First Light: William Optics Flat6A III Flattener & 0.8 Reducer

I have an admission to make: I’ve been running a refractor without a flattener since the day I bought it. Sure, there was edge distortion in my images, but I thought I could live with it and crop the elongated stars out of the finished photo.

My refractor is a William Optics GT-81. It’s a great little scope and I’ve been getting some nice results with it recently. But the elongated stars became a real problem when I got a camera with a bigger sensor - namely, the ZWO ASI1600mm-Pro. I found that I needed to crop a lot of otherwise good data to minimize the distortion.

The upper left area of two images from the same camera shows the amount of distortion on the left, while the image on the right was taken with a flattener in the image train.

Finally, I bit the bullet last month and invested in a William Optics Flat6A III flattener/reducer. It’s made to screw into the draw tube of three different William Optics telescopes, including the GT-81. As it turned out, I had a older model of the GT-81, with no way to attach the flattener. Oops.

I was at a loss. I contacted the vendor, Agena AstroProducts, who in turn contacted William Optics. They confirmed the 6A III would not fit my scope, but said they were going to offer a nose piece adapter soon.

Then I found Precise Parts. Long story short, they custom built an adapter for me, which put me back in the game.

Wait a minute, there are no threads in that draw tube!

Back Focus

The distance between the flattener and the camera sensor is critical. If you want things to be in focus, you’ll need to get that right. Unfortunately, I found different requirements. ZWO says the back focus is 55 mm. William Optics says that it’s 62.1 mm. In the end, I decided to go with 62.1 mm and that turned out correct.

First Light

I did say this was first light for the flattener, right? This is IC 1318 in the Sadr region, taken over the weekend of August 24-25, 2019. I minimally cropped the image after stacking. There is absolutely no distortion, so I’m left with more image. Additionally, the 0.8 reducer reduces the focal length of the GT-81 from 478 mm to 382 mm and brings down the focal ration from f/5.9 to f/4.7. That means I’m getting a larger field of view and shorter exposures.

IC 1318, The Butterfly Nebula in SHO narrowband false colors

Conclusion

Is a flattener/reducer worth the extra money? Yes - especially if your camera has a large sensor. I’ll be interested to try this with my one-shot color camera too.