William-Optics New Adjustable Flattener P-FLAT6AII

William Optics has a new adjustable flattener with (they claim) 97% of full-frame (43mm image circle) coverage for a wide range of refractors from around 480mm focal length, up to 970mm. This is almost certainly the replacement for the old P-FLAT4.

There are several other adjustable flatteners such as the Long Perng one (which is cheaper).

WO publishes some suggested spacings on their web site, which I have summarized here:

There is what I believe to be a typo. Field curvature for refractors (whether doublets or triplets) is proportional to the focal length only (not the focal ratio). However we see in WO's suggested spacings that the Zenithstar 71 and Gran Turismo 71 which have basically equal focal lengths, have significantly different spacings.

If we do a least-squares interpolation using WO's suggested spacings, we get the following:

Notice that the R-squared is 0.9173 which is not very good. However if we drop the Zenithstar 71 data point and keep all the rest, we get this:

A much better fit (almost a straight line) with an R-squared of 0.9926.

In summary: to determine the optimal spacing for your particular refractor, use m=-0.014 and b=14.855 (remember the formula, y = mx + b).

Once I get my copy of the P-FLAT6AII, I will be able to validate if the above formula holds.

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