Saturday, July 6, 2019

What A Spectacular Night

Dark and stable air.  Even the Moon was well behaved, being only a few days old and producing little skyglow.  I rolled out my 8" f/7, which uses a Coulter Optical mirror from when they advertised +-1/25th wave accuracy, and again I see it.  (For those who do not drool over telescope mirrors: how close a mirror's shape is to a perfect paraboloid is measured relative to a wavelength of light.  A minimum acceptable telescope mirror is 1/8th wave.  Before Hubble's optics were fixed, it was about 1/2 wave.)  Tonight, Jupiter's dark cloud bands in both hemispheres were clearly visible, along with just a hint of the turbulent clouds at the boundaries of those bands.  And that was only 112x!

What makes this even more impressive is that the telescope is not well collimated.  It uses a diagonal mirror holder made by Edmund Scientific back in the 1960s, which has no collimation adjustments, so the laser beam is not hitting the center of the mirror and it still performs beautifully.  Properly collimated, this 60 year old scope should do even better!

Because I have been making such a holder for the 3" f/4.5 reflector rebuild (taking forever, it seems), I am planning to make one for the 8" f/7 reflector as well.  This should be much easier.  I do not have the size constraints.  Building a mirror holder that needs to be 1" diameter is really difficult.  The bigger telescope uses a 1.83" minor diameter diagonal mirror, so lots of room.

Speaking of the rebuild that took over my life, I can see the light at the end of the tunnel.  The spider legs are 24 gauge steel.  I used my metal brake bender to put the bend where they meet the inside of the tube.  I had toyed with using epoxy to hold the legs to the 1" square block that holds the collimation screws and mirror holder, but this really did not work very well.  So I did what I should have done in the first place: drilled and tapped 6-32 x .15" holes in each side of the 1" square and drilled through holes for them in the spider legs.  I had some 6-32 x .125" socket head screws hiding in my plastic disorganizer of screws.  I had to make locations of the holes in the spider legs and the 1" square pretty precisely, so I broke down and did it right with an edge finder. 

I often do not do it right because screwing the edge finder on and then off used to be a bit laborious.  I started using an anti-seize compound on all my threads and the edge finder just unscrews by hand now.  I am now getting the precision that distinguishes a fool with a mill, from a beginning machinist.

Holding the spider legs in position in the mill vise was at first a problem. Supporting them with parallels was a problem because without underlying support, the center drill would just bend the steel, not penetrate it.  So I bought a small piece of 1/2" aluminum that is just slightly narrower than the legs.  This provides adequate support and elevation.

1 comment:

  1. A piece of wood works well for backstopping thin material for drilling. Easy to shape or size as needed.