Optical Aberrations
Any deviations from a perfectly flat wavefront indicates optical aberration . There are several types of aberration measured by Quick Fringe:
- Tilt – How much the wavefront is tilted by the interferometer.
- Defocus – Distance from best focus.
- Spherical Aberration – A measure of the curvature of the wavefront.
- Coma – An aberration which results in a pear-shaped image. This occurs in off-axis images on Newtonian telescopes.
- Astigmatism – A measure of the "potato chip" shape of the optic - how much more curvature it has in one direction than another.
The first two of these, tilt and defocus, are due to the setup of the interferometer. The tilt is introduced deliberately to make the measurements possible. As long as the tilt is relatively small, it has no effect on the measurement. Defocus is the adjustment required to reach best focus for the optic. Since these aberrations are due to the adjustment of the interferometer relative to the test optic, neither of these two really have anything to do with the test optic. These aberrations are usually subtracted; this is easily done in Quick Fringe. Note: if you are testing an optical flat, do not subtract defocus; you need to know if the flat has any power.
Coma is often caused by misalignment between the interferometer and the test optic. This measurement can be useful to check off-axis performance.
There are also three measurements of overall quality of the optic:
- Root-Mean-Square – RMS error is a form of average error measure over the entire surface.
- Peak-To-Valley – P-V error is difference between the highest point and the lowest point on the surface.
- Strehl Ratio – The peak intensity of a point-source image produced by the optic compared to that produced by a perfect system.
The P-V rating is commonly used to describe overall optical quality. Lord Rayleigh was the first to point out that if the P-V error is less than 1/4-wave, the image will not be significantly compromised. This is the so-called 1/4-wave "diffraction limit". This is actually a misnomer; for example, the difference between a 1/4-wave telescope and a 1/10-wave telescope can be seen by a discriminating visual observer. The 1/4-wave specification is considered a minimum standard for optical quality in many situations.
The RMS error is a fairer rating of the surface because it is essentially an average error over the whole surface. Note that the RMS rating must be better than 0.075 for the optic to be considered diffraction-limited.
The Strehl ratio gives an indication of the effect of the optical system on a point source image. It is simply the ratio of the peak intensity produced by the optical system versus that produced by an ideal optical system. The "Strehl Limit" of acceptable optical quality is generally considered to be 0.8.
A warning about the various aberration measurements is in order . You might have a mirror which has more waves of spherical aberration than the P-V measurement shows. This is not a problem with the software. The spherical aberration has been canceled out by another aberration. There are "higher-order" aberrations which can do this. An example would be a zone from 80% to the edge which cancels out some of the overall spherical aberration. In terms of overall quality, the only measurements you really need to worry about are the RMS and P-V wave readings or the Strehl ratio.