Sunday, December 31, 2017

Next Steps on Baby Reflector Rebuild

I ordered a new, not chipped diagonal.  I had originally intended to get a 28mm minor axis elliptical mirror, but I was a little concerned that my current 25mm minor axis diagonal was not catching enough of the light cone.  There are three downsides of going bigger on a diagonal:

1. Reduced light gathering power because you are obscuring the mirror.  But failing to capture the entire light cone from the primary also reduces image brightness.  Too big is bad; too small is bad.

2. At low power, with strong ambient light you get a more noticeable black spot in the field of view.   This problem is substantial with any low power eyepiece.  I have an 85mm eyepiece that is unuseful on my big reflector until it is at least dusk for this reason.

3. Reduced resolution.

However, the 25mm diagonal is 10.8% of the primary mirror diameter (a good measure of the problems 1-3, above); 28mm is 13.6%; 31.75mm (which is what I ordered) is 17.2%.

Next step after that, as I mentioned, is how to mount it.  The 1/4"-20 threaded hole in the bottom of the tube does not inspire confidence, especially in PVC.  I looked at the various 144mm ID rings and they were expensive enough to make me look at making my own.  The skills I will develop are ones  that I want to have; what if you need to rebuild industrial civilization by yourself?

Two strategies:

1. Start with a 5" OD 1/4" wall tube 2" long, cut in half, then machine a flat spot on both sides of each hemicylinder to drill and tap holes.  Use 8-32 thumbscrews through those holes  to lock them on the tube.  Machine a flat spot down one side for the 1/4"-20 threaded hole.  This is a very solid material to hold to a camera tripod.

2.  Start with a 5" OD 3/16" wall tube 2" long.  This will allow the telescope to slide inside.  Use 3/8"-16 bolts to secure the telescope tube inside.  Machine a flat spot down one side for the 1/4"-20 threaded hole.  This is a very solid material to hold to a camera tripod.  Less work, not quite as elegant looking. 

Which choice?  Depends what tube size my local metal store, Gem State Metals has in stock.

At some point, this madness is going to draw me into making my own equatorial mount.  Did this last time, largely of oak.  This time, aluminum and steel.

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