William Optics Flattener comparison

I recently acquired the William Optics Adjustable Flattener Reducer 4 as I wasn't too happy with the image quality I was getting from the local astro-club's William Optics FLT-110 telescope with the William Optics Focal Reducer/Field Flattener III.

William Optics FR/FF III
50 minutes of Ha in 10min subs
Cropped view around target.

Although I had used the WO FR/FF III the last season while using the FLT-110, I noticed my star shapes were way off considering how good the scope was supposed to be. I had put the blame on the less than stellar polar alignment of the mount, especially since the mount was secured to the pier and aluminum base plate only by the central bolt. Needless to say, as the temperature during the imaging season fluctuates between +15 C and -35 C, the bolt comes loose due to thermal effects and the mount starts to drift off. During the summer I built a new observatory for a new EQ6 our club had purchased and I made sure the polar alignment won't move. With the mount aligned I discovered that the horrible looking stars were not caused by polar alignment but an optical issue instead. I played around with the flattener to sensor distance and the star images improved a bit, but the stars near border turned from warp effect to cross shapes. Reading thru forums I found out I'm not the only one faced with the same issue on the FLT-110.

William Optics AFR-4
90 minutes of Ha in 10min subs.
Nearly full field of stack.

After a bit of research I placed an order for the William Optics AFR-4, as it had a reasonably long back focal distance, and more importantly, the flattener lens group location can be moved for 20mm by rotating the lockable adjustment ring. The various posts on forums recommended a distance of 73.5mm for the FLT-110, so I made a rough estimate and started waiting for a break in the clouds. A week later I took another set of frames of the Sh2-101 () so I could make a direct comparison on the images produced by the flatteners using the same equipment. Incidentally the sensor distance was quite good with my first guess and now the sensor tilt is the prominent image quality reducing factor. I was able to reach 1.05px HFR stars at center with the stars in corners were around 1.08 pixels HFR. The image scale is a bit tighter than with FR/FF3, the field of view is 2 arcmin narrower at 97.2 arcmin with the AFR-4 compared to 99.0 arcmin with the best star shapes on FR/FF-III. Of course the field of view with reducers depends on the sensor distance, so the final figure after more adjustments is subject to change.

So far the only negative aspect has been a bit of flare around brightest star in the field, but it could be partly caused by the humidity, as the air was far from clean on the test night. Hopefully more to follow on the experience with AFR-4 as the imaging season 2011-2012 progresses and I reach a better optical alignment.