# Presentation: 537894

## Interferometer Characterization 

**Ron Eng**
**NASA / MSFC**
**Mail Stop SD73**
**Huntsville, AL 35812**
**(256)544-3603**
_**[ron.eng@msfc.nasa.gov](mailto:ron.eng@msfc.nasa.gov)**_
**May 10, 2001****	**

***MSFC Science Directorate***
*** ******Space Optics Manufacturing Technology Center (SOMTC)***

## Vibration insensitive phase shifting interferometers

- Errors with phase shifting interferometers when testing mirrors with long radii of curvature
- Coupling of mirror to interferometer is expensive
- NMSD and SBMD tested using AOA Wavescope, a Shack-Hartmann wavefront sensor
- Limited spatial resolution < 150 x 150 pixels
- Painfully slow
- Need for simultaneous phase shifting interferometer
- 4D Vision – PhaseCam
- ADE PhaseShift – IPI; SPSI
- Zeiss - Direct 100

## Vibration insensitive interferometers

**                                   ****	**_**Phase Cam**_
- Manufacturer		4D Vision
- Wavelength		632.8nm
- Spatial resolution 		470 x 470
- Minimum shutter duration  1/30,000 second
- Software			Phase Cam
- 				IntelliWave
- Dimension		20 x 20 x 70cm
- Weight			35 lbs
- Divergers		f/9 and f/33
- Cost			< $100K

** **_**IPI**_
- ADE Phase Shift
- 632.8nm
- 1000 x 1000
- 1/8,000 second
- Opticode (spatial carrier algorithms)
- 19 x 30.5 x 62.5cm
- 67 lbs
- f/1, f/2, f/3, f/5, and f/8
- ~$300K

## Spatial carrier algorithms

- Advantages
- Phase information is recorded on a single frame
- Requires only one interferogram
- Measurements may be performed in adverse environment
- Disadvantages
- Small tilt is necessary and errors may be introduced by interferometer aberrations; lower effective spatial resolution
- Distortion in imaging lens translates to phase error
- Detector sensitivities uniformity over the array
_**Reference:**_
**Interferogram Analysis for Optical Testing, D. Malacara, M. Servin, Z. Malacara, Ch. 8**
**Interferogram Analysis, Ed. David Robinson and Graeme Reid, Ch. 5**
**Fringe Analysis Techniques SPIE short course note by K. Creath**

**            ****IPI**

**Phase Cam**

## Interferometer characterization definitions	

**Interferometer characterization definitions****	**

- Repeatability of P-V: 1**** distribution of P-V values of 20 repeated measurements
- Repeatability of RMS: 1**** distribution of RMS values of 20 repeated measurements
- Measurement repeatability: average of RMS values of 19 difference maps of consecutive measurements
- Uncalibrated accuracy: average P-V value of 20 repeated measurements of a high quality flat (surface error < ****/50 P-V).
- NOTES:
- each measurement shall consists of average of at least 16 data sets, consider more data sets for noisy environment
- P-V value is defined for a 98% cut-off in the histogram

## Characterization procedure

- Perform characterization in two scenarios
- Short optical path
- use high quality flat for uncalibrated accuracy
- use reference sphere for diverger or transmission sphere accuracy
- Long optical path
- with spherical mirrors with >15 meters radius of curvature
- beware of air turbulence and heat source in optical path

##  Characterization results (to date)

**	****	****	****	****	**_**Phase Cam**_
_**Short path**_
- Repeatability of P-V		******/**348
- Repeatability of rms		******/**1711
- Measurement Repeatability 	******/**413 rms
- Uncalibrated Accuracy	******/**11.6 P-V
_**Long path**_
- Measurement Repeatability	 ******/**400 rms

**	**_**IPI**_**	****	****	****	****	**** ********/**685** ****	**
**	********/**2564
- 	******/**870** **rms**	**
**	********/**7.5 P-V** ****	**

## Future plans

- Complete Long Path characterization of IPI & PhaseCam
- Re-characterize IPI after repair
- Characterize UA vibration-compensating interferometer
- Characterize effective spatial resolution of IPI