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A warmer, sunny day provided another chance to work outside, in comfort and good light, on the folded refractor OTA. It reach 43F, 6C after lunch. Faced with replacing the large, plywood, collimation triangle with sturdy 4mm aluminium plate I really wanted to avoid adding all that extra weight. Then it occurred to me that there was no need to use a whole triangular plate. I could use a crossbeam and, provided it was rigidly attached to the tilt strip, I would still have 3D collimation. The active, over-sized, collimation triangle would remain of the exactly same dimensions and should be just as rigid as a full 4mm plate.
The image shows the simple layout using off-cuts of square aerial tubing for the crossbeam and packing piece. I used coach bolts to ensure there would be no rotation of the collimation bolts when the butterfly nuts were turned. Coach bolts have a square shank under the heads to stop them turning. They may have a different name in different countries. A bolt always has a plain length of shank. While a screw is threaded almost all the way to the underside of the screw head. The stiff, collimation resistance springs are seen in the downward view below.
By coincidence the thickness of the tilt strip is slightly under the clamping depth of the top coach screw's square shank. So there is room for slight tilt, without play, at the coach screw head at the top of the vertical tilt strip. If the play should prove annoying in practice I shall add a rubber tap washer behind the tilt strip. I have an extra degree of tilt adjustment independent of collimation by adjusting the nuts on the top coach screw. The tilt strip is gently cranked at top and bottom to make the screw alignment perpendicular to the back plate.
A downward view of the 2nd mirror. I haven't yet decided how best to anchor the lower point of the tilt strip. So it is presently resting on a piece of firm sponge while I play with collimation.
A shot though the objective with the folding mirrors almost collimated. The idea is to center both mirrors in the objective while simultaneously centering their reflections and the focuser in each other. The overlapping circles should all match from the focuser end as well.
The 1st mirror and focuser from the inside of the backplate.
The vertical distance between the centers of the focuser and the 1st mirror should match that of the objective and the 2nd mirror. This would mean that the on-axis rays are parallel.
The OTA is seen on first complete assembly of objective, both folding mirrors and the focuser. The blue line indicates the folded optical path. The OTA now weighs 34 lbs or 15.4kg as shown. Removing the bayonet mounted objective will save having to carry about 10lbs or 5kg. Its removal allows the OTA to be stored compactly on its nose.
The 475 yard, distant trees were much sharper than before but suffering from serious thermal effects in the bright sunshine. I am hoping to find some snap-on lid, food containers which match the
diameters of the mirror retaining shells. These will protect the mirrors
during storage.I later added a short length of Porsa tubing bolted to the lower tube of the 'shoulder.' By using a power tool burr on the backing disk, to make room for the domed coach screw head, and two rubber tap washers the 2nd mirror cell lay flat on the tilt strip. I now had finely adjustable and very stable location of the 2nd mirror. Just as with the 1st mirror there was no shake or slack at all.
By rolling the OTA gently on a thin rod I discovered the balance point is centered on a vertical passing through the base of the 2nd mirror shell. Which I later confirmed by hanging the OTA from the strap of a digital, luggage weighing scale.
I have the option of adding two vertical Porsa tubes dropped from the shoulder to reinforce the balance point for altazimuth, Dobsonian, altitude bearings. This would require additional joints, cutting the long base tubes and some disassembly. So I am really not sure if it is worth it. The alternative is to use plywood or alloy plate, or both, as vertical surfaces for mounting the tubular altitude bearings.Or the OTA could be mounted equatorially on the Fullerscopes MkIV via a plate or crossbars fixed onto the framework. Bottom mounting would bring the heavy objective and 1st mirror nearest the saddle. Though side mounting also minimizes the overhang. The square frame work does not offer the usual convenience of hinged, tube rings. Though a long DIY dovetail rail and twin housings could be adapted to use the MkIV's 24" long saddle.
Click on any image for an enlargement.
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2 comments:
Wow - you're really motoring now, Chris. Whereas in England we have dreary cold and wet weather which is not at all helpful for OTA projects such as finishing the mounting pier which require any outdoor work. Hence I am reduced to fiddling around with flocking material to reduce internal reflections from my tilt plates and hence maximise contrast. Do spare a thought during your construction as to how you are going to achieve the flattest of flat black finishes on the internal surfaces, especially those on the direct light path. Some people talk of crushed walnuts, others of sawdust - and of course all of this needs to be in place well before the precious flats are introduced. At least with your open framework, good access can be achieved.
Onwards and upwards,
Andrew
Hi Andrew
Wet weather is just the amateur astronomer's god reminding you not to plant your pier in a permanent puddle. Only willow and alder are suitable for that.
"Motoring" is not how I would describe my plodding re-invention of the wheel. "Plodding" seems far more appropriate. I find the journey is as important as reaching any particular destination.
When I discovered that Hammerite made black 'smooth' paint I asked them about ground nuts. Their response was not positive and the cost would not have been peanuts. The trick is to avoid glancing reflections. The water based blackboard paint which I have used recently has rather poor mechanical adhesion on bare, shiny aluminium.
"Upwards" may still be a trifle premature given that my "first, folded light" was to admire the local woods.
Chris
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