Some Hong Kong amateur astronomers have
done their own testing of various flatteners with the Lomo 80mm f/6 Super-Apo. Their results can be summarized as such. Note that the Televue TRF2008 performs the worst in their test (24m distance to artificial star).
I recently purchased the
William Optics New Adjustable Flat6A II which supposedly works with a wide variety of refractors. I've collected the recommended spacings from William Optics and
generalized it to any refractor focal length.
For this test, I'm using a Lomo 80mm f/6 Super-Apo with a Russian OK4 air-spaced triplet. Due to poor weather, all testing was done indoors at about 8m distance (admittedly, quite close) and a
Hubble Optics 5-star artificial star. I used the 50-micron (smallest) star for this test.
The test images were captured with a Canon EOS 6D, which is a full-frame sensor and therefore somewhat of a challenge for these flattener/reducers. Do note that because the test was not conducted at infinity, it is not conclusive.
First, the performance of the refractor without any correction at all. We can clearly see that there is quite severe field curvature. This performance is inadequate for even casual imagers.
Compare this to the performance with the inexpensive
Orion 8894 0.8X reducer:
Performance is better than without any flattener, but still not that great. Note that this is at the corners of a full-frame sensor, so on a reduced-frame camera, performance would be much better.
Now for the oldie-but-goodie
Televue TRF2008 flattener/reducer for the TV85, which is designed for 400mm - 600mm focal lengths.
This is a pretty good showing, significantly better than the Orion.
Now for a bad example: the Altair Astro (Long Perng)
0.6X reducer/corrector, which was never designed for full frame:
Suffice it to say, this is barely better than no corrector at all, however there is the 0.6X focal length reduction which may offset the ugly corner stars.
According to William Optics, the GT81 with a 478mm focal length requires 7.9mm of spacing with the Flat6. Here are two attempts with an 8mm spacing:
Neither of them are very good. Definitely worse than the TRF2008. I tried spacings of 9mm and 10mm, with even worse results. With an 11mm spacing, I could not reach focus. This was almost certainly due to the artificial star not being at infinity.
Here's the same Flat6 with a 6mm spacing, which is significantly less than recommended:
Performance is significantly better than at 8mm, and is
almost though not quite as good as the TRF2008.
And with 4.5mm spacing:
Correction is almost the same as at 6mm. In general, the shorter the focal length, the more correction is required. And correction is increased by increasing the spacing. Interestingly, this Lomo 80mm f/6 triplet seems to need
less field flattening than would normally be indicated.
It is somewhat disappointing, however, that after all this drama, the Flat6 cannot surpass the 1999-era TRF2008.
It is not bad at all.
Not too bad a showing for the Canon, I must say, given that the Canon is a 300mm f/4 (75mm aperture). The Lomo is the equivalent of a 384mm f/4.8 so not too far off.
