My recent visit to Allan's Astronomy Lodge revealed a collimation issue with my Takahashi FOA60
(I guess I had bumped it somewhere ..)
Though not visible at lower powers, it became apparent that once magnification went past 100x there was an issue
I finally got a chance to address the problem last night
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For those not familiar with the scope it is a 60mm F8.8 CaF2 doublet with a widely air spaced design. This wide spaced design also means that the tiniest tweak of a push-pull screw has a rather dramatic effect on collimation.
With this scope the sliding dew shield can be removed by unscrewing it, this then allows access to the three set of push-pull screws which are on the counter-cell behind the objective rather than visible on the front like in some scopes.
The Push-Pull arrangement has a pair of 'Push' set screws either side of a 'Pull' screw (3 sets at 120º separations). The 'push' screws need a 2mm Allen key while the 'pull' screw takes a 2.5mm one.
The factory also have an annoying habit of filling the ends of the set screws with clear lacquer. To get to work on these I used a small hypodermic needle and added a single drop of acetone to each screw and after a few minutes cleaned out the softened glue with optical tweezers.
The actual collimation is pretty much the same as any collimation.
I set up the AZ-EQ6GT, and after a 2-star alignment, pointed the scope at Sirius, centred the star then changed to a high power eyepiece, and using only two of the three sets of push-pulls, adjusted them until is had a perfect Airy disc and first diffraction ring. There were many iterations of adjustments as the movements needed were TINY....
Once everything was set and the 3 sets locked off, I pointed at Acrux and saw the Airy discs of the A&B components as perfect as I had ever seen them. A clear separation of the two components though the first ring was an
8
As an aside: If Takahashi ever release an FOA125 or FOA152 I am going to buy two of them.
