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What can this ronchigram tell about this mirror?

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#1 Darren Drake

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Posted 20 May 2019 - 01:56 AM

This ronchigram pic is on file for a mirror that I'm interested in know more about.  It is for a 23 inch f/4.74.  I've never been very knowledgeable about how to interpret such images.  I much prefer a foucault image or certainly a null image.  There appears to maybe be some microripple but can anything be determined about the figure itself from this pic or is more info needed?  Thanks.

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#2 ngc7319_20

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Posted 20 May 2019 - 02:28 AM

Inside focus?  Outside?  Lines per mm?


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#3 Pinbout

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Posted 20 May 2019 - 06:15 AM

The very center has a hole 

 

outside of that some shallow zones

 

but ke is the only way to test the edge.


Edited by Pinbout, 20 May 2019 - 06:29 AM.

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#4 ccaissie

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Posted 20 May 2019 - 07:27 AM

The slight inward kink at the edge tells me it's turned.  An outside focus picture would probably more clearly show the reverse hooks typical of TDE.  The #2 band looks suspiciously uneven.... too sharp a curve near center, too straight as it passes through 70%.

Yeah, microripple.

 

So why are you testing this mirror?  bad images?

 

Someone with the Ronchi skills could do better analysis if you provided the info re: lpm of the grating.  They could do the Bartels technique of generating the ideal Ronchi and overlaying it.



#5 Arjan

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Posted 20 May 2019 - 08:05 AM

The very center has a hole 

 

outside of that some shallow zones

 

but ke is the only way to test the edge.

Agree, but not with your last statement grin.gif



#6 Pinbout

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Posted 20 May 2019 - 08:46 AM

this is what I get in terms of an undercorrection… if and only if....4lines/mm [100lpi] grating was used.

 

I believe the outer 30% is more undercorrect than inside the 70%

 

but still I'm at 2~'s under with this rendering.

 

darrens 23f4_74.jpg



#7 Darren Drake

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Posted 20 May 2019 - 10:38 AM

It is a 50 line ronchigram .  



#8 dogbiscuit

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Posted 20 May 2019 - 10:38 AM

Some astigmatism.

It looks like the mirror might be sitting on a single point at center line bottom edge rather than support points each side of center.



#9 Pinbout

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Posted 20 May 2019 - 01:58 PM

It is a 50 line ronchigram .  

at 50lpi its much much better

 

but the difference between right on and 1/4~ is so small its indistinguishable with a coarse grating.

 

but a 133lpi the difference of a qtr~ is much more obvious.

 

 

 

darrens 23f4_74-qtr.JPG


Edited by Pinbout, 20 May 2019 - 01:59 PM.


#10 scopemankit

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Posted 29 May 2019 - 05:48 AM

I don't think the centre hole is a train smash as it would be shaded by the diagonal ot secondary (if Cass).




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