Last updated on
4/11/
2006

You are visitor number

 

Balancing an SCT

With the exception of the Celestron Ultima 2000, GoTo scopes operate with their drive motors directly connected to the altitude and azimuth axes so if they are unbalanced you probably won’t notice it (with a U2K you will notice it immediately). But all scopes need to be balanced in order for the motors to work properly - an unbalanced scope may cause the motors (especially the slew motors) to fail prematurely. Also, an unbalanced scope is likely to introduce tracking errors that you won’t notice with visual observing but will be very evident when you are doing astrophotography. Balancing a scope is not much of a problem for beginning visual observing, since the counterweight that came with your scope is likely to be heavy enough to balance most 1.25” eyepieces.

Balancing becomes more of an issue if you use 2” eyepieces (which are significantly heavier than 1.25” EPs) and especially an issue (and more difficult) if you add a Rich Field Scope or if you start to do astrophotography - the photo equipment you will hang on the top or rear cell of the scope will be too heavy for the supplied counterweight to offset.

A basic consideration here is that since your scope is free to move in two dimensions -horizontally and vertically - you need to balance it in both those dimensions. Commonly, horizontal balance is called “static balance” and vertical balance is called “dynamic balance”.

Static Balance
If the only additional weight you plan to add to your scope is on the rear cell, such as an electric focuser, or heavier 2” eyepieces and the associated 2” diagonal needed for them, and that additional weight isn’t too large, you are probably fine with establishing static balance. There are several reasonably simple steps you can take to statically balance your scope.

One important step you can take is to purchase a Dew Cap for your scope which will add weight to the front of the scope, and also serve as a lens shade to help to block stray terrestrial light (street lights or your neighbor’s porch light).  And if you use the Celestron Fastar assembly it will protect your correcting lens from potentially expensive damage (click here for more information about that). But at some point you may need to add balance weight to your scope. 

The Celestron U2K comes with a balance weight bar on the bottom of the scope and you can purchase Celestron’s set of additional counterweights but it sells for about $65, which every U2K owner I know considers to be an unreasonable price. If you need more static balance weight than that provided with the U2K you can easily make additional weights for less than $10 in parts from your local hardware store - see the Balance Weights You Can Make page for more information.  Other scopes don’t, for some odd reason, come with a balance weight system.  For NexStar scopes Celestron sells a Counterweight Bar Assembly for $84.  Meade sells a similar balance weight system for LX scopes, for around $90.

However, when mounting significantly heavy items such as a rich-field scope or heavy astrophotography equipment (especially if it is mounted atop your scope) you will really need to establish dynamic (two-dimensional) balance. For illustrations on how to balance a scope, go to the Balancing a Telescope page of the Starizona web site, or the “Balancing an SCT” page of Janet Miller’s Jan’s LX-90 web site.

Dynamic Balance
Dynamic balance means that your scope is balanced vertically.  Another way of looking at this is that when your scope is horizontal there is a balance between the additional weight that is above the optical axis as there is below it. Heavy eyepieces do project weight vertically above the optical axis but not all that much above it, so that’s why I said above that for them a static balance is probably fine.  But if you add a lot of weight on the top of your scope you not only need to add corresponding weight below your scope, but that counterbalance weight needs to project as far below the optical axis as the weight above the scope projects above the optical axis. Anyway, dynamic or vertical balance means that if you aim the OTA straight up, with the Dec drive (or Dec clutch, in the case of a U2K) disengaged it should be balanced and not pivot back to aiming straight down. (Obviously, be careful when checking this <grin>.)

So, with heavy weight mounted atop your scope you can’t achieve dynamic balance by simply adding weight to a counterweight rail nestled against the bottom of the scope.  You will need a two-dimensional, or 2-D counterweight system.

The least expensive 2D counterweight system is the Mounting Rail system sold by Jim Henson’s ScopeStuff.  This system comes in a variety of configurations; a 2D system with a single 2.5 lb. weight sells for about $50.

A second 2D system is made by Losmandy and sells for about $70 with a single 2.5 pound weight included (shown here on the left, mounted on my scope).  Be advised that Losmandy’s numbering system for their model numbers, calls this the WS C8/M8 (for an 8” SCT), using that model number for both Celestron and Meade 8” SCTs.  In fact the counterbalance bar is the same for a Meade or Celestron SCT but Celestrons have a different mounting hole arrangement at the front cell than Meades and thus the bar needs a separate adapter for a Celestron.  So Losmandy does package the counterweight system differently for a Celestron or Meade, and if you order one you need to be sure they plan to send you the correct version for your scope!

A third similar system is the Milburn 2D Counter Weight System (shown here on the right); like those described above its counterweight slides along a (dovetail) rail and is adjustable in two dimensions.  It is sold for both Celestron and Meade SCTs by Ken Milburn’s Bonney Lake Astro Works; the “C8” (Celestron 8” SCT) version sells for $129 plus S&H.  It’s more expensive than the Losmandy counterweight system but does appear to be more substantial (the Losmandy counterweight system is lighter-weight than their dovetail plate although I haven’t found that to be a problem).  The Milburn system is very heavy-duty and is worth checking out even though that polished aluminum finish isn’t in my opinion as attractive as the black anodized finish Losmandy provides.  (Ken Milburn can black-anodize his system for about $25 additional, although you would need to wait until he has a batch of items to be anodized since that’s not the standard finish he produces.)

Note that these two-dimensional counter weights require careful positioning since the counterweight or its shaft may strike the base of the scope’s fork mount when you swing the scope to the down-stop position.

Unfortunately, a problem with both the Losmandy and Milburn systems (but not the ScopeStuff system) is that they apparently assume you always want to balance heavy items.  Unlike the ScopeStuff system , neither the Losmandy nor the Milburn system include a simple attachment that only carries a lighter one-dimensional weight along the bar, for those times when you don’t have something heavy mounted atop your scope. I’ve discussed this with Losmandy (whose 2-D counterweight system I own) but they don’t seem to understand the issue.  I had to purchase an extra sliding block for their counterweight bar and make a small weight to add to it (see the Balance Weights You Can Make page for more information on this).  Presumably you can make a similar light weight system for the Milburn 2D system.
 

___________________________________________________________________________________
 

[Home] [SCT Basics] [Observing] [SCT Tips] [Photo Basics] [Glossaries] [Links] [SITE SEARCH]