Saturday, March 3, 2018

New Finder on Big Bertha

I think that I have mentioned this project, but if not...

Big Bertha has a straight-through 8x50 University Optics finder with illuminated crosshairs.  I wanted a right angle version, because it is a darn nuisance to keep bending your neck from the right angle of a reflector eyepiece to the straight-through finder.

1. Nearly all the RA finders on the market are much shorter tubes than that University Optics finder, so they would not fit in the existing rings.

2. Nearly all the new ones have dovetail bases to make it easy to remove them.  This is mostly of value if you commonly take the OTA off its base.  Not being the Incredible Hulk, this is not a concern, but I might as well use that dovetail base.

3.  It really helps if you know which side of the base is the side where the finder slides into the dovetail.  I got that backwards.  The bottom:

The top:

And this matters, why?  I thought that I would need a spacer to fit into that gap:

So I carefully machined that spacer out of acetal.  I could have done in aluminum, but then would have wanted to paint it black.

So, all that work, and when I try to assemble it, it won't work.  Discard this block that the CNC mill spent three hours making.

4.  This is the old assembly.  DobSTUFF made it as part of the rebuild on Big Bertha several years ago, just before my stroke.  (Very bad timing.)

They do good and clever work there.  The piece of wood with the hole clamps onto a tube on the front assembly.  The wood fits into a recess in the black plate that holds the rings.  When I removed it, I discovered it was made of acetal, not aluminum.  To mount the Celestron dovetail base, I drilled and tapped a 10-32 hole, then put a 10-32 x 1.5 socket head through through the base and into the acetal.  Being paranoid, I then put a wingnut on the far side of the acetal plate.

5. Tightened everything down, then discovered the carriage bolt that holds the acetal plate to the wooden part sticks up too high for the base to be flat on it.  So I used a Forstner bit to relieve the carriage bolt hole 3/16" and now it is flush.

6. Now installed:
The electrical wiring is a safety strap, in case something fails unexpectedly.  I will probably remove it when everything shows itself trustworthy.  By adjusting where it clamps on, I was able to get the same balance as before without changing the balance weights at the rear of the scope.

7. The Celestron RACI (Right Angle Correct Image--not reversed or upside down) 9x50mm finder is lovely.  Crisp, sharp optics.  Instead of the traditional illuminated crosshairs, it has four crosshairs, producing a tiny square in which to center your objet d'amour of the night.   (For my regular readers: there are no mil-dots, nor bullet drop compensator dials.) 

UPDATE: Two annoying aspects: the diagonal can be rotated so it is in the same general plane as your eyepiece; nice, but the thumbscrews that lock it are identical to the ones that adjust point of aim.  It is unlikely you would ever loosen two point of aim screws at the same time so there is little chance you will loosen both diagonal adjustments at the same time and watch it fall out of the finder.  Still, using a different thumb screw would be better.   Also, unlike older finder rings where you have both front and rear adjustment screws, here there are three only, all in the back, and one of them is not a thumbscrew like the other two.  It seems intended to be turned with a screwdriver.  This may be a coarse adjustment.  I do not immediately see how this works although it does seem to.

8. The University Optics 8x50mm finder (which has T. rex tooth marks) has gone into the rings on the 5" Photon Instruments refractor; the old finder was not illuminated.  I tried to align both finders with the main scopes this evening, but the only reasonably distant targets kept disappearing behind snow showers.

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