scope – Portable 16″ f/4.6 Dobsonian

Portable 16″ f/4.6 Dobsonian

Introduction

My first scope came from Sears when I was 15. It was a cheap refractor that my dad and I were able to point at Saturn a couple times, but the views were underwhelming, and required effort to find anything was so great that we promptly returned the scope and Astronomy As Hobby went into remission for a quarter century.

About three years ago, Astronomy As Obsession emerged. There was no great impetus, but I had always been interested in all things related to space, and I now had tenure, so…. I purchased my second scope, a cheap Celestron goto 4.5″ from CostCO. My experience with the CostCo scope was similar to the first, except that this time I had the Internet to tell me that my mistake was in choice of scope, not choice of hobby, so I bought a Hardin 8″ dob and was hooked. The 8″ lasted a year, the 12″ Celestron StarHopper another year, but I had aperture fever bad, and 12″ was just not going to do it. I needed more photons, and the only question was when, not if.

At this point, I admitted my new obsession to friends François Guimbretière and Bobby Bhattacharjee, the latter of whom also had an obsession (a woodshop), and suddenly the idea of building a scope didn’t seem so crazy. Between Kriege and Berry’s The Dobsonian Telescope, the woodworking expertise (and tools) of my friends, the scope expertise freely given the Cloudy Nights forums, articles on both Cloudy Nights and Astromart, and my enthusiasm (read “cash”), this was really going to happen.

Goals

The next question was the type of scope I wanted to build. My goals were big aperture and high portability, so I soon settled on the idea of a collapsible 16″ scope, and soon thereafter I stumbled on Albert Highe’s web pages, and the general form of the scope was determined. This was ideal, Obsessions are beautiful scopes, but they also large and heavy. Albert’s designs are lightweight, and gorgeous. When it turned out that Albert not only had detailed descriptions of his creations on the web, but was ready and willing to answer questions, the course was set. In what follows, I’ll mainly document my digressions from Albert’s canon, as he has so much info online already.

My starting point was Albert’s 17.5″ portable. Differences would be aperture, the number of tubes, and the bearings.

First, the tubes. Albert builds 3-pole dobs. This works because he has built a lot of them, and at this point seems to have an intuitive grasp of the rigidity resulting from pole diameters and wall thicknesses. I don’t have that, so one of my earliest decisions was to go with four poles instead of three. This would hopefully be overkill in terms of stiffness, and and also give me the ability to easily use a shroud (I live on the east coast, where there is dew most months). Albert used 1-3/4″, 1-7/8″, and 2″ tubes, with a wall thickness of 0.049″, letting him nest the tubes within each other for storage. I started with the same idea, but had four, and so went with two 2″ tubes, and two 1-3/4″ tubes, w/ the latter nesting in the former for storage.

My iniital goal was to get the entire rocker base to fit into a box 19’5″ on a side in order to fit it comfortably behind the seats in our Honda Element. Actually, however, only the depth is an issue, so the width grew to 20.5″.

We build the mirror box using ebony star, changing Albert’s design so as to use four tubes.