[Paul Haese]

Having spent quite some time and expense buying adapters in trialling this flattener to determine its ability to flatten the GSO RC12 field and via advice from TS Optics I have come to the conclusion that the flattener does not seem to work on large format sensors. It might work well with smaller sensors but larger format sensors don't seem to be corrected properly.

Upon looking at the flattener it has a 3" aperture and that would indicate it to be ideal for use on large sensors. It has a good stated back focus but I have discovered that is most likely a flattener for a refractor. The flattener looks exactly like the one TS Optics sells for refractors and it looks exactly the same as the one sold by Meade for their refractors. There is a possibility though that there is a different prescription for the refractor flatteners but some how I doubt it. There is nothing out of the ordinary about refractor flatteners being used for GSO RC's. I have used a Tak flattener on my GSO RC8 before and it worked well on an 8300 sensor. That flattener would not work with my current setup.

I found initially that the stated back focus of 106mm produced worse results than without the flattener in play. I was advised to move the sensor further back from 106mm to 109mm. This had the effect of making the field curvature look much worse. Reported curvature went from 37% to 45%-47% on various sub exposures. I was then advised to move to 103mm and found that this improved things a bit but did not eliminate the field curvature beyond 30%.

I have made sure my mirror distance is very close to optimum as stated by GSO under their documentation. Star shapes in the corners of the images without the flattener look like field curvature.

I have included two image that are crops of the top left of the frame of the same field of view for comparison at a back focus distance of 103mm. The first is with the flattener in place and the second is without the flattener. As you can see there is in fact a slightly worse correction with the flattener in place.

So it would be my assessment that this flattener might not work as a flattener for large sensors on GSO RC's. Feel free to ask or discuss.