Saturday, July 29, 2023

Last Night's Discoveries

When I laid out the telescope pad I planned it around the idea that the shed would on the south side of the pad blocking light from my neighbor's garage which really was not that bad.  The downside is that the optical axis of a Dobsonian is quite low; the mirror is almost at ground level.  This means you need an unobstructed horizon to get the whole sky.  The shed pretty blocks the southern sky.

Last night, the gibbous Moon pretty well washed out the deep sky objects I wanted to see but it was also behind the shed so I could not do the long video exposures that I was hoping to use to get sharper images.

In retrospect I should have planned for this by having an area east of the shed available to roll the Dobsonian on to.  Perhaps when the money comes in from the current projects, I will have someone pour me a bit more concrete to get my southern exposure.

In the meantime: the scope was not moving easily in altitude.  The bearing surfaces need regular waxing to move smoothly.  While searching for my box of candles, I found the box was stuck to the shelf.  Yes the heat of the shed had melted them inside the bottom a little seemed to have soaked through the box so I had to pry it off with my knife.

The other annoyance is that the tubes that form the half-Surrier truss are aluminum and the entire assembly is not as stiff as should be.  The tubing is 1.006" OD with a .05" wall and 53" length.  I think the fix is to replace these six aluminum tubes with carbon fiber composite tubes.  This not should be much stiffer, it will reduce weight at the front of the scope allowing to remove some of the clunky counterweights at the rear.   Not that weight is an issue.  They just look dumb.

The only issue besides cost is that the aluminum tubes are held to the mounting knobs by steel screws.  Carbon fiber and steel have an unfortunate corrosion problem over time.  Just look up their electronegativities.  I might need to use aluminum screws instead.


  1. What about steel and aluminum? I read something about needing to insulate them from each other, and this became a problem with the Commemorative Air Force because the airplanes in WW2 were, like so many other things, merely built for the duration.

  2. Aluminum and Carbon fiber don't play well together either. (I used to build 787's, every fastener had to be coated in sealant.) Titanium is cool with it. I don't know about Stainless or Brass.

    1. Thanks. I could not find any carbon fiber tube the right dimensions, so...