The next stage was to turn down the 51mm
secondary holder components to match my 47mm secondary minor axis. The reduction in diameter
will only be about 3mm. Or roughly a 1.5mm cut below the existing
surface. This will not affect the v-shaped slots in the sides of their
vane mounting hub. They can easily be deepened if desired to bring the
screw heads out of the light path.
The image shows the progress so far. It was [almost] freezing in my unheated shed at 33F, +1C! First I tightened the collimation screws against their central spring until everything was only just tight. I was relying entirely on the central screw which holds the secondary holder assembly to the 19mm aluminium stalk. If this screw had stripped or broken then the secondary parts would have flown off the lathe chuck. So I chose a low speed and very fine feed to reduce the impact on the screw. Fortunately there is a small spigot on the alloy stalk to locate the collimation disk. This must have taken the brunt of the forces during the cutting in the lathe.
The entire assembly proved to be wildly eccentric when first turned in the chuck by hand. This required some careful thought and adjustment before I dared to start the lathe under power. The cutting tool wanted to take the most material away from the shortest section of the cylinder just below the 45 degree mirror mounting slope. [i.e. The curved surface facing the camera in this image.]
So I adjusted the collimation nuts again to produce a perfect witness mark along the longest section of the secondary holder cylinder. I then wound the tool out and rotated the cylinder until the shortest section was facing the cutting tool. I could now feed in the tool until I produced a very light cut. From that point on I tool only took a couple of thousands of an inch per cut using the finest feed. I kept this up until the entire cylinder was machined all over. This was achieved at about the 49mm diameter.
From that point on, every time a cut was completed I would turn the chuck by hand to bring the longest section of the secondary holder cylinder to face the tool. I would then run the tool along the cylinder without any tool adjustment. Just to ensure the witness mark was perfect each time. Had the line been broken at one end or the other I would immediately know that the assembly was moving out of perfect alignment. Fortunately it did not.
Having completed the machining of the secondary cylinder down to 47mm I moved onto the collimation disk This was also very eccentric at first but was cleanly cut all over the circumference by the time I reached 49mm diameter. I stopped at 48mm as the washers under the adjusting nuts were just beginning to strike the cutting tool. Which, by the way, was a small, diamond-shaped ceramic bit clamped into a CNC type holder. The tool cut the plastic very cleanly. Leaving a smooth surface and producing a fine and continuous thread of plastic swarf. My wife commented on the strong smell during machining but my nose was so cold I could not smell anything.
Cleaning the sharp edges left by the machining was a simple matter of running some emery paper along. The grey colour suggests PVC bar but I have no real idea. [An update: According to a reliable source the spider assembly is by Barrie Watts at Beacon Hill telescopes and the material is machinable nylon.]
The next stage is to turn down the vane disk to a concentric 48mm diameter. Then make a new brass shroud to hold the secondary mirror gently onto the 45 degree slope of the secondary holder. My thinking is that the extra 1mm in diameter, over the 47mm m.a. of the secondary mirror itself, will help to provide a clean obstruction disk. Any screws sticking out beyond this disk would have produced their own diffraction effects. So masking them with a larger disk is likely to be more beneficial than worrying about achieving the absolute minimum obstruction.
The trend these days is to glue the secondary to a smaller mounting block. So that the secondary forms its own circular obstruction. I prefer to hold the secondary with a shroud for greater security. This will inevitably be of slightly large diameter than the mirror itself. There must also be fixing screws on the circumference of the shroud. The 48mm diameter of the collimation disk should help to mask them out of the light path. There is certainly logic to making the secondary support quite a bit smaller than the mirror itself. This means that any misalignment of the support components does not increase the size of the secondary's shadow on the primary mirror.
I checked the old shroud on the secondary holder and found the angle was completely wrong. A quick check with a 45 degree set square proved the point. So the secondary holding cylinder will have to go back in the lathe on the vertical slide to have the 45 degree slope milled true. At this point I haven't a clue whether the angle was wrong to start with or I have made it wrong by turning the cylinder down at a slight angle. Though it really isn't important at this stage as it is easily rectified.
The image shows the progress so far. It was [almost] freezing in my unheated shed at 33F, +1C! First I tightened the collimation screws against their central spring until everything was only just tight. I was relying entirely on the central screw which holds the secondary holder assembly to the 19mm aluminium stalk. If this screw had stripped or broken then the secondary parts would have flown off the lathe chuck. So I chose a low speed and very fine feed to reduce the impact on the screw. Fortunately there is a small spigot on the alloy stalk to locate the collimation disk. This must have taken the brunt of the forces during the cutting in the lathe. The entire assembly proved to be wildly eccentric when first turned in the chuck by hand. This required some careful thought and adjustment before I dared to start the lathe under power. The cutting tool wanted to take the most material away from the shortest section of the cylinder just below the 45 degree mirror mounting slope. [i.e. The curved surface facing the camera in this image.]
So I adjusted the collimation nuts again to produce a perfect witness mark along the longest section of the secondary holder cylinder. I then wound the tool out and rotated the cylinder until the shortest section was facing the cutting tool. I could now feed in the tool until I produced a very light cut. From that point on I tool only took a couple of thousands of an inch per cut using the finest feed. I kept this up until the entire cylinder was machined all over. This was achieved at about the 49mm diameter.
From that point on, every time a cut was completed I would turn the chuck by hand to bring the longest section of the secondary holder cylinder to face the tool. I would then run the tool along the cylinder without any tool adjustment. Just to ensure the witness mark was perfect each time. Had the line been broken at one end or the other I would immediately know that the assembly was moving out of perfect alignment. Fortunately it did not.
Having completed the machining of the secondary cylinder down to 47mm I moved onto the collimation disk This was also very eccentric at first but was cleanly cut all over the circumference by the time I reached 49mm diameter. I stopped at 48mm as the washers under the adjusting nuts were just beginning to strike the cutting tool. Which, by the way, was a small, diamond-shaped ceramic bit clamped into a CNC type holder. The tool cut the plastic very cleanly. Leaving a smooth surface and producing a fine and continuous thread of plastic swarf. My wife commented on the strong smell during machining but my nose was so cold I could not smell anything.Cleaning the sharp edges left by the machining was a simple matter of running some emery paper along. The grey colour suggests PVC bar but I have no real idea. [An update: According to a reliable source the spider assembly is by Barrie Watts at Beacon Hill telescopes and the material is machinable nylon.]
The next stage is to turn down the vane disk to a concentric 48mm diameter. Then make a new brass shroud to hold the secondary mirror gently onto the 45 degree slope of the secondary holder. My thinking is that the extra 1mm in diameter, over the 47mm m.a. of the secondary mirror itself, will help to provide a clean obstruction disk. Any screws sticking out beyond this disk would have produced their own diffraction effects. So masking them with a larger disk is likely to be more beneficial than worrying about achieving the absolute minimum obstruction.The trend these days is to glue the secondary to a smaller mounting block. So that the secondary forms its own circular obstruction. I prefer to hold the secondary with a shroud for greater security. This will inevitably be of slightly large diameter than the mirror itself. There must also be fixing screws on the circumference of the shroud. The 48mm diameter of the collimation disk should help to mask them out of the light path. There is certainly logic to making the secondary support quite a bit smaller than the mirror itself. This means that any misalignment of the support components does not increase the size of the secondary's shadow on the primary mirror.
I checked the old shroud on the secondary holder and found the angle was completely wrong. A quick check with a 45 degree set square proved the point. So the secondary holding cylinder will have to go back in the lathe on the vertical slide to have the 45 degree slope milled true. At this point I haven't a clue whether the angle was wrong to start with or I have made it wrong by turning the cylinder down at a slight angle. Though it really isn't important at this stage as it is easily rectified.
Click on any image for an enlargement.
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