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Has anyone set up a Dall Null test in OSLO?

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11 replies to this topic

#1 ccaissie

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Posted 28 June 2022 - 07:32 PM

Looking to see if a Dall Null will be accurate enough...or rather, what the aberrations will be for a 10" f/10 mirror and a 16" f/4.5.  Interested in seeing what the Astigmatism will be and if there is any way to mitigate that.

 

I'm using a Newport PCX193 lens as the Dall Lens.

 

Any Oslo setup or drections for setting this up?

 

 



#2 Steve Dodds

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Posted 28 June 2022 - 07:35 PM

Why are you using the Dall instead of the Ross?


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#3 Mike I. Jones

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Posted 28 June 2022 - 11:26 PM

Looking to see if a Dall Null will be accurate enough...or rather, what the aberrations will be for a 10" f/10 mirror and a 16" f/4.5.  Interested in seeing what the Astigmatism will be and if there is any way to mitigate that.

 

I'm using a Newport PCX193 lens as the Dall Lens.

 

Any Oslo setup or drections for setting this up?

I can't find PCX193 in the Newport Corp. catalog.  They have a KPX-193, 50mm diameter, 149mm FL.  Ross has a L-PCX193 lens, 1" diameter, 300mm FL.  Setting either up in OSLO is easy, but what's the lens you have?

 

Here's an OSLO file using the Ross L-PCX193 lens with your 10" f/10 mirror.

 

Attached File  Dall Null with Ross L-PCX193.len   545bytes   14 downloads


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#4 ccaissie

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Posted 29 June 2022 - 04:22 AM

I can't find PCX193 in the Newport Corp. catalog.  They have a KPX-193, 50mm diameter, 149mm FL.  Ross has a L-PCX193 lens, 1" diameter, 300mm FL.  Setting either up in OSLO is easy, but what's the lens you have?

 

Here's an OSLO file using the Ross L-PCX193 lens with your 10" f/10 mirror.

 

attachicon.gifDall Null with Ross L-PCX193.len

Oops.  It's KPX-193

Thanks for the setup.


Edited by ccaissie, 29 June 2022 - 04:23 AM.


#5 ccaissie

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Posted 29 June 2022 - 05:22 AM

Why are you using the Dall instead of the Ross?

'Cuz I sold the Ross lens I had.   Was it to you?



#6 BGRE

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Posted 29 June 2022 - 05:53 AM

In principle clocking the test surface can be used to separate the field astigmatism from polished in astigmatism.
However, a quantitative method such as interferometry is required to allow this.

#7 MKV

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Posted 29 June 2022 - 09:56 AM

In principle clocking the test surface can be used to separate the field astigmatism from polished in astigmatism.
However, a quantitative method such as interferometry is required to allow this.

Isn't there also a beam separation factor contributing to astigmatism in the Dall method that needs to be for removed as well?



#8 BGRE

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Posted 29 June 2022 - 05:35 PM

That astigmatism component remains stationary as the test surface is clocked whilst polished in astigmatism moves with the test surface allowing any polished in astigmatism to be separated from such fixed orientation astigmatism contributions.
The easiest way to do this is with interferometry. In which case for the relatively small mirrors you are considering using a numeric null with a Bath interferometer is more effective than using a Dall null element with unknown residual errors due to fabrication tolerances etc..

#9 Mike I. Jones

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Posted 29 June 2022 - 06:52 PM

Looking to see if a Dall Null will be accurate enough...or rather, what the aberrations will be for a 10" f/10 mirror and a 16" f/4.5.  Interested in seeing what the Astigmatism will be and if there is any way to mitigate that.

 

I'm using a Newport PCX193 lens as the Dall Lens.

 

Any Oslo setup or drections for setting this up?

The Dall Null is a great test for high f/# mirrors like your 10" f/10.  It fails miserably for that 16" f/4.5.  You've got to be on-axis like a Ross or other null lens test for that one.  You've got the OSLO file now, try it for yourself!



#10 ccaissie

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Posted 30 June 2022 - 09:08 AM

The Dall Null is a great test for high f/# mirrors like your 10" f/10.  It fails miserably for that 16" f/4.5.  You've got to be on-axis like a Ross or other null lens test for that one.  You've got the OSLO file now, try it for yourself!

Yes, I was pretty sure from the literature that the larger short focus mirrors under test show astigmatism from the lateral separation in the standard Dall test.  

 

I was interested in looking at a beam splitter to avoid this, or otherwise look at solutions for the off axis issue.   I had considered making a Dall Null unit that could be dialed in...like the barrel on a camera lens focus...to set separation from the integral source for various optics/conics.  A laser diode for the source....it would be easy to set up and do Null figuring.

 

Question: in your *.len file, I notice you had worked "reverse" in setup....source, mirror, then lens, then KE?  It does give a 1/30 wave null, but how else could it be set up...i.e. the usual source, lens mirror then KE?

I'm working on it.


Edited by ccaissie, 30 June 2022 - 09:21 AM.


#11 Ed Jones

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Posted 01 July 2022 - 11:37 AM

On test where you could use your lens for the 16 inch.


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#12 ccaissie

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Posted 02 July 2022 - 10:01 AM

On test where you could use your lens for the 16 inch.

I had commented on that video...I had the same ideas then on Dall nulling.  The lens I have is 1/2 that f.l. but I will use your calcs in the video and go through it. 

 

In ATMT, there's discussion on this sort of nulling used by opticians trying different lenses from the "junk box"...should be able to find one that works to get a reasonable null...mostly to detect zones, roughness...not meant to be Quantitative.

 

I'll set this up in Oslo also.

 

Thanks for the reminder.




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