MAIN MIRROR BEARING SUPPORT
Description:
Mirror shift (or 'flop' or whatever else you wish to call it) is a problem mainly associated with commercial SCTs which use a moving main mirror to achieve focus. In these designs the center part of the mirror mount is a tube, which itself slides along the primary baffle tube and is held in axial alignment by it. This design demands a minimum clearance between the two tubes to prevent binding due to shrinkage or expansion caused by changes in temperature, and also simple mechanical clearance for the moving parts. This clearance gap is normally filled with a special viscous lubricant which (when new) does a reasonable job of preventing the mirror moving freely from one side to the other within the limits of said clearance. However, only a very slight movement of the mirror is enough to cause a noticeable shift in the image position, and this becomes worse over a period of use as the normal focussing action causes the lubricant to be scraped off and deposited at the extreme ends of the primary baffle tube. A quick 'n dirty remedy for noticeable image shift can sometimes be achieved by racking the focussing mechanism from one end to the other, thus re-distributing the lubricant (in theory anyway, I had no success whatsoever with this). Altogether this is a quite unsatisfactory mechanical arrangement. The image shift produced by mirror movement is not too serious for visual use (at low-medium powers anyway), but for photography or CCD imaging at any power it results in a ruined exposure. The LX200 is no different in terms of this problem than any other make of SCT, even expensive telescopes like Takahashi suffer from it to some degree.
Strategy:
What I have tried to do here is control the off-axis mirror movement which is the source of the image shift. My strategy here was to use three pairs of pre-loaded bearings attached to three pivoting levers at the front end of the mirror support tube. These bearings run on the surface of the baffle tube and act as guides to keep the mirror on-axis. Natural 'spring' in the lever maintains the pressure of the bearings in contact with the baffle tube (the 'pre-load'). The diameter of the device had to be as small as possible to prevent obstruction of the light path, thus very tight tolerances were essential as only small movements were available in the confined space.
These pictures show the assembled device. The main components are the central tube, which is made in 2 parts which screw together, the front part holds the bearings and levers, whilst the rear has a cone section at the front end. The 3 levers are spaced at 120 degrees around the front part of the tube, and the rear of the levers which support them bear against the cone section at the back. As the 2 parts are screwed together the wedging action of the cone forces the rear of the levers to move upwards - and thus the bearings on the front of the levers are forced downwards into contact with the surface of the baffle tube. The rear part of the tube has six socket screws which clamp it firmly to the sliding mirror tube. So, with the rear part clamped turning the front part pre-loads the bearings to whatever degree necessary to inhibit unwanted mirror movement. The 6 tiny roller bearings (3/16" O.D. x 1/8" bore) provide a friction-free movement which does not interfere with the focussing action.
Here the device is shown fitted in place on the scope, and the bearings have been adjusted to remove the clearance. The angle of this shot suggests the device is of larger diameter than it actually is, I do not believe there is any significant light loss attributable to fitting the device (it is smaller in diamter than the secondary baffle anyway).
In Use:
Well, at this point all I can say is 'watch this space'! Certainly, there appears to be no detrimental effect on the focussing action, but whether the image shift has been reduced in any way will have to wait until this interminable bad weather clears away. At this point I have only applied support to the front of the mirror tube - whether this is sufficient alone, or whether it would be necessary to also add support to the rear of the tube I don't know.
Addendum:
First test was not very encouraging. Visually, there was a large amount of image shift due to the focussing action which I attribute to increased resistance of the mirror to move (i.e., the mirror mount flexed and took up the clearance before the focusser would push it along). Interstingly, focussing in one direction shifted the image one way, and and reversing the direction shifted it in the other - but with a noticeable delay ('dead spot') between the two. The image would stay in place once it had 'shifted' and further tweaks of the focusser in the *same* direction caused no more shift. Tom me, this symptom indicates there is still excessive clearance at the rear end of the mirror mount which the device is unable to correct. I will have to remove the device but I will also continue to experiment...
These pictures show the assembled device. The main components are the central tube, which is made in 2 parts which screw together, the front part holds the bearings and levers, whilst the rear has a cone section at the front end. The 3 levers are spaced at 120 degrees around the front part of the tube, and the rear of the levers which support them bear against the cone section at the back. As the 2 parts are screwed together the wedging action of the cone forces the rear of the levers to move upwards - and thus the bearings on the front of the levers are forced downwards into contact with the surface of the baffle tube. The rear part of the tube has six socket screws which clamp it firmly to the sliding mirror tube. So, with the rear part clamped turning the front part pre-loads the bearings to whatever degree necessary to inhibit unwanted mirror movement. The 6 tiny roller bearings (3/16" O.D. x 1/8" bore) provide a friction-free movement which does not interfere with the focussing action.
