This weekend I took delivery of an Alt-Az mount (after 3 months of waiting) which finally enabled me to use my Tak Sky 90 II in its grab-and-go configuration. I was a bit surprised by the outcome and I wanted to share my experience to counter the 'conventional wisdom' that the Tak shows lots of false color.
I acquired the Tak three years ago as a narrowband astrograph and it performed those duties very well. But eventually dark nebulas caught my attention and I quickly learned that the doublet was not very happy imaging L data (bloated stars, ...). The solution was simple but expensive: get a TV127is, and sell the Tak to offset the cost. The first part - buying the 127is - was simple, but selling the Tak proved impossible due to a defect in the front element - a coating defect 1x2 mm^2 in size at the extreme outer edge of the front lens. After a couple of days on the market without any interest at all, I decided to do the next best thing - keep the Tak as a visual instrument when the weather isn't good enough for imaging. So I sell the Tak's photo accessories (CAA, FR) and use the money to buy some eyepieces and an Alt-Az mount.
Today the weather clears and after letting the scope cool down for an hour I take a look at the moon. At 75x, there was an obvious yellow fringe along the edge of the limb. I swing over to Jupiter and I see a red fringe on one side and a blue fringe on the other. Rotating the eyepiece (ES 6.7mm 82 degree) did nothing to the CA, so I was certain it was coming from the scope. I was not let down a lot. After all, there are numerous accounts of the Tak's poor color correction all over the internet. Returning to the moon, I adjusted the eyepiece position using one of the Tak's adapters and voila, the CA is gone. Jupiter too had lost its red/blue cast. Only when I positioned my eye well off center from the eyepiece did the CA reappear. I guess the CA was due to the diagonal+eyepiece being pushed off center from the light cone, not from the lenses. It made me think how many observers critiqued the Tak's color correction without checking if there wasn't anything crooked with the light path.
Of course seeing was poor, so I after 10 minutes of destroying my night vision on the moon I called it a night.
This post isn't meant to prop up the Tak doublet as a color-free instrument. I just wanted to share my subjective experience and cast some doubt on the 'common knowledge' that the Tak doublet shows every object as a rainbow. The scope certainly has its shortcomings (price) but when you consider the target audience (travelers) it's hard to fault the little scope.
And I am well aware that I only went up to 75x. There is no doubt in my mind that eventually some CA will show up at higher powers especially when compared to scopes with a longer focal length.
Bottom line: I'm glad I kept the Tak!