Some more progress on the secondary assembly this weekend. I'm waiting on a couple of parts that I am having made up (ie. vanes and vane hub plates), but I have assembled the parts I have ready to give an idea of how it will work. I took a long time thinking about the whole
secondary assembly and basically designed it from the ground up. Having used Newtonians in the past (including the one I made recently), I wanted to avoid the problems inherent in typical secondary mirror spider/holders; my main objection being that they are an exercise in
frustration when it comes to collimation!
I wanted a secondary mirror support system that holds the mirror firmly in place with no tendency to move over time, and also provides simple and precise collimation.
In thinking through the design I came up with the following basic criteria:
- Since my secondary mirror has a minor axis of 100mm it is thus similar in size to a small Newtonian's main mirror, and that I should keep this in mind in order to break free of the secondary hub paradigm; viz. to avoid collimation points that act on the base of an extended 45 deg hub, where by adjusting the mirror angle it also moves the mirror laterally as well.
- I realised that 3 point tip/tilt positioning is not even necessary provided that fine adjustment in rotation is provided. You only require tip/tilt in one axis (being parallel to the minor axis of the secondary mirror); this together with fine control in rotation provides full collimatable functionality.
- I wanted to incorporate an offset vane configuration (described in Texereau's "How to Make a Telescope") as it provides maximum stiffness (including resistance to rotation).
- I wanted to design out any residual forces that act to pull the mirror out of collimation; ie. any cantilevering forces that the mirror may exert on the assembly and rotational forces (less important) due to the off-centering of the mirror / hub design.
You can see (in the first image) a closeup of my implementation of the tip/tilt supports. Note the assembly is in mirror face down
orientation (though the mirror is not attached; it will be siliconed on to the bottom plate in the image):
- There are two non-adjustable support points (seen on the left). Note that contact is made in a line between the two points to avoid stress on the bottom plate in particular.
- The adjustment in the third support (on the right) is provided by the nut in combination with a wave spring washer between the plates. This provides approximately +/-2mm of adjustment. This should be more than adequate provided the vane holes, etc are set accurately in place; if more adjustment is needed this can be accomplished by inserting extra washers.
- Once collimation is achieved the 2 fixed support point nuts can be tightened, which makes the whole thing extremely rigid.
The other two images show the assembly (less mirror and vanes / vane hubs). Also shown are the two titanium 8mm rods that will be
supported by the vane hubs and hold the vanes:
- One of the images shows the end of the 8mm rod held in the push/push screws of the micro-collimator. This will provide fine adjustment in rotation.
- Near the end of the main tube you can see a horseshoe shaped stainless steel counterweight that I made yesterday. This will bring the centre of mass of the adjustable sub-assembly (ie. mirror / plates / tube / micro-collimator) in line with the vanes, thus avoiding any tendency of the mirror to tilt due to cantilevering forces. Further, the horseshoe shape will be oriented to negate the weight bias on one side and so avoid any tendency to rotate.
Well, that's the theory anyway ...