Aligning the Optical and Polar Axis

Here is a technique used by Russell Whigham from Montgomery, Al.

I stumbled across a discovery the other night that might be of some interest to other Schmidt Cassegrain owners, especially those S-C's with detachable fork arms such as Celestron's C-11 and C-14. It may be a problem for fixed forks, I'm not sure.

The problem that I had struggled with was the coaxial alignment of the mechanical and optical axes. Misalignment causes improper polar alignment and field rotation. It's virtually impossible to verify that the declination circles are set correctly using the"eyeball" method. In the past, I've tried tricks like leveling the tube assembly and setting the declination circles to my latitude. This and other attempts just were not accurate enough.

During polar alignment, I noticed that while rotating the fork in R.A., that the star trails around the Celestial Pole were not concentric with the view in the eyepiece. It's not even a problem for visual work, especially for a "star hopper" like me, but it can be a real aggravation during guided astrophotography.

After fumbling with this for part of an afternoon and the early dark hours last weekend, the comic strip incandescent 100 watt lit up in my head. I attached the tele-extender (without an eyepiece) to the rear cell. Then after centering and affixing a paper "hole reinforcer" to the plastic cover of the polar axis at the base of the fork and pointing the open end of the tele-extender toward the polar axis cover, I looked down through the corrector plate and made fine adjustments in declination until I had concentric circles of (1) the secondary assembly, (2) the opening of the tele-extender, and (3) the polar axis cover with "hole reinforcer". Now rotating the fork/tube assembly 360 degrees in R.A. should show optical/mechanical coaxiality. If the "reinforcer" should wander about as the fork is rotated, adjustments to the fork arms will be straight forward. There is less than an inch clearance between the end of the tele-extender and the polar axis cover, making preliminary alignment very close.

Now, so that I don't have to do this all over again next time out, I set the declination circles on both sides to 90 degrees.

I hope this helps anyone who has been frustrated with the same problem.

The fork arm adjustment Russell mentioned above can be accomplished by loosening up the 2 bolts that secure the forks to the base plate. You should discover the holes for these bolts are a bit larger than the bolt diameter giving a small amount of adjustment.

The most important thing is that the declination circles be set to 90 degrees when the "wander" of the reinforcing ring is at its smallest.

 

Here is another technique for aligning the polar and optical axis. This will work just fine out in the field and can be done in a couple minutes. I wrote these up for Celestar users with the wedgepod. For those with a regular wedge, you can make your adjustments there.

Ok... set up the scope and point the fork arms towards Polaris. If you have set up your DEC circle using Russell's instructions, point the scope to 90 deg. DEC. Moving the wedgepod itself and not the scope on its axes, center Polaris in a fairly widefield eyepiece.

If you have not set your DEC setting circle, you can do it at this time by rotating the scope in RA. If Polaris moves out of the field of view or makes a big circle, fuss with the latitude adjustment until Polaris makes the smallest circle you can get. This process will require adjusting the wedge and recentering Polaris with the Dec control. Keep adjusting, recentering and turning the RA axis until Polaris sits still (it probably won't) or makes a very small circle.

The reason Polaris will not stop moving is because your optical and mechanical axes are probably not parallel. If you can get Polaris to make a small circle in the field of view while rotating the scope in RA, you should be ok. Once you get Polaris as stationary as possible, set the DEC axis to 90 degrees. Lock down the DEC circles and you should never have to go through this procedure again.

 

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