How to remount a Schmidt-Cassegrain tube

Wondering what to do with a perfectly good 1990s LX200 scope with a dead drive? Re-mounting the optics on a German equatorial head can give it a new lease of life.

Written by Martin Mobberley

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Image Credit: 
Martin Mobberley
Before and after...

The 1990s saw Meade mass produce thousands of fork-mounted Schmidt-Cassegrains on their infamous LX200 fork. These instruments, known as the ‘Classic’ LX200s, offered the only truly affordable Go-To system at the time. Many of these old telescopes can now be found for sale at giveaway prices on internet forums and eBay.

The only problem with Meade’s package is that the drives burn out and cease to work over time, so that the scope loses its Go-To capability. Optically, the scopes are fine. But the tiny plastic moving gears and glued-on encoder wheels, which monitor motion and position in the mount, suffer from wear and tear. The heaviest models, with tubes weighing up to 30kg (66lb) and loaded with CCD cameras, piggyback scopes and dew caps, took their toll on the drives. When slewing at a maximum 5° per second with spin rates up to 14,000rpm, these didn’t last long.

What’s more, the 1992 vintage motherboards have components that have been obsolete for more than a decade. With the one-year warranty long expired and the drives on the blink, it will set you back £500 to fix the problem, because you’ll need a re-conditioned motherboard and both sets of motors.

However, there’s another way around this, and that is to re-mount the old scope tube on an equatorial mount. Many people prefer these to fork mounts. They enable you to view the polar regions and are far less prone to vibration. What’s more, the telescope can be precisely balanced in right ascension so that there’s less strain on the drive, and the same mount can be used for more than one telescope.


What you'll need to get started

Aluminium and steel - Our dovetail was milled from a solid block of aluminium, which is easily acquired from ironmongers or scrap metal dealers. The same goes for the steel bands. Gluing felt to the inside of the bands will avoid scratching the tube.

Assistants - Telescope tubes can be heavy and slippery things, especially when they are being prised out of a tight fork unit. A beefy assistant or two to help with the tube removal and re-installation phases can be a real help.

Bolts and taps - To screw the tube strap’s bolts tightly down to the dovetail bar, you’ll need to cut threaded holes into the bar. Screw tapping sets for a variety of hole sizes are readily available from hardware stores, along with metal-cutting lubricants and compounds.  

Milling machine - Making a custom dovetail does require some machining with a mill. But it is surprising how many people have access to such equipment, either via knowledgeable friends in local astronomical societies, or friendly engineering company employees willing to help out.

Tools - You’ll need screwdrivers, drills, spanners and maybe even a crowbar for the project. A couple of stepladders and a plank will also be useful for supporting the weight of the telescope tube when it is being prised out of the fork mount.


Weight watchers

To rescue your tube in this manner, you’ll need to find a suitable equatorial mount. Consider the weight issue carefully. Compared to an 11-inch Celestron 11 tube, you might not think the 12-inch Meade Classic tube would be heavier, but it is. A modern Celestron 11 tube weighs around 12.5kg (27.5lb), whereas the Meade 12-inch Classic weighs around 18.5kg (40.7lb). It’s a similar story with the 14-inch models. The Celestron weighs 20.5kg (45lb) while the Meade weighs 31kg (68lb).

You’ll need an equatorial head that is going to be heavy-duty enough to cope with the weight. We opted for a Sky-Watcher EQ6 Pro. With one of these, you can resurrect a 12-inch scope and get all the advantages of an equatorial mounting for under £1,000. At 18.5kg (40.7lb), the 12-inch LX200 tube is towards the upper weight limit of what this mount can take, but in practice the EQ6’s drive gears and bearings can easily take a balanced 20kg (44lb) payload. It’s the dovetail bar and tube interface that are the weak points.

With our new mount ordered, we turned our attention to the old one. LX200 systems aren’t designed to be disassembled but, if you’re determined enough, the tube and fork can be separated. Fortunately, the older Classic models have a black cradle that connects to the fork tine bearings and simply screws to the tube at the mirror end. Once three vital screws are removed you just need to prise the tines apart and slide the tube out. Modern LX200 AutoStar GPS models are far trickier to remove. However, a US amateur called David Illig has a website detailing how to do just this with these models.

There’s no dovetail bar on the Classic LX200, and a simple Vixen-style dovetail plate wouldn’t safely hold the weight. So we asked a local machine shop to make a few components, which cost £20. The first item was a job for their milling machine: a friction-fit dovetail bar that would slide snugly into the EQ6’s mount head. This should be a milled slab of aluminium that is such an exact fit that it never tilts sideways out of the channel.

The second item we asked them to make was some steel bands to fit tightly around the tube and bolt onto the dovetail bar. Find out where the centre of gravity of your tube is, because this will affect how long the dovetail bar is and where the bolt positions for the bands will be. Get the machine shop to make a right-angle bend at each end of the band and drill two holes so you can screw it to the dovetail.


Welcome home

Once you have these items, fit the bands around your tube, screw them to the new dovetail bar and slot this into the mount head to secure your Classic tube.

Conversions like this can be carried out on many fork-mounted scopes. If the tube is heavier than the LX200 at 18.5kg (40.7lb), it would be wise to employ thicker steel bands or use a Homeyer Cradle to avoid the tube flexing too much. This will ensure your views aren’t blurred while you’re enjoying Go-To views with your resurrected scope.


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Step 1

Check whether your tube has the three screws on the rear arm, shown here. If so, removing it from the fork cradle should be fiddly but not too tricky. If these aren’t accessible, removing the tube will be harder and the declination drive may need disassembling.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Step 2

Unscrew the screws on both sides. Get an assistant to hold the tube and use a plank to support the weight. Even when the screws are removed the tube will still be clamped tight. Prising the fork tines apart with brute force should enable you to slide the tube out.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Step 3

Measure the dovetail slot dimensions on the equatorial mount and determine where the balance point of the tube will be when all the accessories are in place. Decide how long the dovetail needs to be and find a friend with a milling machine.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Step 4

With your freshly milled dovetail block, drill and tap some screw holes into it. These will be used to tightly screw down the steel hoops that hold the weight of the tube. Ensure the bolts won’t foul the mount head or its screws.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Step 5

Measure the circumference of the LX200 tube very carefully and bend the bands of 2-3mm thick steel to screw onto the new dovetail block. The bands are shown here with the tube removed. Any slack will be disastrous when the weight shifts around.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Step 6

Assemble the EQ6 head with the counterweights in place and pointing downwards. With the dovetail and straps tightened firmly around the telescope, slide the tube in and tighten the locking screws. Finally, check for any movement of the tube in the rings.

 


This 'How to' originally appeared in the January 2008 issue of BBC Sky at Night Magazine.

 

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