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Orion 150 MCT Seeing 9/10 300x 36% CO (~50mm) 10 Apr 2012

    That surely looks excellent! I think you can be very happy with these optics! Thanks for sharing with me!
    Thank you, Mert. It's really hard to evaluate ring brightness due to the eye's response, but it seemed fairly straightforward to evaluate relative brightness on one side of focus, particularly the inner bright ring, the middle ring, and the outer bright ring. It was more difficult to compare relative brightness on the other side of focus, but the same relative pattern could be observed. Also, the inner most rings do appear brighter outside focus. All in all, some under correction is noted and expected. Somewhat crude Ronchi testing confirms this. The patterns resemble almost exactly those for 1/8th wave LSA in Suiter's book. But, I doubt the scope is that good. It's definitely not L/4, but closer to L/8. However, fudging by adding in residual astigmatism and coma (not observed on axis) brings down the final P-V to about 1/6th. I suspect this is a good estimate due to the level of care taken in a mass produced scope. Removing my primary baffle, however, reduced the contrast outside of focus. Initially this was a real shocker. Outside is softer and less distinct, but the rings can still be discerned and they hold the same relative brightness pattern. I /suspect/ the blur is caused by residual HSA and transverse error of the marginal rays. It cannot be eliminated without an aspheric surface. Question is, does this 'blur size' speak to the level of balanced 6th/4th order SA, if any? I think it does. The blur size with reduced transverse aberration tends to be smaller and more concentrated in brightness resulting in less contrast outside focus. (I suspect this is in line with Roland's article on the topic of asymmetrical star tests.) Pure HSA is much more diffuse and may offer improved contrast. The tests a 4 waves defocus seem supportive, but not conclusive, of near 0.15 P-V of balanced high/low order SA. Of course, the shadow size is not accurately depicted here. It's difficult to measure them visually. However, break out testing yields results between 1:1.5 and 1:2. That seems a touch better than L/4 LSA, again consistent with star testing. Anyway, the mass produced MCTs can be very good.

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