Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Why are ronchis sometimes blurry slitless?

  • Please log in to reply
30 replies to this topic

#1 Lognic04

Lognic04

    Viking 1

  • -----
  • topic starter
  • Posts: 849
  • Joined: 21 May 2017
  • Loc: Melbourne, Aus

Posted 21 January 2019 - 05:02 AM

Hi all,

i've noticed that all my slitless ronchis are blurry towards the sides outside ROC and completely blurry inside. Anyone know why this is?

Thanks.



#2 TOMDEY

TOMDEY

    Mercury-Atlas

  • -----
  • Posts: 2799
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 21 January 2019 - 08:23 AM

Well, without looking at the actual image (camera should be focused on the mirror without the ronchi in there first)...

 

The Ronchi acts as a Lateral Shearing Interferometer, with the shear in the direction perpendicular to the ruling. The finer the ruling, the more the shear. So that will make the edge of the mirror (and everything else) appear smeared/convolved in that direction.

 

PS: This also explains why e.g. "correction" (which is spherical aberration as seen from the Ronchi's COC location) looks like traditional TWG coma fringes ... because sheared spherical indeed IS coma.

 

BTW, in that sense, Ronchi is desensitized relative to traditional interferometry by the shear fraction (shear/diam). This is why most labs have at least one Zygo or other such in their test room.

 

The beauty of Ronchi is that the geometrical optics analysis and the full wave-optics predictions give nearly the same answers, with the wave optics (of course) explaining the subtle shading and ringing effects, as well.

 

Anyway, I think what you may be describing is that coherent shear manifestation.

 

CONFIRMATION: Just hold the Ronchi right up to your eye, and look around. You will see that shear in everything. It's acting like a coarse grating.    Tom


  • MKV and Augustus like this

#3 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 21823
  • Joined: 22 Feb 2010
  • Loc: Montclair, NJ

Posted 21 January 2019 - 08:51 AM

Well, without looking at the actual image (camera should be focused on the mirror without the ronchi in there first)...

 

The Ronchi acts as a Lateral Shearing Interferometer, with the shear in the direction perpendicular to the ruling. The finer the ruling, the more the shear. So that will make the edge of the mirror (and everything else) appear smeared/convolved in that direction.

 

PS: This also explains why e.g. "correction" (which is spherical aberration as seen from the Ronchi's COC location) looks like traditional TWG coma fringes ... because sheared spherical indeed IS coma.

 

BTW, in that sense, Ronchi is desensitized relative to traditional interferometry by the shear fraction (shear/diam). This is why most labs have at least one Zygo or other such in their test room.

 

The beauty of Ronchi is that the geometrical optics analysis and the full wave-optics predictions give nearly the same answers, with the wave optics (of course) explaining the subtle shading and ringing effects, as well.

 

Anyway, I think what you may be describing is that coherent shear manifestation.

 

CONFIRMATION: Just hold the Ronchi right up to your eye, and look around. You will see that shear in everything. It's acting like a coarse grating.    Tom

 

I think he wants to know if its an indication of correction error...if its blurry on inside RoC does that mean the focus is too long, making the inside bands more blurry.

 

I would not try to read into it like that.


Edited by Pinbout, 21 January 2019 - 08:52 AM.


#4 MKV

MKV

    Fly Me to the Moon

  • *****
  • Posts: 7440
  • Joined: 20 Jan 2011
  • Loc: Florida, USA

Posted 21 January 2019 - 09:58 AM

Hi all,

i've noticed that all my slitless ronchis are blurry towards the sides outside ROC and completely blurry inside. Anyone know why this is?

What's the frequency of your screen, i.e. lines/inch or lines/mm? And what did you test with it?


  • Augustus likes this

#5 MKV

MKV

    Fly Me to the Moon

  • *****
  • Posts: 7440
  • Joined: 20 Jan 2011
  • Loc: Florida, USA

Posted 21 January 2019 - 10:59 AM

The Ronchi acts as a Lateral Shearing Interferometer, with the shear in the direction perpendicular to the ruling. The finer the ruling, the more the shear. So that will make the edge of the mirror (and everything else) appear smeared/convolved in that direction.

The Ronchi test is both geometric and interferometric. At lower frquencies (coarser screen), it's almost purely geometric (i.e. the bands are shadows). At very high frequencies, it's a shearing interferometer, and the bands are interference fringes). The former is qualitative; the latter quantitative.

 

If the rulings are sufficiently coarse there will no no significant interference between the rulings. A rule of thumb that gives very clean ronchigrams is 6 times the focal ratio.  That's where edge diffraction just becomes noticeable.

 

Thus for an f/8, a Ronchi screen with 48 lpi should provide perfectly clean bands all the way to the edge. Higher frequencies begin to give misleading edge appearance. Notice how the edge gets worse with higher frequencies, and mimics a turned edge error. 

 

roanchi frequency.jpg

 

If one is to use higher frequencies, then it's best to use a very high frequency Ronchi screen as a shearing interferometer. The interference fringes are tack sharp to the edge, and the test is quantitative.

 

ronchi shearing if.jpg


  • Jon Isaacs, PrestonE, Dale Eason and 4 others like this

#6 Steve Dodds

Steve Dodds

    Owner - Nova Optical

  • *****
  • Vendors
  • Posts: 170
  • Joined: 02 Apr 2007
  • Loc: Utah

Posted 21 January 2019 - 02:13 PM

I get the  same result using glass Ronchi's.  Inside of focus is fuzzy and indistinct, but outside is crisp and clear, with film Ronchi's it's a little better inside.


  • Pinbout likes this

#7 mark cowan

mark cowan

    Vendor (Veritas Optics)

  • *****
  • Vendors
  • Posts: 9009
  • Joined: 03 Jun 2005
  • Loc: salem, OR

Posted 21 January 2019 - 03:23 PM

With glass be sure you have the grating surface towards the mirror.



#8 Steve Dodds

Steve Dodds

    Owner - Nova Optical

  • *****
  • Vendors
  • Posts: 170
  • Joined: 02 Apr 2007
  • Loc: Utah

Posted 21 January 2019 - 04:15 PM

It always is, I think that's why inside images are so fuzzy, maybe I will try it with the Ronchi inverted.



#9 Mark Harry

Mark Harry

    Cosmos

  • *****
  • Posts: 7819
  • Joined: 05 Sep 2005
  • Loc: Northeast USA

Posted 21 January 2019 - 04:39 PM

"and the bands are interference fringes..."

Sorry Mladen, I wish this was true, since we have been seeing "eye to eye" the last few years.

I have been somewhat spoiled recently, having access to 2 IF's that can measure the radius to 1/10th micron, and wave ratings to whatever one can dream up, with the proper software.

This is all I'm gonna say, to avoid raising the ire of the majority of the ATM's out there. (also to avoid the moderators to clean up a crapload of posts.)

In no way, is "Raunchy" a truely acurate representation, or to be considered an equivalent, of a higher class IF.
Even to have a coached few jobs and training, it's stupendously amazing what you could do with metrology related to a true IF.  I can't stress this enough.

********
I think I have been blessed to see this difference.



#10 mark cowan

mark cowan

    Vendor (Veritas Optics)

  • *****
  • Vendors
  • Posts: 9009
  • Joined: 03 Jun 2005
  • Loc: salem, OR

Posted 21 January 2019 - 06:41 PM

Oh god, are we back to "Ronchi bands are interference fringes?"  Somebody please put that horse out of its misery.

 

Ronchi is good for quick and dirty evaluation, and it will show you overall curve.  Strange I don't see these "fuzzy lines" maybe it's due to using a 50 micron pinhole for the source so far. A slitless source makes no sense for Ronchi, but a large (3-5mm) source works fine.

 

Could be what looks blurry is the changing appearance of the diffraction fringes at the various distances from actual zonal ROC that is evident when looking at a strong asphere with Ronchi.  Here's what a fast mirror (f/3.57) looks like under 133 LPI  Ronchi, 50micron source.  Note the fine detail - all of which is predicted by the theoretical simulation as well.  Some of it is due to that source not matching the spacing of the grating, but it gets the job done nevertheless.

 

session 39 ronchi 100%.jpg


Edited by mark cowan, 21 January 2019 - 06:42 PM.

  • Mike I. Jones likes this

#11 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 21823
  • Joined: 22 Feb 2010
  • Loc: Montclair, NJ

Posted 21 January 2019 - 06:50 PM

But it’s his inside RoC bands that look more fuzzy


Edited by Pinbout, 21 January 2019 - 06:51 PM.


#12 MKV

MKV

    Fly Me to the Moon

  • *****
  • Posts: 7440
  • Joined: 20 Jan 2011
  • Loc: Florida, USA

Posted 21 January 2019 - 08:07 PM

"and the bands are interference fringes..."
Sorry Mladen, I wish this was true, since we have been seeing "eye to eye" the last few years.
I have been somewhat spoiled recently, having access to 2 IF's that can measure the radius to 1/10th micron, and wave ratings to whatever one can dream up, with the proper software.
This is all I'm gonna say, to avoid raising the ire of the majority of the ATM's out there. (also to avoid the moderators to clean up a crapload of posts.)
In no way, is "Raunchy" a truely acurate representation, or to be considered an equivalent, of a higher class IF.
Even to have a coached few jobs and training, it's stupendously amazing what you could do with metrology related to a true IF.  I can't stress this enough.

Mark, I am not disagreeing with you. If you read closely I never said a Ronchi test using high frequency grating is a higher class IF. 

 

Not all interferometers are the same, but all interferometers have one thing in common: the separation between fringes is a multiple of a wavlength and the results are quantifiable. This makes any interference pattern quantifiable.

 

The Ronchi test is both geometric and interferometric. The difference is in the ruling frequency and order overlaps. In lower frequencies it's geometric; in higher interferometric. Daniel Malacara's book Optical Shop Testing has a lengthy chapter on the theoretical aspects of both. And there's no doubt that the professional optical community recognizes that as true. Here's an illustration from Malacara's book which can be found in PDF format online.

 

ronchi if.jpg

 

All one has to do is search for Ronchi test interferometer online and you'll find numerous articles in American Optics and similar professional magazines and books on the this topic. A recently published book intended for ATM readership by William Zmek (Interferometry for Amateur Telescope Makers) also has a whole chapter dedicated to the Ronchi test used as an interferometer, and an image (page 123) similar to my example.

 

As to the accuracy of such an interfometer, you know very well, there are many ways to evaluate a ronchigram. Not so long ago, the only way was to manually measure distances between fringes and extrapolate the value. It's a far cry of what you have at work, but it's much better than a qualitative guess of ordinary Ronchi tests used by most ATMs with relatively low frequency screens.

 

Manual fringe analysis described in Ingalls's ATM books shows that it cna be done to 1/10 wave PV wavefront, if not better. It may not be close to modern Zygo's working at 1/50 wave PV  wavefront accuracy, but it's not bad either if we consider that very little improvement will be gained by making a mirror better than 1/10 wave PV wavefront.

 

All the best, Mark.

Mladen


Edited by MKV, 21 January 2019 - 08:09 PM.


#13 mark cowan

mark cowan

    Vendor (Veritas Optics)

  • *****
  • Vendors
  • Posts: 9009
  • Joined: 03 Jun 2005
  • Loc: salem, OR

Posted 22 January 2019 - 12:16 AM

But it’s his inside RoC bands that look more fuzzy

I never look at that. ;)  Besides he said "blurry towards the sides outside ROC"



#14 Mark Harry

Mark Harry

    Cosmos

  • *****
  • Posts: 7819
  • Joined: 05 Sep 2005
  • Loc: Northeast USA

Posted 22 January 2019 - 05:26 AM

"a qualitative guess of ordinary Ronchi tests used by most ATMs with relatively low frequency screens."

Well yes, this is the problem. Almost always, the most frequent grids used here are either 100, or 133. 
Totally inaccurate for interferometric analysis. (IME)



#15 MKV

MKV

    Fly Me to the Moon

  • *****
  • Posts: 7440
  • Joined: 20 Jan 2011
  • Loc: Florida, USA

Posted 22 January 2019 - 11:29 AM

"a qualitative guess of ordinary Ronchi tests used by most ATMs with relatively low frequency screens."

Well yes, this is the problem. Almost always, the most frequent grids used here are either 100, or 133. 
Totally inaccurate for interferometric analysis. (IME)

Exactly. I thought I made that clear in the first paragraph:

 

"At lower frequencies (coarser screen), it's almost purely geometric (i.e. the bands are shadows). At very high frequencies, it's a shearing interferometer, and the bands are interference fringes). The former is qualitative; the latter quantitative."

Maybe I left something out. shrug.gif


Edited by MKV, 22 January 2019 - 11:31 AM.


#16 Mark Harry

Mark Harry

    Cosmos

  • *****
  • Posts: 7819
  • Joined: 05 Sep 2005
  • Loc: Northeast USA

Posted 23 January 2019 - 04:56 AM

OK, we're cool.


  • MKV likes this

#17 a__l

a__l

    Viking 1

  • -----
  • Posts: 978
  • Joined: 24 Nov 2007

Posted 23 January 2019 - 06:09 AM

This is all I'm gonna say, to avoid raising the ire of the majority of the ATM's out there. (also to avoid the moderators to clean up a crapload of posts.)
In no way, is "Raunchy" a truely acurate representation, or to be considered an equivalent, of a higher class IF.
Even to have a coached few jobs and training, it's stupendously amazing what you could do with metrology related to a true IF.  I can't stress this enough.

Why only ATM's?

For example here:

https://www.cloudyni...to-build/page-3

 

This is a commercial production.
What for is it given to the client?



#18 Mark Harry

Mark Harry

    Cosmos

  • *****
  • Posts: 7819
  • Joined: 05 Sep 2005
  • Loc: Northeast USA

Posted 23 January 2019 - 06:05 PM

"Why only ATM's?"

Because they aren't aware of 2 things.

1. the proper approach/procedure.
2. Clueless to the relative value of numbers.

This applies 99.5% of the time, combined with lack of practical experience. (IME, and others)



#19 Mark Harry

Mark Harry

    Cosmos

  • *****
  • Posts: 7819
  • Joined: 05 Sep 2005
  • Loc: Northeast USA

Posted 23 January 2019 - 06:11 PM

I have tested one of Carl's mirrors; one of his initial conics past a paraboloid. - Did very well, after a consulting session.. It was for a 17" Chief. (one of the few examples)

 

"What for is it given to the client?"


I need you to clarify this inquiry in English, please. (?)

*******
The link shows the supporting structure; has nothing to do with the actual glass fabrication.

Please make your point.

M.



#20 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 21823
  • Joined: 22 Feb 2010
  • Loc: Montclair, NJ

Posted 23 January 2019 - 06:25 PM

Ronchi’s are qualitative- showing nice polish and no zones or tde. That’s why a maker would give the image.


  • sopticals likes this

#21 MKV

MKV

    Fly Me to the Moon

  • *****
  • Posts: 7440
  • Joined: 20 Jan 2011
  • Loc: Florida, USA

Posted 24 January 2019 - 05:17 AM

Ronchi’s are qualitative- showing nice polish and no zones or tde. That’s why a maker would give the image.

The no-mask Foucault test does the same and is more sensitive. 


  • Mark Harry likes this

#22 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 21823
  • Joined: 22 Feb 2010
  • Loc: Montclair, NJ

Posted 24 January 2019 - 09:19 AM

The no-mask Foucault test does the same and is more sensitive. 

it only nulls the zone its focused on

 

but adding that with the ronchigram would be nice

 

I prefer outside RoC ronchi's better that inside...



#23 scopemankit

scopemankit

    Messenger

  • -----
  • Posts: 438
  • Joined: 27 Jan 2009
  • Loc: south africa

Posted 24 January 2019 - 10:29 AM

Use a slit. 12" Cass mirror

Attached Thumbnails

  • 20180104.jpg

Edited by scopemankit, 24 January 2019 - 10:34 AM.


#24 MKV

MKV

    Fly Me to the Moon

  • *****
  • Posts: 7440
  • Joined: 20 Jan 2011
  • Loc: Florida, USA

Posted 24 January 2019 - 12:07 PM

it only nulls the zone its focused on

The Foucault does more than that, Danny. It also shows if the edge is intact or turned, as well as surface finish, and then some. 



#25 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 21823
  • Joined: 22 Feb 2010
  • Loc: Montclair, NJ

Posted 24 January 2019 - 12:10 PM

but it doesn't elude to the quality of correction, where a ronchi can.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics