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Help! WO GT81 Field Curvature Issues

08 Feb, 2019 16:26
Hi all. I'm having some serious field curvature issues with my William Optics Gran Turismo 81. I have the WO Flat 6AII adjustable field flattener/reducer and have set it to roughly 7.9mm like the sales page suggests (, but the stars are still very oblong in the corners. I experimented last night for about 3 hours, moving the field flattener about a full rotation in each direction from my starting point but didn't see much difference in the curvature at all.

This is causing a lot of problems when trying to stack subs that are on different sides of the meridian flip. The curvature is different in each corner so the stars don't line up well when the camera is rotated 180°. It gets even worse when I try to align my narrowband frames in Photoshop. For example, I took Ha and OIII before a flip, and SII afterwards. After aligning them, stars in the corners are biased towards red (SII) or green/blue (Ha/OIII) depending on how misaligned they are from each other.

I can see the benefit to a camera rotator here, but I feel like that's a band-aid.

Can anyone lend some advice? Am I maybe just expecting a perfectly flat field that I will just never get?

08 Feb, 2019 18:11
I have not gotten a perfectly flat field but have come close on full frame. I’m using a GT71 tho @ 9mm
08 Feb, 2019 18:13
I'm using a 4/3 sensor ( ZWO ASI1600MM). I wonder if that has something to do with it.
08 Feb, 2019 21:43
i use this its oem for the gt81
maybe the new adjustable type has to many variables .
09 Feb, 2019 16:10
It should be much easier with the smaller 4/3 sensor, you should be able to get pin point stars across the entire frame. If the adjustable part is not tight you will get some tilt and one corner will be bad
09 Feb, 2019 16:37
I did find that some of my extender tubes were loose last night. I think in adjusting the FF, I possible loosened things and caused more tilt. I also discovered that I have ~56.7mm of backfocus between the rear flange of the FF and the camera sensor, when William Optics specifies that there should be 54.8mm. I'm going to adjust the FF to account and try again the next chance I get.
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