PV What?

With respect to the optical quality of a triplet-lens refractor, what does the phrase "certified as minimum 0.25PV" mean? Would "0.25PV" be good, bad, or mediocre? I think I know, but I'd rather have someone more knowledgeable explain this to me.

Thank you.

Bill

William Green said:

With respect to the optical quality of a triplet-lens refractor, what does the phrase "certified as minimum 0.25PV" mean? Would "0.25PV" be good, bad, or mediocre? I think I know, but I'd rather have someone more knowledgeable explain this to me.

Thank you.

Bill

Bill,

I suspect it means 1/4 wave peak-to-valley (PTV). It simply means that the maximum deviation from perfection is 1/4 the wavelength of light. It is important to know what wavelength it applies to, and whether it is at the wavefront or at the optical surfaces (although I suspect the optical surface readings are usually done for single surface optics like Newtonians).

A 1/4 wavefront is usually considered the minimum to provide good performance, or be labeled "diffraction limited" with performance only impaired by the ill behavior of those pesky photons. Unfortunately, a single measure is not helpful. If the optic has a lot of rough smaller variations it will not perform as well as a smooth surface with only minor and gentle smaller variations. Often an RMS measure will be given to measure these problems. It should be a four or five fold improvement on the PTV measure.

Clear skies, Alan

A classic .25 PV will get you 81% of perfection. However, it is quite possible to have .25 PV defect over just a tiny portion of the aperture, and have an optic that performs to 92% of perfection. It could also produce less than 80% if the defect is recurring, or irregular over the entire surface. Therefore, the PV measurement is sort of meaningless without other measurement details.

RMS and P-V can be used together to determine the overall correction, or the closeness to perfection of an optic (otherwise known as Strehl ratio).

Rolando