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Balancing the LX's OTA is important to both tracking performance and pointing accuracy. An unbalanced OTA will cause additional flexure in the fork mount which varies according to where the telescope is pointed, this will confuse the LX200's internal electronic 'Map' of the heavens and pointing suffers as a result. T-Point software can help get around this by 'mapping' the flexure at many points across the entire sky, but this compensation is then only valid for the particular load the telescope carried at the time. Accurate balancing would probably be almost as effective effective, and provide balance is maintained the improvement is valid for any instrument load. Good balance is also essential to prevent undue strain being applied to the drive system. An out of balance OTA will cause greater mechanical wear and tear in the worm/worm-wheel sets due to the increased torque required to move the telescope. In addition, the particular design of DC servo-motor used in the LX200 responds to commands by attempting to drive the telescope at a fixed speed to reach it's target, if for some reason the telescope can't move (perhaps because of gross imbalance or an obstruction) then more and more current is supplied to the motors until either they do - or something melts! If you are lucky, in this situation the fuse blows, but more often than not the motherboard heroically sacrifices itself to save the fuse. In any event, the best philosophy should always be to minimise the electrical power required to move the telescope, and this is best achieved by accurate balancing.
Balancing a complex system such as the 2-axis equatorial wedge-mounted LX is simple in theory, but it does need to be tackled in a logical and progressive way, starting with the optical tube itself and working through each axis in turn. It often appears easy to achieve good balance for one particular pointing position only to find that the telescope is then out of balance when pointed to another part of the sky. This will not happen if the telescope is balanced such that the center of gravity lies at a point midway between the two DEC bearings. To achieve this, each component needs to be balanced separately about it's pivoting axis, and it thus becomes clear that a single weight cannot accomplish this and a number of separate weights need to be distributed at various points.
The strategy for achieving balance should follow this order of preference:
It's likely that you have more than one configuration of additional instruments to consider (perhaps different cameras, piggy-back loads, guidescopes etc.,) and in this case you will need to go through the balance process for each one to determine how best the load can be carried. It will usually be possible to arrive at a compromise position for each, or you may end up carrying an instrument you perhaps don't need every time to simplify the number of balance weight configurations. Of course, to simplify this pricess you need accessory mounting hardware that allows you to mount instruments and also counter-balance weights in the most flexible manner. Simple brackets (e.g., piggy-back camera mounts) often give you little choice about where it's placed, relying as it does on the existing accessory screw positions. My own system was designed specifically to offer the greatest number of options for mounting any type of instrument load, that is why there are two dovetail bars (one above and one below the OTA), and facilities to mount additional weights most anywhere on the telescope.
Just how accurately balanced does the telescope need to be? A fairly crude test is to release the RA and DEC clutches and randomly point the telescope around the sky, this will show up gross imbalance. Ideally, it should stay where it's put, but there are a number of factors that might affect this. The most obvious is friction in the DEC bearing (the RA is usually not a problem). The DEC bearings are greased nylon sleaves and there is normally sufficient friction here to overcome a slight imbalance, and in some cases even significant imbalance, but slight pressure with one finger on the rear cell should still set the telescope moving. The pressure required to move the telescope in opposite directions is also a good indicator of imbalance. If the DEC bearings on your scope show appreciable friction then you need to investigate the cause and correct it, they may be mis-aligned or in need of lubrication. The opposite scenario is where the nylon DEC bearings have been replaced by roller or ball bearings whereupon the friction is very much reduced. In this situation the OTA moves so easily in the DEC axis that only a very slight imbalance will cause the tube to swing freely. Yet another method is to listen to motors when the telescope is slewing, and to watch the current draw on the LED block as the scope is moved back and forth in each axis, but this is less sensitive than looking for free movement with released clutches.
I offer my 10" LX200 as a working example. Note that the method described to achieve balance refers to my own dovetail bar / balance weight system, you will have to make other arrangements if you have a different system.
For Astrophotography:
For CCD imaging:
For Visual:
Before going into practical details of how to achieve correct balance it might be useful to look at the options provided by my own weight system. The diagram below shows the three principle components (Bias, Trimming and 2-D weights) together with their optional positions pn the OTA.
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Note that the trimming weight can be placed in one of four positions beneath the OTA, to the left or right of center line and front or back of the tube. In addition, this weight can be extended some 5 inches beyond the ends of the OTA thus maximising the leverage for a given weight. The bias weights can be attached to one of two 1/2" diameter tubes located either side of the lower dovetail bar, and anywhere along the length of the OTA. The 2-D weight is attached to a sliding bracket and can be mounted on either of the dovetail bars, the weight's distance from the center of the optical axis can be adjusted.
Of course, with no instrument load there is little need for a sophisticated balancing system! It may be necessary to compensate for exceptionally heavy eyepieces, a metal dew-shield, or a heavy camera body (like the Nikon F) mounted at prime focus. These simple needs can generally be acommodated using the trimming weight alone.
Problems arise when a substantial instrument load such as a guidescope is mounted piggy-back, here the loading is off-axis with regard to the center of the OTA and it is this off-axis position that causes difficulties. The cardinal rule of balancing is that the weight distribution has to be equal either side of *every* axis of movement.
To continue the example of my own 10" LX200, the heaviest item is the 90mm Vixen refractor guidescope. This has to go on top of the OTA for reasons of accessibility (I need to look through it) and if it were mounted on the lower DT bar the 201XT won't clear the base of the forks. Problem number one is that the DEC bearing is not actually in the center of the tube but positioned somewhere towards the rear, this is because the primary mirror is the heaviest item and the DEC pivot was placed at approximately the center of gravity with no other load on the OTA. Therefore, any additonal weight which spans the length of the OTA is going to make it front-heavy (there being more weight added in front of the DEC pivot than behind it). For this reason, my dovetail bars have a substantial cutaway on the underside at the front ends which returns the net effect back towards a neutral balance. This trick won't work with the guidescope of course, so again, this load will tend to make the OTA front-heavy unless it's weight was positioned equally either side above the DEC pivot - which would make it stick out the back so far it would be very inconvenient to use. To counteract this then, some additional weight needs to placed below and to the rear of the DEC pivot, and this is the purpose of the Bias Weights (Note that I use 2 bias weights because the 90mm refractor on it's ALT-AZ ring mount is quite heavy).
The Bias Weights perform two functions:
The first step for the astrophotography configuration above, is to install all the instruments and accessories in their respective positions on the OTA:- the Guidescope, Piggy-back camera +lens, dewshield, trimming weight (at rear of OTA), and also the Nikon F. The piggy-back camera can be adjusted along the length of the OTA depending upon which lens is attached as this affects the total weight of the camera and also how far towards the rear it can be positioned without the OTA itself getting into the picture frame. I normally position it towards the front to enable the use of wide-angle lenses. With the OTA horizontal, it's now possible to estimate how heavy the bias weights need to be to counter-balance the refractor. This can be by trial and error, but remember that the bias weights are restricted to about 2" diameter and they will be placed somewhere along the auxilliary mounting tubes. The weights do not need to be an exact measure, it is possible later to fine-tune the balance by adjusting placement of the bias weight(s) with respect to the DEC pivot (they will slide on the mounting tubes), by moving the piggy-back camera back and forth along the lower dovetail bar, by adjusting the trimming weight, or by installing the 2-D weight behind the piggy-back camera on the lower dovetail bar. What *is* required is that the total weight attached below the OTA is equal to that attached to the top of the OTA, and this will be checked now. By adjusting the weights as described, with the OTA in horizontal position you will be able to achieve balance such that it stays horizontal with the DEC clutch released. Try moving the OTA up and down just a few degrees by gently pushing it, the effort required to set it moving should be the same for either direction. Now, point the OTA at the zenith and check that it stays there. If it doesn't, but instead tends to continue to fall backwards beyond the zenith, then the *total* weight mounted below the OTA is less than that above it. On the other hand, if it won't stay pointed at the zenith and tends to drop back down towards horizontal, then the *total* weight mounted below the OTA is greater than that above it. Both of these conditions are corrected by adjusting the weight of the bias-weights. The diagram below shows how each of these items contribute to the total weight loading on the mounting in both horizontal and vertical positions:
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Note that the Nikon F mounted at prime-focus and also the dew-shield are far less of a problem because their weight is equally distributed about the longitudinal center line of the OTA (i.e., it is on-axis), which means they only shift the center of gravity along the length of the OTA and not above or below the center line (which would move the center of gravity off-axis). This is easy to compensate for.
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