William-Optics New Adjustable Flattener P-FLAT6AII on AP130GTX

I've attempted to create a general formula for spacing of the William-Optics New Adjustable Flattener P-FLAT6AII here.  I've also done some rudimentary testing of this flattener on a Lomo 80mm f/6 triplet as well. The key takeaway at the 480mm focal length is that this William Optics flattener performs better than the half-priced Orion flattener/reducer, but is marginally outperformed by the 20-year old design of the Televue TRF2008, at least in my testing with an artificial star that was quite close by, not at infinity.

What about on the Astro-Physics AP130GTX, which has an 819mm focal length?

Based on William-Optics' tables, the closest focal lengths are the 970mm Z126, with a 1.4mm spacing, and the 711mm Z103, with a 4.6mm spacing. Neither is particularly close to 819mm, but using my least-squares approximation with m=-0.014 and b=14.855 a spacing of 3.4mm is obtained.

However, based on my previous experience with the Lomo, where the best spacing was around 2mm shorter than indicated by the table (may be caused by the artificial star not being at infinity) I decided to try several spacings of the Flat6 to determine the best one.

First the AP130GTX with no flattener, on the corners of a Canon EOS 6D. Performance is decent, actually: the field curvature is much less than with the Lomo 80mm.

We do observe that there still are comet-shaped stars, although they are pretty tight.

The Televue TRF2008 did very well with the Lomo, in spite of its 20-year vintage (it was released in 1999). It does not do quite as well with the AP130GTX, however, in spite of (supposedly) being designed for 400mm - 600mm focal lengths.

The stars are not comatic, but they are quite eggy.

The Flat6 supposedly is best at 3.4mm spacing. Here's 3mm. It's not great. The stars are round (ish), but quite diffused compared to without the flattener. This could also be down to my technique or lack of it (that's a typo, the spacing is 3mm, not 3 meters).

 I decided to try an even shorter spacing of 1.5mm which would correspond to 900mm of focal length.

While the stars are tighter, they are less round than at 3mm, and show a bit of a comatic shape. I suspect that 3.4mm or 4mm is a better choice for spacing for the AP130GTX. But performance is still only tolerable.

I guess that's why AP gets away with charging $825 for their dedicated flattener.

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