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

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#26 raal

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Posted 29 March 2015 - 06:45 PM

Nils Olof, thank you for pointing out the Suiter's article.

 

To me, it means that far away, thick spider is no more destructive than a hairline spider. My question relates to how destructive it would be closer to Airy disk.

 

Speaking of that model, at which spider thickness would his model of sqaring the energy would completely suck everything out of Airy disk?

 

I've read at Vladimir Sacek's site that it's not the same modeling that can be used for spiders and larger obstructions and that is logical to me...

Also, Sacek mentioned that a spider vane that is D/100 will shoot 100 Airy disk diameters away (with some meaningful significance of energy, I take it,or maybe it's the first lobe).

 

If my single stalk is 1/5th aperture thick, shouldn't it's significant influence spread to 5 Airy disk diameters, or some such number (and let it taper down to equal energy of a hairline spider, I don't mind)?

 

In another direction, my CO+stalk would cover 60% of aperture and effectively reduce it to 40%.

 

So, what happens in that case?



#27 raal

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Posted 29 March 2015 - 07:01 PM

BTW, that Mark Twain quote is so true in my case! :grin:



#28 mark cowan

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Posted 29 March 2015 - 09:36 PM

To me, it means that far away, thick spider is no more destructive than a hairline spider. My question relates to how destructive it would be closer to Airy disk.

 

Yeah, that's pretty much what's missing here.  Thicker obstructions, thicker curved vanes, etc, rob contrast close to the target.  Maybe the effect is subtle in practice, but that diffracted energy is going somewhere, and the spatial frequencies say it's going closer to the target.



#29 Oberon

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Posted 29 March 2015 - 10:09 PM

Several of the images show a pretty clear relationship between thickness of vanes and diffraction. The OP shows that very clearly. However that effect is most noticeable with straight vanes, which is consistent with the observation about parallel surfaces having a significant effect.



#30 mark cowan

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Posted 29 March 2015 - 10:40 PM

Or a lack of comparative resolution in the simulations.  The actual behavior is subtle and there's no free lunch here. :shrug:



#31 Nils Olof Carlin

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Posted 30 March 2015 - 02:52 AM

Nils Olof, thank you for pointing out the Suiter's article.
 
To me, it means that far away, thick spider is no more destructive than a hairline spider. My question relates to how destructive it would be closer to Airy disk.
 
Speaking of that model, at which spider thickness would his model of sqaring the energy would completely suck everything out of Airy disk?
 
I've read at Vladimir Sacek's site that it's not the same modeling that can be used for spiders and larger obstructions and that is logical to me...
Also, Sacek mentioned that a spider vane that is D/100 will shoot 100 Airy disk diameters away (with some meaningful significance of energy, I take it,or maybe it's the first lobe).
 
If my single stalk is 1/5th aperture thick, shouldn't it's significant influence spread to 5 Airy disk diameters, or some such number (and let it taper down to equal energy of a hairline spider, I don't mind)?
 
In another direction, my CO+stalk would cover 60% of aperture and effectively reduce it to 40%.
 
So, what happens in that case?


As you can see from Suiter's diagram, most of the diffracted light will come inside the first minimum which is, in arcsec, lambda*3600*(180/pi)/width of vane in mm - or 113/width arcsec (each side of the star, assuming lambda=550 nm)- close to the radius of the Airy disk of that aperture.
Doubling the width of the vanes crams twice the light into a peak half the width, meaning the peak close to the star (or e.g. Jupiter, 30-50 arcsec) is 4 times as high.
Thus thick vanes may affect contrast within Jupiter's disk but likely not much. An example: Assume a 150 mm mirror with a 37.5 mm secondary (25% dia), and a 5 mm wide single stalk support (rather wide, I know):
The secondary will obstruct 6.25% of the light and diffract another 6.25% in the vicinity of the Airy disk (most of it inside the Jupter image). The stalk, about 1.7% of the remaining unobstructed area, will diffract 1.7% of light, mainly within one Jupiter diameter of the field (a curved stalk of the same area would do much the same). So is planetary contrast an issue, vs the ugliness of spikes?

Nils Olof

#32 Oberon

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Posted 30 March 2015 - 06:57 AM

The issue for me is that the human brain isn't wired to permit spikes at different angles as when viewed through a binoscope. I'm happy with thin vanes for an ordinary scope, and really like the trivial impact of my thin wire vanes on Merope. But I'm assured by several users that I'll want to use curved vanes for my planned binoscope AND I'll want less movement than ever, so I'm doing all this to get my head around the options and their impacts.

 

Basically I'm trying to get the smoothest result within the limits of a practical and rigid design.


Edited by Oberon, 30 March 2015 - 07:03 AM.


#33 Oberon

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Posted 30 March 2015 - 07:11 AM

And I'm playing.

 

Here is another series of rather weird shapes we wouldn't normally associate with spiders, as they are not parallel vanes. Some work really well and some are hopeless. What I found interesting and consistent between these designs is:-

 

1. curves that overlap create ugly artifacts

 

2. the less "edges" the better

Yes, yes, I hear you that I'm still doing that at low resolution so that Maskulator introduces ts own effects. Which is why I always include two images without vanes, and one without an obstruction for reference. But this is just a quick and dirty first pass to see what ideas may be worth developing further and what is not.

 

gallery_217007_4746_20590.jpg



#34 dag55

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Posted 30 March 2015 - 10:18 AM

Nils Olof, thank you for pointing out the Suiter's article.
 
To me, it means that far away, thick spider is no more destructive than a hairline spider. My question relates to how destructive it would be closer to Airy disk.
 
Speaking of that model, at which spider thickness would his model of sqaring the energy would completely suck everything out of Airy disk?
 
I've read at Vladimir Sacek's site that it's not the same modeling that can be used for spiders and larger obstructions and that is logical to me...
Also, Sacek mentioned that a spider vane that is D/100 will shoot 100 Airy disk diameters away (with some meaningful significance of energy, I take it,or maybe it's the first lobe).
 
If my single stalk is 1/5th aperture thick, shouldn't it's significant influence spread to 5 Airy disk diameters, or some such number (and let it taper down to equal energy of a hairline spider, I don't mind)?
 
In another direction, my CO+stalk would cover 60% of aperture and effectively reduce it to 40%.
 
So, what happens in that case?


As you can see from Suiter's diagram, most of the diffracted light will come inside the first minimum which is, in arcsec, lambda*3600*(180/pi)/width of vane in mm - or 113/width arcsec (each side of the star, assuming lambda=550 nm)- close to the radius of the Airy disk of that aperture.
Doubling the width of the vanes crams twice the light into a peak half the width, meaning the peak close to the star (or e.g. Jupiter, 30-50 arcsec) is 4 times as high.
Thus thick vanes may affect contrast within Jupiter's disk but likely not much. An example: Assume a 150 mm mirror with a 37.5 mm secondary (25% dia), and a 5 mm wide single stalk support (rather wide, I know):
The secondary will obstruct 6.25% of the light and diffract another 6.25% in the vicinity of the Airy disk (most of it inside the Jupter image). The stalk, about 1.7% of the remaining unobstructed area, will diffract 1.7% of light, mainly within one Jupiter diameter of the field (a curved stalk of the same area would do much the same). So is planetary contrast an issue, vs the ugliness of spikes?

Nils Olof


OK, here is what I am considering and according to the info so far will this be a good design for a 8" f-7.5 planetary and globular scope? Your post on a single post stalk if I am understanding shows promise with a few different parameters involved, I am no engineer so please bear with me.
A 203mm primary area of 32349mm, a sec. of 38.61mm area of 1170.2 at 19% CO. Actual area of optics minus sec.= 31178.8. Now the single stalk which is 3.97mm wide and 82.2mm from edge of primary to edge of sec. area of 301.67mm at .97% obstruction of remaining pri. mirror area.
If I am understanding the info so far, would his be a good to excellent design?
I am asking because I was going to incorporate a .007" wire spider design, but if this will perform as well it is much easier to install and maintain. I have the components to do either method, but after some testing on my Dob. it appears that the wires induce more diffraction that I expected and the single post seemed to do well at least visually.
Thanks for your input, Dane

#35 Nils Olof Carlin

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Posted 30 March 2015 - 12:45 PM

Dane,

Compared to your single stalk, a 1 mm four vane spider will diffract as much total light. In the far field, due to 4 times as long total length, twice as much and in two directions. In the near field, near the star, the single stalk (sorry for being unclear before this edit!) gives 4 times the intensity (16x from each vane, but 1/4 as many vanes). As my calculated example suggests, the effect on planet contrast can be expected to be small (I've never heard complaints about that), and it is really a matter of aesthetics. And seeing of course, but I should believe (on theoretic grounds) the single stalk will be a quite good choice for you. Maybe it ought to be more popular - it does not need tension.

Nils Olof

Edited by Nils Olof Carlin, 30 March 2015 - 02:12 PM.


#36 mark cowan

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Posted 30 March 2015 - 12:55 PM

Doing it at low resolution the results are pretty much misleading as there's so much artifact that the actual patterns are being lost.  This is always the problem modeling without reference to sampling theory, and a serious problem trying to model ultra thin vanes to see a result that comes even close to reality.  Obstruction is uniformly damaging. :shrug:

 

 

Yes, yes, I hear you that I'm still doing that at low resolution so that Maskulator introduces ts own effects. Which is why I always include two images without vanes, and one without an obstruction for reference. But this is just a quick and dirty first pass to see what ideas may be worth developing further and what is not.



#37 raal

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Posted 30 March 2015 - 12:59 PM

Nils Olof, Glenn, Christian,

 

Thank you for explanations and examples. I understand thing a lot better now.

 

In Maskulator, I have reduced the aperture pupil to about just 10% of the mask size and reduced brightness to 0.5 to prevent overexposure. Also, I reduced the contrast of resultant bitmap so it would be easier to see what I got.

In general, my aperture pupil is of interest is 317mm, CO is 66mm (~21%).

 

 

For starters, here's the result I got for unobstructed aperture:

(I will downsize the JPG for in the following posts, this is just to see what comes out)

Attached Thumbnails

  • unobst 3000 iv.jpg

Edited by raal, 30 March 2015 - 01:02 PM.


#38 raal

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Posted 30 March 2015 - 01:07 PM

This is only 21% CO, no spider.

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#39 raal

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Posted 30 March 2015 - 01:11 PM

4 straight vanes, 0.7mm thick at 317mm aperture with 21% CO.

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  • 4 x 07 exposure 05 iv.jpg


#40 raal

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Posted 30 March 2015 - 01:15 PM

Single stalk 5mm

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  • ss 5mm iv.jpg


#41 raal

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Posted 30 March 2015 - 01:18 PM

Single stalk in full width of CO.

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  • ss iv.jpg


#42 raal

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Posted 30 March 2015 - 01:20 PM

Rounding the corners...

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#43 raal

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Posted 30 March 2015 - 01:24 PM

Just for fun...

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#44 raal

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Posted 30 March 2015 - 01:31 PM

So much about fat stalk idea.

No some serious stuff.

 

3 curved vanes, 63 degrees each, "normal" to both CO and aperture stop.

 

First 0.7mm thick:

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  • 3x07mm 63deg  21percent iv.jpg


#45 dag55

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Posted 30 March 2015 - 01:49 PM

Thanks Nils



#46 raal

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Posted 30 March 2015 - 01:53 PM

same, but vanes 1.4 mm thick.

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Edited by raal, 30 March 2015 - 01:56 PM.


#47 raal

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Posted 30 March 2015 - 02:13 PM

In comparison, 21% CO without spider is barely distinguishable to 21% CO with 3 curved vanes of 0.7mm. I wouldn't even call that a "halo".

CO of 21% is a far greater disturbance than curved 3-vane spider with vanes <0.7mm at 317mm aperture (D/450 or 0.36% of obstruction)

 

This is to be expected from Suiter's and other models. A very small amount of energy is spread 360 deg, far enough to be so "thin" that it just can't be visible.

 

According to all this, it can be said that curved vane spider does not disturb the PSF distribution in any meaningful way, and that the benefits of it are not just esthetical.



#48 mark cowan

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Posted 30 March 2015 - 02:25 PM

Done properly, impressive indeed.  Will 0.7mm work for a 317mm aperture?



#49 jim kuhns

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Posted 30 March 2015 - 03:00 PM

Personally I will stick to 4 thin vanes and 20% or less obstruction, row 1 #2.   Been well proven over the many years of the past. 

I Know 20% is not practical when you start getting below F4.5 or even F5 unless your are willing to start losing  light at the eyepiece due to a undersized secondary, though I suspect many of you know this already.

Many of the images starting with row 2  #2 through  all on row 3,  the vanes appear to spread the light much more than the traditional 4 thin vanes type. In the past theses type of vanes were called light spreaders.



#50 dag55

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Posted 30 March 2015 - 03:06 PM

same, but vanes 1.4 mm thick.

raal, if you don't mind do one with .007" wire four vane with 20% CO

thanks Dane




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