Re: [ATM] Interpreting DK secondary interference test

[Aplanatic@aol.com]

Greetings,

I used FringeXP to analyze the first interferogram.  Relative to the test plate, and across the full diameter, the secondary has a Strehl of ~0.75 if astigmatism is included, and a Strehl of ~0.8 if astigmatism is removed from the analysis.  Not surprisingly, the major error is spherical aberration, which has a Zernike coefficient of 0.15 waves on the wavefront.  I can't tell the sign of the error without further information.

Most of the error is from a turned edge.  If the edge is not used except for baffling then the mirror may be quite good as it is.  If the full diameter is used (generally a bad idea) then I'd work on it further.

Nice interferograms.

Dave Rowe


In a message dated 12/14/2004 5:02:28 AM Eastern Standard Time, "Rod Brackenridge" <rjbr@lycos.com> writes:

>Hi Mike and Bob,
>
>Thanks for the suggestions.  I have posted some pictures at:
>
>    http://www.geocities.com/penbra/techie.html
>
>They were taken in neon light with a digital camera.
>
>Let me know what you think.
>
>Rod.
>
>
>> Rod,
>>
>> Rod Brackenridge wrote:
>> > There is one other error.  I worked for some time to reduce a depressed
>>  > central zone.  It is virtually gone.  With 7 fringes it is not visible.
>>  >  With 3 fringes I can see it as almost like a 'fuzziness' in the centre
>>  > when using neon light as the light source.  It is almost invisible when
>>  > I use a laser pointer and lens shining through a tissue.
>>
>> Great job continuing on the work and getting the fringes straightened out.
>>
>> I'm assuming that the mirror and test plate are completely polished
>> out.  How large is the area of 'fuzziness'?
>>
>> Any chance you could post a picture of the fringes somewhere?
>>
>> > My own thinking is that the central area is not really used as it will
>>  > just reflect the central obstructed area of the primary so there is not
>> >  much point trying to remove this residual error.  Does that sound
>>  > right?
>>
>> It may be right, but I don't like the sound of it.  I'd keep on
>> going and try to remove the thing completely.
>>
>> > One other question.  Is there any way to check for surface smoothness
>>  > when testing with interference fringes against a testplate?
>>
>> As Bob May already said, if the fringes are indistinct when you are
>> set up to only view a few fringes, then you probably have some
>> roughness.
>--
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