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Spider and Secondary Diffraction: what to do, what to avoid

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#51 raal

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Posted 30 March 2015 - 04:10 PM

Thanks,Mark! I don't know for sure, but I beleive it should, and easily.

 

Dane, I'll gladly do it, but tomorrow.

The way the mask size vs. matrix size is set, even these 0.7mm (0.0275") vanes are on the edge resolution.

I'll set a different mask in order to be sure that 0.007" vanes are within resolution, and then show you the results from the beginning, first the unobstructed, then with just 20% CO, then with CO and spider, because I don't really know would those results would be comparable to what I already showed due to different setup.

Well, I'll throw in something of what is already shown here and we'll see how it compares.

What is your aperture?

 

Cheers,



#52 mark cowan

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Posted 30 March 2015 - 04:13 PM

My request also.  :bounce:



#53 abberation

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Posted 30 March 2015 - 05:23 PM

A little mechanical info on a carbide stalk:

 

I found an elastic beam calculator and I think I did the right thing to get the deflection of a single stalk.

 

First, the design:  a 1/4" carbide rod thinned in to a rough elipse (cross section: 0) 1/8" wide.  The calculator uses metric so to be conservative, I'll say 3mm.

 

An elipse will be much stiffer to bending to forces applied in the plane of the long axis so to get a worst case, you calculate for a 3mm round bar and halve the deflection because it's going to act like two 3mm bars bound together (cross section: 8).  It will actually be better than that because 0 has a bit more section than 8.

 

Carbide has a Youngs Modulus of 550GPa, the moment for a 3mm bar is 3.9761e-12 m^4 and I put in a 200mm beam length with a .25N force.

 

The resulting deflection was 0.3048mm, divide by 2 to approximate an elipse and you get 0.1524mm or 0.006"

 

I think that is decent but I would still minimize the weight of the secondary, minimize the length of the stalk and consider widening the elipse based on how heavy it is.

 

If you put this in a Dobson with the stalk prependicular to the elevation axis, it gets better because the force of gravity will always be acting on the stiff axis of the elipse.

 

If you incorporate the sag of the secondary stalk in to a Serrurier truss design, you can cancel it out.

 

Any way, I think a single stalk is half way decent until your aperture and secondary get huge, for my 12" f8 project, it looks promising.



#54 dag55

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Posted 30 March 2015 - 05:43 PM

Thanks,Mark! I don't know for sure, but I beleive it should, and easily.

 

Dane, I'll gladly do it, but tomorrow.

The way the mask size vs. matrix size is set, even these 0.7mm (0.0275") vanes are on the edge resolution.

I'll set a different mask in order to be sure that 0.007" vanes are within resolution, and then show you the results from the beginning, first the unobstructed, then with just 20% CO, then with CO and spider, because I don't really know would those results would be comparable to what I already showed due to different setup.

Well, I'll throw in something of what is already shown here and we'll see how it compares.

What is your aperture?

 

CheThanks

Thanks Aleksandar, my aperture is 203mm. I am comparing the results to a single stalk although mine  is 3.97mm wide. instead of the 5mm but still should give me an idea since yours was done with a 317mm aperture.

Dane


Edited by dag55, 30 March 2015 - 05:47 PM.


#55 mark cowan

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Posted 30 March 2015 - 07:41 PM

I'd like to see it for the original 307mm at .007", that's closer to the reality of what I build and a direct compare to the stalk.  

 

Next clear night I'll test a stalk shadow on my 8" that has .007" wires just to see what happens.  I know what to expect but I'd like to verify it with my eyeballs.   ;)

 

Thanks,

Mark



#56 DesertRat

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Posted 30 March 2015 - 08:39 PM

A 7 mil wire will not even cover a single pixel unless you sample the pupil at almost 2k pixels across. That ratio is approx 1 to 1726. Pad that with enough black space for an accurate analysis and you need almost 64 megapixels. It can be done but why? Such a spider has a near zero impact on optical performance.

 

But pressing on. You could draw a physically proportionate pupil on a large canvas and resize using you favorite IP program. Then the wire spider will likely take on a value brighter than 0. Since resizing is almost a black art I really don't want to get into all the details now. You could simply draw the pupil at half size using 128 gray for the wire instead of 0, assuming you are drawing with 256 gray levels.

 

Good luck!

Glenn



#57 mark cowan

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Posted 30 March 2015 - 11:08 PM

Why?  Because that's a totally workable spider. :lol:

 

I'm well aware it's hard to simulate correctly, but if the comparison is between buildable curved vane designs (the usual competition) and wire spiders it'd be nice to have accurate simulations showing the relative effect. 



#58 abberation

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Posted 30 March 2015 - 11:35 PM

Failing to simulate a build able design tells you nothing about the design or the simulation.



#59 mark cowan

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Posted 31 March 2015 - 12:07 AM

Failing to simulate accurately tells you that the simulation is inadequate for some reason, which is actually quite useful.  

 

But more to an actual point, just now before it clouded over (roughly one half hour after I started) I had a look at straight obstructions of 1/8", 3/16", and 1/4" rod, as well as briefly with a 1/2" ruler, on my 8" with 3 .007" wire spider legs.  Years of viewing with that scope have showed me that the spikes it generates are innocuous under almost all conditions.

 

However, just now, on Jupiter at least, with the moon too close, one would be hard pressed to tell the difference between the faint wire "smears" and either the 1/8" or 3/16" for a half-aperture span.  And of course there's only the double spike to be seen.  Anything thicker than that begins to become annoying.  I didn't have a chance to try this on Sirius but since that's the target on which the .007" wires actually generate visible spikes under most conditions I don't expect the stalk to fare as well under a fully dark sky, when it's much easier to pick out subtle contrast differences, but the performance is surprisingly good in this case.

 

Vibration may be a problem for thin stalks - at least in a properly designed wire spider it can be effectively damped, and holding collimation really isn't an issue at all for the latter either - but that's another topic. 

 

Best,

Mark

 

PS I would add that one of the most revealing experiments I've done is to lay a flat ruler on the OTA end under ideal conditions while looking at Sirius at high power.  Twist the ruler slowly from flat to edge on and observe what happens to the fine detail in the polychromatic spike structure close to the star image, you can see exactly what the effect is as the width of the "vane" varies. This helps you to see the power distribution in the lobes of the diffraction spikes, and why minimizing the thickness of the vane is useful, because it reduces the power in the central diffraction lobe, which is the only one that really matters...the extended spikes are always visible (but not harmful) because they extend far away from the target and aren't nearly as responsive to reductions in vane thickness as one might think...as Suiter's diagram illustrates quite well.


Edited by mark cowan, 31 March 2015 - 01:30 AM.


#60 Oberon

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Posted 31 March 2015 - 01:27 AM

Again...the central diffraction lobe is not the only one that matters, it is the spike that matters and matters most.  :crazyeyes:



#61 mark cowan

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Posted 31 March 2015 - 01:44 AM

The central lobe of the spider diffraction pattern (the spike) is what I'm talking about.

 

The spider diffraction has lobes that are much wider than the Airy disc, and the central lobe of the spider diffraction pattern is typically arcseconds in size. The extended spike (the secondary lobe and beyond) has little or no effect on contrast performance on the target, but to some people it may matter more or most, I suppose, but that's an aesthetic complaint and doesn't affect the contrast of the object under view.  For your binos you won't want spikes that don't line up, that's definitely true, and curved spiders guarantee that.


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#62 han7720

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Posted 31 March 2015 - 02:33 AM

In my case,

3 curved vanes seems to be poor when used with the Cassegrain telescope.

curved vanes were susceptible to vibration.

Cassegrain telescope secondary mirror was very sensitive to vibration

Could affect the vibration caused by wind and equatorial.

 

31212.jpg

 

I think it seems to be more sensitive because of my telescope vanes curve angle.

May be due to my mistakes....


Edited by han7720, 31 March 2015 - 02:58 AM.


#63 Oberon

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Posted 31 March 2015 - 03:16 AM

Each of the vanes seem to curve about 120-150 degrees. That far more than the 60 degrees required, making them longer and more susceptible to flexure and vibration. :(



#64 han7720

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Posted 31 March 2015 - 03:57 AM

Each of the vanes seem to curve about 120-150 degrees. That far more than the 60 degrees required, making them longer and more susceptible to flexure and vibration. :(

 

It seems that your thinking is right.
My curve spider is too large angle.
Vibration problem had not thought of.

 

It need repairs...for 60 degrees curve or Typical 4 Spider.

 

thanks


Edited by han7720, 31 March 2015 - 04:04 AM.


#65 raal

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Posted 31 March 2015 - 08:54 AM

Glenn, I'd sure like to have the option of doing 64MP or 256Mp simulations! Makes no sense, I know, as I'll still build what is the thinnest possible spider, wire or curved or both, so I'll get the practical minimum of diffraction, but still...in this day and age of FEM, BEM, CFD, multiphysics sims, this should be peanuts. Our expectations have risen since these tools became "ordinary"...They help decision making process a lot and I'm spoiled by using some of them.

 

Anyhow, what you are about to see works with just what Glenn said. Subpixel features have been assigned a whole pixel, only shaded. I have no idea whether or not the shading is proportional and how shaded stuff works in diffraction sims, so don't believe too much the improvement that you will see for thinning the spider.

 

The unobstructed 317mm aperture:

 

 

 

 

 

 

 

 

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  • unobst 1500 iv.jpg


#66 raal

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Posted 31 March 2015 - 08:56 AM

317mm 20%CO

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  • 20 percent 1500 iv.jpg


#67 raal

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Posted 31 March 2015 - 09:00 AM

For comparison with previous sims, here's a 317mm 20%CO (it was 21%CO previously, but it's not relevant to what the spider does), with 4 x 0.7mm vanes:

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  • 4x07 1500 2048 iv.jpg


#68 raal

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Posted 31 March 2015 - 09:04 AM

Here's the improvement with 4 vanes of just 0.18mm:

 

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  • 4x02 1500 2048 iv.jpg


#69 raal

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Posted 31 March 2015 - 09:06 AM

Same 0.18mm spider at 203mm, 20% CO, aperture:

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  • 203 4x02 str 1000 iv.jpg

Edited by raal, 31 March 2015 - 09:07 AM.


#70 raal

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Posted 31 March 2015 - 09:14 AM

Now the F1 of all spiders:

3x 0.18mm wire. No overlap of spikes, so already dim becomes even dimmer...

 

 

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  • 3x02 str 20p 1500 iv.jpg


#71 raal

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Posted 31 March 2015 - 09:23 AM

For comparison with yesterday sims, here's a curved 3 vane 0.7mm spider.

 

I can easily see the halo now. All contrast reducing setting are the same as before, but now the aperture is 20% of mask size, instead of 10%.

Still, this mask shows some halo even on unobstructed and 20%CO spiderless sims, so I don't know which is closer to reality and whether this one better compares thinner straight spiders to thicker curved ones.

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  • 3x07 62 1500 2048 iv.jpg


#72 Oberon

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Posted 31 March 2015 - 09:51 AM

Meanwhile back in binana land we're approaching the semi-finals with a last ditch attempt to squeeze the minimum length of edge and non-repeating angles. The results are so close I've had to re-order them to make it easier to compare like with like...

gallery_217007_4746_301608.jpg



#73 Oberon

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Posted 31 March 2015 - 09:59 AM

Taking the pick and following the earlier suggestions for avoiding artificial artifacts, I've provided two results for each, one at half brightness, and the other at 5 x brightness. The wire spider is 0.3mm, the thin curved vane is 3mm and the thick curved vane is 8mm

gallery_217007_4746_67772.jpeg

 

gallery_217007_4746_55806.jpeg

 

gallery_217007_4746_33324.jpeg

 

gallery_217007_4746_21000.jpeg



#74 raal

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Posted 31 March 2015 - 10:33 AM

Jonathan, it's not just the brightness. For really nice results that are mostly free of artifacts, make the aperture pupil to be just 10% of the whole mask (and set the brightness to 0.5).

This large margin is a problem for simulating thin vanes, but it's not going to be a problem for the thick stuff and you will actually see the Airy disk, rings around it and a proper diffraction pattern.

 

After you get the result, use any photo editor to reduce contrast to taste. It will bring the dim features of diffraction out.

 

I'm waiting in anticipation to see what you'll end up with for that binozilla!

 

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  • ss d.jpg


#75 dag55

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Posted 31 March 2015 - 01:41 PM

Thanks Aleksandar , nice work, wow looks like the .007" wire spider is back in the running.
Mark thanks for the input, I didn't take vibration into the single post design. Can the effects be simulated?
I hate to install the post just to test, don't want a bunch of holes in my aluminum OTA.I can install navcon rubber shims to reduce vibration as long as it is to bad.
Thanks again for your work Aleksander and your input Mark as you already have a wire spider set up.
Dane


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