I was fortunate enough to be able to acquire another Sky 90. Its the very same sample that was discussed in this thread here:
https://www.cloudyni...212-tak-sky-90/
As I understand it, It was a one-owner scope that was on consignment at an Internet astro-retailer. A fellow CN member obtained it from the retailer which was local to him on a "try prior to buy" arrangement. When he used it, he discovered a collimation issue and returned it. I inquired with the dealer, who is also somewhat local, and made arrangements to purchase it. If I had not done this it was to be sent to TNR/Takahashi America for a tune-up costing in excess of $500. To me, I just did not see the value in that considering I am able to do the same things and, perhaps, more then TNR can do...
First impressions are the scope is in really good condition. And its the latest version - The Sky90-II with a fully collimatable lens cell containing 3 collimation screws and 6 centering screws.
Upon first inspection on the optical bench, I confirmed the initial reports that that the lens was out of collimation and displayed flaring (coma) off to one side as was described in the thread linked above. Basically, it reminded me of an out of collimation SCT displaying a similar diffraction pattern.
In preparation for lens collimation, I disassembled the OTA, dewshield and lens cell. Each painted surface was meticulously buffed then sealed with a high quality synthetic automotive wax. The OTA is mint with only a miniscule - 1mm paint chip at the finder-scope mount section of the focuser. The lens is spectacularly clean and required absolutely no work to it.
The retracting dew shield is threaded on to a sliding ring that can be difficult to remove on TAK OTA's. This one only took slow and deliberate pressure and it about 90 seconds started to unscrew. I applied a tiny amount of lightweight synthetic grease to the threads to prevent galling and allow for easy removal in the future.
I spent about 4 hours collimating and testing the Sky90. Its in perfect collimation now.
The camera picked up some chromatic effects that are not necessarily visible to the eye. The overall color correction is quite good for such a fast doublet. I mean, its amazingly good. There is some unavoidable spherochromatism - spherical aberration as a function of wavelength. Unfortunately this is a trade-off in a doublet that is designed at such a short focal ratio. To do markedly better, you would need a triplet and at least one of the 6 surfaces of the triplet would have to be aspherized. The Sky90 is a spherical fluorite doublet. Its about as good as the design allows. There are no free lunches.
Since I do not need another short focal length APO, I will probably sell this Sky90 and pass the considerable savings on to the next owner.
The lens is VERY smooth and only a hint at very minor undercorrection in white and green. These Ronchi images were taken close to focus in double pass, In single pass, no curvature could be detected. This is a fine lens and is very similar to one I tested earlier and wrote about on CN.
The images below show the current collimation and the overall correction.
