testing parabola with center hole


Peter Chen
 

Hi folks. 
Does anyone have any suggestions on how to use a Bath to test a parabola with a center hole?  With a minimum of auxiliary optics.

Thanks,
P.C. Chen


George Roberts (Boston)
 

I'm not sure I understand the question.  With a bath, you test parabolic and spherical mirrors the same whether they have a hole (aka perforation) or not.

The DFTFringe lets you setup an ignore region where the hole is.  It's in the same page as when you setup the outline.  In the upper right there are 3 radio buttons where you can setup the "mirror outside" "central hole" and ignore "region"s.  The "central hole" option is just for this purpose you seem to be describing.


George Roberts (Boston)
 

Oh - maybe you are asking about the reference beam.  The reference beam doesn't have to bounce off the center of the mirror - you can aim it at any part of the mirror.


Dale Eason
 

The hole needs to be less than 30% the diameter in order for the Zernike values to be correct.  DFTFringe does not use the annular set of polynomials needed to test rings with large central holes.


Peter Chen
 

Hi George:
                  Thanks for the reply and suggestion.
                  Yes, I was concerned about the reference beam from the Bath.
                   My Bath is set up with a 6 mm separation between the reference and test beams. The center hole is much bigger, about 20 mm diameter. My concern is that
                    If the test beam is centered on the mirror while the reference beam is diverted to somewhere else, the reference beam would not be able to retrieve its path
                    If both beams are kept parallel and pointed to a reflective part of the mirror, then I am effectively testing an off axis parabola. That sounds hairy in that I now need to deal with several more variables including tip, tilt and offset, The mirror is not perfect, so some of the errors may not be easy to disentangle. 
                    Thank you and Dale for the learned discourse.

Regards,
P.C. Chen
         


On Wed, Jul 29, 2020 at 11:47 AM George Roberts (Boston) <bb@...> wrote:
Oh - maybe you are asking about the reference beam.  The reference beam doesn't have to bounce off the center of the mirror - you can aim it at any part of the mirror.


Dale Eason
 

Peter,

It's ok to not center the reference beam.  I do it all the time.  You will not cause errors with that kind of setup until the beams become very not parallel.  On my 16 F5 I have tried reference near the edge compared to the center and could not see differences related to that.

So give it a shot.  You can experiment with the placement to see how the changes the results just by trying it on one side then the other.

Dale


Bruce Griffiths
 

No, there is no change in the test conjugates in such a setup, you are merely using a different part of the spherical wave test beam to illuminate the test surface. A spherical wave has no axis merely a CoC.

Bruce 

On 30 July 2020 at 05:59 Peter Chen <peterchenmd@...> wrote:

Hi George:
                  Thanks for the reply and suggestion.
                  Yes, I was concerned about the reference beam from the Bath.
                   My Bath is set up with a 6 mm separation between the reference and test beams. The center hole is much bigger, about 20 mm diameter. My concern is that
                    If the test beam is centered on the mirror while the reference beam is diverted to somewhere else, the reference beam would not be able to retrieve its path
                    If both beams are kept parallel and pointed to a reflective part of the mirror, then I am effectively testing an off axis parabola. That sounds hairy in that I now need to deal with several more variables including tip, tilt and offset, The mirror is not perfect, so some of the errors may not be easy to disentangle. 
                    Thank you and Dale for the learned discourse.

Regards,
P.C. Chen
         

On Wed, Jul 29, 2020 at 11:47 AM George Roberts (Boston) < bb@...> wrote:
Oh - maybe you are asking about the reference beam.  The reference beam doesn't have to bounce off the center of the mirror - you can aim it at any part of the mirror.

 

 


Michael Peck
 

Peter:

If you get some decent quality interferograms would you mind sharing one or two? I've been working on automated edge finding routines and have almost no examples of interferometry of obstructed/annular apertures.

Mike P.


Peter Chen
 

Sure, Mike. I'll send some data as soon as I've figured out how to work the Bath on an off-axis paraboloid.
Assuming I live to tell the tale, that is.       -:) smile

The amount of knowledge and brain power in this forum is awesome.  Thanks everyone for helping.

Regards,
P.C. Chen


On Fri, Jul 31, 2020 at 9:58 AM Michael Peck <mpeck1@...> wrote:
Peter:

If you get some decent quality interferograms would you mind sharing one or two? I've been working on automated edge finding routines and have almost no examples of interferometry of obstructed/annular apertures.

Mike P.