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Value of Ronchi Test

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#1 Steve Allison

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Posted 12 February 2020 - 02:16 AM

Hello all-

 

I have a DPAC tester I cobbled together that seems to work well with most of my telescopes. However, my Brandon f/15, three inch refractor is a little long for my current set-up, so I tested it with a Ronchi eyepiece instead.

 

Looking at a bright star, the lines were ruler straight. I adjusted the eyepiece until only one line was visible and slowly moved the line to, and past, the edge of the field of view. On both sides of focus, I could not see the slightest indication of bowing, or hooks that might indicate a turned edge. My Brandon has an internal diagonal mirror, so I was actually testing the optical quality of the objective and the mirror together.

 

I know that a DPAC test is twice as sensitive as using a Ronchi grating alone, but wouldn't at least some curvature be visible in the Ronchi test as performed above if any significant spherical aberration were present?

 

I notice in DPAC that as the number of visible lines are reduced and the spacing between the lines increases, any curvature present gets worse. That is why I reduced the number of lines visible in the Ronchi eyepiece to just one during my test.

 

I have a lens that shows significant bowing of the lines under DPAC testing and if I mentally reduce the bowing by half, it would still seem it would be noticable.

 

So is a Ronchi test of some value when assessing the quality of an optic? At least for gross defects?

 

Thank you.

 

Steve


Edited by Steve Allison, 12 February 2020 - 02:18 AM.

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#2 markb

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Posted 12 February 2020 - 06:53 AM

Why not use the Ronchi tester on the 'lens that shows significant bowing' and see for yourself, and share the results. Do the same for any other scopes as well.

 

I don't recall having read about such a direct comparison, other than the 'twice as sensitive' information, although I may have. I am sure the testing 'community' has done this many times. One DPAC attraction, besides sensitivity, is the simpler interpretation of different optical designs as I  understand it (but I only have refractors and SCTs).

 

I have my flats and all parts needed to set up for DPAC, but cannot do so until I relocate, but have used Ronchi testing to confirm visual impressions of scope quality and extrafocal comparisions, successfully. I look forward to using the DPAC setup and would like to see your impressions of the result differences.


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#3 G-Tower

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Posted 12 February 2020 - 08:33 AM

To equal the DPAC use a 266 lpi. Ronchi. 



#4 t.r.

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Posted 12 February 2020 - 08:48 AM

The paperwork that came with my 133 lpi Ronchi eyepiece states that the sensitivity of the test can be doubled by adding a 2x Barlow when testing. I’ve only ever used it native and is said to give diffraction limited (1/4 wave results).
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#5 TOMDEY

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Posted 12 February 2020 - 09:06 AM

Double Pass AutoCollimation is singly-sensitized to imperfections of the AC Flat Mirror and doubly-sensitized to imperfections of the System Under Test. So, just make sure you are using a good plano.

 

Ronchi may be interpreted in the traditional geometrical sense or as lateral-shearing interferometry. Both give the same interpretation result.

 

Ronchi is desensitized (relative to traditional interferometry) by the shear % (typically 5%-10%). So Ronchi "fringes" will look deceptively encouraging (not very unstraight) unless the aberrations are substantial.

 

When you go to focus and peek inbetween two Ronchi Bars... that is (almost) the same as performing a traditional Foucault Knife Edge test on the system, so can and should be interpreted as such.

 

I always use a coarse Ronchi for focusing and assessing the system and atmospherics. My superb Coulter 12.5-inch newt would always show spherical that would subside to perfection once the blowers had brought it to ambient.

 

That's really all there is to it.

 

Malacara's book covers this stuff thoroughly. As an alumnus, I audited his full-semester course "Astronomical Optics" way back when! We later conspired on a joint white-paper "Telescope Quality - How Good is Good Enough?" Sounds like your scope is wonderful!   Tom

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#6 Mike Spooner

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Posted 12 February 2020 - 09:08 AM

I posted some single pass vs double pass examples here. https://www.cloudyni...08mike spooner"

 

Also have some star test photos.

 

Mike Spooner


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#7 mikeDnight

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Posted 12 February 2020 - 09:10 AM

I've used a ronchi grating for many years and find its a very good indicator of a lens or mirror figure. It's effectively multiple knife edge tests across a surface, but is easier to interpret. 


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#8 Steve Allison

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Posted 12 February 2020 - 02:23 PM

Thanks, everyone!

 

Wow, I can't believe I didn't think of trying a Ronchi eyepiece with the telescope that shows spherical aberration under DPAC testing. It's been cloudy out so it may be a while until I can do the test, but I'll post the results here.



#9 Jeff B

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Posted 12 February 2020 - 03:38 PM

To equal the DPAC use a 266 lpi. Ronchi. 

Not really as the light does not pass through the system twice.   In DPAC, my experience is that doubling the grating increases the "noise" that I see but does not tell me that much more concerning the condition or performance of the optic.

 

Jeff


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#10 markb

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Posted 12 February 2020 - 07:33 PM

Mike, thanks for the link.

 

The defects were not as obvious on the 100 lpi Ronchi, but jumped out at 250lpi on the Ronchi test..

 

The DPAC as expected was way easier to 'read', but the simple Ronchi test really is a quick and valuable evaluation tool, as has been my limited experience so far. My local atmospheric conditions tend towards mediocre at best, making objective testing tools particularly helpful. I think I'm done buying scopes but you never know, they do pop up.

 

I might even give testing some recently acquired camera lenses (bought for both camera and AP tryout fun) just to see what pops up. Setting up for Ronchi will be tough but DPAC will be easier.

 

I can't thank you enough for a simple A:B:C comparision, and seeing the tracking of the refiguring was fascinating.



#11 peleuba

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Posted 13 February 2020 - 09:27 AM

To equal the DPAC use a 266 lpi. Ronchi. 

 

Not really as the light does not pass through the system twice. 

 

 

Amen Jeff.  bow.gif

 

The sensitivity is not doubled with more Ronchi lines per inch.  



#12 peleuba

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Posted 13 February 2020 - 11:58 AM

Double Pass AutoCollimation is singly-sensitized to imperfections of the AC Flat Mirror and doubly-sensitized to imperfections of the System Under Test. So, just make sure you are using a good plano.

 

 

I want to clarify something regarding the accuracy of the optical flats used in Double Pass Autocollimation.  Optical flats are specified one of two ways:

 

(1) surface finish - described as "waves" and speaks to the regularity (smoothness) of the optic.

 

(2) Optical power - "fringes" describes power.  A perfectly plano optic has no power.    

 

You can - with success - utilize an optical flat that is many waves from flat (but highly regular) in Double Pass Autocollimation.   Mladen (MKV) here on CN has highlighted the work done by C.F. Burch in the mid 1930's.  Burch came up with this equation to determine the accuracy of an autocollimation flat.  Its expressed as:  3.6(focal ratio)²  where F = focal ratio of the mirror/lens under test.

 

So for a typical F/8 ED APO we can use a flat that is, maximally, 115.2 waves from flat.

 

The calculation is as follows:  3.6(8)² = 230.4  to convert to waves in green light we divide by 2 which is 115.2.

 

The longer the focal ratio, the more the flat can deviate from perfect as the error on the flat only contributes fractionally to the overall result in DPAC.  HOWEVER, if using the flat in an interferometer, ALL errors are additive and the flat, really needs to be flat.

 

My advice it to purchase the absolute best flat you can afford, but the 1/20 wave flat you see for a few thousand dollars is not absolutely necessary - you can get by with less for DPAC.


Edited by peleuba, 13 February 2020 - 11:59 AM.

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#13 TOMDEY

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Posted 13 February 2020 - 02:35 PM

I want to clarify something regarding the accuracy of the optical flats used in Double Pass Autocollimation.  Optical flats are specified one of two ways:

 

(1) surface finish - described as "waves" and speaks to the regularity (smoothness) of the optic.

 

(2) Optical power - "fringes" describes power.  A perfectly plano optic has no power.    

 

You can - with success - utilize an optical flat that is many waves from flat (but highly regular) in Double Pass Autocollimation.   Mladen (MKV) here on CN has highlighted the work done by C.F. Burch in the mid 1930's.  Burch came up with this equation to determine the accuracy of an autocollimation flat.  Its expressed as:  3.6(focal ratio)²  where F = focal ratio of the mirror/lens under test.

 

So for a typical F/8 ED APO we can use a flat that is, maximally, 115.2 waves from flat.

 

The calculation is as follows:  3.6(8)² = 230.4  to convert to waves in green light we divide by 2 which is 115.2.

 

The longer the focal ratio, the more the flat can deviate from perfect as the error on the flat only contributes fractionally to the overall result in DPAC.  HOWEVER, if using the flat in an interferometer, ALL errors are additive and the flat, really needs to be flat.

 

My advice it to purchase the absolute best flat you can afford, but the 1/20 wave flat you see for a few thousand dollars is not absolutely necessary - you can get by with less for DPAC.

Good point and yes, of course... Pure Zernike Power in the AC "flat" just changes the far-field test conjugate a bit, from the nominal infinity to the radius of the AC mirror R = D2/8s, where D is the diam of the flat and s is it's sag. Convex is finite positive conjugate and concave is negative. We would actually build our aerospace AC flats ever so slightly convex, matching the nominal slant range of the test article. For something like Hubble it would be nominally flat, for an equivalent orbital earth-imager it would be the intended nominal slant-range. That would allow us to precisely characterize the focus drives as well as the usual wavefront etc., without having to "go outside".

 

Same subject: The Newtonian Folding Flat also needs not be flat, provided it has just the right blend of astigmatism and curvature to cancel at the nominal used 90o deviation angle. The one I am using on my 36-inch Dobsonian is like that. I tested it at the 90o used angle and then (legitimately) backed out the Zernike Power. Really, all folding flats should be tested that way. And, sure enough... first light on the 36 and... no astigmatism; Yippee!   Tom


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#14 MikiSJ

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Posted 13 February 2020 - 03:14 PM

Just joining the conversation, but - what is DPAC?

 

I did search the site for a definition but, alas did not find one.



#15 peleuba

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Posted 13 February 2020 - 03:36 PM

Just joining the conversation, but - what is DPAC?

 

 

 

Double Pass Auto Collimation

 

A qualitative method of testing a complete telescope using an LED, a Ronchi Screen and an optical flat.  

 

The light emitted by the LED shines through the Ronchi screen, through the telescope and is reflected off of the flat back through the telescope and the Ronchi screen to its point of origin.  The tester observes the return beam for aberrations which show up in the distortion (from perfect) of the Ronchi bands.  The test is sensitive as the errors it shows are twice what is actually in the lens/mirror.

 

 

***edited for clarity


Edited by peleuba, 13 February 2020 - 05:56 PM.

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#16 markb

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Posted 13 February 2020 - 05:58 PM

Wow, great brief description Paul

 

I would only add that a strength of DPAC testing is only having to look for straight, parallel-sided, even lines because of the nature of the test (the optic itself is the null?).

 

Total newbie, just getting the hang of what I've read, I'll build mine after I move.

 

For examples, this thread had examples, discussion of results and little of the arguing that can pop up in testing threads

 

https://www.cloudyni...llimation-test/



#17 peleuba

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Posted 14 February 2020 - 09:31 AM

I would only add that a strength of DPAC testing is only having to look for straight, parallel-sided, even lines because of the nature of the test (the optic itself is the null?).

 

 

Yes - a strength of testing in double pass with a Ronchi grating is that you can, quite quickly, determine if the telescope is Excellent, Good, Bad from a spherical correction and turned edge standpoint.  Astigmatism takes a little longer to assess in DPAC.    

 

However, to me, where DPAC is truly useful is looking at the surface of the lens under test.  You can - with practice - see surface features that are 1/20 of a wave high/low.

 

Combined with the star test, DPAC makes a very useful tool.  I know, for me, there is now never a doubt if one of my telescopes is a top performer or a dog.  What can take weeks of star testing outdoors to determine can be realized on the bench in a single night of testing.



#18 TOMDEY

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Posted 14 February 2020 - 03:17 PM

Just joining the conversation, but - what is DPAC?

 

I did search the site for a definition but, alas did not find one.

Yeah, see my comment #5 up there. Note also that the double pass autocollimation configuration is also fine (indeed preferred) using an interferometer as the feed. Like a Zygo (Fizeau) Interferometer with appropriate F# transmission sphere. Here is a snapshot of the fringes I got doing that on my 6.25-inch folding flat, tested at 90o fold off a superb AC flat. The Zernike Power gets appropriately backed out, and the wavefront divided by two, because AC is double-pass, whereas the scope is (of course) used single-pass. It's a superb fold and performs magnificently in the scope.   Tom

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