I realized I didn’t test the combination of 90FL and 7704 – 0.65x super reducer. Although I was told it was not optimized, I wanted to see how it wasn’t anyway. This is the gorgeous 6-element optical system including one fluorite and one ED. BORG-proprietary quadruplet reducer system was originally designed in film era to fully cover medium format film (image circle 90mm diameter) and this 3rd generation system was more tweaked for digital sensors by incorporating ED element while downsizing. Here is the setup I tested last night. Very compact!! This is 90FL F3.6 optical system. The system configuration is 2590 – 7102 – 7801 – 7835 – 7704 – 5005 for Canon
The result was much better than what I anticipated. Following is the full resolution taken with Canon 6DH. No cropped, as it is just after converting to JPEG from RAW. I only adjusted the background color.
Click for full resolution (1.1MB)
I’m not an optical expert. So I can’t judge perfectly. But what I thought from the image are
- Spherical aberration is looking very minimal as expected. The objective lens should be produced in textbook quality as designed.
- Surprisingly, no coma, and no noticeable distortion and astigmatism either.
- Only issues are curvature of field and illumination drop.
- And finally this result reminds me how this unique 0.65x quadruplet reducer is the premium quality. It’s not low cost. But I believe the performance justifies the cost in my opinion. So the collaboration of fluorite and ED create very clean star edge.
The illumination curve was expected since native focal ratio was F5.6. So as shown in a picture, unfortunately hot spot was obvious.
I don’t mention this is fully optimized for full frame sensor camera. But I don’t hesitate to describe it works with APS-C or smaller sensor.
While we are waiting for the dedicatedly-designed reducer for 90FL, I will ask BORG optical designer to study the possibility to optimize current 7704 for 90FL too. I hope they can do something with minimal modification and cost. Then, it will turn out one of the best astrograph refractors in the current market – 90mm F3.6 with fluorite and ED. I wonder the larger aperture front reducer element helps to make the illumination flatter.
Anyway I will try to shoot several targets at dark site with this system in coming new moon. I should have realized much earlier…….. I’m curious how I can work for hot spot by the flat. In any cases, I felt last night the optics was very fast – F3.6!!, which feels like F2.8 or faster in conventional telephoto lenses.