Sunday, February 14, 2021

Baby is Pretty Much Done

 I have been rebuilding a 3" f/4.5 Newtonian over the last few years, probably averaging about 10 minutes a week.  I made the mirror in the late 1980s, and initially I feared that I had it aluminized when it was really not a very good paraboloid.  In retrospect, the problem was that the mechanical parts (mirror cell and diagonal holder) were really bad.  With the available tools and materials, I am still trying to remember how I managed to cut a reasonably round set of discs of oak for the mirror cell. 


The diagonal holder was one of those utterly non-adjustable stalks that went through a hole in the Edmund's rack and pinion focuser's base.  This focuser was obsolete when I bought it.  Friction alone held the eyepiece in the tube and when I say friction, I mean friction.  Any notion of removing an eyepiece and putting in another without moving the scope was delusional.  The only part that I kept when it was time to do a better job on the mechanicals was the 4.5" OD PVC tube.  Fiberglass and carbon fiber tubes this size are remarkably expensive and the weight difference is trivial for this size.  (Square-cube law.)


Now I had a lathe and a CNC mill.  I turned the new mirror cell from 6061 aluminum.  I made it .01" smaller than the inside of the tube.  That was maybe a little close of a tolerance; getting it in and out is a pain.  Edge mounting holes and adjustment bolt holes are exactly 120 degrees apart.  I used a rotating table on the mill to get within tenths of a degree.  


The diagonal holder I made from 1" diameter aluminum, some springs, some socket head screws, and a 1/4"-20 bolt that went through a spider made specifically for this tube's dimensions. (I have forgotten the spider maker's name.)  The actual holder I turned from aluminum, then cut to a 45 degree angle on a tilting table on the mill.  Again, a very precise cut.  Then I bored out the interior on the lathe through which to put the1/4"-20 bolt.


I was horrified at how expensive 1 1/4" focusers were.  I am sure they are very good, but for a scope this small and cheap, it seemed unreasonable to spend >$200.  Besides, I was fantasizing about making my own.  A Crayford looked to be more work than I wanted to expend, so I decided to use a drawtube for coarse focus, with a helical focuser for fine adjustment. The results are attached.  It was not very satisfying and it seemed nearly hopeless to get my range of eyepieces to all hit the focal point.


I would have preferred the basic Orion focuser because it was inexpensive, but it was backordered, so I bought the GSO focuser from Agena.  It was $79, so still reasonably priced.  It is a very nice piece of work.  Everything is metal, as near as I can tell.  The name concerned me; the People's Republic of China has pretty well burned all bridges with its handling of the COVID-19 disaster and it is also the country we are most likely to be at war with in the next 20 years.  Putting any money into their pockets, even indirectly through a private company, ran against my grain.  Fortunately it is labeled as made in Taiwan.  Concerns alleviated.


There are two thumbscrews; one controls tension of the operating rod against the drawtube; one prevents motion of the tube.  It is not really a lock; it just disables the focus knobs in some way that seems magical to me at the moment.  When trying to punch and drill the mounting holes, I found myself wishing for a much longer drill bit and tap for making the 10-24 holes, especially on the focuser side.  Solution: using a really tiny Allan wrench, I removed the knob from the shaft, and now I had enough clearance (barely).  The height of the focuser fully racked in is about 3", so I had to move the mirror cell up the tube about 1/2" to use my shortest F.L. eyepiece, an Orion 35mm that they called their Super-Plossl back 20 years ago.


I wanted dust caps, and to my pleasure B&H Photo had some 114mm dustcaps that are exact press-on for a 4.5" OD tube.


There are now many holes in the tube that are unsightly but not dysfunctional.  The next steps are using a black epoxy glue to fill those holes.  The focuser is not quite as fine as I would like, so I think that I will machine some larger knobs out of 3" diameter acetal and use the existing set screws to hold them to the axis.  Being more than twice the diameter of the 1.25" knobs, I am expecting slightly finer precision of motion.  The primary needs recoating; it was done in the early 1990s.

No comments:

Post a Comment