A friend recently asked me to help fix up his Sky 90 as it had been determined that it was out of collimation. After getting that all done I decided to compare it head to head with my WO 80 FD. I tested them a number of times under different conditions and found the results somewhat interesting.
The WO has a fl of 555mm with an f/ratio of 6.9 and has fpl-53 glass and the Tak Sky90 has a focal length of 500mm and an f/ratio of 5.6 with a front flourite element.
The WO has always been a tremendous preformer for an ED doublet. The star test is quite good with only a slight hint of spherical aberration and virtually no color in focus. The Tak surprisingly showed significant spherical aberration in the form of undercorrection. To determine if it's significant to affect performance I slowly moved the focuser through focus and it was apparent that all the zones of the lenses were not quite focusing at the same exact point. The inside focus rings were very sharp and the outside focus rings were quite diffuse and il defined. While the focused image is respectable it didn't seem to deliver images that a first class apo should dliver.
While examining lunar and terrestrial features at high power the extra aperture of the Tak seems to deliver a slightly higher resolution image when compared to the WO but the contrast is definately better in the WO. This is confirmed by examining the the airy disk patterns at high power. The WO has a clean disk with a very dim first diffration ring on the brighter stars. The TAK definately has a brighter ring and hence a lowered contrast on extended objects. These are critical observations under high power it should be mentioned here. At most mags the Tak should perform quite nicely. The color is a little more noticeable in the Tak but is not at all significant while the WO is basically color free.
While it seems that the Tak isn't quite measuring up it should be mentioned that it is an f/5.6 which is extremely fast for a refractor. I do wonder how the 1.6X extender would affect the Tak. I know that color will be reduced but does it help SA? Anyways I thought I'd mention my findings here on the mart
The WO has a fl of 555mm with an f/ratio of 6.9 and has fpl-53 glass and the Tak Sky90 has a focal length of 500mm and an f/ratio of 5.6 with a front flourite element.
The WO has always been a tremendous preformer for an ED doublet. The star test is quite good with only a slight hint of spherical aberration and virtually no color in focus. The Tak surprisingly showed significant spherical aberration in the form of undercorrection. To determine if it's significant to affect performance I slowly moved the focuser through focus and it was apparent that all the zones of the lenses were not quite focusing at the same exact point. The inside focus rings were very sharp and the outside focus rings were quite diffuse and il defined. While the focused image is respectable it didn't seem to deliver images that a first class apo should dliver.
While examining lunar and terrestrial features at high power the extra aperture of the Tak seems to deliver a slightly higher resolution image when compared to the WO but the contrast is definately better in the WO. This is confirmed by examining the the airy disk patterns at high power. The WO has a clean disk with a very dim first diffration ring on the brighter stars. The TAK definately has a brighter ring and hence a lowered contrast on extended objects. These are critical observations under high power it should be mentioned here. At most mags the Tak should perform quite nicely. The color is a little more noticeable in the Tak but is not at all significant while the WO is basically color free.
While it seems that the Tak isn't quite measuring up it should be mentioned that it is an f/5.6 which is extremely fast for a refractor. I do wonder how the 1.6X extender would affect the Tak. I know that color will be reduced but does it help SA? Anyways I thought I'd mention my findings here on the mart
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