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10λ intra and extra defocus distance for star test

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#1 Joe1950

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Posted 12 May 2018 - 04:51 PM

I recall the star test is recommended to be done at a defocused distance of 10λ inside and outside focus. Is there a way (a formula perhaps) to determine this distance for an f-6 scope (I also recall the distance was focal ratio dependent)? Suiter had a table in his book, but I no longer have it.

 

Also is there any latitude in this distance as the shorter focal ratios call for shorter distances and thus smaller defocused images? I know the rule of thumb of using, for example, a 6mm eyepiece or shorter for an f-6 focal ratio scope.

 

I'm not a star test fanatic since interpreting it is, IMO, difficult to do unless you are very familiar and experienced with it. But, my son has our 6", f-6 DOB which I recently replaced the secondary mirror with a better one from Antares, and I want to get an idea of its relative quality.

 

So any info is appreciated. More of a curiosity thing to determine if the better diagonal helps. 

 

 


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#2 Pinbout

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Posted 12 May 2018 - 04:57 PM

my fourmula

 

open abberator and let it tell me 10~ is 1.58mm from best focus

 

not critical, more critical to get same - same.

 

1.58 around a 1/16"


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#3 Joe1950

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Posted 12 May 2018 - 05:03 PM

Much better than a formula, Danny.

 

Some of those short distances hardly give you an image with any information between the outer ring and the secondary obstruction  - the Fraunhofer rings? Doesn't he sell bread also? 

 

Thanks Danny!



#4 TOMDEY

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Posted 12 May 2018 - 05:24 PM

That's a reasonable rule of thumb. A series through focus is even more telling. For a badly-aberrated mirror, the wavefront (surfaces of equal phase) actually ~volutes~ as one passes thru focus, but is NOT voluted, sufficiently far from focus (on either side). The 10-wave assumes that is outside of the voluted regions, where interpretation has more than one possible wavefront solution, and poles in the unraveling analysis. With bad mirrors (e.g. severe zones) more than 10 waves makes sense, for superb mirrors, less than 10. The unraveling of such is called "phase diversity analysis", like they would have done on Hubble and other remote imagers.  Tom


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#5 Joe1950

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Posted 12 May 2018 - 06:03 PM

Thanks Tom!



#6 sec4aa

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Posted 12 May 2018 - 06:40 PM

On my f6 I remember using a star with a magnitude around 2 and around 1.6mm in and out...


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#7 Joe1950

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Posted 12 May 2018 - 07:47 PM

Very good Vlad! That matches up exactly with Danny, post 2.

 

With the DOB unable to track, I use Polaris, right at magnitude 2, and it moves very little.

 

Thanks Vlad!



#8 MKV

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Posted 13 May 2018 - 12:34 AM

So any info is appreciated. More of a curiosity thing to determine if the better diagonal helps. 

Joe,

 

A 1-wave defocus d = ±8·λ·(f#)²/n

 

where d is the defocus distance, λ is the wavelenght of light used, f# is the focal ratio of the optic tested, and n is the refractive index of the medium (substrate).

Based on this equaiton any deofcus wave value can be calculated either as a fraction or a multiple thereof.

 

For your f/6 mirror f#² = 36,  n ≈1.00 (air, in this case), and let the wavelenght be green light of 0.00055 mm (the human vision is most sensitive around that region).

 

Thus we have 8·0.00055·36/1.00 = 0.1584 mm = 1-wave defocus.

 

And a 1/4 wave = 0.1584/4 = 0.0396 mm, and 10-wave defocus = 10·0.1584 = 1.584 mm (as already mentioned above)

 

Cheers!

Mladen


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#9 Joe1950

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Posted 13 May 2018 - 06:21 AM

Mladen, thank you! It’s always good to have the formula, which I will save in my optics formula document.

 

Hope you are doing well. Been a while, but always good to hear from you.

joe



#10 MKV

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Posted 13 May 2018 - 09:48 AM

Hi Joe, glad to see you're having too much fun! :o)


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