With incoherent light, the complex amplitude is squared (individually) in the pupil and summed up at the focus, so doubling the flux doubles the peak intensity. Hence, a four times wider vane will produce as many times brighter spike, not 16 times brighter as on Suiter's graph. Also its full half-length in this case is closer to 3 arc minutes.




Fraunhofer diffraction of a spider vane is indeed modeled by a rectangular aperture (Babinet's principle) illuminated by temporally incoherent but *spatially* coherent light. The image plane receives an accordant spike of widely diffracted light in the form of a spike having a sinc-squared intensity profile (if filtered to narrow the wavelength band enough to throw out the patterns from other wavelengths that fill in the dark fringes.)*

In such a profile, 90% of the light diffracted by the vane falls between the first dark fringes to either side of the peak. If a star of four times the brightness relative to another is observed, the image of the diffraction spike (same vane width) will indeed be four times as bright at the peak of the spike relative to that from the first star. However, if the same brightness star is observed but the vanes are quadrupled in width, then the *total* light contained in the diffracted spike will be (almost exactly) four times that from the narrow vanes. However, the *peak intensity* of the pattern will indeed be 16 times greater. This is because four times the total power (due to the greater obscuration or "inverse aperture" as you so aptly and descriptively stated) will be concentrated into one fourth the area (due to the first dark fringes spanning one fourth the spacing, as demanded by diffraction theory). The two factors of 4 create the combined increase at the peak of 16.

This effect works in a similar way as the impact on Airy disk peak intensity from a star when masking an existing aperture to 1/2 the original diameter. The peak intensity goes down by a factor of 16 in that case as well. The first factor of 4 is due to the area obscuration (half the diameter gives ¼ the area) and another factor of 4 is due to the fact that the transmitted light is spread over a 4x greater area (twice as wide = 4x the area).

Similarly, placing a mask of one quarter the diameter over a telescope aperture drops the peak intensity at the focal place by a factor of 16 squared, or 256 times. The equation for the “intensity” at the peak of the Airy disk (e.g., see Born & Wolf, 6th Ed., eq. 8-5 (14),) is proportional to the square of the aperture area for a constant illumination. This is the same result as that of a rectangular aperture, though in the case of vane obscuration, the length is constant so area changes linearly with the changing parameter, not by the square.

I am confident that Dick Suiter's plot is not a plot based on "narrow" vane width in the sense that the width is close to the wavelength. In our article on spider vanes, we were careful to sample the vane sufficiently (which was a bit of a challenge for a PC in those days.) I also believe that the plot in Dick’s website is completely correct for a constant uniform illumination (for monochromatic light) on common telescope spider vanes of varying width. My recollection from lab measurements of wire diffraction from decades ago is consistent with this, but I will repeat the experiment in the next couple of months and report back. (I’m working on a related investigation and can easily take some pertinent data.)

Cheers,
Bill

*This means that the Fraunhofer diffraction pattern between the temporally-and-spatially coherent point source (e.g., laser) and the spatially-coherent-only point source (star) are identical, but that the treatment of *extended* sources must be done differently between the two, in precisely the way you describe.


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wzmek
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Re: Diffraction spikes and secondary vanes new [Re: wh48gs]
      #5367561 - 08/13/12 11:32 PM

Vla,

Thanks for the image in your 5:13 PM image. Similar 'real' images can be found by browsing through the HST image archives. Often in such images, when the Airy disk is saturated, features of the spider vane diffraction can be seen in wonderful detail, including the first diffracted 'line' to either side of the main spike. (The spike appears as three parallel lines with sinc-squared modulation along their length. There are of course an infinite number of such parallel lines arrayed to either side of the main spike, but they have no significant number of photons in them and therefore so not show.) Often these are in color as well, with red outside and blue inside as one looks further from the star image.

Best,
Bill


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Starman1
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Re: Diffraction spikes and secondary vanes new [Re: wh48gs]
      #5367638 - 08/14/12 01:04 AM

Quote:

Don,

Your math is correct, and the relation is what the theory says (if the sinc function is the right one, the spike length would be somewhat less, D/1.22W times the Airy disc). But if you see that far from the star (assuming zero magnitude), that is at least the 12th subsidiary maxima, which is according to Suiter's graph - which should apply in that respect regardless of the degree of light coherence - some 1,600 times fainter than the central spike maxima. Since the maxima itself is, for this vane area (~160sq.mm) about 500 times fainter than mirror maxima (about 1/10 of the first bright ring in aberration-free aperture, or about 3rd bright ring level, assuming intensity proportional to the area), that makes it some 800,000 times - or 5.9 logarithmic units - fainter than the mirror maxima. For zero magnitude star, this translates into 14.8 magnitude down for that last maxima.

That would be comparable to the brightness of the 38th bright ring in monochromatic light. Since the starlight is not monochromatic, the ring structure is suppressed quite a bit, and one probably can't see more than 10 rings, or so, under any circumstances. But if you really se that far down, you should be able to see nearly ten rings with zero magnitude stars (with the end ring still significantly brighter than the spike end).

Also, the spike should be much (several magnitudes) fainter in its outer portion. You should clearly see the maximas separated (in about a dozen sections), since the minimas are of much lower intensity. Does this fit what you see?

Vla



Yes, at least a dozen minima in the spike (I was calling it waves). At the minimum, the spike intensity is nearly invisible.
For stars like Vega and Deneb and Altair, the spike only dies when the star passes out of the field of view of the scope, and then it abruptly and instantly disappears entirely. If my UTA had a larger opening, my secondary were larger, and the baffles in the mirror box had a larger opening, chances are likely I could still see the spike much farther out. I have little trouble picking up stars in the 16.5-17.0 range, so I would expect to see the spike fade into oblivion at about that magnitude with averted vision.

Oddly, I never really noticed this phenomenon until I owned a primary mirror from Carl Zambuto. The contrast between the sky and faint features in extended objects is truly phenomenal.
Bright star images have substantially less flare and are noticeably smaller than I have seen in other scopes of similar aperture. And the spikes are quite intense. I recently noticed some spikes on a star of eighth magnitude (though they were short) and now you have me interested in counting the minima in the spikes for Vega to see how far out I can see them.

This has been a true learning thread. Thanks.


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Starman1
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Re: Diffraction spikes and secondary vanes [Re: wzmek]
      #5367640 - 08/14/12 01:08 AM

Quote:

Vla,

Thanks for the image in your 5:13 PM image. Similar 'real' images can be found by browsing through the HST image archives. Often in such images, when the Airy disk is saturated, features of the spider vane diffraction can be seen in wonderful detail, including the first diffracted 'line' to either side of the main spike. (The spike appears as three parallel lines with sinc-squared modulation along their length. There are of course an infinite number of such parallel lines arrayed to either side of the main spike, but they have no significant number of photons in them and therefore so not show.) Often these are in color as well, with red outside and blue inside as one looks further from the star image.

Best,
Bill



Really?
Is that why the spikes on bright stars often appear prismatic?
Incredible.


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careysub
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Re: Diffraction spikes and secondary vanes new [Re: wh48gs]
      #5367760 - 08/14/12 05:19 AM

Quote:

Don't forget that eye brightness response is not linear; doubling the intensity will result in a significantly lesser increase in the apparent brightness. Also, a thinner vane will produce somewhat fainter spike, but it will be longer, which may in part offset the brightness decrease, as perceived by the eye.




Thanks, this is an important point to remember if we are talking about visual - not imaging - scopes. Regardless of the underlying physics, we are really interested in what the eye can detect.

Similarly I have seen it argued that curved vanes are pointless since they "only" distribute the same amount of scattered light (everything else being equal) over the image instead of being concentrated in relatively intense spikes. While physically true, if the eye cannot detect the light distributed this way (or its ability is greatly diminished) this is a win.


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Jon Isaacs
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Re: Diffraction spikes and secondary vanes new [Re: careysub]
      #5368018 - 08/14/12 10:15 AM

Quote:

While physically true, if the eye cannot detect the light distributed this way (or its ability is greatly diminished) this is a win.




It is there and it decreases the overall image contrast the same way any diffraction effect does, the energy is less concentrated, it is spread out, smeared. It is a small effect and only matters when viewing the planets.

Jon


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wh48gs
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Re: Diffraction spikes and secondary vanes new [Re: mark cowan]
      #5368167 - 08/14/12 11:45 AM

Quote:

The design owes something to native american petroglyphs that I've seen ("she who watches").





Fascinating; talking about modern art Wonder if "she" is God, or mother - or it comes to the same?

Vla


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mark cowan
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Re: Diffraction spikes and secondary vanes new [Re: careysub]
      #5368478 - 08/14/12 02:37 PM

Quote:

Similarly I have seen it argued that curved vanes are pointless since they "only" distribute the same amount of scattered light (everything else being equal) over the image instead of being concentrated in relatively intense spikes. While physically true, if the eye cannot detect the light distributed this way (or its ability is greatly diminished) this is a win.




I don't think its a win. Being necessarily thicker they distribute more diffracted light closer to the airy disc. True, if done right they distribute it in a circular pattern - but nevertheless the impact is greater overall, if less obvious visually. Contrast is affected more strongly.

Best,
Mark


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