Wednesday, August 15, 2018

I Should Have Planned Ahead

A friend visiting me wanted to do some astrophotography with Big Bertha.  I have had pretty good luck with my Pentax K10D and tube balance because Pentax cameras are small and light.  His Canon is aptly named.  Once plugged into the eyepiece focuser, it was so heavy that it dragged the telescope down from the target.  Solution: provide a balance adjustment.  My Pentax KS-2 weighs 1 lbs. 10 ozs.  The prime focus adapter adds 2.2 ozs.  (The eyepiece projection part adds another 3.2 ozs.)

Just to give you more information that you do not need about astrophotography: The focal point for most telescopes is just where the eyepieces rest in the focuser.  For astrophotography, you need some in focus travel to get the telescope's focal point inside the camera, at the image sensor (or film plane in previous centuries).  You could look for a focuser with 5"-6" of focus travel, but not find one.

To deal with this problem, I had my telescope rebuilder put the focal point far enough outside the normal position to add a 3" long eyepiece tube extender in the focuser.  This means my eyepieces focus properly and by removing that 3" long tube extender, the focal point ends up in the camera.  The balance problem requires adding camera body weight + prime focus adapter weight - tube extender weight (or about 1 lb. 9 oz.) to the back of the telescope tube.  Clumsy looking but this uses two 15 oz. metal donuts from another project with a piece of electrical wire and a bungee strap.

 For the next time a visitor brings a hefty camera, I will need to add more weight at the rear, by adding more steel donuts.


  1. Kluge to the max!
    You might consider a sliding weight to counter the differing weights of the various cameras you would mount to the scope. A weight that has a rod or tube that runs through the middle to hold it and allow shifting of location, with appropriate clamps or nuts to fix the position.
    The articulated surgical laser arms were counterbalanced this way, with enough travel to compensate for different handpieces and adapters the surgeons might add. I've got one you could have, but the weight is a steel cylinder about 6"x6", probably overkill for your application. Threaded adjustment, just rotate the weight.

  2. Not to the max! There is no hamster wheel with running rodents. I used to use a weight on a tube in an earlier incarnation of this scope, and I may look for a way to do this again, going up and down the truss tubes.