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10 Waves Defocus

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#1 Asbytec

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Posted 08 November 2018 - 08:39 PM

Star testing my 8" f/6 Dob gave me pause and confusion as to what Suiter (or anyone) means by 10 waves defocus. As I understand it, there is a difference between 10 waves defocus distance and 10 waves of defocus error. 

 

In focus, my focuser graduated scale reading is right at 30mm (I believe the measure is in millimeters) as a starting point using a 6mm TMB II. When I scroll inside to what I believe is 10 waves defocus (distance?), I get an image I am familiar with that looks similar to the +/- 10 wave simulations we see in Suiter's book and other references. That is, a bright marginal diffraction ring, a bright paraxial diffraction ring bordering the shadow, and a single ring between them. This is the pattern I recognize as 10 waves defocus, same as the simulations show. The test results look pretty good on both sides of focus. So, I am happy. 

 

But, when I read the focuser travel on the scale, it shows I am inside focus by about 2mm with a reading of right at 28mm. Same with outside focus, the reading is 2mm beyond best focus right at 32mm. Two millimeters either side gives me the pattern I am looking for. However, according to Suiter, that amount of defocus distance should be 12 waves distance and 10 waves distance is just over 1mm. So, I scroll back toward focus so the focuser scale reads 1mm from best focus at 29mm inside and 31mm outside. I do that visually, anyway, just to see when the paraxial ring disappears on both sides. Then I take a reading of focuser travel, it turns out to be close to 1mm travel and 1mm from best focus on both sides. Again, that corresponds to 10 waves focuser travel.  

 

At this shorter defocus distance the paraxial diffraction ring is barely discernible inside focus, but it can be seen. A touch more toward focus and the paraxial ring gives way to a bright marginal ring with rings decreasing in brightness toward the center. This happens right near where Suiter says is 10 waves defocus distance or about 1.2mm. Outside is quite different at near 1mm focuser travel. At 1mm outside best focus, the bright paraxial ring is clearly seen and brighter than the now reduced marginal diffraction ring. There are only these two rings visible outside the shadow and it does not resemble the +/- 10 wave simulations, anymore.

 

At 1mm defocus both sides, the test looks like a classic 1/4th PVW under correction as the paraxial ring is gone inside focus and brighter outside focus. I am not as happy, the under correction looks more dramatic (more sensitive, I guess). However, when I scroll back out to 2mm from best focus, the defocused patterns look much better corrected. They still show some indications of minor under correction. So, when evaluating the optic, which setting do I trust? The one with a measured 1mm focuser travel consistent with 10 waves defocus distance as show in Suiter's book, or the better looking ones at 2mm defocus that /may be/ 10 waves error and resemble the +/- 10 wave simulations?

 

I have always trusted the latter. And I still do because the better corrected patterns are consistent with the favorable shadow break out and sizes. The shadows are well broken out by 2mm and even at 1mm, and they are not grossly different with the outside being marginally smaller. I scrolled through focus several times trying to see the difference, it was not that easy to determine. But, what is going on here? Am I really at 12 waves defocus travel per Suiter and 10 waves defocus error per simulations? If so, which defocus do we test for correction error?

 

Oh, my obstruction happens to be right at 65mm or 33% for some reason. Very tall focuser.

 

Thanks in advance. 


Edited by Asbytec, 08 November 2018 - 09:15 PM.


#2 MKV

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Posted 09 November 2018 - 03:47 AM

It's a linear distance that produces a 10-wave error.

 

1 λ  (i.e. 1 wave) defocus distance (in mm) = (8*λ*F2)/n, where λ  is the wavelength of light in mm , F is the focal ratio of your telescope, and n the index of refraction of the ambient medium (in this case air ≈ 1 for all practical purposes).

 

Thus, for a number of waves (N) the defocus distance d = (N)*(8*λ*F2)

 

For λ = 0.00055 mm (green light), and focal ratio = f/5, and N = 10 waves, d = (10*8*0.0005*25) =1.1 mm

 

For a 1/4 wave error d = 0.0275 mm

 

Mladen


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#3 Asbytec

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Posted 09 November 2018 - 04:54 AM

Mladen, okay, thank you. Crunching your equation with my numbers and referencing Suiter, I see one glaring mistake I made. I misread Suiter's chart on defocus distances. At f/6, 1.2mm is only 8 waves not 10 waves as I misread it. His figures are consistent with the equation you provided above. At 10 waves, I get 1.584mm much closer to my actual defocus and maybe within my ability to read the scale accurately enough to within a few tenths of a millimeter. The latter is probably my second source of error. So, I'm sure I really was at 10 waves defocus and not 12 waves (~2mm). So, the smaller more sensitive pattern I observed was closer to 8 waves, not 10 waves. I am confident my observations were at 8, 10, and 12 waves. I can better associate what I observed with the proper defocus distance and the simulations at that distance. Thank you. 



#4 MKV

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Posted 09 November 2018 - 05:36 AM

Hi Norme, I'm glad you got it all sorted out. Clear skies!


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#5 Pinbout

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Posted 09 November 2018 - 06:01 PM

 

As I understand it, there is a difference between 10 waves defocus distance and 10 waves of defocus error.

I use abberator.

 

 10waves of a 8in f6 as 1.58mm

 

but in a star test its not extremely critical, 1mm is too close, 2mm shims would do you better.


Edited by Pinbout, 09 November 2018 - 06:02 PM.

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#6 Asbytec

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Posted 09 November 2018 - 06:17 PM

I agree, Danny. Just got confused reading the graduated focuser scale. Shoulda shimmed it. Actually, I just scroll out about the right distance to get close to 10 waves. That's what I did then read the focuser's scale. There's room for slop in the scale as a few 10ths of a millimeter are hard to read from it. That's probably what happened, then comparing to Suiter's (misread) chart. Crunching the numbers in the equation Mladen posted showed the error of my ways. Shoulda shimmed it. 


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#7 Mark Harry

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Posted 10 November 2018 - 12:26 PM

Ahem, not to throw a wet towel on anything, I prefer a sharp in-focus pattern, where the airy and first diffraction ring is plainly evident.
(for evaluation purposes)


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#8 totvos

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Posted 10 November 2018 - 01:29 PM

I use abberator.

 

 10waves of a 8in f6 as 1.58mm

 

but in a star test its not extremely critical, 1mm is too close, 2mm shims would do you better.

Just to toss in a random question...how do you shim, precisely?



#9 Mike Spooner

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Posted 10 November 2018 - 02:27 PM

Ahem, not to throw a wet towel on anything, I prefer a sharp in-focus pattern, where the airy and first diffraction ring is plainly evident.
(for evaluation purposes)

Renegade! lol.gif



#10 Pinbout

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Posted 10 November 2018 - 02:48 PM

Just to toss in a random question...how do you shim, precisely?

Feeler gauges 

 

I used some alum bar stock I had on hand... not exactly 10~ but it’s close enough for demo  lol.gif

 

https://youtu.be/QxUQJjjsdW4

 

for refractors

https://youtu.be/r3gbGxgz8cM


Edited by Pinbout, 10 November 2018 - 02:56 PM.

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#11 Pinbout

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Posted 10 November 2018 - 03:02 PM

You leave a lot of information on the table only evaluating in focused stars - eyepiece selection very critical.

 

https://youtu.be/DtdYBmSapio

 

 

wink.gif



#12 Asbytec

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Posted 10 November 2018 - 03:58 PM

Just to toss in a random question...how do you shim, precisely?

Danny recommends 2 quarters. I cut some spacers in the past. Come to focus, then shim the eyepiece so it rises out of the focuser by the given amount. You will be outside focus by the right amount. Return to focus and remove the shim, the eyepiece will drop inside focus by the same amount. This allows one to observe the defocused pattern the same known amount on both sides. 

 

I like a good looking in focus pattern too. Clean, high contrast, and uniform. And of course, circular and concentric. Some good amount of snap to focus. All that. If it's not, in focus performance should be part of the star test, too. 


Edited by Asbytec, 10 November 2018 - 04:03 PM.


#13 Pinbout

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Posted 10 November 2018 - 07:11 PM

 

Danny recommends 2 quarters.

that's for in a pinch...

 

if your better prepared, cut up a feeler gauge.


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#14 Mark Harry

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Posted 11 November 2018 - 07:07 AM

All I need is ~15x/inch.



#15 Asbytec

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Posted 11 November 2018 - 07:40 AM

All I need is ~15x/inch.

To star test at defocus or in focus. I suspect the latter. I need more than that. Gotta see it. I suppose a good observer could do it, but I need 25x per inch.  :) 



#16 Mark Harry

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Posted 11 November 2018 - 08:18 AM

I suspect the latter.

Yes.


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#17 Pinbout

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Posted 11 November 2018 - 10:29 AM

i don't conside mag,

 

I consider exit pupil.

 

1mm desired, 2mm doable but not as sensitive, but if its "badder" than 1/4~ you'll see it with 2mm exit pupil



#18 totvos

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Posted 11 November 2018 - 01:54 PM

Yikes, the defocus distance is dependent on focal ratio squared. That sure is harsh for fast scopes!


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#19 MKV

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Posted 11 November 2018 - 05:11 PM

Yikes, the defocus distance is dependent on focal ratio squared. That sure is harsh for fast scopes!

Yup, that's why I always chuckle when I read that so and so could tell 1/8 wave pv error on the wavefront for an f/4 using a star test. Even if the seeing were perfect and the optics perfectly thermally stable (which is never!), a 1/8 wave error for an f/4, the defocus distance is 0.009 mm (9 μm)! 



#20 Asbytec

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Posted 11 November 2018 - 05:57 PM

Mladen, that's interesting. But, even defocused that tiny amount in a fast aperture provides a defocused image large enough to see (at some magnification), yea? Personally, I like to use enough magnification to see the pattern comfortably, both in and out of focus. I think a 1mm exit pupil is minimal, generally, to see the in focus image and comfortably observe the defocused image. If I could go higher without using a Barlow, I would. 

 

Edit: You mean being able to get that amount of defocus accurately. Yea, I might agree. I don't have a feeler gauge that thin or a focuser that smooth. smile.gif

 

It's certainly best when seeing and thermals are not a problem, but (despite any distortion due to a cooling mirror) we can kind of see what is going on through a mild boundary layer and modest seeing. Of course, it's much harder when seeing is "average" or worse. There is a certain level of noise we can read through. Until we can't. 

 

I have had seeing so good in the tropics I could actually watch the Poisson spot morph from a tiny spot to a very small donut and back to a spot while scrolling slowly through focus. That was amazing to watch. One night I was stunned to see what we might take as the first normally bright ring (very close to focus, one bright ring and the Poisson spot) was actually comprised of several very thin ones. That shocked me. One night I had to sketch Arcturus (through my MCT) very close to focus, it as beautiful. I think I got the colors in the right place. 

 

color.jpg


Edited by Asbytec, 11 November 2018 - 06:10 PM.

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